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Multi-storey timber frame buildings a design guide Rob Grantham and Vahik Enjily BRE Centre for Timber Technology and Construction

Contributing authors Martin Milner – Chiltern Clarke Bond Mostyn Bullock – Chiltern International Fire Geoff Pitts – The Palmer Partnership

constructing the future

ii Prices for all available BRE publications can be obtained from: BRE Bookshop 151 Rosebery Avenue London EC1R 4GB Tel: 020 7505 6622 Fax: 020 7505 6606 email: [email protected]

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BR 454 ISBN 1 86081 605 3 © Copyright BRE 2003 First published 2003 BRE is committed to providing impartial and authoritative information on all aspects of the built environment for clients, designers, contractors, engineers, manufacturers, occupants, etc. We make every effort to ensure the accuracy and quality of information and guidance when it is first published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain. Published by BRE Bookshop by permission of Building Research Establishment Ltd and TRADA Technology Ltd Requests to copy any part of this publication should be made to: BRE Bookshop Building Research Establishment Watford WD25 9XX

Cover photographs: front: a seven-storey timber frame building on Brook Street, Nottingham. Structural frame by Prestoplan Purposebuilt. back: TF2000 building at Cardington.

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iii

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Contents Preface Scope Readership Acknowledgements

iv iv iv iv

Foreword

vi

Chapter 1 Introduction 1.1 The feasibility study 1.2 The TF2000 project 1.3 The TF2000 building 1.4 Multi-storey timber frame market

1 1 1 2 3

Chapter 2 Structural stability and robustness 2.1 Introduction 2.2 Building layouts and structural stability 2.3 Minimum stability requirements: for general strength and stiffness 2.4 Minimum stability requirements: for robustness 2.5 Building Regulation requirements for accidental damage 2.6 Timber frame design against disproportionate collapse 2.7 Lessons learnt from TF2000 test building

5 5 6 6 8 9 9 13

Chapter 3 Fire safety 3.1 Introduction 3.2 TF2000 compartment fire test 3.3 TF2000 stair fire test

16 16 17 17

Chapter 4 Differential movement 4.1 Introduction 4.2 Regulations, codes and standards 4.3 Sources of movement 4.4 Research conducted on the TF2000 building 4.5 Reduction in movement by design 4.6 Provision for movement

19 19 19 20 22 23 24

Chapter 5 Achieving performance 5.1 Introduction 5.2 Timber frame benefits in multi-storey construction 5.3 Benchmarking at TF2000 5.4 Construction process lessons from TF2000 5.5 Build tolerances 5.6 Safe construction procedures

26 26 26 27 29 30 32

Appendix A Regulatory guidance for fire safety Introduction Means of escape Internal fire spread (linings) Internal fire spread (structure)

33 33 34 36 38

References and further reading

48

iv

Preface

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Scope Although this publication will be of general interest to those concerned with the design, construction and performance of timber frame structures the text focuses on aspects specific to multi-storey buildings of platform frame type construction. Design and best practice guidance is provided on: ❐ Structural stability and robustness (disproportionate collapse) ❐ Fire safety ❐ Differential movement ❐ Construction benchmarking ❐ Construction process and building tolerances These subjects, which are of particular importance for multi-storey timber frame structures, were investigated as part of the TF2000 project, a collaborative R&D venture lead by BRE and TRADA Technology with joint funding from the Government and industry. Lessons from the TF2000 project are covered in the guidance to provide context to the recommendations and principles of best practice provided. Guidance is also closely linked to codes, standards and building standards for England and Wales, Scotland and Northern Ireland. General principles of timber frame design and construction are not included in this document unless in support of the issues outlined above. Common specifications for the construction and design of timber frame buildings are given in the TRADA publication Timber Frame Construction. Readership This book will be of interest to all building professionals responsible for the design and construction of multi-storey timber frame buildings. Building control, local authorities and insurance companies will also benefit from the normative guidance provided for timber frame buildings. Since this publication documents the results and lessons learnt from research conducted on the world’s first six-storey timber building using the platform frame technique of construction, the information contained within will also have global appeal for regulators and code writers. Acknowledgements The TF2000 partnership is unique; international competitors throughout the supply chain have united to create a peerless example of the 90’s approach to innovation, pooling state-of-the-art knowledge on technology, product and process development. This project would not have proceeded without the support of the following organisations acknowledged as Partners and Associates to the TF2000 project. This recognises the greater contributions received from TF2000 Partners. The Building Research Establishment Ltd (BRE) and TRADA Technology Limited (TTL) would like to thank them for their financial support and technical co-operation without which this design guidance could not have been provided. Two major contributors have been jointly in charge of the project from the onset: Vahik Enjily of BRE Centre for Timber Technology and Construction (CTTC), and Simon Palmer of Palmer Partnership (previously TRADA Technology Ltd).

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v

TF2000 Partners ❐ Office of the Deputy Prime Minister, previously Department of Transport and Local Regions (DTLR) ❐ P J Steer Consulting Structural Engineer ❐ Stewart Milne Timber Systems

❐ Century Homes ❐ Prestoplan Purpose Built ❐ Walker Timber

TF2000 Associates ❐ Andy Collett Associates ❐ British Gypsum ❐ Chiltern Clarke Bond ❐ Crown Timber ❐ David Carr Consulting Engineer ❐ Devon Fire & Rescue Service ❐ F R Shadbolt & Sons Ltd ❐ Hazlin Doors Ltd ❐ Highfield Consultancy ❐ ITW Paslode ❐ Nexfor Ltd ❐ PACE ❐ Panel Agency ❐ Rugby Joinery UK ❐ Simpson Strong-tie ❐ The A Proctor Group ❐ Trus Joist MacMillan ❐ UKFPA ❐ Woodbridge Timber

❐ Brick Development Association ❐ British Gypsum Isover ❐ Chiltern International Fire ❐ Cullen BP ❐ Department of the Environment for Northern Ireland ❐ Forestry Commission ❐ Hanson Brick ❐ HM Fire Service Inspectorate ❐ Luton & Bedfordshire Fire & Rescue Service ❐ Marley Building Materials ❐ NHBC ❐ Palmer Partnership ❐ Pinewood Structures ❐ Scottish Executive ❐ SODRA Timber ❐ TRADA ❐ TTF (UKTGC) ❐ UKTFA

Design Guide steering group Production of this design guidance has been scrutinised by a steering group of leading industry experts on timber frame construction; many of them have been involved actively in the TF2000 project. The contribution of the following group members is gratefully acknowledged: ❐ Steve Ashton. Prestoplan Purpose Built ❐ Mostyn Bullock. Chiltern International Fire ❐ Andy Collett. Andy Collett Associates ❐ Paul Graver. BRE Bookshop ❐ Geoff Harding. ODPM ❐ Phillip Key. PACE ❐ Steve Limb. British Gypsum ❐ Hugh Mackay. Stewart Milne Timber Systems ❐ Paul Marsh. UKTGC ❐ Seamus McCrystal. Building regs NI ❐ Simon Palmer. The Palmer Partnership ❐ Bob Selmes. Forestry Commission ❐ Paul Stollard. Building Standards, Scotland ❐ Rab Taylor. Wren & Bell ❐ Bryan Woodley. UKTFA

❐ Barbara Bedding. TRADA Technology ❐ Antony Burd. Fire Building Regulations, ODPM ❐ Charles Grant. Walker Timber, UKTFA ❐ Peter Grimsdale. Peter Grimsdale Associates ❐ John Haynes. NHBC ❐ James Lavender. Chiltern International Fire ❐ Ian Loughnane. Prestoplan Purpose Built ❐ Jonathan MacMullen. Greenframe ❐ Jim McBride. Century Homes ❐ Martin Milner. Chiltern Clarkebond ❐ Geoff Pitts. The Palmer Partnership ❐ Peter Steer. P J Steer Consulting Engineer ❐ David Sulman. UKFPA ❐ Mark Wilson. Panel Agency

vi

Foreword The construction industry has undergone a major step change in its bid to improve the quality of buildings and reduce their environmental impact.

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One of the leading protagonists in this area has been the timber construction industry. The Timber Frame 2000 (TF2000) project is a shining example of the commitment by industry and Government to the technological progress of timber frame buildings. Situated in the Building Research Establishment’s (BRE) large building test facility at Cardington, the TF2000 six-storey experimental building has been subjected to a rigorous programme to test the performance of both UK and overseas multi-storey timber frame constructions. Issues such as construction process benchmarking, stability, differential movements, disproportionate collapse, compartmental fire and timber stair performance have all been assessed. Test results have proven that timber frames are well suited to multi-storey construction. The TF2000 building was a world-first and is already having a huge impact on the advancement of multi-storey timber frame construction. The Government’s commitment to sustainable development places great importance on the use of construction materials with low environmental impact, such as timber, and further encourages the use of sustainable resources of timber. This guidance document gives the industry the necessary tools for designing and constructing timber frame buildings in a safe and economic manner and also provides industry with a new and lucrative market. As sustainability moves from being a ‘buzz-word’ to reality, I hope that timber frame buildings will play an increasing role in buildings of the future.

Brian Wilson MP Minister of State for Energy and Construction

1

Chapter 1

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Introduction

Modern timber buildings constructed using the platform frame technique were first introduced to the UK from Sweden in the 1920s. Since then, their popularity has grown and so has our knowledge and experience of their performance in service. It was the government’s confidence in their performance that lead to a change in the fire safety regulations in 1991, allowing for the first time the number of storeys to potentially reach eight (in England and Wales) without any additional fire resistance requirements other than those existing for many four-storey buildings.

Beck Street, Nottingham

This created new opportunities for the timber frame industry to expand its market in private and social housing, compartmented flats and commercial projects for hotels, nursing homes, student accommodation and other commercial buildings. Four-storey timber frame buildings have been constructed extensively since the mid-eighties for various occupancy classes. British Standards, meanwhile, have been or are being updated or revised, where appropriate, to cover such practices. However, in certain respects current code recommendations are extrapolated. For this reason, the need of guidance for buildings with five or more storeys has recently been recognised as vital. Consequently, a co-operative project was set up in October 1995 by the Office of the Deputy Prime Minister (ODPM) then Department of the Environment, Transport and the Regions (DETR), the UK timber frame industry, BRE and TTL, entitled Timber Frame 2000 (TF2000). It followed a feasibility study on such buildings, involving close consultation with the construction industry, ODPM and building professions [1].

1.1 The feasibility study The aim of the feasibility study was to review the design issues relating to safety, and to examine the construction requirements for multi-storey timber frame buildings. Case studies involving seven real projects were investigated during 1994 and 1995. The feasibility report [1] reviewed and summarised design and construction requirements. Much emphasis was placed upon the economical and technical issues and potential of taller timber frame buildings (four to eight storeys) as a desirable method of construction. The research requirements and design options for full-scale tests on at least a five-storey test building were identified.

1.2 The TF2000 project Consequent to the feasibility study's findings and conclusions, the TF2000 project started in October 1995, preparing the way for proposed tests on the first six-storey timber frame building of its kind in the world. Two main committees were set up to deal with managerial and technical issues of the project [2]. The UK timber frame industry and ODPM, together with representatives from TTL and BRE, constituted the Management and

2

Multi-storey timber frame buildings – a design guide

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Technical Committees of TF2000. Operational Task Groups were set up to deal with specific areas of investigation. The test building (see left) was a six-storey platform frame type of residential construction, with four flats per floor around a service core built in BRE’s Cardington hangar. Brick cladding was selected and C16 timber was used. To assess productivity targets, prefabrication and in-situ construction methods were used for comparison.

Construction of the TF2000 building at BRE Cardington

The programme of research work is defined in the table below. The main responsibility of the Management and Technical committees were: ❐ Schematic architectural and engineering design of fullscale test-building. ❐ Definition of the test-building to enable costing and tender action. ❐ Definition of test programmes ❐ The design and construction of the six-storey timber frame test-building [3]. ❐ Conduct identified test programmes [4][5]. ❐ To bring all aspects of construction together, from Regulations to Research to Design to Construction and to include whole Building Evaluation. ❐ Regulatory harmonisation between English/Welsh, Scottish and Northern Ireland Building Regulations. ❐ Produce authoritative guidance documents. Recommended research programmes Value engineering and process benchmarking Differential movement. ● Structural performance: whole building stability (racking stiffness). ● Fire: stairs and compartmentation performance. ● Acoustics: walls and floors [6][7]. ● Disproportionate collapse. ● Guidance documents. ● ●

1.3 The TF2000 building The TF2000 project team concluded the following, in relation to the broad principles of the test-building: ❐ Concentration upon multi-occupancy residential construction. ❐ The test building should simulate typical compartmental flats. ❐ Platform-frame construction and cellular layout, along with the use of communal lifts and stairways should be used. ❐ Building height considerations and Fire Regulations in England, Wales, Scotland and Northern Ireland should be harmonised. ❐ The proposed test building should be six storeys and should be clad with clay brick. ❐ The test-building should be a real-life building incorporating all the current design and construction practices. ❐ Fire performance of communal stairways and lift shafts made of timber should constitute one of the major assessments in the test building. Three alternative architectural schemes were considered from which a ‘commercial’ brief was derived [8]. The scheme was then reviewed, storey-bystorey, to remove elements not required to meet the needs of a prototype building and the core research programme. The building comprised:

Chapter 1: Introduction

3

❐ six storeys; ❐ four flats per storey, each with two bedrooms, kitchen, bathroom, living room and hall; ❐ a plan-aspect ratio of approximately 2:1; ❐ platform-type timber frame; ❐ timber stair and lift shaft; ❐ single timber stair; ❐ brick cladding.

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The following general specifications for key elements were agreed: ❐ The ground floor is notionally a concrete slab-on-ground. ❐ External walls consist of two layers of 12.5 mm standard (Type I) plasterboard with a vapour control layer and 89 x 38 mm C16 homegrown timber studs with glass wool insulation in between. The sheathing is 9 mm Oriented Strand Board (OSB/3). The cavity is 60 mm with single-leaf 1/ -brick cladding connected with stainless steel ties to the timber frame. 2 ❐ Internal loadbearing walls consist of C16 homegrown timber stud groups at 600 mm centres with two layers of 12.5 mm plasterboard and 9 mm OSB/3 sheathing to one side, where needed for wind bracing. The internal non-loadbearing walls consist of C16 homegrown timber studs with one layer of 12.5 mm plasterboard to each side. The compartment walls are twin-leaf using C16 timber studs with glass wool insulation in between and OSB Type 3 sheathing is used where wind resistance is required. ❐ Compartment floors consist of two layers of plasterboard ceiling (19 mm Plank 12.5 mm plasterboard) on joists with mineral wool in between. OSB Type 3 is used as a floor deck. Floating floors contain proprietary resilient battens with plasterboard and Type P5 chipboard. Timber joists with low moisture content of 12% were specified for floors 1 to 4; the fifth floor comprises timber I-beams and metal webbed beams. ❐ The roof comprises trussed rafters with hipped ends supporting concrete interlocking tiles on felt and battens.

1.4 Multi-storey timber frame market The UK construction market, in common with most construction markets world-wide, has come under increasing pressure to reduce costs and enhance client value, at the same time maintaining or improving quality and performance standards. Maximising land usage, particularly on brownfield (inner city) sites, is also dictating increased building heights for residential 45% buildings, from two to three storeys to four to eight storeys. In the five to eight33% storey markets, research showed that a significant market existed for five and six storeys but there was a much lower demand for seven and eight storeys at the 15% time (see left). The construction processes involved in 2% 5% adopting steel and concrete framed 7 or 8 6 5 4 3 buildings are well known to the Number of storeys construction supply chain. The drive for Market potential for timber frame buildings in 1996 market demands of improved efficiency, Number of storeys and relative share of potential market better quality and performance, faster construction and better-cost control has

4

Multi-storey timber frame buildings – a design guide led to the questioning of how we build. Other pressures to change the way we build have come from increased concerns about the impact of the construction process on the environment and local communities. This has forced responsible and leading clients to take stock of the sustainability impacts of how we build, as well as the operation and maintenance of buildings. The market response has been to investigate the use of timber frame as the solution to these demands.

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The considerable savings that can be gained from off or on-site prefabrication and the use of efficient erection techniques make multi-storey timber frame very attractive. The professional designers and surveyors, and large sections of the contracting and building industry, still have to be made aware of timber frame's potential and be convinced of its advantages. This has and continues to be achieved through Timber Frame 2000, which has brought enormous publicity for timber frame buildings. The UK construction demands for residential accommodation, from housing, student accommodation and hotels is significant. For housing alone the Government predictions suggest that more than 3.5 million new dwellings will be required from 2002 to 2016. Currently, the market for buildings of four storeys and above is estimated to be 25% and it is predicted that the market will increase to as high as 45%. For the student accommodation and hotel markets, the trends are already evident for five-storey inner city highdensity units. This presents a significant challenge to the UK construction supply chain with its diminishing labour force and increased business performance demands. Furthermore, client requirements for higher building standards and the increasing demand from regulatory improvements, particularly in thermal insulation, acoustic performance and health and safety issues are pushing the industry to reconsider on-site methods of construction and to investigate other ways of building homes. Trinity Village, Preston

courtesy Westbrook Design

5

Chapter 2 Structural stability and robustness

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2.1 Introduction This chapter provides guidance for timber frame engineers on the stability design issues that are not covered in current British Standards or other guidance documents. Structural stability guidance is supported by tests conducted on the TF2000 building [9] and is limited in scope to currently publicised timber platform frame construction up to seven storeys in height.

Ion building

courtesy Chiltern Clarke Bond

Advice is provided for three different stability design conditions that the timber frame structural engineer considers during the engineering of a multistorey building: ❐ Strength and stiffness design ❐ Robustness design based on good practice but not case specific calculations ❐ Robustness design based on case-specific calculations The approach to the first stability check is covered in BS 5268 which, with the exception of timber frame racking resistance, is not restricted to the storey height of a building. The second and third stability design conditions can be dependent on the storey heights. The second stability condition is achieved through good practice and is applicable to all building types. For most timber platform frame layouts, the robust detailing is inherent in the method of construction and the engineer’s duty is to ensure that the layouts are not vulnerable from poor construction or minor accidents that would cause failure disproportionate to the cause. For multi-storey frames, it may be necessary to check assumptions of diaphragm action and general tying forces to ensure that the building design is appropriate for anticipated load and construction conditions. General principles of timber frame design, construction and detailing are given in Timber Frame Construction [10] although some caution is required as the details presented in that document relate to buildings up to four storeys. Specific design at each level is required to transfer forces. The last robust design condition is typically called ‘disproportionate collapse design’. The Building Regulations cover the type of buildings that are to be checked to ensure that disproportionate collapse does not occur when subjected to a defined set of load and support conditions. Guidance given in this chapter also refers to the responsibility of the timber frame engineer for informing the building design team of the implications of frame stability as it can influence the design of claddings, services and supporting structures.

6

Multi-storey timber frame buildings – a design guide

Principles for achieving a robust building Eurocode general principles on building robustness Select a structural form which has low sensitivity to the hazards considered.

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Select a structural form and design that can survive adequately the accidental removal of an individual element or a limited part of a structure, or reasonable localised damage.

Timber frame response Timber platform frame techniques are inherently robust through the interconnectivity of walls and floor panels. Backed by over 30 years of experience and further tested and demonstrated on the TF2000 building through full-scale tests [11][12]. Trussed rafter roofs have proven robustness against damage

[13].

TF2000 demonstrated the integrity of platform frame construction and the natural robustness of the structural form given by the conventional Avoid as far as possible structural systems connectivity of walls and floors. which may collapse without warning. Timber frame buildings typically use compatible materials materials that Provide structural forms that can be tied are easily fastened together. Care is needed to understand the interaction together. with other parts of the building such as cladding. Ensure that layouts and plan arrangements provide returns and intersecting walls and floors. Adopt compatible materials used in the structure and ensure adequate interaction.

2.2 Building layouts and structural stability Structural layout and form of a multi-storey building will influence the stability of framing elements under normal vertical and horizontal loading. The layout will also influence the overall building robustness. Based on guidance presented in the BS 5268 and Eurocodes, the following section presents general principles on how timber frame can provide robust design solutions.

2.3 Minimum stability requirements For general strength and stiffness The timber codes, BS 5268 and Eurocode 5, provide appropriate guidance for strength and stiffness of the components that make up the multi-storey timber frame. However, clarification is needed on how horizontal resistance, referred to as racking resistance in timber frame, can be calculated using the current codes BS5268: Parts 6.1 and 6.2 as they currently state that they are limited to four storeys only. This limit has not been given for strength reasons (the wind forces can be increased through location as much as height) but related to the lack of information on robustness and experience of the use of platform timber frame above four storeys. There have been concerns that the deflection of timber frame under horizontal forces requires additional limits for buildings above four storeys. The current limits suggested by BS 5268: Parts 6.1 and 6.2 take a stiffness limit of the panel height divided by 300 as being acceptable. This value is clearly theoretical as such movements are likely to lead to cracking of finishes that have not been experienced in timber frame. The TF2000 tests demonstrated that the actual stiffness of a timber frame building is significantly higher than the code limitations. The TF2000 findings were:

Chapter 2: Structural stability and robustness

7

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Examples of building layouts and the influence on a building stability Open-plan layouts

The open plan nature with no transverse structure is not appropriate for platform timber frame and additional structure is required for stability design.

Open-plan with additional structure

The introduction of portal frame elements can provide solutions to open plan layouts but attention is needed to stiffness limits and connectivity of the framing types as well as differential movement of different materials.

Cellular layouts

Cellular layouts are best suited to multi-storey platform timber frame. Internal load bearing walls may need to be strengthened to carry horizontal forces.

Where party walls separate the structure into separate units, the engineer should ensure that the horizontal forces can be taken by each unit or transferred across the

party walls. For reasons of acoustic performance, there is a limit to the quantity of structural ties permitted across a party wall as shown in Timber Frame Construction [10].

❐ The addition of plasterboard lining to a sheathed timber frame building increased the lateral stiffness by a factor of 3.3. ❐ After applying masonry cladding, the building stiffness was recorded as 17.7 times that of the bare timber frame with sheathing. While these test results are specific to TF2000 and must not be used in any other layout, the TF2000 building has provided evidence that BS 5268 racking design principles are adequate for strength and stiffness for buildings higher than four storeys and that there is no reason for normal cellular platform frame to have additional deflection limits imposed. It is considered, therefore, that the use of BS 5268: Parts 6.1 and 6.2 can be extended to the design of platform frame timber buildings up to eight storeys.

8

Multi-storey timber frame buildings – a design guide For structures or layouts that are unable to provide sufficient racking resistance, the use of structural engineering calculations and bespoke designs solutions, such as portal frames, can be considered. The deflection limits should be appropriate for the structure and finishes and justified on a case-by-case basis. In these situations, the deflection limit may be at least height / 500 for the framing to keep to the same stiffness as the timber frame elements. Consideration of differential movement is also required – see Chapter 4.

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Multi-storey buildings are more likely to include tall panels for ground floors, such as restaurants and reception areas at hotels, or top floor rooms, such as penthouse apartments or plant rooms. Since BS 5268: Part 6.1 is limited to 2.7 m height panels, BS5268: Part 6.2 guidance shall be used for panels above 2.7 m (but limited to 4.8 m) on all forms of building. In any horizontal load condition, the wind is likely to be the dominant load. However, any building should have sufficient horizontal strength and stiffness for resisting a minimum horizontal long-term force equivalent to 2.5% of the vertical dead + live load. Transfer structures

It is common to have transfer structures below multi-storey timber frame construction, such as basement car parks or open plan reception areas. The substructures, sometimes made from different materials, such as concrete and steel, are to be designed for adequate support to the timber frame. Consultation with the timber frame engineer and substructure designer is needed to ensure appropriate support conditions. Design for the construction period

Broadway Plaza Birmingham

courtesy

Caution is required in timber frame multi-storey construction where the racking resistance requires plasterboard or vertical load from the roof to contribute to the stiffness and strength. During construction the building can be exposed to wind loads before vertical loads are applied or any plasterboard is fixed. It is acceptable to reduce the wind load in accordance with BS 6399 for the construction period. The stability of the building during construction shall be considered as a design requirement. It is good practice to ensure that at least the lower half of the framing has adequate racking resistance without contributions of plasterboard, as reliance on temporary bracing in large multi-storey construction has proven to be more complex than low-rise projects.

Chiltern Clarke Bond

A design check may be required on the vertical elements, such as studs in party walls where the design of the stud assumes lateral stability is achieved through the fixity of the plasterboard. On multi-storey, the construction process period can be increased and construction loading should be considered prior to the plasterboard being fixed. It is also possible that significant vertical loads can occur during construction through storage of construction materials, such as plasterboard packs, on the building. This aspect shall be considered in the design process with temporary restraint or additional members added as appropriate.

2.4 Minimum stability requirements For robustness For general robustness of timber frame it is accepted through experience and case histories that timber platform frame robustness is achieved through the application of standard detailing that has become established

Chapter 2: Structural stability and robustness

9

over the last 30 years and more. Timber Frame Construction [10] provides typical detailing and material sizes to achieve proven robust solutions. For multi-storey timber frame, checks on the nailing schedules are required to ensure that appropriate transfer of the horizontal and shear design loads can be achieved at each platform level.

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Specific design checks for accidental damage require the engineer to determine whether the building form is not unduly sensitive to damage, caused accidentally or otherwise, such that collapse or partial collapse is not disproportionate to the original cause. The principle for design against accidental damage is primarily based on ultimate strength and not serviceability. In addition, as with all construction materials, there is no defined period for the stability of the building subject to accidental damage. Increased design factors for accidental damage conditions are provided in BS 5268: Part 2. The extent of the damage to be considered is not material dependent but one of agreed philosophy. Specific design checks may be required under the Building Regulations, commonly referred to as design against disproportionate collapse.

2.5 Building Regulation requirements for accidental damage Background and compliance

Regulations exist for accidental damage design limitation based on the principle that in the event of damage occurring to a building partial collapse is acceptable providing it is proportionate to the cause. The UK Building Regulations, (Part A for England and Wales, Part C for Scotland and Part D for Northern Ireland), cover this aspect under the heading of Disproportionate Collapse. At the time of the TF2000 project, and writing of this publication, the UK Building Regulations require that only buildings with a height above four storeys are to include a check in the design process to ensure that any collapse is proportionate to the cause. Revised Building Regulations may alter this approach but the principles presented here will cover all building heights if applicable. While timber platform frame construction has a history of excellent performance against instability and there are no known examples of accidental damage disproportionate to the cause, it is the duty of the engineer to ensure that the structure being designed is not unduly sensitive to minor accidental damage or misuse.

2.6 Timber frame design against disproportionate collapse In common with guidance documents for steel and concrete it is neither practicable nor necessary to provide a definition of the cause of the event that could lead to disproportionate collapse. As stipulated by Approved Document A, the structural engineer’s approach to achieve robustness design for timber frame buildings compliant with Building Regulations is to check the structure against either the notional removal of a defined length or specific member of the structure or to design key elements to resist a force of 34 kN/m2 over that element.

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Multi-storey timber frame buildings – a design guide

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Options for disproportionate collapse design Option and compliance 1 Provide vertical and horizontal ties to resist defined forces

Timber frame option Timber platform frame can be considered as a structural form of close centred posts and beams with nominal tie resistance provided at each joist to wall stud junction. The regulations suggest high tie forces to take account of the typical wide spacing of post and beam structures using steel or concrete materials. While these tie forces could be designed to spread through the timber frame elements it is not considered a practical option, although not impossible.

2 Consider notional removal of a support member, one at a time in each storey in turn, to check that upon its removal the rest of the structure would bridge over the resulting lack of support, albeit in a substantially deformed condition, or that the risk of collapse of the remaining structure due to the removal of the member is limited to 15% of the area of the storey or 70m2 within the storey or immediately adjacent storeys, whichever is less.

This option provides the appropriate route for platform timber frame structures. Guidance on the length of notional member to be removed is provided in the following sections.

3 If it is not possible to bridge over a Can be adopted for timber frame structures when required. missing member or to limit the area at risk, the member should be designed as a protected element. The protected members (sometimes referred to as ‘key elements’) should be designed for a load of 34 kN/m2 applied in any direction.

Disproportionate collapse test

11

Chapter 2: Structural stability and robustness

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Definitions of the extent of structure being considered under the robustness design Loadbearing element type Beam

Definition of element

Extent of structure

Compliance

Primary structural support member acting alone

Clear span between supports

Notional removal or protected element

Column

Primary structural support member acting alone

Clear height between lateral restraints

Notional removal or protected element

External wall

All loadbearing walls that form the perimeter and external face of the building but not party walls

Length between intersecting walls (party walls, return walls, internal room dividers), or between key element columns. Minimum length of wall to be considered 2.4 m. There is no maximum length of wall.

Notional removal

Internal wall

All loadbearing walls within the building including party walls

Length limited to between intersecting walls or key element columns or 2.25H, where H is the clear height between lateral supports (eg floor-to-floor).

Notional removal

Floor/roof

Structural beam and decking: joists, rafters etc.

2.4 m width of the floor/roof for the clear Notional removal span between designed supports.

Clarification of structural elements used for robustness compliance Intersecting walls

Minimum length of intersecting wall to be 1200 mm in total (framed openings can be permitted but are in addition to the 1200 mm length). All intersecting walls may be either racking walls or non-loadbearing walls.

Typical intersecting wall framing detail Intersecting wall

External wall

Support walls

Substantial non-loadbearing partitions can be used where the resultant load paths can be proven and where the walls are not to be removed.

Removing floors and roof panels

Additional consideration should be given to check that if a roof or floor panel is removed, wall panels adjacent to the removed elements are stable.

Key elements: load paths

The resultant horizontal force from a key element is to be designed and detailed to be carried by the remaining structure.

Trussed rafter roof

Trussed rafter roofs have proven robustness against damage

[13].

12

Multi-storey timber frame buildings – a design guide

Definition of failure limits for the timber frame elements

Licensed copy from CIS: atkins, Atkins Plc, 01/07/2014, Uncontrolled Copy.

Under accidental actions failure of any timber structural element occurs when the following is exceeded: Element Floor/wall

Failure limit Deflections > L/30 or when deflection of the floor under consideration prevents safe egress from the building, whichever is smaller.

Comments For timber members the element will be overstressed before deflection limits are reached.

Protected element

Member over-stressed under accidental load conditions from applied load of 34 kN/m2 in any direction.

The common approach is to use engineered wood products such as LVL or Glulam.

Support member within a wall, eg studs

Member over-stressed owing to additional Additional studs may be required for load following the notional removal of one short-term strength purposes. adjacent member.

Compatibility of other materials used in the structure

The UK Building Regulations requirements for robustness refer to the whole building and not just the structural frame or loadbearing elements. Therefore, the check for robustness shall include the effect on elements supported or restrained by the timber framing. In considering cladding, the area of cladding to be checked for disproportionate collapse should be treated the same as the timber frame. This applies to brickwork as demonstrated by the TF2000 test programme. Special consideration is required for cladding, particularly the effect of window openings and movement joints that form natural breaks in brickwork. Other issues: Where the lower storeys are constructed from other structural materials (concrete, masonry or steel frame), the effect of loss of support from the underlying structure on the timber frame shall also be considered. Coordination of the removal or deflection of support structure must be allowed for in the timber frame design check. Where two engineers are responsible for different sections of the building, such as concrete frame engineer for the ground floor and timber frame engineer for the upper floors, coordination is needed to ensure that each party is aware of the impact of their robustness stability design on each other’s structure. Stability robustness designs should consider the following structural loads applied simultaneously: ❐ Full dead load (including any finishes or fixed M&E plant). ❐ 1/3 imposed floor load without floor reductions due to number of storeys. ❐ 1/3 imposed roof load. ❐ 1/3 wind: typically not needed in a timber frame check due to method of resisting wind forces through the racking resistance of walls. The removal of members each in turn is unlikely to result in the loss of more than 1/3rd of the racking resistance being removed.

13

Chapter 2: Structural stability and robustness

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Timber design modification factors for accidental damage checks

Timber design check Member strength

Strength factor Comments Duration of load factor from BS 5268 can Applicable to bending, shear and bearing. be taken as K3 = 2.00

Serviceability checks

For all members mean values

Connections

Duration of load factor on basic loads for nailed joints can be taken as K3 = 2.00, ie all long-term design values can be multiplied by 2.

Applicable to nails, screws and bolts for timber element to timber element. Not to be used on the racking resistance calculations. If BS 5268: Parts 6.1 and 6.2 are to be used under accidental load checks, an additional factor of 1.2 can be used for strength design checks.

2.7 Lessons learnt from the TF2000 test building Testing for robustness

The TF2000 test building provided proof of the inherent robustness and availability of secondary load paths in platform timber frame. Two loadbearing walls were removed, one internal and one external, to check the structural integrity of the building when required to span over a missing supporting member. The results [11] demonstrated the potential of standard platform frame timber buildings to span with only small movement over gaps formed by the removal of loadbearing supports. The following conclusions have been drawn from the TF2000 tests.

Wall panels: standard platform frame 4.2 m-long wall panels as tested on the TF2000 building were found to have sufficient strength created by the plasterboard/timber board sheathing to span unsupported. Conclusions drawn from this are that sheathed walls with no openings designed to BS 5268: Part 6.1 can be regarded as deep beams with vertical shear taken in the panel-to-panel connections and tension taken out through the sheathing material in continuation with any timber framework across panel junctions. Sheathed walls 2.4 m high and 4.2 m long can be assumed satisfactory without calculation.

The following diagrams summarise other observations and conclusions drawn from the tests.

Panel above the gap redistributes forces as a 'deep beam'

Outline of external wall panels

Diagrammatic elevations on panels spanning over a gap for a typical five-storey timber frame

Panel above the gap redistributes forces as a cantilever

4 3 2 1

Panels removed

14

Multi-storey timber frame buildings – a design guide

Floor built into wall

External wall panel

A

A

Support wall

Support wall Floor span

Nailing transfers forces

Plan on floor As initially designed

Licensed copy from CIS: atkins, Atkins Plc, 01/07/2014, Uncontrolled Copy.

Support wall Removal of support wall Section A-A Floor decking built into external wall panel

Floor decking acts as a plate element

Plan on floor Loadbearing wall removed

Diagrammatic plan of the TF2000 test floor behaviour

Floor panels: the standard platform frame detail is to build the floor such that the floor is supported on all sides, even though it is designed to span from one wall to another. The design of floors ensures that there is sufficient strength and stiffness in the direction of the span. TF2000 tests demonstrated that the floor has additional strength through the transverse capacity of the floor that is supported on the walls parallel to the span. The TF2000 test was to remove the designed loadbearing support walls. For the removal of an internal loadbearing wall, the floors spanning 3.6 m and 4.2 m wide deflected at the unsupported edge by up to 24 mm. For the removal of an external wall, the unsupported floor deflected by 4 mm. Both test results demonstrated the inherent robustness of platform timber frame. TF2000 beams

15

Chapter 2: Structural stability and robustness Using rim beams to increase resistance

On the TF2000 building, external wall panels had an additional support system of rim beams that were continuous around the building perimeter at each floor level. These can be designed to support the floor and wall above as part of the robustness design. Support conditions were detailed such that removal of the panel below does not remove the beam’s support.

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Load from floor and panel

Elevation on rim beam with the notional removal of wall below Header joist/rim beam spans across gap supported by adjacent/transverse panels or designed key elements support columns

Intersecting wall

Intersecting wall

Wall to be removed

Plan on wall panels

Plan on rim beam and supporting structure below Exploded view – support from the notional removal of walls provided by intersecting walls

Plan on rim beam

For most buildings, a rim beam will be required to provide support of the structure when notional loadbearing panels are removed. Alternatives are to have floor systems such as on the TF2000 where encastré support was demonstrated, or to use continuous or cantilever floor frame systems over loadbearing walls.

16

Multi-storey timber frame buildings – a design guide

Chapter 3 Fire safety

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3.1 Introduction To provide evidence to support multi-storey timber frame dwellings, a series of full-scale fire tests was conducted on the TF2000 building [14][15]: ❐ to demonstrate compliance with the functional fire safety requirements as defined by the relevant regulations and technical standards; ❐ to provide evidence in order to support the harmonisation of the different regulations and technical standards.

* Extracts taken from the Consultation Document on the Sixth Amendment to the Building Standards (Scotland) Regulations 1990, proposals to Amend Parts D & E of the Technical Standards, July 2000, Scottish Executive Development Department. Both pieces of legislation are now in force.

Appendix A gives a comprehensive guide to the appropriate sections of the relevant statutory guidance. Evidence in support of this harmonisation is provided within the Appendix by the cross comparison of the different regulations and technical standards. Two critical legislation harmonisations that resulted from the TF2000 compartment fire test were as follows: ❐ The use of combustible materials in separating walls now permitted up to 18 m rather that just 11 m, based on the Timber Frame 2000 tests.* ❐ The use of combustible materials in external walls within 1 m of the boundary now permitted up to 18 m rather than just 11 m, based on the Timber Frame 2000 tests.* As an outcome of the testing programme, several essential generic building practices were identified as requiring attention, a summary of these practices is highlighted below: ❐ As with any construction project, care must be taken to ensure that the plasterboard is attached using all the required fixings. Especially when more than one layer of board is being utilised, each layer must be independently fixed. Failure to ensure the correct fixing of plasterboard would result in reduced performance in a real fire scenario. ❐ Building Regulations Approved Document B states: ‘The external envelope of a building should not provide a medium for fire spread if this is likely to be a risk to health and safety.’ Correct location of cavity barriers and fire stopping is, therefore, important to maintain the integrity of the structure wherever the cavity of a building provides a medium for fire spread. ❐ With the increasing number and variety of different construction methods, not only specifically related to timber frame but to all construction technologies, the training of fire brigades to instil an understanding of construction methods, fire performance and risk would reduce the risk of service personnel being injured. ❐ As with all types of construction, the current common design practices for buildings do not address the issue of vertical flame spread from floor to floor via the windows and/or other external openings. ❐ All construction methods are sensitive to the quality of workmanship. Workmanship in relation to the success of fire safety provisions in any building is of vital importance with all efforts being made to ensure and maintain these provisions.

17

Chapter 3: Fire safety

3.2 TF2000 compartment fire test A detailed summary of the test is contained in a BRE report

Licensed copy from CIS: atkins, Atkins Plc, 01/07/2014, Uncontrolled Copy.

TF2000 Building

[16].

The primary objective of the TF2000 compartment fire test was to evaluate the fire performance of a medium-rise six-storey timber frame building subject to a severe natural fire exposure. Specific fire resistance issues of structural integrity and compartmentation were assessed during the test. Fire loads in the compartment, comprising dried timber battens, provided a realistic assessment of fire spread during the test. This enabled a quantitative appraisal of the true performance of this form of construction tested to the Fire Resistance test methods. The compartment fire test demonstrated that this construction can meet the functional requirements of the Building Regulations for England and Wales and the Building Standards for Scotland and Building Regulations for Northern Ireland for such buildings. The compartment fire test met the stated objectives of the programme. The following conclusions were drawn from an analysis of the data and from observations during and after the test. ❐ Derived values of time equivalence demonstrated that the performance of a complete timber frame building subject to a real fire is at least equivalent to that obtained from standard fire tests on individual elements. ❐ Results indicated that fire conditions in the living room of the flat represented an exposure approximately 10% more severe than a standard 60-minute fire resistance test. ❐ The test demonstrated that timber frame construction can meet the functional requirements of the Building Regulations for England and Wales, the Building Standards for Scotland and Building Regulations for Northern Ireland in terms of limiting internal fire spread and maintaining structural integrity.

3.3 TF2000 stair fire test A BRE report

[17]

gives a summary report for this fire test series.

The TF2000 building was fitted with a single stair of timber construction that was located in a stair shaft, the walls of which were of timber frame construction. The intention was that the results of the stair tests would provide data that would assist regulators in the United Kingdom to consider changes leading to a possible harmonisation of the technical guidance in support of their Building Regulations.

Post flashover fire in the living room of the fire flat

It was necessary to define at an early stage the fire performance objectives for a stair in such a residential building during a fire situation. In meeting the fire safety requirements of the Building Regulations, the fundamental consideration for the stair is as follows: The stair has to remain usable for firefighting after initial evacuation of occupants immediately at risk and for subsequent evacuation by the other occupants of the flats who are initially advised to remain in their dwellings.

18

Multi-storey timber frame buildings – a design guide In the event of a fire in the TF2000 building, the stair will be used by the fire brigade, upon attendance at the scene, to gain rapid access to the building to remove any people who are immediately at risk from the fire. Upon completion of this duty, the stair will provide the access for the fire brigade to fight the fire from inside the building. Once the fire has been brought under control or extinguished the stair would then be used to complete the safe evacuation of other occupants.

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The stair must remain usable during all these operational phases. It must continue to support its design load for the duration of the incident and must not itself contribute significantly to a state of fire development that would render the stairwell space inaccessible to firefighters. In terms of fire, the most onerous situation was regarded as one where the fire is actually in the stair itself. A fire that starts and grows in the stair may arise due to materials being left or stored in the stairwell that are then either accidentally or purposefully ignited. It was recognised that a large fire load in the stairwell could result in the development of untenable conditions for means of escape from heat, smoke and toxic fumes. This could happen at an early stage independent of the inherent fire load of the actual stair enclosure and stairs. With these points in mind, it was proposed that the stair in the TF2000 building should demonstrate a significant resistance to becoming involved in a fire when subjected to an appropriately severe fire source that is in intimate proximity to exposed timber components of the stair. The resulting fire (including any contribution from the stair construction) should not cause the loadbearing capacity of the stair to be reduced below a serviceable level and should not cause a breach of the compartmenting elements of structure enclosing the stair. The test demonstrated that the specific timber type and treatment used for the experiment together provided an appropriate level of fire performance to satisfy functional fire safety objectives for a stair in the residential building. More detailed information is given in reference [17]. As a consequence of this research project, regulatory authorities may wish to consider the use of an appropriately treated timber stair as adequate in terms of meeting the functional requirements of the UK Building Regulations, on a case-by-case basis.

19

Chapter 4 Differential movement

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4.1 Introduction

Brick cladding on TF2000

With all structural and non-structural components of a building, detailing should allow for any potential movement across connections. This movement between two connected parts of a building is known as differential movement. Allowance for differential movement is of particular importance in a timber frame building where dry internal environments cause the timber to shrink across the grain and therefore reduce the overall height of the construction. When timber is delivered to site it may have a moisture content of up to 20% that reduces down to 12% and below after the building has been occupied for a few years [18]. Other building components, such as brick or stone cladding, may increase in height during the building’s design life, owing to both reversible and irreversible moisture related movement and thermal expansion. This differential movement between the cladding and timber frame, and even between the timber frame and internal stairwells of mixed construction, must be allowed for in design. Buildings less than four storeys height can be easily designed to accommodate differential movement using the guidance and standard details given in other publications and standards [13][19]. Because differential movement is cumulative with increasing height, accommodating this movement is much harder in multi-storey buildings. Designs for differential movement in timber frame buildings of four or more storeys should attempt to reduce the sources of movement as much as possible. This chapter provides a summary of the sources for movement in the timber frame and cladding and gives guidance on designs and detailing for multi-storey buildings. Sections 4.2 to 4.4 provide background material; the actual guidance for conducting designs is contained in sections 4.5 and 4.6.

4.2 Regulations, codes and standards Designing for differential movement within the fabric of multi-storey timber frame buildings in the UK will normally be required for satisfying building control. Although differential movement may occur between many different types of connected parts within the building, both structural and nonstructural, the main areas of concern will be movement between the timber frame and cladding, service shafts and structures of dissimilar movement such as lift shafts. Failure to accommodate such differential movement in the building may tend to indicate non-compliance with different parts of the Building Regulations 2000, Building Standards (Scotland) Regulations 1990 or Building Regulations (Northern Ireland) 2000. Only the Building Regulations (England

20

Multi-storey timber frame buildings – a design guide and Wales) 2000 provide specific guidance on the problem of differential movement in Section 2 of Approved Document A9. This refers the reader to two British Standards, BS 8200: 1985 Design of non-loadbearing external vertical enclosures of buildings and BS 5628: Part 3: 2001 Code of practice for use of masonry – Materials and components, design and workmanship, which provide detailed information concerning the movement of different types of cladding but do not provide the level of detail required for assessing the vertical movement in a multi-storey timber frame building. The following sections expand upon the principles for movement of the timber frame and the differential movement that should be accommodated at various interfaces in multi-storey timber frame buildings.

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4.3 Sources of movement Whilst some movement within buildings is to be expected, it will greatly depend upon variations in the environmental conditions and applied loads. These conditions and loads may change during the construction and lifetime of the building and will need to be considered for individual elements of the building. It is important to gain a good estimation of the range of conditions and loads that different parts of the building will experience and understand the propensity for movement of different materials and constructions. 4.3.1 Moisture movements

Many of the materials used in the construction of buildings are hygroscopic. That is to say the amount of moisture contained in the material will be in equilibrium with the relative humidity of surrounding air in steady-state conditions. Changes in the air relative humidity will cause gradual changes in the moisture content of the material. Changes in the material moisture content are often accompanied by dimensional changes, but these will not necessarily occur equally in all three principal axes of the material. For example, the dimensional movement of timber with respect to changing moisture content is negligible along the grain but should be considered in both directions perpendicular to the grain, tangential and radial. Tangential movement is generally twice as large as radial but will vary from species to species. Normal cutting patterns for saw logs produce timber with varying grain orientation that could be purely tangential, radial or a mixture of both. The effects this can have on timber members is shown on left. Design calculations should consider the average of tangential and radial movement, which will provide a good approximation of the actual moisture movement Since bricks are more homogeneous, their expansion due to the uptake of water will occur to the same degree in all three axes, although this expansion will not necessarily be fully reversible when conditions cause the moisture content to reduce.

Shrinkage of timber with tangential and radial grain orientation

Some manufactured materials, such as fired clay products and cementitious products exhibit irreversible (permanent) changes in size owing to the uptake of moisture [21]. This movement may occur over a period of months or years. For clay bricks, water absorbed from the atmosphere as they cool after firing in the kiln causes the bricks to expand by about 0.2 to 1.5 mm/m over the life of the bricks; about half of this occurs in the first week [22]. Further movement of the bricks may also occur as seasonal variations due to the cyclic wetting and drying. These additional movements are reversible and less dominant than the long-term permanent expansion.

Chapter 4: Differential movement

21

4.3.2 Thermal movements

All materials are subject to some increase in size as their temperature rises, and vice versa. The amount by which they change in size for a given temperature difference varies widely, as does the range of temperatures to which different parts of the building fabric may be subjected.

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To estimate the thermal movement of construction materials, the temperature range that the material will experience in service is needed. This is not necessarily the range of ambient temperatures expected over the life of the building since the material temperature will depend on its thermal characteristics (surface absorptivity and emissivity, conductivity, diffusivity, capacity) as well as the characteristics of its environment (air temperatures, radiant gains and losses, evaporative potential). These factors have been taken into account in other texts that provide a detailed description and design data for this type of movement [23][24][25]. When considering the thermal movement of hygroscopic materials, such as timber, it is important to consider the effect on other types of movement, such as moisture movements. Although an increase in temperature causes thermal expansion of the timber, it will also cause the moisture content to reduce and so induce shrinkage. The dominant effect of these two opposing movements when equilibrium is reached will be the moisture movement. This will be the movement type considered in design for internal timber members that are not subjected to rapid changes of temperature and moisture content. Bare external timber that is used for cladding without a painted finish may be subjected to rapidly changing temperature. This can cause thermal expansion before the more dominant shrinkage movement occurs. 4.3.3 Movements from induced stresses

Consideration should also be given to the movement occurring from induced stresses due to the material self-weight and any serviceability loads. Since timber frame buildings are generally constructed using a platform frame technique, vertical movements in the timber frame may be considered as unrestrained. The vertical movement of studs is negligible, even for multistorey structures where axial shortening of studs may account for only 1 mm of total movement at eaves level. The stiffness and movement of timber loaded perpendicular to the grain requires in depth knowledge of the mechanics involved and is not covered in this section. The current draft of Eurocode 5 gives a more detailed account of the requirements for design. Although the elastic compression of a timber frame building can account for a significant proportion of the overall movement, delayed compression (creep) can be greater than the initial elastic deflection with time. To estimate the delayed compression deformation, its value should be assumed to be equal to the elastic compressive movement for the total dead and imposed loads on the building. 4.3.4 Bedding in

Timber frame wall and floor panels are constructed to tight tolerances in factory conditions to produce a building with good dimensional accuracy. There will, however, be some variation in the cut length of studs, and hence the height of wall panels, which cause small gaps between the walls and floors. As the construction proceeds, these gaps are reduced owing to the self weight of the supported structure and it is commonly accepted that once the roof has been constructed, including the tile or slate covering, all of the bedding in movement will have occurred.

22

Multi-storey timber frame buildings – a design guide

4.4 Research conducted on the TF2000 building 6

Cumulative downward movement at

5

each floor level

Storey of building

4

3

2

0

Relative movement due to shrinkage and elastic compression for the TF2000 building

For the timber frame, the majority of vertical movement comes from the floor zone above wall panels and rim beams on external walls (as described in Chapter 1). Figure left, below, shows the relative movement of structural members with measurements taken from the TF2000 building. The movement of joists would have been much greater if they had not been pre-conditioned to 12% moisture content when delivered to site.The dominant movement of the joist is mainly due to the depth of section when compared with other members. Vertical movement of timber framing members

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1

The movement that occurs in a timber frame is often referred to as shrinkage. Although this statement is a good approximation for low-rise timber frame structures, research conducted on the TF2000 building highlighted that there are other significant and sometimes more dominant movements that occur in multistorey timber frame structures [26]. For example, the compressive movement of the sole plate and bottom rail on the ground floor panels is greater than shrinkage of the same members in Compressive movement medium-size structures. Figure left shows Shrinkage movement the results of tests conducted on the TF2000 building that identified the Movement relationship between shrinkage and elastic compression for multi-storey structures. For lower storeys, the two types of movement are similar over the short-term although compression perpendicular to the grain can produce much higher movement over medium- and long-term time-scales when creep effects contribute.

Summation of 1 to 4 (total movement)

4 Joists - 64% of total

1 Sole plate and bottom rail 2 Studding

- 25% of total

3 Top rail - 7% of total - 4% of total

Time: construction start to occupied building

1 year

Movement of the timber frame on the ground floor of the TF2000 building

The amount of differential movement may also be reduced through control of the site construction process. Construction of the TF2000 building ensured that the roof construction and tiling were completed before commencement of the brickwork cladding. Sequencing of the construction such that the dead weight of internal linings is already supported by the structure before construction of the brickwork cladding can also reduce the provision required for compressive movement of the timber frame. Admittedly the design sequence will not always be known at the design stage but timber frame buildings that adopt closed panel systems will benefit from additional dead weight early on in the construction sequence.

23

Chapter 4: Differential movement

4.5 Reduction in movement by design

Options for reducing the amount of differential movement are: ❐ Use timber joists and headers with a low target moisture content, typically 12% or lower so that there is very little shrinkage in service. ❐ Substitute timber joists and headers with engineered wood products with low moisture contents. ❐ Reduce the amount of timber loaded perpendicular to the grain, such as multiple-sole plates and header plates. ❐ Use clay bricks with low movement characteristics for cladding. ❐ Specify alternative claddings such as timber boarding or tiles. To demonstrate the effect some of these options have on reducing differential movement, predictions for a typical six-storey timber frame building are shown below. This clearly demonstrates the benefits of using materials with low movement characteristics for multi-storey buildings; taking full account of this in design can save expensive detailing requirements around openings and for connections such as wall ties. Movement tolerant wall ties should be used for multi-storey timber frame.

storeys

Timber movement

6 5 4 3

Brickwork movement

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Simply determining the amount of expected differential movement may not be enough for more stretching designs in multi-storey structures. The accumulation in differential movement on upper storeys of the building may be too great around openings and liftshafts and will need to be reduced to a tolerable level. When the design value for differential movement has been determined, this should be accommodated in the constructed building by providing adequate movement joints and suitable connections such as flexible wall ties and sliding wall ties.

2

Expansion of the brickwork

Contraction of the timber frame

1 0

Cavity

-30

-20

-10

0

10

20

30

40

50

Vertical downward movement for a typical six-storey building, mm OPTIONS: NORMAL JOISTS

SUPER DRY JOISTS

HIGH MOVEMENT BRICKWORK

LOW MOVEMENT BRICKWORK

Example of differential movement for a multi-storey brick-clad building

ENGINEERED JOISTS

24

Multi-storey timber frame buildings – a design guide For design, these values can be assumed in the absence of more precise data Options for building components Normal timber installed at 20% mc (1) Super dry timber installed at 12% mc or below

Allowance for movement 1.40 mm / 50 mm of cross-grain timber (1)

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Engineered wood products installed at 12% mc or below

(1)

(2)

0.6 mm/ 50 mm of cross-grain timber

(2)

0.4 mm/ 50 mm of cross-grain timber

(2)

Brickwork cladding

1 mm/ 1m height

Hung tiles, slates and timber cladding

Negligible

Other claddings

Varies

(3)

(4)

Notes (1) Design values assume that the timber moisture content (mc) is kept at

(3) A conservative value for use in the absence of

or below the target value.

manufacturer’s data.

(2) The total depth of cross-grain timber should include all sole plates,

(4) Other publications provide details

[2][12]

bottom rails, top rails, joists and any other timber loaded perpendicular to the grain that adds to the total building height at eaves level. This includes I-beams.

The above movement values for the timber frame account for all shrinkage and compression (both elastic and creep) but do not allow for bedding in of the timber frame. The common construction sequence for timber frame ensures that the timber frame, prior to construction of the brick cladding, supports the majority of the design dead load including the roof load. If a different construction sequence is adopted, an additional 3 mm movement per storey should be allowed for in design. The majority of the timber frame movement will be expected to have occurred during the first 36 months of the building’s occupation. Further small movements in the building will occur with seasonal variation in conditions but these will be minimal by comparison. If other target moisture contents are specified for timber components, the design allowance for movement will have to be adjusted.

4.6 Provision for movement Designs suitable for differential movement of the external wall construction will inevitably require the provision of expansion gaps at eaves level between the cladding and around window openings. When the timber frame is connected to any dissimilar material that will move differentially in-service, the connectors or materials should accommodate such movement without transferring load. If mastic sealant or other compressible material is used to fill gaps provided for differential movement, the designer should ensure that there is sufficient room left for movement once the sealant is compressed. Designs often wrongly assume that such sealants compress to nothing. Weather sealants provided around vertical interfaces between openings and cladding should also accommodate movement or be maintained to ensure good performance – see figure on page 25.

25

Chapter 4: Differential movement

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Window fixed to studs of frame

Vertical movement at joint as timber frame shrinks

Differential movement around windows

If masonry is adopted for the cladding, the design of wall ties shall take account of the differential movement as well as the transfer of horizontal forces. Common areas on timber frame buildings that require special detailing are: ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐

windows; doors; openings for services; soffits; cavity ties [27]; battens across floor zones; junctions for mixed cladding designs; lift shafts and stair wells of mixed construction.

Many of these details are included in Timber Frame Construction other publications [19][28].

[10]

and

Good detailing at these intersections that accommodates the design differential movement can ensure that costly maintenance is avoided during the service life of the structure. Common failures occur through poor detailing, insufficient provision for movement or poor construction on site. Good designs must be communicated correctly on site and checked to ensure that costly failures do not occur.

26

Multi-storey timber frame buildings – a design guide

Chapter 5 Achieving performance

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5.1 Introduction The TF2000 project recognised the need for market based facts as well as technical research. Part of the activities was to undertake a productivity benchmarking exercise on the test building [29]. Benchmarking can be defined as ‘the continuous process of measuring products, services and practices against the toughest competitors or those companies recognised as industry leaders’. Hence a study was undertaken on building construction performance using timber frame, concrete and steel framing techniques. Since full account of this study has been previously published [30] this chapter merely summarises the report and expands on the key issues for improving the performance of timber frame for the multi-storey residential based market. The benchmarking investigations have proven that platform timber frame provides the construction industry reduced build times and high in service performance. These benefits can be realised through management of the processes involved within the project’s development. This chapter provides specific guidance relating to achieving good performance for the building. The key issues are presented as: ❐ Construction process lessons ❐ Build tolerance ❐ Safe Construction procedures

5.2 Timber frame benefits in multi-storey construction The TF2000 building provided the first full scale test for buildings above two storeys and was the first 6-storey residential timber frame building. Since TF2000 there have been a number of six-storey buildings that have proven on a commercial scale the benefits identified on the TF 2000 project. Timber frame provides four proven benefits for the multi-storey construction market:

● ● ●

Supply proven supply chain design and build solutions speed of erection

Reduced support loads

● ● ●

ground works transfer slabs foundations

● ● ● ●

reduced environmental footprint renewable resource reduced waste reduced impact on local community

UK timber frame

● ●

factory standards site build improvements

Quality

Sustainability

27

Chapter 5: Achieving performance

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Multi-storey buildings under construction

courtesy Pinewood (left) and Chiltern Clarke Bond (right)

TF2000 was targeted to deliver these improvements to the way buildings are procured:

● ● ●

Construction crane utilisation knowledge transfer speed of erection

Design

● ● ● ● ●

early integration of supply chain understanding of the processes layout impacts fire engineering acoustics

TF2000 – target investigations for improvements

● ● ●

panel design to maximise repetition floor cassettes roof prefabrication Fabrication

Sustainability

● ● ●

better in-service performance waste minimisation use of renewable resources – timber

5.3 Benchmarking at TF2000 Following an industry review and a timber frame performance review forum a number of key competitive benchmarks were established. The full results and breakdown is given in Medium-rise timber frame – a best practice benchmarking guide [30].

The following best practice benchmarks were used to determine the key issues:

● ● ● ●

m2/week Speed of construction on site arrival watertight timber frame framing solution

Measured competitive benchmarks ● ● ● ● ●

Lead time

● ● ● ● ●

Best practice benchmarks

£/m2 total timber frame value structural frame costs erection costs Construction value removing costs that add no value

Quality

start point – customer contact end point – delivery of frame to site design freeze framing solution agreement early involvement

Buildability

● ●

● ● ●

level of customisation and production optimise design impact of speed of construction

delivery of finished building in-service performance

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Multi-storey timber frame buildings – a design guide

The following best practice benchmarks were used to determine the key issues Benchmark Productivity rate

Current best practice 253 m2/week

Target 750 m2/week

TF2000 performance 656 m2/week

Erection value

100%

70%

60%

Structural frame value

100%

70%

75%

Lead time

6 weeks

5weeks

5 weeks

The productivity rate can be considered as total floor area rates. These are based on 1999 values.

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The values presented are percentage values based on current practice as 100%. Typically erection costs were found to be 14% of the structural frame costs. The timber frame productivity and value are based on delivering the complete structural shell for which insulation, plasterboard and floating floors are to be completed in follow-on packages.

In reviewing these benchmarks, it is important to understand that the TF2000 benchmark values are influenced by the following: Influence Building under cover

Impact No weather problems – winds etc

No site access problems

Ease of delivery of materials and safe storage

Little interference from other site operations

No waiting for other trades – foundations etc

Regular layout and standard design

Repeat layouts and ease to swap panels with structural problems

Fixed design

No late design changes

The results from TF2000 have highlighted area where improvements can be achieved in practice. The following table provides an outline of a four step guide to improvement. Design standardisation Simplified design leads to right first time ethos

Reduced construction times

Efficient use of materials: minimum cutting and waste Simplified panel design Economies of scale

Reduced costs

Reduced engineering Reduced checking time

Reduced lead in times

Greater potential for Reduced defects understanding the build process

continued ...

Chapter 5: Achieving performance Wall panel prefabrication Increase size (large panel)

Reduced erection times but reliant on crane availability

Reduced size (small panel)

Increased erection costs but less reliant on crane

Closed panels (internal and external boards and insulation)

Reduce overall construction programme but increase weight of panel

Panel size and weight

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Floor assembly Structural type

Cassette floors

Roof assembly Structural Type

Prefabricated

29

Impact on health and safety Impact on erection crew and teams: target for two erectors and one labourer 450 m2/day typical floor area rates

Sawn timber joists: difficult to control waste, strutting increases construction time: assembly rate 100 m2/day Proprietary engineered joists: ease of fixing, delivered to length but premium cost: assembly rate 300 m2/day Reduce on site processes Increasing productivity by up to 60% Design freeze needed at an early stage

Trussed rafters: allows on site ground level prefabrication: TF2000 gave 40% improvement Panel roof systems: for complete insulation assembly Reduced health and safety risks Requires crane Reduces up to 80% of high level work Minimal temporary working platforms

5.4 Construction process lessons from TF2000 The TF2000 project provided a number of essential techniques to deliver successful build projects. Recommended approach identified from TF2000: ❐ Engaging the timber frame fabricator design team at an early stage. ❐ Investing 'up front' time in developing the project on paper with the timber frame design team prior to a design freeze. ❐ Identifying holistic cost and value for money of the methods being considered. ❐ Integrating the timber frame design and build team with the project’s design team and other trade contractors to ensure a clear flow of information and to reduce interface problems. ❐ Ensuring that sufficient training is in place for site management and site labour to understand the construction methods and processes. Site management is especially important to cover the information flows and ensure logistics of the site to cope with the speed of the off-site assembled units. ❐ Identification and agreement across all parties in the project of tolerance issues such as foundation levels. ❐ Integration and method of build knowledge to the follow on trades to the timber frame such as plasterboard fixers and service trades.

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Multi-storey timber frame buildings – a design guide

5.5 Build Tolerances The TF2000 building was monitored for construction tolerances [27]and it became clear that multi-storey timber frame requires a different set of tolerances to avoid conflict with lift shaft designs and cladding cavity issues. The understanding and communication of tolerances will support faster build times and improved quality.

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For timber frame buildings less than four storeys high, guidance on erection tolerances is already provided [10]. However for multi-storey timber frame buildings of four or more storeys, there are additional considerations that are presented as follows:

Tolerances and construction note checklist for timber frame buildings above four storeys Support structure/foundation

Tolerances ❐ Lengths of support structure beneath the timber frame sole plate to be within ± 10 mm. ❐ Diagonals should be equal. Acceptable deviation: up to 10 m: ± 5 mm more than 10 m: +/- 10 mm ❐ Walls/beams or slab supporting sole plates are levelled to ± 5 mm. Construction notes ❐ Underground ducts for plumbing stacks to rise clear of floor joists, disproportionate collapse columns/beams, and trussed rafters. ❐ Position and secure disproportionate collapse key element column (if adopted), metal fixing plates to slab if specified; check they are adequately protected by hot dip galvanizing or have equal corrosion resistance. ❐ Steel goal post frames to be detailed so not to cause problems with sole plate setting out, cavities or finished floor constructions.

Sole plates or bottom rail of panel if no sole plate

Tolerances ❐ Sole plates should be set out within ± 10 mm in length or width defined by the drawings. Diagonals should be equal. Acceptable deviation: up to 10 m: ± 5 mm more than 10 m: ± 10 mm ❐ The sole plates should not overhang the support structure/ foundation by more than 10 mm. ❐ Sole plates must be level within ± 5 mm. Fully bed the plates on mortar (and structural shims if necessary) if base is not level. Bedding and shims should be approved by a structural engineer. Construction notes ❐ Sole plate/bottom rail should be surveyed and corrected for line and level at each storey height. ❐ Where appropriate, check that sole plates are pressure treated with preservative where specified and brush apply preservative to site cut ends and drilled holes. ❐ Where appropriate, ensure that there is a dpc under sole plates lapped 100 mm at joints and overlapping the dpm.

Fixing down sole plates (or bottom rails of wall panels)

Tolerances ❐ Type, number and centres of fixings as specified by the structural engineer. Construction notes ❐ If mild steel brackets, shoes, straps and/or disproportionate collapse plates have been specified, check that they are adequately protected by hot dip galvanising or have equal corrosion resistance. ❐ Replace any plates that are split or damaged during fixing. continued ...

Chapter 5: Achieving performance

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Walls

31

Tolerances ❐ Plumb within ± 10 mm over any storey height but not more than 10 mm cumulative over the building height. Line within ± 3 mm on the sole plate as the set-out template. Inside faces of panels to be flush. Tightly butt-joint panels together. Use solid packing to close panel junctions; packing to structural engineer’s specification. ❐ Level to top of the wall frames in each storey to be: up to 10 m: ± 5 mm; more than 10 m: ± 10 mm. Construction notes ❐ For compartment walls use only light metal straps to connect wall leaves during construction; rigid connections will impair sound performance of completed wall. Metal straps maximum 3 mm thick; one row per storey height every 1200 mm with fixings in accordance with fixing schedule for the specific multi-storey building. Take care not to compromise acoustic requirements. ❐ Ensure any specified seals (air leakage etc) between wall panels and at panel/floor junctions are fitted to specification. ❐ Ensure scaffolding supports to timber frame structure are approved for the specific multistorey building: specification to structural engineer’s requirements. ❐ Non-loadbearing walls must be adequately supported; ensure that the walls are not loaded by subsequent floors or roof structures; structural engineer to specify fixing. ❐ Ensure walls are accurately aligned, braced temporarily and stable before installing floors. Temporary bracing should remain until the floor is complete. ❐ Ensure joist notching and drilling is within limits specified by structural engineer. ❐ Ensure there is sufficient clearance between services and walling to allow for long-term shrinkage of timber frame.

Floors

Tolerances ❐ Relative to each storey as a datum, structural floor decking level to be: up to 10 m long: ± 5 mm; more than 10 m: ± 10 mm. Maximum deviation of floor levels to ground level floor level to be ± 20 mm with no more than 20 mm difference in level over 10 m. ❐ Finished floor decking levels to be: up to 2 m long: 5 mm deviation, no step between boards more than 2 mm or to specification; up to 10 m long: ± 5 mm; more than 10 m: ± 10 mm. Construction notes ❐ For compartment floors ensure: there is a gap at the floating floor perimeter with a resilient material in the gap; there is a gap between the skirting and the floating floor deck; there are no fixings penetrating the floor deck and resilient layer; balcony restraint straps do not compromise acoustic performance of the floor. ❐ Ensure services will rise clear of floor joists and disproportionate collapse beams. ❐ Ensure header joists and/or disproportionate collapse beams align with wall panels and do not protrude into the cavity. ❐ Floor deck fixings should be in accordance with fixing schedule for the specific multi-storey building. ❐ All ends and edges of sheets other than tongue and grooved to be fully supported on joists or noggings; ensure specified expansion gap is around perimeter of chipboard and OSB decks. ❐ Ensure metal joist hangers are of the correct type for their location and are firmly fixed in accordance with manufacturer’s recommendations. ❐ Ensure joist notching and drilling is within limits specified by structural engineer. ❐ All short edge joints should be staggered. ❐ Glue sheet material decks to joists; glue all tongue and grooved joints of sheet material decks and ensure all fixings are driven below the surface of the boards. ❐ Weather protection of decking to be specified and maintained. ❐ Check clearance of timber components adjacent to flues and chimneys.

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Multi-storey timber frame buildings – a design guide

5.6 Safe construction procedures

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The Approved Code of Practice to the Construction (Design and Management) Regulations 1994 requires that a risk assessment of the method of construction is undertaken. The following key elements should be part of any submission from the timber frame design and build team: ❐ Description of the project. ❐ Description of the environment in which the activities are to take place and requirements on access. ❐ Description of the design, including design loads taken, and a breakdown of the components. ❐ Method statement of the construction process to erect the timber frame and its associated components. ❐ Risk assessment of the method of erection, indicating control methods and how information is being presented to site operatives. ❐ The role of each company involved and names of the persons responsible for undertaking action, supervising and overall responsibility.

Kenavon Drive, Reading

courtesy Guildway Ltd, Walker Timber Group

33

Appendix A Regulatory guidance for fire safety

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Introduction This chapter provides assistance to the designer in terms of finding and interpreting relevant statutory guidance as well as providing information relating to new developments in the field of fire safety in timber frame buildings specific to the design process. This Appendix deals with those aspects of fire safety precautions that primarily affect the structural design concept of a timber frame building. For instance, it is not intended to reproduce information relating to requirements for fire alarm systems unless such published requirements are likely to impact on structural design. The designer should therefore view the guidance provided within this document as supplementary to statutory guidance and not as a replacement to statutory guidance. The guidance is split into a number of sections that cover the various functional fire safety objectives embodied in the Building Regulations of England & Wales, Northern Ireland and Scotland. Guidance is provided for means of escape and internal fire spread for both linings and structure. Commentary is not provided for external fire spread, access of facilities for the fire brigade and other fire safety issues. The Building Regulations of England and Wales, Northern Ireland and Scotland do differ in structure but the functional objectives of the regulations are broadly the same. The intent of this document is to provide as much design guidance as possible in a form that allows for a common approach that will satisfy enforcing authorities within each District of the United Kingdom. The aim is also, as appropriate, to point out where important differences apply that will affect the structural design of timber frame buildings. Some of the published statutory guidance is, by necessity, relatively complex and cannot be generally simplified. It is not the intention to reproduce such statutory guidance. Where aspects of this guidance have a significant effect on the structural design concept for a timber frame building, this document will highlight and discuss these issues. As a final point, it is important to stress that the relevant Building Regulations within England & Wales and Northern Ireland are fully functional. In England & Wales the technical provisions published in support of the regulations takes the form of guidance that, if followed, would tend to demonstrate compliance with regulations. In Northern Ireland, deemed-tosatisfy technical provisions support the regulations and, if followed, the regulations are deemed to have been satisfied. These technical provisions are not mandatory; if a designer wishes to adopt an alternative approach, this can be done provided that appropriate steps are taken by the designer to demonstrate that the alternative approach still satisfies the Building Regulations. The situation in Scotland is slightly different in that the technical

34

Multi-storey timber frame buildings – a design guide provisions in the published statutory instruments are mandatory with respect to the standards that are set. Provisions that are deemed to satisfy the standards are, however, optional and other approaches can be used provided that they can be demonstrated by the designer to meet the same standard. For instance, each country’s technical provisions stipulate that dwellings shall be separated from each other by fire resisting construction. In England & Wales this is guidance, in Northern Ireland it is deemed to satisfy and in Scotland it is mandatory. Consequently the term ‘statutory guidance’ has a different meaning depending on the technical provisions being referred to. In order to help the user appreciate the difference between technical provisions, the word ‘require’ and its derivatives are in bold text where they relate to technical provisions that are mandatory.

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Means of escape

Table A1 State

England & Wales

B1

Source of published statutory guidance affecting structural design Approved Document B. Section 3

Northern Ireland

E2

BS 5588: Part 1

Scotland

Regulation

12 & 13

Technical Standards D & E

The sources of statutory guidance listed in Table A1 provide design guidance that will allow layouts to be developed that comply with the building regulations for providing adequate means of escape. Many of the recommendations given in statutory guidance aim to control the maximum travel distances within the building and to ensure that the final escape route is not compromised by poor layout concepts. With regard to implications on structural design of multi-storey timber frame buildings, it is the aspect of common escape routes that has the greatest impact in the context of the providing adequate means of escape. Table A2 distils the relevant information from the statutory guidance and highlights how the different requirements may affect structural design.

Appendix A: Regulatory guidance for fire safety

35

Table A2 Common escape stairs

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England & Wales A single stair is permissible in the guidance provided that the top floor of the building is no more than 11 m above ground level, there are no more than three storeys above the ground level storey and the stair does not connect to a covered car park. Where an automatic opening vent (1.5 m2) is provided in the protected lobby/corridor access to the escape stair, there is no limitation on the height of the building that can be served by a single stair.

Northern Ireland In Northern Ireland BS 5588: Part 1 is the deemed to satisfy provision for means of escape.

Scotland A single stair is permissible in the guidance for flats and maisonettes of any height.

A single stair is permissible provided: ❐ The floor of the highest storey does not exceed 11m above ground level ❐ There are no more than four storeys above ground level ❐ The stair does not connect to a covered car park unless the car park is provided with permanent crossventilation.

Where no storey is at a height greater than 7.5m, there are not more than four dwellings on each storey and the stair does not serve more than eight dwellings in total, a protected lobby is not required within the protected zone between the escape stair and the accommodation provided that each dwelling has within it a protected enclosure.

Section 6 of the statutory guidance recommends that a single stair serving a four-storey building or greater should have flights or landings constructed from materials of limited combustibility.

For all other cases where dwellings are served by a single stair, a protected lobby with an automatic opening vent (1.5 m2) is required. The topmost storey of the building is exempt from this requirement.

Buildings provided with more than one escape stair are free from this limited combustibility requirement up to a height of the top storey above ground level of 18 m.

All escape stair flights and landings are required to be constructed from noncombustible materials. The requirement also applies to floors of protected lobbies.

Notes The British Standard fire test for ‘non-combustibility’ and ‘limited combustibility’ is BS 476: Part 11. Timber will not pass this test even when treated to modify its reaction to fire performance. Therefore, the recommendations for limited combustibility (non-combustibility in Scotland) if followed would prevent the use of timber frame construction in the stairs which would then need to be of steel or masonry construction. In Scotland the non-combustibility recommendation is also extended to the construction of the stair shaft and floors of protected lobbies. Experimental fire tests have been carried out to provide data on this issue as part of the TF2000 research programme carried out on a six-storey timber frame building provided with a single escape stair of timber frame construction. The results of the research showed that the functional fire safety objectives may alternatively be achieved by utilising treatments to modify the reaction to fire performance of the timber components of the stair. The summary for these experiments is available in a BRE report [17]. One of its recommendations is: As a consequence of this research project, regulatory authorities may wish to consider the use of an appropriately treated timber stair as adequate in terms of meeting the functional requirements of the UK Building Regulations, on a case-by-case basis. Limited combustibility does not exist as a definition in Scotland.

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Multi-storey timber frame buildings – a design guide

Internal fire spread (linings)

Table A3 Internal fire spread (linings)

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State

Regulation

England & Wales

B2

Source of published statutory guidance affecting structural design Approved Document B. Section 7

Northern Ireland

E3

Technical Booklet E, Sections 2 and 6

Scotland

12

Technical Standards D

The control of building linings has an impact on the structural design concept for a timber frame building insofar as the wall linings contribute to the structural action of the timber frame panels. The situation with regard to the recommendations made by published statutory guidance is a good deal simpler than published guidance relating to means of escape; it is summarised in Table A4.

Appendix A: Regulatory guidance for fire safety

37

Table A4 Control of internal building linings England & Wales Small rooms not greater than 4 m2 in plan area can be fully lined with Class 3 material.

Northern Ireland Small rooms not greater than 4 m2 in plan area can be fully lined with Class 3 material.

Other room linings should be Class 1 except for a maximum wall area of 20 m2 or 50% of the floor area (whichever the less) which may be Class 3.

Other room linings should be Class 1 except for a maximum wall area of 20 m2 or 50% of the floor area (whichever the less) which may be Class 3.

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Circulation spaces in dwellings are Circulation spaces within dwellings are recommended to have Class 1 linings. recommended to have Class 1 linings.

Scotland Small rooms not greater than 4 m2 in plan area can be fully lined with High Risk material (kitchens are specifically excluded by this guidance). Other room linings are recommended to be Medium Risk except for a maximum wall area of 20 m2 or 50% of the floor area (whichever the less) which may be High Risk. Protected routes are recommended to have Low Risk linings.

Common circulation spaces and Common circulation spaces and protected routes are recommended to protected routes are recommended to have Class 0 linings. have Class 0 linings. Exemptions include doors, windows, Exemptions include doors, windows, architraves, picture rails, skirtings and architraves, picture rails, skirtings and exposed beams exposed beams

Notes Class 3, 2 and 1 surfaces are evaluated by means of the British Standard BS 476: Part 7 surface spread of flame. Class 0 is a combination of the best result from the Part 7 test combined with a good result from an additional BS 476: Part 6 fire propagation test. Timber materials are regarded as deemed to satisfy a Class 3 performance. Plasterboard is regarded as deemed to satisfy a Class 0 performance. A variety of commercial treatments are available for timber that will allow Class 1 or Class 0 levels of performance to be attained – see References and further reading. It is important to note that new harmonised European fire tests have been developed that will replace the current British Standard Reaction to Fire tests. These new BS EN tests are fundamentally different from the old BS tests and require a completely new classification system. Thus material classifications of Class 3, Class 0 etc will soon disappear and be replaced by a completely new set of classifications that will be embodied in revised published statutory documents. As can be seen from the above, Scotland has already made the change. In Table A3 of the Scottish Technical Standards, there is deemed to satisfy guidance that allows the transposition of BS and BS EN classes as follows: Risk Non-combustible

British Standards Satisfies appropriate test criteria of BS 476: Part 4 or BS 476: Part 11

European standards Class A1 or Class A2

Low Risk

Class 0

Class B

Medium Risk

Class 1

Class C

High Risk

Class 2 or Class 3

Class D

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Multi-storey timber frame buildings – a design guide

Internal fire spread (structure)

Table A5 Internal fire spread (structure)

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State

Regulation

Source of published statutory guidance affecting structural design Approved Document B. Sections 8, 9, 10, 11

England & Wales

B3

Northern Ireland

E4

Technical Booklet E, Sections 3 and 6

Scotland

12

Technical Standards D

Appendix A: Regulatory guidance for fire safety

39

Licensed copy from CIS: atkins, Atkins Plc, 01/07/2014, Uncontrolled Copy.

Table A6 Loadbearing elements of structure England & Wales Structure that supports only a roof does not need to be fire resisting provided that the roof does not perform the function of a floor, eg for the support of plant or is used for means of escape etc.

Northern Ireland Structure that supports only a roof does not need to be fire resisting provided that the roof does not perform the function of a floor, eg for the support of plant or is used for means of escape etc.

Typically, the structural frame for a medium-rise timber frame building would require a fire resistance of 60 minutes.

Typically, the structural frame for a medium-rise timber frame building would require a fire resistance of 60 minutes.

Based on a floor/floor height of nominally 2.6 m, an eight-storey medium-rise timber frame building would possess a height to the top storey in excess of 18 m. Statutory guidance would then dictate a fire resistance for the structural frame of 90 minutes. If a basement storey deeper than 10 m is used, the requirement also becomes 90 minutes for elements of structure within the basement.

Scotland Structure that supports only a roof does not need to be fire resisting. It is then, however, a requirement that the roof does not support plant or is used for means of escape or parking etc. Typically, the structural frame for a medium rise timber frame building would require a fire resistance of 60 minutes.

The actual period of fire resistance required depends on the height to the The guidance in respect of fire floor of the top storey above ground resistance steps from a 60-minute to level: 90-minute requirement at a floor height up to 7.5m = Short, of more than 20 m. Hence, in N. >7.5m and up to 18m = Medium Ireland, the eight-storey medium rise >18m = Long. timber frame building described above would possess a height to the top For all elements of structure in a storey less than 20 m and would be basement, the requirement is Medium. expected to have a loadbearing frame with 60 minutes fire resistance. The Technical Standard recommends the following fire resistance periods: If a basement storey deeper than 10 m Low = 30 minutes is used then the requirement also Medium = 60 minutes becomes 90 minutes for elements of Long = 120 minutes structure within the basement. and refers to both British and European fire test standards.

Notes Fire resistance in the context of loadbearing elements of structure means that the structural elements must continue to support the design load under the conditions of the fire resistance test (BS 476: Parts 20/21) for the designated period. Failure criteria for loadbearing capacity relate to collapse or unacceptably large deflections. The latest design details for timber frame panels and connection details are contained in Timber Frame Construction[10]. Chapter 6 of the document deals with intermediate floors (ie those located wholly within a single occupancy and that therefore are not also compartment floors) and Chapter 7 of the document deals with floors that also have to provide compartmentation. The guide shows typical structural solutions for up to and including 60 minutes fire resistance. Where higher periods of fire resistance are required then the British Gypsum Whitebook [32] provides a useful point of reference for wall and ceiling lining combinations that will provide the necessary performance. Where alternative solutions are adopted, some changes to joist/studwork and fixing specifications may be necessary. It must be noted that any floor that is not also a compartment floor will still need to be fire resisting since it is loadbearing. The same applies to loadbearing walls.

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Multi-storey timber frame buildings – a design guide

Table A7 Compartmentation

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England & Wales The following are recommended as being constructed as compartmenting elements: ❐ all floors between individual dwellings; ❐all walls separating individual dwellings from the remainder of the building; ❐all walls enclosing refuse storage chambers.

Northern Ireland The following are recommended as being constructed as compartmenting elements: ❐all floors between individual dwellings; ❐all walls separating individual dwellings from the remainder of the building; ❐all walls enclosing a communal waste container; ❐all walls common to two or more buildings; ❐all walls provided to divide a building into separated parts; ❐all walls or floors provided to separate different purpose groups.

Notes The fire resistance ratings required for compartmenting elements of structure are derived as for loadbearing elements unless a specific minimum value is stipulated depending on the nature of the compartmentation. Compartmenting elements of structure are required to provide fire resistance in terms of integrity and insulation when exposed to the conditions of the fire resistance test BS 476: Parts 20/21.

Scotland The Scottish statutory guidance retains special forms of compartmentation known as separating walls and separating floors. These separating walls and floor are required to have a Medium duration of fire resistance (see Table A6) and are required as follows: ❐all floors between individual dwellings; ❐all walls between individual dwellings. For buildings with a height to the floor of the top storey not in excess of 18 m separating walls are not required to be constructed from non-combustible materials. This is on the required proviso that insulation and surfaces within the wall cavity have at least a Low Risk (see Table A4) surface spread of flame performance or are non-combustible and contains no pipes, wires or other services. The structural frame may therefore be timber but care should be taken in respect to the specification of sheathing boards that may be desired for racking resistance. The guidance requires by implication dealing with non-combustibility requirements for separating floors that the floor over solid waste storage rooms should be constructed as a separating floor of non-combustible construction irrespective of the overall height of the building.

The use of combustible materials in separating walls is allowed in the Scottish guidance up to 18 m (as a result of the proposed 6th amendment). As stated in Table A2, timber will not pass the non-combustibility test however treated, and the limitation of 11 m for separating floors comprising of timber frame construction would preclude the construction of multi-storey timber frame buildings in Scotland above five storeys. The latest design details for timber frame compartment walls and compartment floors are contained in Chapter 5 an 6 respectively of Timber Frame Construction [10]. The guide shows typical solutions for up to and including 60 minutes fire resistance. Where higher periods of fire resistance are required, the British Gypsum Whitebook [32] provides a useful point of reference for wall and ceiling lining combinations that will provide the necessary performance. Where alternative solutions are adopted, some changes to joist/studwork and fixing specifications may be necessary.

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Appendix A: Regulatory guidance for fire safety

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Table A8 Roof voids England & Wales The guidance recommends that compartment walls are carried through the roof void to the underside of the roof membrane.

Northern Ireland The guidance recommends that compartment walls are carried through the roof void to the underside of the roof membrane.

Scotland The guidance requires that separating walls are continued through the roof void to the underside of the roof membrane.

Up to a building height of 15 m, combustible materials used as a substrate to the roof covering may be continued over the compartment wall provided that they are embedded in mortar or other cementitious noncombustible material over the width of the wall.

Up to a building height of 15 m, combustible materials used as a substrate to the roof covering may be continued over the compartment wall provided that they are embedded in mortar or other cementitious noncombustible material over the width of the wall.

For buildings in excess of 15 m, fire rated roof coverings (classification AA, AB or AC) on a deck of a material of limited combustibility are required to a distance of 1500 mm to either side of the wall.

For buildings in excess of 15 m, fire rated roof coverings (classification AA, AB or AC) on a deck of a material of limited combustibility are required to a distance of 1500 mm to either side of the wall.

The guidance recommends the separating or compartment wall is continued through the roof by a distance of at least 375 mm except: ❐ where the roof substrate is noncombustible; ❐ the junction is fire-stopped and the roof covering is low vulnerability for a distance of at least 1.7 m to each side of the centre line of the wall; ❐ in a pitched roof covered by slates that are nailed directly to sarking and underlay where the junction between the sarking and the wall head is fire stopped; ❐ in a pitched roof covered by slates or tiles fixed to tiling battens and any counter battens, where only the tiling battens and underlay are carried over the wall and are fully bedded in mortar (or similar) at the wall head.

Notes Typical solutions for roof void compartmentation details provided in Timber Frame Construction

[10].

Classifications for roof coverings are derived from the British Standard fire test BS 476: Part 3. An alternative approach may be to consider designing the ceiling of the uppermost storey as fully fire resisting, ie as a non-loadbearing compartment floor with a rating of 60 minutes (for a medium rise building). This could let an unoccupied roof void be treated as a separate void compartment that may then only require lightweight cavity barriers (if at all). A precedent for this type of approach has already been set by instances where the regulatory authorities have allowed compartmentation between the units in shared residential accommodation (eg student halls of residence) to be terminated at the soffit of the uppermost storey. The compensatory measure has been the specification of a 30-minute fire rated ceiling lining to the uppermost storey. The reason for this alternative approach has been to simplify the amount of work necessary in the roof void.

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Multi-storey timber frame buildings – a design guide

Table A9 Enclosures to protected corridors, lobbies and stairs that are not also compartment walls separating dwellings from these zones

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England & Wales The guidance recommends a fire resistance for enclosing walls of 30 minutes.

Northern Ireland The guidance recommends a fire resistance for enclosing walls of 30 minutes.

Scotland The guidance requires the following fire resistance levels for the enclosing structure of: Protected Zone = Medium (60) Protected Lobby = Short (30) Protected Enclosure = Short (30)

Note Such walls need not necessarily be constructed to the specification for compartment walls given in Chapter 5 of Timber Frame Construction [10] but instead may be more akin to the specification given in Chapter 4 of the document for internal walls. Where the specification in Chapter 4 of the document is used, care must be taken to establish that the quoted fire resistance rating of 30 minutes relates to integrity and insulation performance and not just loadbearing capacity.

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Table A10 Protected services and shafts England & Wales The guidance recommends that a protected service shaft that conveys piped flammable gas should be provided with ventilation openings to outside air at the top and bottom of the shaft. The fire resistance of the protected shaft should be as derived for compartmenting elements of structure.

Northern Ireland The guidance recommends that a protected service shaft that conveys piped flammable gas should be provided with ventilation openings to outside air at the top and bottom of the shaft. The fire resistance of the protected shaft should be as derived for compartmenting elements of structure.

The guidance allows for suitably firestopped pipes and fire resisting doors to penetrate compartment walls separating occupancies. Therefore, by implication, it is recommended that other services such as ventilation ducts and cables do not penetrate these compartment walls.

The guidance allows for suitably firestopped pipes and fire resisting doors to penetrate compartment walls separating different occupancies. Therefore, by implication, it is recommended that other services such as ventilation ducts and cables do not penetrate these compartment walls.

Scotland The guidance makes no reference to protected shafts and therefore places no special requirements on construction and makes no recommendations with respect to deemed to satisfy solutions. The approach is simply then to treat service risers etc as creating holes in separating floors that need to be addressed in order to maintain the compartmentation offered by the separating floor.

Notes A protected escape stair is a form of protected shaft (see Table A2). A firefighting shaft (if required) is also a form of protected shaft. Service risers do not necessarily have to be designed as protected shafts but there may be benefits to be gained in reducing the amount of firestopping if a large number of services penetrate compartment floors with a few of these services branching out at each floor level. Maintenance and the addition of new services can also be facilitated by employing service risers that are designed as protected shafts. Should the concept of a protected service shaft be designed for a project in Scotland, the approach for designing the enclosing walls of the shaft should be approached as for separating walls, ie the enclosing walls would be required to be of non-combustible construction if the floor height of the top storey is greater than 18 m. Chapter 10 of Timber Frame Construction [10] shows solutions for maintaining the fire resistance rating of compartment/separating walls where service risers pass through compartment floors. The guide also shows a detail for maintaining the fire resistance of compartment/separating walls at the location of socket outlets. TRADA Technology Report RR1/2001 [33] describes the results of fire resistance testing that demonstrated that electrical power socket outlets may not compromise the fire resistance of timber frame compartment/separating walls and this data could potentially be used on a case by case basis to demonstrate an alternative approach.

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Table A11 Waste chutes England & Wales The guidance recommends by implication in Clause 9.35c that refuse chutes should be of non-combustible construction.

Northern Ireland Scotland The technical provisions allow for Makes no specific requirements or waste chute systems constructed of recommendations non-combustible materials to penetrate compartment floors. Regulation J3 requires waste chute systems to be constructed of noncombustible materials.

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Notes Waste chutes are effectively treated as a special form of protected shaft. BS 5906 gives specific guidance with respect to the design of on-site storage/treatment of solid waste from buildings. The provisions of this standard for waste chute systems are considered as deemed to satisfy in the Northern Ireland Regulations. A non-combustible refuse chute can be provided in the form of an insulated steel duct. Where the chute passes through compartmentation boundaries, it would be required to be fire resisting in terms of integrity and insulation performance.

Table A12 Cavity barriers England & Wales Cavity barriers are recommended to be provided: ❐ at the junction between the external cavity wall and every compartment wall and compartment floor; ❐ at the junction between a cavity wall and every compartment floor, compartment wall, or other wall or door assembly that forms a fire resisting barrier; ❐ in a protected escape route, above and below any fire resisting construction which is not carried full storey height, or (for the top storey) to the underside of the roof covering (this is relaxed if the cavity is fully enclosed throughout the compartment on its lower side by a ceiling with a fire resistance of 30 minutes); in the void behind the external face of rainscreen cladding at every floor level, and on the line of compartment walls abutting the external wall of buildings that have a floor 18 m or more above ground level; ❐ at the edges of cavities (including around openings): guidance allows for window and door frames to form cavity barriers in this regard (a PVC continued ...

Northern Ireland Cavity barriers are recommended to be provided: ❐ at the top of an external cavity wall; ❐ at the junction between the external cavity wall and every compartment wall and compartment floor; ❐ at the junction between a cavity wall and every compartment floor, compartment wall, or other wall or door assembly that forms a fire resisting barrier; ❐ in a protected escape route, above and below any fire resisting construction which is not carried full storey height, or (for the top storey) to the underside of the roof covering (this is relaxed if the cavity is fully enclosed throughout the compartment on its lower side by a ceiling with a fire resistance of 30 minutes); ❐ in the void behind the external face of rainscreen cladding at every floor level, and on the line of compartment walls abutting the external wall, of buildings that have a floor 20 m or more above ground level.

Scotland Cavity barriers are required to be provided: ❐ around the edges of the cavity where a wall, floor or other part of the building which is required to be fire resisting, other than a wall which is required to be fire resisting only because it is loadbearing, abuts a structure containing a cavity: it must be installed so as to extend the line of the fire resisting structure; ❐ where the cavity is above a ceiling and continues over a wall between the accommodation and an escape route, or above a fire door across an escape route: the barrier must be installed in the same plane as the wall and door; ❐ where required to maintain the distance between cavity barriers to a maximum of 10 m: where the surfaces in the cavity are Low Risk (see Table A4) or non-combustible, the distance can be extended to 20 m; ❐ within the ventilated void between a rainscreen panel and an external wall at every floor level, and on the line of compartment walls abutting the Cavity barriers are recommended to external wall, of buildings that have a have a fire resistance of 30 minutes floor 11 m or more above ground continued ... continued ...

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Table A12 Cavity barriers ... continued window or door frame should not be considered as performing as a cavity barrier).

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Cavity barriers are recommended to have a fire resistance of 30 minutes integrity and 15 minutes insulation. Deemed to satisfy cavity barriers for stud walls or partitions include: ❐ steel sheet with a minimum thickness of 0.5 mm; ❐ timber with a minimum thickness of 38 mm; ❐ polyethythene sleeved glass wool or glass wool slab placed under compression when installed; ❐ calcium silicate, cement or gypsum based boards at least 12 mm thick. The guidance also recommends that cavity insulation material in external walls should be of limited combustibility if any storey has a floor height in excess of 18 m above ground level (this recommendation is made in Section 13 of the statutory guidance dealing with the control of external fire spread).

integrity and 15 minutes insulation. However, cavity barriers for stud walls or partitions may include: ❐ steel sheet with a minimum thickness of 0.5mm; ❐ timber with a minimum thickness of 38 mm; ❐ polyethythene sleeved glass wool or glass wool slab placed under compression when installed; ❐ calcium silicate, cement or gypsum based boards at least 12.5mm thick. The guidance also recommends that cavity insulation material in external walls and any materials used to support cladding should be of limited combustibility if the building has a storey with a floor height in excess of 20 m above ground (this guidance is given in Section 4 of Technical Booklet E).

level. If a rainscreen is formed by overcladding with Low Risk (see Table A4) surfaces attached to a masonry external wall and the cavity contains no combustible material, such cavity barriers may be avoided The guidance also requires that cavity insulation material in external walls should be non-combustible if the building has a storey with a floor height in excess of 18 m above ground level. Cavity barriers are required to have a fire resistance of 30 minutes for integrity only. Recommended deemed to satisfy cavity barriers for stud walls or partitions include: ❐ steel sheet with a minimum thickness of 0.5 mm; ❐ timber with a minimum thickness of 38 mm; ❐ polyethythene sleeved glass wool or glass wool slab placed under compression when installed; ❐ calcium silicate, cement or gypsum based boards at least 12mm thick.

Notes The deemed to satisfy cavity barriers recommended by all the three sets of statutory guidance relates to timber stud walls and partitions. Cavity barriers between external masonry work and timber frame wall panels should, therefore, be tested and approved because the deemed to satisfy solutions are not deemed to satisfy in that location. Class 0 is a reaction to fire performance classification that is derived from the British Standard fire tests BS 476: Part 6 and BS476: Part 7 (both tests have to be carried out). Scottish guidance allows the insulation performance criterion for large cavity barriers to be waived. This allows for large cavity barriers to be installed that are fabricated from such materials as woven glass fibre cloth. Whilst the E&W and N. Ireland guidance does not contain the same waiver, it is possible that the insulation criterion could be accepted for these cavity barriers in specific cases. Chapter 5 of Timber Frame Construction [10] shows details of cavity barrier solutions at the junctions of compartment walls and compartment floors. Chapter 8 of the document shows details of cavity barrier protection at the eaves of the building and Chapter 9 contains specific details relating to the design treatment of cavity barriers in external walls. The research work on the TF2000 building has demonstrated the importance that workmanship plays in the adequacy of cavity barriers; they must be fitted in the correct location and not dislodged during other subsequent construction works. For example, there is a risk of the build up of excess mortar from bricklaying operations on the top of the cavity barriers causing the cavity barriers to be dislodged from their required location. This would produce a breach in the fire resisting line of the cavity barrier. Further advice on the use of thermal insulation material is given in BRE Report BR135

[31].

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Multi-storey timber frame buildings – a design guide

Table A13 Lift shafts

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England & Wales A lift well may be provided in a protected stair provided the stair is not also a firefighting stair.

If not located within the confines of a protected stair, it is recommended that the lift well is designed as a protected shaft.

Northern Ireland Scotland A lift well may be provided in a There are no special requirements but protected stair provided the stair is not the design must preserve the also a firefighting stair. compartmentation that is provided by separating floors. If not located within the confines of a protected stair, it is recommended that the lift well is designed as a protected shaft.

It is recommended that lift motor rooms are sited over the lift well. Where the lift well is within a protected stairway, that is the only stair serving the building and, if the motor room cannot be located above the lift well, it is recommended that the motor room is located outside the stairway to prevent smoke from a fire in the motor room affecting viability of the escape route.

It is recommended that lift motor rooms are sited over the lift well. Where the lift well is within a protected stairway, that is the only stair serving the building and, if the motor room cannot be located above the lift well, it is recommended that the motor room is located outside the stairway to prevent smoke from a fire in the motor room affecting viability of the escape route.

If located in a protected stair, the walls of the lift shaft do not need to be provided with fire resistance if they are not also part of the fire resisting walls enclosing the protected stair. Lift landing doors in the stair do not need to be fire resisting.

If located in a protected stair, the walls of the lift shaft do not need to be provided with fire resistance if they are not also part of the fire resisting walls enclosing the protected stair. Lift landing doors in the stair do not need to be fire resisting.

Where the lifts are located in a protected lobby or protected corridor, the walls of the lift will need to be constructed as compartment walls and the lift landing doors are recommended to have half of the period of fire resistance recommended for the walls subject to a minimum FD 30 rating.

Where the lifts are located in a protected lobby or protected corridor, the walls of the lift will need to be constructed as compartment walls and the lift landing doors are recommended to have a minimum FD 30S rating.

Notes There are some subtle differences in how the issue is approached in the Scottish statutory guidance. In E&W and N. Ireland, a lift that is located wholly within the protected stair enclosure will not have to have fire resisting walls between the lift well and the stair enclosure and will not have to have fire resisting lift landing doors. In Scotland, there would be a requirement for 30-minute walls and 30-minute lift landing doors. In E&W and N. Ireland, a lift well located in the protected lobby approach to a protected stair would be designed as a protected shaft with 60-minute walls and 30-minute doors (for a building with a top storey height less than 18 m for E&W, and 20 m for N.Ireland). In Scotland, a lift in the same location would be required to have 30-minute walls and 30-minute doors.

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Table A14 Fire doors England & Wales Recommendations with respect to fire doors can be summarised as follows: ❐ separating dwellings from a space in common use = FD 30S; ❐ enclosing a protected stairway = FD 30S; ❐ enclosing a protected lobby or protected corridor to a stairway = FD 30S; ❐ providing access to a protected service shaft = FD yy, where yy = half the period of fire resistance required by the wall of the shaft, ie FD 30 for a 60-minute shaft and FD 60 for a 90-minute shaft (subject to a minimum performance of FD 30); ❐ sub-dividing corridors giving access to alternative exits = FD 20S; ❐ within a cavity barrier = FD 30; ❐ forming part of the protected enclosure to a hall or landing in a dwelling = FD 20; ❐ access to an external escape route = FD30; ❐ in any other wall or floor = FD yy, where yy = the fire resistance of the wall or floor.

Northern Ireland Recommendations with respect to fire doors can be summarised as follows: ❐ separating dwellings from a space in common use = FD 30S; ❐ enclosing a protected stairway = FD 30S; ❐ enclosing a protected lobby or protected corridor to a stairway = FD 30S; ❐ providing access to a protected service shaft = FD 30; ❐ sub-dividing corridors giving access to alternative exits = FD 20S; ❐ sub-dividing a dead-end portion of a corridor from the remainder of the corridor = FD 20S; ❐ within a cavity barrier = FD 30; ❐ forming part of the protected enclosure to a hall or landing in a dwelling = FD 20; ❐ access to an external escape route = FD30; ❐ in any other wall or floor = FD yy, where yy = the fire resistance of the wall or floor.

Scotland Fire doors are required to have the same fire resistance as for the wall in which they are installed. Unless the door gives access to an area where pressurisation techniques for smoke control are utilised, it is recommended that the door also has smoke control performance.

Notes Explanation of classification FD xxS: FD = Fire door xx = fire resistance rating in accordance with the British Standard fire resistance test BS 476: Part 22 (now superseded by European Standard BS EN 1634: Part 1) S = smoke seals should be incorporated in the door unless pressurisation techniques complying with BS 5588: Part 4 are used. A smoke-sealed door is expected to have a smoke leakage rate not in excess of 3 m3/m/hour at the head and vertical door edges when subjected to the conditions of the British Standard fire test BS 476 Part 31.1. The Scottish Technical Standards refer also to the European test standard for smoke control doors as an alternative to the British Standard. In terms of design and installation information for fire doors with timber leaves, BS 8214 is a useful guidance document to help suppliers, specifiers and installers avoid potential problems. Fire doors need to be self-closing.

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References and further reading

[1] Enjily, V, and Mettem, CJ. Medium-rise timber frame buildings: disproportionate collapse and other design requirements. Garston, BRE, 1995. [2] Enjily, V, and Palmer, S. Timber frame 2000. Phase I: Summary of commercial and technical findings. Garston, BRE, 1996. [3] Steer, PJ. Design of TF2000 Building. Proc COST Action E5 Workshop: Constructional, structural and serviceability aspects of multi-storey timber frame buildings. Garston, BRE,1998. [4] Enjily, V, Palmer, S and Pitts,G. TF2000 – an update. Proc COST Action E5 Workshop: Constructional, structural and serviceability aspects of multi-storey timber frame buildings. Garston, BRE, 1998. [5] Enjily, V, and Palmer, S. The current status of Timber Frame 2000 (TF2000). Proc Pacific Timber Engineering Conf, Rotarua, New Zealand. Forest Research Bulletin 212. New Zealand Forest Research Institute, 1999. [6] BRE. Specifying dwellings with enhanced sound insulation. BRE Report BR406. Garston, BRE, 2000. [7] Pitts, G. Acoustic performance of party floors and walls in timber framed buildings. TRADA Technology Report 1/2000. High Wycombe, 2000. [8] Enjily, V, and Mettem, CJ. The current status of medium-rise timber frame buildings in the UK. Proc International Wood Engineering Conference. Ed, Gopu, VKA. Omnipress, Madison, USA, 1966. [9] Milner, MW, Edwards, S, and Enjily, V. Verification of the robustness of a six-storey building to accidental damage. Proc COST Action E5 Workshop: Constructional, structural and serviceability aspects of multi-storey timber frame buildings. Garston, BRE, 1998. [10] TRADA Technology. Timber Frame Construction. TRADA, High Wycombe. 2001. [11] Milner, MW, Edwards, S, Turnbull, DB, and Enjily, V. Verification of the robustness of a six-storey timber frame building. The Structural Engineer, 1998, 76, 16. London, UK. [12] Mettem, C, Milner, M, Bainbridge, R, and Enjily, V. Timber frame engineering for mediumrise structures. Proc World Conference on Timber Engineering, WCTE 1998. Montreux. Presses polytechniques et universitaires romandes. Lausanne, Switzerland, 1998. [13] TRADA Technology. Disproportionate collapse of timber structures. Part 2: Permissible stresses in fasteners and behaviour of timber connections under short duration loading. Part 3: Fullscale testing programme simulating accidental events on a trussed rafter roofed building. And video of test programme. TRADA Research Report RR3/93. TRADA, High Wycombe, 1993. [14] Lennon, T, Bullock, M and Enjily, V. The fire resistance of medium-rise timber frame buildings. Proc World Conference on Timber Engineering, WCTE. University of British Columbia, Canada, 2000. [15] Enjily, V. The fire performance of a six-storey timber frame building at BRE Cardington (GB). Proc Brandschutz im Holzbau. 23, 24 October, Würzburg, Germany. Deutsche Gesellschaft für Holzforschung e. V. (DGfH). 2001. [16] Lennon, T, Bullock, M and Enjily, V. The fire resistance of timber frame building. BRE Report No 79486-1. Garston, BRE, 2000. [17] Lennon, T, Bullock M, and Enjily, V. Medium rise timber frame 2000 stair fire test. BRE Report No 200-711. Garston, BRE, 2000. [18] Covington, S, Braver, A, and Wynands, R. Moisture conditions in the walls of timber framed housing. BRE Report BR228. Garston, BRE, 1992. [19] BRE. External walls: brick cladding to timber frame – the need to design for differential movement. Defect Action Sheet 75. [20] BRE. The movement of timbers. PRL Technical Note N 38. BRE, Garston, 1982. [21] Morton, J. Designing for movement in brickwork. BDA Design Note 10. Windsor, Brick Development Association, 1998. [22] BRE. Clay bricks and clay brick masonry. Digest 441 (in two parts).

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References and further reading

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[23] BRE. Estimation of thermal and moisture movements and stresses: Part 1. Digest 227. [24] BRE. Estimation of thermal and moisture movements and stresses: Part 2. Digest 228. [25] BRE. Estimation of thermal and moisture movements and stresses: Part 3. Digest 229. [26] Grantham R, and Enjily, V. Differential movement between the brick cladding and timber frame of the TF2000 building. Proc World Conference on Timber Engineering, WCTE2000. University of British Columbia, Canada, 2000. [27] BRE. Ties for masonry walls: a decade of development. Information Paper 11/00. [28] Keyworth, B. Brick cladding to timber frame construction. Windsor, Brick Development Association, 1992. [29] Palmer, S, and Enjily, V. TF2000 – Performance targeting. Proc COST Action E5 Workshop: Constructional, structural and serviceability aspects of multi-storey timber frame buildings. Garston, BRE,1998. [30] Palmer, S, and Herriot, R. Medium size timber frame: a best practice benchmarking guide. TRADA Technology, High Wycombe, 1999. [31] BRE. Fire performance of external thermal insulation for walls of multi-storey buildings. BRE Report BR135. Garston, BRE, 1988. [32] British Gypsum. Technical manual of building products. The White Book. Nottingham, British Gypsum Ltd, 2001. [33] TRADA Technology. Timber frame walls and floors: fire resistance of service penetrations. Report 1/2001. TRADA, High Wycombe, 2001. BRE Defect Action Sheet 75 External walls: brick cladding to timber frame - the need to design for differential movement BRE Digests 227 Estimation of thermal and moisture movements and stresses: Part 1 228 Estimation of thermal and moisture movements and stresses: Part 2 229 Estimation of thermal and moisture movements and stresses: Part 3 441 Clay bricks and clay brick masonry (in two parts) BRE Information Paper 11/00 Ties for masonry walls: a decade of development

Building Regulations Department of the Environment and the Welsh Office. The Building Regulations 1991. Statutory Instrument 1991 No 2768. London, HMSO, 1991. Approved Documents Approved Document A: Structure (1992 edition). 1991. Approved Document B: Fire safety (1992 edition). 1991. Approved Document C: Site preparation and resistance to moisture (1992 edition). 1991. Approved Document E: Resistance to the passage of sound (1992 edition). 1991. Approved Document F: Ventilation (1995 edition). 1994. Approved Document L: L1 Conservation of fuel and power (1995 edition). 1994. Approved Document to support Regulation 7: Materials and workmanship (1992 edition). 1991. Scottish Office, Environment Department The Building Standards (Scotland) Regulations 1990. Statutory Instrument 1990 No 2179 (S187). London, HMSO, 1990. Approved Documents Part C: Structure Part D: Structural fire precautions Part G: Preparation of sites and resistance to moisture Part K: Ventilation in buildings Part J: Conservation of fuel and power Department of the Environment for Northern Ireland The Building Regulations (Northern Ireland) 2000. Statutory Rules of Northern Ireland 2000 No 389. Belfast, The Stationery Office Ltd, 2000. Approved Documents Technical Booklet C: Preparation of site and resistance to moisture. 1994. Technical Booklet D: Structure. 1994. Technical Booklet E: Fire safety. 1994. Technical Booklet F: Conservation of fuel and power. 1998. continued ...

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Multi-storey timber frame buildings – a design guide British Standards Institution BS 476 Fire tests on building materials and structures BS 5268 The structural use of timber Part 2: 1996 Code of practice for permissible stress design, materials and workmanship Part 3: 1985 Code of practice for trussed rafter roofs Part 6: 1996 Code of practice for timber frame walls Section 6.1 Dwellings not exceeding four storeys Section 6.2 Buildings other than dwellings not exceeding four storeys BS 5588 Fire precautions in the design, construction and use of buildings BS 6399: Loading for buildings Part 1: 1984 Code of practice for dead and imposed loads Part 2: 1995 Code of practice for wind loads Part 3: 1988 Code of practice for snow loads BS 8200: 1985 Design of non-loadbearing external vertical enclosures of buildings.

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BS 8214: 1990 Code of practice for fire door assemblies with non-metallic leaves CP3: Code of basic data for the design of buildings Chapter V: Part 2: 1972 Wind loads

European Standards Eurocode 5: Design of timber structures. General rules. Structural fire design (together with United Kingdom National Application Document) BS EN 338 Loading for buildings. Structural Timber. Strength Classes BS EN 1634 Fire resistance tests for doors and shutter assemblies CEN –2001- EN 1990: 2001 Eurocode. Basis of structural design – Section 2