All Technical Guidance Notes in Level1

󲀺      

www.thestructuralengineer.org

Technical

TheStructuralEngineer

39

January 2012

Technic echnical al Guidance Guidance Notes: an introduction In his editorial of 18th October 2011, Managing Editor Lee Baldwin heralded the introduction of a series of ‘Technical Guidance Notes’. Sarah Fray - Director: Engineering and Technical Services provides an introduction to the series. Fray  Director Sarah Fray

The Technical Guidance Notes are one of a range of initiatives planned by the Institution’s Engineering and Technical Services Department to increase the practical support offered to members. The notes have been conceived to provide technical guidance to both undergraduates and those in the early stages of their careers, with the intention of helping them to gain skills and technical competence in the workplace and hence increase their individual value to the businesses they contribute to. Experienced

LEVEL 󰀳 Complex design concepts LEVEL 󰀲 Element design and communication LEVEL 󰀱 Core design concepts

Technicians may also find these notes helpful when looking to develop a greater understanding of structural design - which may bring benefits to the overall quality of structural detailing and also enhance an individual’s career. The Technical Guidance Notes are intended to be easily accessible. Each note is designed to form part of the foundation of a personal technical reference library which can be continuously conti nuously referred to. In developing the strategy for the Technical Guidance Notes, we have been conscious of the need to provide sound foundations from which design skills can be developed, and so basic structural engineering fundamentals are presented initially; i nitially; the implementation plan for the series has identified second and third tier subjects which, in the medium term, will address structural engineering principals at increasingly complex levels. It should be noted that we have decided to adopt titling using traditional UK terms such as ‘loading’ rather than Eurocode terms. Whilst the notes have been written to adhere to the Eurocodes we have taken I HOPE THAT MANY the view that adopting the OF YOU WILL Eurocode titles would not BENEFIT FROM aid the accessibility for the  THE SER SERIES IES AND relatively inexperienced. WILL MAKE USE OF I hope that many of you will benefit from the series and will  THOSE  THO SE GUI GUIDAN DANCE CE make use of those guidance NOTES THAT ARE notes that are particularly PARTICULARLY applicable to your field. The  APPLIC  APP LICABL ABLE E TO TO first two, entitled: ‘Principles of

 YOUR  YOU R FIEL F IELD D

design’ and ‘Derivation of dead loads’ follow. 󒀢

󲀺     Note 1 Level 1  

40

TheStructuralEngineer

Technical

January 2012

Technical Guidance Note

Principles of design ICON LEGEND

Introduction This Technical Guidance Note acts as an introduction to the core design concepts that are found within the current codes of practice used within the UK. It also explains the relationship between between each of the other guidance notes and how the reader is to navigate and use them. All of the subsequent notes make reference, be they direct or implied to this core guide; it is therefore imperative that anyone seeking to use these guides must be fully conversant with what is contained within this note.

􂀢  Applied practice practice

􂀢 Worked example

􂀢 Further

 All of the guidance notes in this series have an icon based navigation system designed to aid the reader.

Design principles The current codes of practice used in the UK are the Eurocodes. The design d esign assumptions, criteria and terminology used throughout these documents are explained in this note. These explanations serve as a reference to all subsequent notes, as they make regular reference to the terms and concepts that are defined below.

Definition of Terms and Concepts The following principles are the core components of design of structural elements. They are the basis from which all design is carried out. Many of the terms listed below are also found in Eurocode – Basis of Structural Design, which is sometimes referred to as ‘Eurocode 0’. Action: In the most simplistic of terms, an Action: action is a load that is applied to a structure. It can also however be an effect on the structure via an external source. Examples of such sources include; change in temperature, differential settlement of foundations, earthquakes and moisture variation. Limit State Design: Limit state design is the guiding principle upon which all of the current codes of practice are based. The concept centres on the placing of an extremis upon a structure and all actions that are placed upon it during its design working life should not

exceed this defined point. Ultimate Limit State: The ultimate limit state, sometimes abbreviated to ULS, is the point at which a structure will collapse when subjected to actions that cause it to exceed this limit. These actions are multiplied by partial factors that are defined in Eurocode 0.

􂀢 Design principles

􂀢 Web

reading

resources

Local to element axial or ‘Torsion Axis’ is designated as ‘x-x’ w

See Figure 1 for 1 for further clarification on the geometric axis notation protocol:

Serviceability Limit State: The serviceability limit state, sometimes abbreviated to SLS, is a defined point at which the structure fails to comply with pre-defined criteria. These criteria are normally related to the movement of the structure that occurs after it is subjected to an action. These movements can be of the structure as a whole or elements within it e.g. the mid-span vertical deflection of a simply supported beam. Equilibrium: Equilibrium is an analysis state that checks for instabilities within a structure. It is designated as ‘EQU’ within the Eurocodes and has its own set of partial factors that are applied to loads for when equilibrium analysis is being carried out.

1 Local Axis notation Figure 1 Local

Design Assumptions and Criteria

Local Geometric Axes: The nomenclature for the local axes to structural elements has been standardised within the Eurocodes.

There are a set of key design assumptions and criteria described within the Eurocodes that structures are subject subject to. They must be referred and adhered to when carrying out

This notation is defined as follows:

any analysis and design of structures.

Local to element horizontal axis or ‘Minor Axis’ is designated as ‘z-z’ w Local to element vertical axis or ‘Major Axis’ is designated as ‘y-y’

Competency: All design and construction of the structure is to be carried out by people with the appropriate level of skill and experience.

w

 

www.thestructuralengineer.org

41

Supervision: Adequate supervision must be provided to all personnel who are involved with the design and construction of the structure. Maintenance and usage: The structure shall be maintained and used in a manner that is in accordance with the design assumptions. Materials: The materials used to build the Materials: The structure will be in accordance with relevant codes of practice and manufacturer’s specifications. Design life: All structures have a design life period. The designer m must ust take this into account when considering any element within a structure in terms of its ability to achieve that design life period. The relevant UK National Annex lists 5 categories for design life of structures. They are reproduced in Table 1 below: Category No.

Design working life (years)

Typical structures

1

10

Temporary structures e.g. access platforms

2

10-30

Replaceable structural parts e.g. bearings

3

15-25

Agricultural buildings

4

50

Common building structures not listed under categories 1,2,3 & 5

5

12 0

Monuments, bridges and civil engineering structures

Environmental protection: The primary methods by which elements of the structure are to be protected against environmental effects e.g. moisture, are specified by the designer of the structure. An example of this would be the corrosion protection coating to steel elements. elements. Note that this does not include secondary and tertiary methods of protection, such as damp proof membranes within floor slabs. Fire resistance requirements: All elements within a structure that require additional fireresistance that is over and above what it is inherently able to resist, must be highlighted. A good example of this is a steel column that requires some form of fire-resistant material applied to it. Note that it is not the requirement of the designer of the structure to determine the form of fire protection; they must only highlight what needs to be protected. Materials: A description of the materials Materials: A used within the structure is normally included within the specification. This document describes the materials that are to be used to build the structure and the standards they are expected to meet. Examples of such materials include concrete mix, steel grade and timber type and strength class.

1 Design life categories from Table NA.2.1 Table 1 Design

 Applied practice

of UK National Annexe to Eurocode – Basis of Structural Design

Essential Information As part of the design process, an expected level of information is required to be produced and shared by the designer of the structure within any given project. What follows are descriptions of what design information is typically expected to be delivered. It must be noted that this is not an exhaustive list: Design loads:The loads:The designer must describe what loads the structure has been designed for. This is then communicated to the rest of the design team and the end user so that they know what loads the design of the structure has been based on. From this they can then have a good understanding of what limits have been imposed upon room use, e.g. where it is possible to use a room for storage. Structural member sizes and location: All load bearing elements within a structure must be sized and located. This includes lintels for openings within non-load bearing masonry walls, as although the element they are installed within is non-load bearing, the lintel itself is.

The applicable codes of practice for basis of structural design are as follows: BS EN 1990: Eurocode 1990: Eurocode Basis of Structural Design

give a curtailed instruction on aspects of structural engineering design practices. The reader is therefore urged to use these guides in conjunction with the current codes of practice. With regard to navigation, the guidance notes are published in an approximate order of increasing complexity. A higher level (1, 2 or 3) denotes a significant increase in 2   complexity from the previous level. Figure 2 explains this system as well as the reliance on prior knowledge of the guides:

Figure 2 Navigation 2 Navigation of Technical Guidance Note example

Glossary and further reading Glossary : The list below is of terms used in this Technical Guidance Note. All notes will have such a list to aid the reader. Code of Practice – Practice – A set of rules that need to be followed in order to complete a task to achieve an appropriate standard. In the UK, the British Standards Institute authors and distributes the codes of practice for the design of structures.

BS EN 1990: UK 1990: UK National Annex to Eurocode: Basis of Structural Design

 Worked example Each subsequent guidance note will include a worked example that seeks to explain further the concepts defined within it. This particular note however acts as more of an aid to those reading all of the other guides, and as such does not include such an example. It is important that these notes are navigated correctly. Crucially, they must not be treated as a replacement for codes of practice. They are guides that aid the reader in the design of structures. With this in mind, they have been developed to

Eurocodes – A set of European-wide codes Eurocodes – of practice for the design of both building and civil engineering structures. Technical Guidance Note – Note – A brief guide on core aspects of the design of structures and the elements they are built from.

Further Reading Manual for the design of building structures to Eurocode 1 and Basis of Structural Design –Institution of Structural Engineers – April 2010

 Web resources For more information on this subject, please visit the Institution’s website: www.istructe.org/knowledge/library

󲀺    

www.thestructuralengineer.org

Note 2 Level 1

 

Technical

TheStructuralEngineer

Technical Guidance Note

January 2012

43

Derivation Deriva tion of dead loads Introduction This Technical Guidance Note concerns the derivation of dead dead loads. This is a core guidance note and as such, subsequent notes will make reference to this one. It is therefore important to understand understand the contents of this note before attempting to digest any of the others.

ICON LEGEND

􂀢 Design principles

􂀢  Applied practice practice

􂀢 Worked example

 All of the guidance notes in this series, have an icon based navigation system designed to aid the reader.

􂀢 Further

􂀢 Web

Design principles Dead load is defined as the weight of static materials contained within a structure. This includes the self weight of the structure as well as the materials it is supporting that are fixed to it. Within Eurocode 1 it is defined as a Permanent Action.

Definition of Dead Load Elements  Within a Structure Structure Building elements that can be considered to be part of the self weight of the construction form the dead load of the structure. Examples include: self weight of the structure elements i.e. beams, columns and floor slabs •  finishes e.g. screeds and ceilings and ceiling finishes. These are als also o sometimes referred to as ‘super imposed dead loads’ •  building services installations, such as ducts, cable containment, small pipes and lighting •  soil •  fixed partitions, i.e. those that cannot be demounted and placed elsewhere and non load bearing walls

• 

There are many elements that are often oft en mistaken as being a part of the self weight of construction when they are imposed load elements (see Technical Guidance Note 3, Level 1). These include the following:

moveable partitions plant room installations, such as air handling units and boilers •  cradles for maintenance access, but not the secondary support beams and rails they are fixed to •  sports and gym equipment such as goal posts, basketball hoops and punch bags •  audio and video equipment installations, including speaker clusters, televisions and lighting rigs •  • 

reading

resources

Some dead loads can be expressed as an area load. Table 2 2 lists  lists such loads: Material

Area Load (kN/m2)

Building services†

0.25 kN/m2

Insulatio Insulation n per 25mm th thickne ickness ss 0.005 0.005 kN/ kN/m2 m2 Suspended ceilings†

0.25 kN/m2

Raised floors†

1.00 kN/m2

Roof tiles

0.39 – 1.00 kN/m2

† These are typical values and can therefore vary.

Table 2 Typical area dead loads

Densities for Common Materials Found in Structures Table 11 is  is a list of common materials found in

(γ 󰁇 ) for Dead Loads Partial Factors (Permanent Actions)

building structures along with their attributed densities:

Partial factors are used within Limit State design methodology. They are applied when designing elements based on their capacity to resist stress, be it shear, bending, torsion or a combination of the three. They are also used when checking for stability within the structure. The Eurocode’s approach to these factors considers the nature of the load before any factors are applied to it. Dead loads, or ‘Permanent Actions’ do not have the duration and frequency factors defined in Eurocode 1 applied to them. These factors are however expanded upon within Technical Guidance Notes 3, 4 & 5 (Level 1) as they cover imposed, wind wi nd and

Material

 (γ)

Density

Soil *

17-25 kN/m3

Reinforced Concrete

25 kN/m3

Mass Concrete

24 kN/m3

Steel

78.5 kN/m3

Timber (Softwood)

5.5 kN/m3

Plywood sheeting

5 kN/m3

Brick (facing) *

13-22.5 kN/m3

Brick (engineering) *

21-24 kN/m3

Block (lightweight) *

8.5-11 kN/m3

Block (medium density)

14 kN/m3

Block (dense/architectural) Plaster

19-21 kN/m 19.2 kN/m3

Glass

25 kN/m3

3

* Varies depending upon material type/manufacturer  type/manufacturer 

Table 1 Typical dead load densities

snow loads respectively. All of which are subject to duration and frequency factors. The only variance in the partial factor for dead loads occurs when it is acting ‘favourably’ in a certain loading pattern or is an element of a sub-structure.

 

󲀺     Note 2 Level 1

44

TheStructuralEngineer

Technical

January 2012

Technical Guidance Note

Stability Analysis Partial Factors (γ 󰁇 )

Consider figure 1 below, Consider  1 below, where ‘G  ’ refers to 󰁫  ‘ ’ the imposed load: the dead load and  Q 

󰁫 

Figure 1 Equilibrium 1 Equilibrium partial factors

It can be seen that the load in the main span is acting favourably to resist the load being applied to the cantilever section. This cantilever load is causing instability in the structure, as support ‘A’ is unable to resist an upward vertical force. The factors that are applied to loads are dependent upon the nature of the analysis being carried out. In the case of the above, the stability of support point A is being checked for uplift. This falls under the category of checking for equilibrium in a structure and hence the following partial factors (γ 󰁇 ) apply:

been applied to the loads placed upon it. This analysis concerns the design of the element and its supports. In this instance the worst case bending moments and shear forces are based on the maximum partial factors (γ 󰁇 ) that the Eurocode 1 will allow, which are: ‘Favourable’ and ‘Unfavourable’ dead load – 1.0 or 1.35 G  󰁫  This is on the basis of the single-source principle that requires all dead loads have a single partial factor applied to them. Therefore in the case of multi-span beams, the value of the partial factor (γ 󰁇 ) can only be 1.0 or 1.35 and not a mixture of the two when analysing the beam. Again the  Q  ‘ ’ refers to imposed load (also 󰁫  covered in Technical Guidance Note 3 (Level 1).

 Worked example Determine the characteristic and ultimate dead loads that are applied at the base of the columns at Level 1 of the structure shown in  in Figure 4. 4.

Partial Factors for Sub-Structure Element Design 3 is of the same load condition with Figure 3 is partial factors that apply to elements within sub-structures:

Figure 3 Sub-structure 3 Sub-structure partial factors

‘Favourable’ dead load – 0.9 Gk ‘Unfavourable’ dead load – 1.1 Gk

These factors apply when considering foundation design, specifically for the loads being applied to the footings. Hence the partial factor (γ 󰁇 ) is:

The ‘G  ’ refers to imposed load that is covered 󰁫  in Technical Guidance Note 3 (Level 1).

‘Favourable’ and ‘Unfavourable’ dead load – 1.0 G 

󰁫 

Partial Factors (γ 󰁇 ) for SuperStructure Element Design In the design of structural elements, the worst case loading pattern is used to determine the maximum shear and bending forces the structure is to be subjected to. Figure 2 2 is  is of the same structure for the stability analysis above. The appropriate partial factors to Ultimate Limit State (ULS) design of super-structure elements have

 Applied practice The applicable codes of practice for the derivation of dead loads are as follows: BS EN 1991-1-1 Eurocode 1: Actions 1: Actions on Structures – Part 1-1: General actions – densities, self weight, imposed loads for buildings BS EN 1991-1-1 UK National Annex to Eurocode 1: Actions 1: Actions on Structures – Part 1-1– densities, self weight, imposed loads for buildings BS 648: 1964 648: 1964 Schedule of Weights of Building Materials

Figure 2 Ultimate 2 Ultimate Limit State partial factors

4 Worked example structure Figure 4 Worked

 

www.thestructuralengineer.org

45

Glossary and further reading There are a total of 6 storeys to this building above Level 1; the 400mm square column layout is a grid measuring 8m x 12m. Levels 2,3,5,6 have raised floors and suspended ceilings with services installed within them. Level 7 is the roof and has a suspended ceiling and services fixed to its soffit and a 50mm screed and 15mm thick t hick tiles on its external surface. Level 4 is a plant room with no finishes to the floor or suspended ceiling at level 5. There are building services hung from the soffit of level 5.

Initially the self weight of the elements that form the structure is determined:

Action – The consequences of an applied Action – load. Characteristic load  load – A load that has had no partial factors applied to it. Favourable load – load – A load that does not increase the bending/shear stresses within an element or create instability, but instead acts to resist failure. Super-imposed dead load – load – Load from applied finishes and building services. It does not include plant e.g. air handling units and boilers. Ultimate load – load – A load that h has as had partial factors applied to it. Unfavourable load – load – A load that does increase the bending/shear stresses within an element or generates instability within a structure.

Further Reading

Then the super-imposed dead load from finishes and building services are calculated:

Manual for the design of building structures to Eurocode 1 and Basis of Structural Design –Institution of Structural Engineers – April 2010

 Web resources

For more information on this subject, please visit the Institution’s website at: www.istructe.org/knowledge/library

Finally the total characteristic and ultimate dead loads acting at the base of the column on Level 1 are calculated:

󲀺     Note 3 Level 1  

46

TheStructuralEngineer

Technical

February 2012

Technical Guidance Note

Derivation Deriva tion of imposed loads Introduction This Technical Guidance Note concerns the derivation of imposed loads. This is a core guidance note and as such, subsequent notes will make reference to this one. It is therefore important to understand the contents of this note before attempting to digest any of the the others. Please be aware that this note does not cover lateral loads onto barriers, balustrades and axle loads from vehicles. These will be covered in a forthcoming note.

ICON LEGEND

Design principles

􂀢 

 Applied practice practice

􂀢 

Worked example

􂀢 

Further reading

􂀢 

 All of the guidance notes in this series have an icon based navigation system, designed to aid the reader.

Design principles

Web resources

􂀢 

building services installations, such as ducts, cable containment and lighting mass of soil fixed partitions, e.g. those that cannot be demounted and placed elsewhere and non load bearing walls Please see Technical Guidance Note 2, Level 1 for more details on dead loads/permanent actions. •

• •

Imposed load is defined as the load that is applied to the structure that is not permanent and can be variable. In Eurocode phraseology, quasi-permanent nent variable it is described as a quasi-perma action.

Definition of Imposed Load (quasi-permanent (quasi-perma nent variable action) The items listed below can be considered to be imposed loads: moveable partitions furniture and occupancy livestock plant room installations, such as air handling units and boilers cradles for maintenance access (but not the supporting structural elements they are fixed to such as rails and secondary support beams) sports and gym equipment such as goal posts, basketball hoops and punch bags audio and video equipment installations including speaker clusters, televisions and lighting rigs •

Typical Imposed Loads Imposed loads are sub-divided into categories A-H in Eurocode 1-1 and their values can be found in Tables NA.2 to NA.7 (inclusive) in the UK National Annex. They are based on the structure’s use.

• • •

•

•

•

Elements that can often be mistaken for imposed loads when they are in fact dead load elements include the following: finishes, such as screeds and suspended ceilings. These are sometimes referred to as ‘super imposed dead loads’ •

Table 1 is 1 is a list of common imposed loads that are applied to building structures: Imposed load t ype

Area load (kN/m2)

Concentrated load (kN)**

Residential

1.5

2

Office (above ground)

2. 5

2

Car park ††

2. 5

10

Retail

4

3.6

General storage areas †

2

1.8

Restaurants

2

3

Plant rooms

7.5

4.5*

Theatre/stages Staircases

5 4

3.6 3

Roof access

0.6

0.9

Partitions

1

n/a

Table 1 List 1 List of typical imposed loads

* This load is taken from the now withdrawn BS6399 Loading for Buildings Part 1: 1996 Code of practice for dead and imposed loads, Table 1. 1. The Eurocodes currently advise to determine actual plant loads from building services engineers rather than apply an assumed blanket load. In the event this information is not available, the area load suggested in Table 1 can be used. ** Concentrated loads are point loads that can be supported anywhere on the structure to the t he exclusion of the corresponding area imposed load. They should only be considered when checking local effects that would be induced by such loads. † General storage refers to category E11 i.e. for static equipment that does not include book and paper storage or other specific types of items listed in Table NA.5 †† For car parks that are limited to vehicles with a gross weight that is less than 30 kN. The Table contains only a sample of the most common types of imposed loads. The reader is directed to Eurocode 1: Actions on Structures – Part 1-1: General actions – densities, self weight, imposed loads for buildings and the relevant National Annexe for a more comprehensive list.

Partial Factors (γ  Q  ) for Imposed Loads (Quasi-permanent variable actions) Partial factors are used within Limit State design methodology. They are applied when designing elements based on their capacity

 

www.thestructuralengineer.org

47

to resist stress, be it shear, bending, torsion or a combination of the three. They are also used when checking for stability within the structure. The Eurocode’s approach to these factors considers the nature of the load before any factors are applied to it.

discussed previously. The appropriate partial factors to Ultimate Limit State (ULS) design of super-structure elements have been applied to the loads placed upon it.

The following partial factors apply to imposed loads that are commonly found within building structures:

Where n is the number of storeys of structure above the level that is being considered, excluding the roof. This factor is applied to imposed loads only, prior to any partial factors being applied to them.

Imposed Load Reduction for Element Design

2 Ultimate Limit State partial factors Figure 2 Ultimate

Stability Analysis Partial Factors

󰁫 

= 0.5 for n > 10 10

Note: Reduction factors can only be applied to imposed load categories A-D, as defined in Eurocode 1-1 and the UK National Annex. Annex.

1.5 (γ q ) is the base partial factor for imposed loads for superstructure element design when it is ‘unfavoured’ 1.3 (γ q ) is the base partial factor for imposed loads for substructure element design when it is ‘unfavoured’

Consider Fig. 1 below, 1 below, where ‘G  ’ refers to the 󰁫  dead load and  Q  ‘ ’ the imposed load:

αn 

This analysis concerns the design of the element and its supports. In this instance the worst case bending moments and shear forces are based on the maximum partial factors (γ   Q  ) that Eurocode 0 will allow, which are: ‘Unfavourable’ Imposed Load – 1.5 Q  󰁫  Again the ‘G  ’ in Figure 2 refers to dead load 󰁫  that is covered in Technical Guidance Note 2, Level 1.

Eurocode 1-1 allows for the reduction of applied imposed load to be applied when designing elements within a structure. The UK National Annex states that the area being supported by the element can be reduced using factor A, that is defined in Equation NA.1:

α a

= 1.0 - A/1000 > 0.75

Where  A ‘  ’ is the area of structure that is being supported. It iselement not the tributary areato. of imposed load the is subjected Figure 4 below 4 below explains this further:

Partial Factors for Substructure Element Design Figure 1 Equilibrium 1 Equilibrium partial factors

Finally consider Figure 3 below 3 below and note the same load condition with reduced partial safety factors:

It can be seen that the load in the main span is acting favourably to resist the load being applied to the cantilever section. This cantilever load is causing instability in the structure, as support ‘A’ is unable to resist an upward vertical force. Figure 1 indicates what factors are to be applied to loads that are dependent upon the nature of the analysis being carried out. In this instance the stability of support point A is being checked for uplift. This falls under the category of checking for equilibrium in a structure, and hence the following partial factors (γ   Q  ) apply: ‘Favourable’ Imposed Load – 0 Q  󰁫  ‘Unfavourable’ Imposed Load – 1.5 Q  󰁫  The ‘G  ’ in Figure 1 refers to dead load that is 󰁫  covered in Technical Guidance Note 2, Level 1.

4 Imposed load area reduction factor; Figure 4 Imposed supported area vs. tributary area

3 Sub-structure partial factors Figure 3 Sub-structure

These factors apply when considering foundation design, specifically for what loads are being applied to the footings. Hence the partial factor (γ   Q  ) is: ‘Unfavourable’ Imposed Load – 1.3 Q 

󰁫 

Imposed Load Reduction in Multi-storeyStructures

Partial Factors (Q) for Super

Imposed loads in multi-storey structures can be reduced, based on the likelihood of all of the floors in the structure being fully occupied. The factor that can be applied to the imposed loads as they are considered from a lower

Structure Element Design

level in the structure, are as follows:

In the design of structural elements, the worst case loading pattern is used to determine the maximum shear and bending forces the structure is to be subjected to. Figure 2 2 is  is of the same structure for the stability analysis

αn  αn 

= 1.1 – n/10 for 1< = 0.6 for 5 <

n < 5  5 

n < 10  10 

 Applied practice The applicable codes of practice for the derivation of imposed loads are as follows: BS EN 1990: Eurocode 1990: Eurocode Basis of Structural Design BS EN 1990: UK 1990: UK National Annex to Eurocode: Basis of Structural Design BS EN 1991-1: Eurocode 1: Actions on Structures – Part 1-1: General actions – densities, self weight, imposed loads for buildings BS EN 1991-1: UK 1991-1: UK National Annex to Eurocode 1: Actions on Structures – Part 1-1– densities, self weight, imposed loads for buildings

 

󲀺     Note 3 Level 1

48

TheStructuralEngineer

Technical

February 2012

Technical Guidance Note

 Worked example Determine the characteristic and ultimate imposed loads that are applied to the columns at Level 1 of the structure in Figure 5 5 below:  below:

There are a total of 6 storeys to this building above Level 1; the 400mm square column layout is a grid measuring 8m x 12m. Levels 2,3,5,6 have raised floors and suspended ceilings with services installed within them. Level 7 is the roof and has a suspended ceiling and services fixed to its soffit and a 50mm screed and 15mm thick tiles on its external surface. Level 4 is a plant room with no finishes to the floor or lowered ceiling at Level 5. There are building services hung from the soffit of Level 5.

The first loads to determine are the imposed loads from each level:

Then the reduction factor is applied to the column at Level 1:

Finally the characteristic and ultimate imposed loads are calculated:

Figure 5 5 Worked  Worked example structure

Glossary and further reading

certain types of imposed loads that allows for the likelihood of full occupancy of a structure.

Action – The consequences of an applied Action – load.

Ultimate load – load – A load that has had partial factors applied to it.

Characteristic load – load – A load that has had no partial factors applied to it.

Unfavourable load – load – A load that does increase the bending/shear stresses within

Favourable load – load – A load that does not increase the bending/shear stresses within an element or create instability. Reduced imposed load – load – Factor applied to

an element or generates instability within a structure.

Further Reading Manual for the design of building structures to Eurocode 1 and Basis of Structural Design – Institution of Structural Engineers – April 2010

 Web resources For more information on this subject, please visit: http://www.istructe.org/resourcescentre/library

󲀺     Note 5 Level 1  

22

TheStructuralEngineer

Technical

March 2012

Technical Guidance Note

Derivation Deriva tion of snow load Introduction This Technical Guidance Note concerns the derivation of snow load onto structures. It is based on Eurocode 1: Actions on Structures Part 1-3; General  Actions – Snow Loads. With this Eurocode being focused on an action that is sensitive to environmental effects, the UK annex to it plays a significant role, as it makes reference to projected snow falls that are unique to the British Isles. There are a large number of variations and conditions the designer must be aware of when determining snow loads onto structures. As such, the reader is referred to the code text more frequently than in other Technical

ICON LEGEND

Design principles

󒀢 

 Applied practice practice

󒀢 

Worked example

󒀢 

Further reading

󒀢 

Guidance Notes.  All of the guides in this series have an icon based navigation system, designed to aid the reader.

 A is the altitude in meters of the ground level

Design principles The derivation of snow load requires the designer to make judgements on the environment the structure is placed in as well the form of the structure itself. The potential for the build up of snow must be allowed for when determining the magnitude of the resulting persistent snow load onto a structure. Eurocode 1-1-3 addresses these issues by establishing a base load and then applying factors to that load that represent snowdrift.

Determining Determinin g Basic Snow Load (s  k ) The basic snow load  (s  k ) is defined as the amount of snow on the ground based on an altitude of 100m above mean sea level. Clause 1.6.1 of Eurocode 1-1-3 sets the probability of exceeding this value at 1 in 50 per year. Clause NA.2.8 defines (s  k ) by the following expression:

  [= 0.15 + (0.1 Z  +0.05)]  +0.05)] +

 

–100 525

1

Shape Coefficient (

)

To allow for snowfall thickness variances depending upon the shape of roof

1

structures, Eurocode 1-1-3 uses coefficient  to take geometric changes into account. It is applied to the basic snow load s  k  and  and is based on the type of roof the structure has; be it flat, mono-pitch, duo-pitch or a multi-span roof. The proximity of protrusions such as taller elements of the structure or chimney stacks also has an impact when determining the value of shape coefficient .

1

1 � 1 1 1

Shape Coefficient ( ) for Flat, Mono- & Duo-Pitch Roofs The relative angle ( ) of the roof pitch affects the shape coefficient  value. This is shown in Figure 5.1 of Eurocode 1-1-3. For flat roofs the value of  is 0.8.

 A s  k 

of the site where the structure is situated above mean sea level. This part of the expression is ignored when considering sites near coastal regions that are below 100m above mean sea level.



Where: Z  is  is the zon  zone e numbe number  r  (i.e.  (i.e. 1,2,3 etc) taken from Figure NA.1 in the UK Annex to Eurocode 1-1-3 and not  the ground snow load figure at 100m above mean sea level.

�

For mono-pitch roofs, it’s the pitch angle   that is plotted against , referenced in Figure 5.1 or Table 5.2 of Eurocode 1-1-3. For roofs with a pitch angle of 60° or more the value of  is 0.

1

Web resources

󒀢 

�1

In the case of duo-pitched roofs, pitch angle  of each side of the roof is read against . In cases where there are differing pitch angles, each section of the roof will have its own unique shape coefficient. In this condition a series of snow load patterns must be considered before arriving at a definitive snow load. The patterns that need to be reviewed are:

1

a) Both sides loaded using the   coefficient for each side of the roof drawn from Figure 5.1 or Table 5.2 b) One side loaded using NA Figure NA.2 and Table NA.1 c) Other side loaded using NA Figure NA.2 and Table NA.1 The worst case from the conditions listed above is considered to be the persistent snow load. For more information on this, see Clause 5.3.3(3) and Figure 5.3 of Eurocode 1-1-3. It’s important to note that load cases (ii) and (iii) cited in Clause 5.3.3(4) have been replaced by Clause NA 2.17 in the UK National Annex of Eurocode 1-1-3.

1

Shape Coefficient ( Roofs

) for Multi-Span

Snow formation is not often uniform when it lands upon on multi-span roofs. To allow for this, two loading conditions are considered to determine the persistent snow load case.

www.thestructuralengineer.org

 

23

The first requires the separate pitches of each part of the roof to have the relevant coefficient applied to them, in a similar fashion to duopitched roofs previously described. The second condition allows for the build up of snow within the valley of the multi-span roof. Annex B2 of Eurocode 1-1-3 should be used to determine the value of  for drifts forming in the valleys.

1

Taking the lowest value from the following:

1 1 3 1  1  = 2h / s  k 

 = 2b  / (l  s   +l  s  )  = 5

Figure B1 in Annex B of Eurocode 1-1-3 provides a definition of the above variables. It is very important to note that these load cases are exceptional in that they are

conjunction with Table B1 in Eurocode 1-1-3, which provides definitions for the variables shown. Again as with snowdrift in multi-span roofs, snow loads defined by guidelines stated within Appendix B of Eurocode 1-1-3 are exceptional and must not be considered in conjunction with the blanket snow load.

For the derivation of snowdrift shape factors near parapets, the reader is directed to Clause B4(3) and Figure B4 in Annex B of Eurocode 1-1-3. The snowdrift factor ( ) is defined as the lowest value from the following expressions:

1

 = 2h / s  k 

1

Shape Coefficient ( and Obstructions

) for Projections

Small projections from the side elevation of structures, such as canopies and obstructions within roofs, including chimneys and parapets have an impact on snow load. They are barriers around which snow can drift and create localised heaped areas of snow. When assessing snow loads onto a structure, shape codes that represent these increased volumes of snow are applied to the base load. For snowdrifts due to obstructions such as chimneys, the reader is directed to Clause B4(1) and Figure B3 of Annex B of Eurocode 1-1-3 for the derivation of shape coefficient .

11

 = 2b / l  s   = 8

1

The length of drift l  s  is  is either 5h, b  or a maximum of 15m. All of the variables mentioned in these expressions are defined in Figure B4.

Partial Factors for Snow Loads

k

Eurocode 1-1-3 defines snow loads as variable fixed actions. The partial factors (Q ) for snow loads are dependent on the likelihood of the snowfalls that are projected by the Eurocode, actually occurring. In the case of flat, mono- and duo-pitched roofs, the loads

 in exclusion

considered  toEurocode the blanket snow load. See Clause B1(2) in 1-1-3 for further guidance on this.

1

Shape Coefficients ( ) & ( ) for Roofs Adjacent to Tall Structures

For canopies over entrances, Clause B4(2)b in Eurocode 1-1-3 needs to be followed. This clause states that where it is not possible for more than 1m depth of snowdrift to form, no shape factor needs to be applied.

When a roof is situated adjacent to or within 1.5m of a vertical element, a barrier exists against which snow can build up. To address this, alternative shape coefficients  and   need to be applied to the basic snow load as a separate case.

For smaller doorways that are less than 2m wide, the depth ‘h’ of the snowdrift is limited to the lesser of the height of the projection or its width that is perpendicular to the wind direction.

1 3 1 3

1

For flat roofs abutting taller structures, the shape factor  is 0.8. The snowdrift that appears on top of the snow covered roof is arrived at via the application of shape coefficients. Figure 1 explains 1 explains how the extent of the snowdrift is defined. It should be read in

1

The shape coefficient  for canopies is defined by Clause B4(2)c in Eurocode 1-1-3 thus:

 = 2h / s  k  or 5, which ever is the lesser.

1

1

d

k

Partial factor for snow load in isolation: 1.5Q , Partial factor for snow load in conjunction with dead and imposed loads:

ψ k kψ ψ1

(0.5

) 1.5Q

(0.2ψ1 k

Partial factor for snow load in conjunction with dead, imposed and wind loads:  ) 1.5Q Where Q  is  is the partial factor for the snow load and  and  and  are the combination  are factors to be used when snow load is considered with others. The numbers stated adjacent to the factors above are their respective values.

For canopies over doors that do not project more than 5m, the value of  cannot exceed  s  , with ‘ ’ being the larger value of b   b 2b / l  and b . See Figure 2 for 2 for clarification of the previously referenced variables.

1

are considered toloads be transient/persistent actions. All snow derived using Annex B of Eurocode 1-1-3, are deemed to be an extreme condition and are therefore classified as accidental actions (A ).

1

When snow loads are derived using Annex B of Eurocode 1-1-3 no partial factor is applied to them. This is because they are considered to only occur in extreme cases and are therefore classified as accidental.

 Applied practice The applicable codes of practice for the derivation of snow loads are as follows: BS EN 1991-1-3 Eurocode 1: Actions 1:  Actions on Structures – Part 1-3: General Actions – Snow loads Figure 1 Snowdrift condition for roofs abutting tall structures shown with basic snow coverage of roof. Note that these two conditions are never applied simultaneously.

2 Variables definition of snowdrift shape Figure 2 Variables factor for entrance canopies

BS EN 1991-1-3 UK National Annex to Eurocode 1: Actions 1: Actions on Structures – Part 1-3: General Actions – Snow loads

󲀺    

www.thestructuralengineer.org

Note 5 Level 1

 

24

TheStructuralEngineer

Technical

March 2012

Technical Guidance Note

 Worked example An indoor sports hall is to be constructed adjacent to an existing further education college. It is located 1 km south of Inverness city centre and is 90m above mean sea level. Calculate the characteristic snow load on the roof and entrance canopy to the new sports hall. The roof pitch angle  to the sports hall is 8°.

�

Initially, the basic snow load is calculated using Figure NA.1:

The shape factor for the overall snow load on the duo-pitch roof for the new sports hall is then determined and the corresponding snow load is calculated.

k

Note that this load is a variable static action and therefore would have a partial factor of 1.5Q  if  if it were being considered in isolation to other loads. The shape factor for the canopy entrance is determined and the projected snow load onto it is calculated using Clause B4(2)b in Eurocode 1-1-3.

3 Isometric view of new sports hall Figure 3 Isometric

This is considered to be an accidental action as it is classified as an extreme condition. Therefore in Ultimate Limit State (ULS) and Equilibrium (EQU) analyses, no partial factor would be applied to this load.   Finally, we consider the snowdrift load onto the main roof due to the adjacent existing structure, which is significantly taller than the sports hall. It is at this point when Table B1 in Eurocode 1-1-3 is used.

Like the canopy load, this is deemed to be an accidental action and therefore no partial factors are applied to it within ULS and EQU analyses.

Glossary and further reading

Partial factor – factor – A factor that is applied to characteristic loads when carrying out design of structures and the elements they are constructed from.

Action – An applied load, both due to a direct application or as a consequence of an indirect effect such as thermal expansion of the structure.

Variable static action – A load that is static and variable in magnitude. Loading due to snowfall is typical of this type of action.

Accidental action – action – A loading condition that is unlikely to occur. Partial factors are not applied to them within ULS analysis.

National Annex – Annex – A part of the Eurocode that has been written specifically for a particular region.

Further Reading  Manual for the design of building structures to Eurocode 1 and Basis of Structural Design — Institution of Structural Engineers – April 2010

 Web resources

Characteristic load – A base load that has not had any partial factors applied to it.

For more information on this subject, please visit: www.istructe.org/resources-centre/ library

󲀺    

www.thestructuralengineer.org

Note 6 Level 1

 

Technical

TheStructuralEngineer

Technical Guidance Note

March 2012

25

Notional loading ICON LEGEND

Introduction This Technical Guidance Note concerns the concept of notional loading,  which the Eurocodes classifies as Equivalent Horizontal Forces. Forces. These are loads that exist due to inaccuracies and imperfections introduced into the structure during its construction. The following text explains how notional lateral loads are incorporated into the design process.

Design principles

󒀢 

 Applied practice practice

󒀢 

Worked example

󒀢 

 All of the guides in this series have an icon based navigation system, designed to aid the reader.

Further reading

󒀢 

Web resources

󒀢 

Design principles A notional load is based on a proportion of the vertical load the structure is supporting. Typically they are applied in conjunction with other loads during analysis.

Generic Notional Horizontal Load

(F  hn) Eurocode 1-1-6 concerns loading during the construction of structures. Within Annex A, Clause A1.3 of Eurocode 1-1-6 there is a generic definition of a notional horizontal load (F  hn) that can be applied to all structures. The magnitude of this force is 3% of the vertical loads from the worst case load combination for a given structure. This can be adopted for all structures, regardless of the material they have been constructed from.

Material Sensitivity to Notional Load Notional loads represent forces that come about due to imperfections in the structure. Some materials are more sensitive to this phenomena than others and it is for this reason that notional loads are linked directly to the material a structure is constructed from. The Eurocodes for steel and concrete structures have sections within them that are dedicated to deriving notional horizontal loads within structures. The following sections explain how each material addresses notional loading.

Notional Loads in Steel Frames Steel frames are very sensitive to notional loads. This is because imperfections within the fabricated elements and their connections are inevitable as they are impactful. It is for this reason that any design of a steel frame structure must take them into account. Eurocode 3-1-1, Clause 5.3.2(3) covers this by creating coefficient (ф ), which the vertical load of a structure is multiplied by. This replaces (F  hn) notional load from Eurocode 1-1-6 described previously.

It is defined as



m =

     

0.5 1+

1 m



where m is the number of columns in a row that are connected to the bracing system being considered. These columns must also be supporting at least 50% of the average vertical load of those columns in the row being considered (Figure (Figure 1): 1):



Coefficient (ф ) is determined thus:  ф  =  = ф 0  0  h  m (Equation 5.5, Eurocode 3-1) Where:

ф 0  0 is the sway angle at which the structure rotates due to notional loads and has a base value of 1/200



h is the factor that is related to the height of

vertical elements within the structure. This is defined as



h =

 

2 h

,



Figure 1 Extent of columns that influence the value

m

where ‘h’ is the height of the structure.

of

This factor can only be within the range

Clause 5.3.2(4)B in Eurocode 3-1 states that



of 0.66 < h