Hybrid Concrete Construction.pdf

Hybrid Concrete Construction

Maximising the potential of concrete by combining precast and in-situ concrete

Hybrid Concrete Construction

Introduction Table 1: Benefits of hybrid concrete construction

Hybrid construction combines the most appropriate materials and methods of construction. The search for greater economy, in terms of material costs and reduced construction time, has resulted in innovative approaches that seek to combine construction materials and methods to optimum effect. Hybrid concrete construction (HCC) is one such development that combines in-situ and precast concrete to maximise the benefits of both forms of concrete construction. Hybrid concrete construction embraces a number of different forms of structural frame, but in all cases precast concrete and cast in situ concrete elements are used where they are most appropriate for the project. HCC produces simple, buildable and economic structures which result in faster, safer construction and reduced costs. There are many benefits of concrete which are shared by both precast and insitu concrete. Many of these are listed in Table 1 and described in the Benefits of Hybrid Concrete Construction section (page 3).

Precast concrete

Precast or in-situ concrete

In-situ concrete

Economic for repetitive elements

Inherent fire resistance

Economic for bespoke areas

Long clear spans

Durability

Continuity (structural efficiency)

Speed of erection

Sustainability

Inherent robustness

Buildability

Acoustic performance

Design flexibility

High-quality finishes and consistency of colour

Thermal mass

Services coordination later in programme

Accuracy

Prestressing

Locally sourced materials

Reduced propping on site

Mouldability

Short lead-in times

Reduced skilled labour on site

Low vibration characteristics

Contents Benefits of hybrid concrete construction 3 Hybrid options

8

Design and procurement

12

Case study 1: Jubilee Library, Brighton

13

Case study 2: Hilton Hotel Tower Bridge, London

14

Case study 3: West Quay car park, Southampton

14

Case study 4: Homer Road, London

15

References

15

The Ideas Store on Whitechapel Road, London is a hybrid precast and in-situ concrete structure. The project, which was completed in 24 weeks, was a combination of cast in situ beams and columns and precast ribbed soffits slabs (as shown above). The designers deliberately exposed the concrete to provide a high-quality visual interior finish, which also provides thermal mass efficiency. Courtesy of Adjaye Associates.

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Cover images Main: Ideas Store, London, courtesy of Hanson Top inset: Homer Road courtesy of Foggo Associates Bottom inset: West Quay Car Park, Southampton This page: Ideas Store, courtesy of Adjaye Associates

Hybrid Concrete Construction

Benefits of hybrid concrete construction Hybrid concrete construction produces simple, buildable and economic structures. It delivers increased prefabrication, faster construction and consistent performance. HCC can achieve very significant cost savings and can satisfy the requirements of the most demanding of clients. Buildability

Speed

The key advantage of HCC is its buildability. Because precast and cast in situ concrete are used where most appropriate, construction becomes relatively simple and logical. The use of HCC encourages design and construction decisions to be resolved at design stage. This means, for example, that precast elements can be manufactured, stored at the factory and delivered ‘just-in-time’ to site. They can then be lifted from delivery truck to final position in a single crane movement, eliminating the need for site storage and reducing crane hook time.

Speed of construction depends on designs which are easy to procure and construct. HCC takes a proportion of work away from the site and into the factory, reducing the duration of operations critical to the building programme on site. The precast process takes place in a controlled environment, unaffected by weather. Rigorous inspection before installation removes causes of delay on site. Developments and innovation in formwork systems and concrete technology mean that in-situ elements of a HCC structure can also be completed within tight programme constraints.

Traditional formwork typically accounts for 40 per cent of in-situ frame costs and is dependent on weather and labour. The use of HCC means that a percentage of the frame is manufactured in a weather-proof factory, resulting in faster construction.

Some HCC techniques can reduce or eliminate following trades, e.g. installing ceilings and finishes. This enables even faster programme times but requires greater co-ordination and care in detailing and protection on site.

Cost Although the structural frame of a building represents only 10 per cent of the total construction cost, the choice of material has dramatic consequences for subsequent processes. Hybrid construction can reduce frame costs by using precast concrete for the repetitive elements, or to act as permanent formwork. In-situ concrete is more cost-effective for large volumes (due to reduced transport costs) for tying the frame together and for bespoke areas. Using the two together maximises the cost efficiency.

Safety A high proportion of hybrid concrete construction is carried out in the precast factory by experienced personnel. On site, the innovative use of HCC and the improved buildability helps ensure that each safety plan is prepared on the individual project’s merits. HCC can reduce the potential for accidents by providing successive work platforms and a tidier site. If precast spandrel beams are used they can provide immediate edge protection.

Home Office Headquarters, London. The HCC frame was designed specifically for the project. This image shows the installation of the precast beams.

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Hybrid Concrete Construction

Benefits of both in-situ and precast concrete Precast or in-situ concrete Inherent fire resistance Durability Sustainability Acoustic performance Thermal mass Prestressing Mouldability Low vibration characteristics

Sustainability Concrete is a local product to the UK, manufactured from plentiful resources under strict regulations ensuring the highest environmental and social standards. Therefore the sector has been able to embrace responsible sourcing and manufacturers have gained accreditation at the highest level for their concrete products. This is recognised in sustainability assessment methods, enabling designers to gain maximum credits by choosing concrete. Thermal mass Buildings with concrete frames have embodied energy and CO2 of a similar order to equivalent buildings constructed from other materials. For all buildings the operational energy consumption is far more significant than that during construction, but concrete buildings utilising thermal mass can reduce this impact on the environment by moderating building temperatures, delaying the peak temperatures to later in the day and thus minimising the need for air-conditioning. Use of thermal mass as part of passive solar designs can also reduce energy demands for heating during the winter, particularly in residential and education sectors. Further information is available from the document Utilisation of Thermal Mass in Non-Residential Buildings [3].

Fire resistance Concrete has inherent fire resistance, which is present during all construction phases, and is achieved without the application of additional treatments. The fire resistance is also maintenance free. Concrete has the best European fire rating possible because it does not burn and has low heat conductance. Further information can be found in Concrete and Fire Safety [1] available from www.concretecentre.com/publications.

Internal temperature with high thermal mass

External temperature

Internal temperature with low thermal mass

Peak temperature delayed by up to six hours

Up to 6-8oC difference between peak external and internal temperature

30oC Durability A well-detailed concrete frame is expected to have a long life and require very little maintenance. It should easily be able to achieve a 60-year design life and, with careful attention to the specification of the cover and concrete properties, should be able to achieve 100 years even in aggressive environments. BS 8500 [2] is the British Standard for durability and gives advice for various environments.

15oC Day

Night

Day

Figure 1: Stabilising effect of thermal mass on internal temperature.

An award winning hybrid structure. Jubilee Library, Brighton. Courtesy of Bennetts Associates. For the full case study, see page 13.

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Hybrid Concrete Construction

Acoustic performance Concrete is a very good sound insulator, even when the source of noise is an impact on the face of the concrete. For this reason concrete floors and walls are often used in residential accommodation, including flats, hotels and student residences, to prevent the passage of sound between units. Concrete can also be used to prevent the sound escaping into or out of a building. A good example would be the use of concrete floors beneath mechanical plant on the roof of a building to prevent the noise penetrating to the habitable areas. Prestressing Prestressing concrete, using tensioned high-strength steel, reduces or even eliminates tensile stresses and cracks. This gives rise to a range of benefits that exceed those found in normally reinforced concrete sections. Benefits include increased spans, stiffness and watertightness, and reduced construction depths, self-weights and deflections. Prestressing can be carried out before or after casting the concrete. Tensioning the prestressing steel before casting (i.e. pre-tensioning) tends to be carried out in factories e.g. in producing precast floor units. Post-tensioning is more usually carried out on site using in-situ concrete. Mouldability Concrete can be formed into any shape and this can be achieved with either precast or in-situ concrete. Concrete provides the opportunity to create unusual shapes at a small cost premium. Repetition of elements can make even complex shapes affordable for projects which are costdriven. This can be particularly beneficial if circular columns are required for aesthetic reasons or where columns need to be contained in walls, e.g. for apartments. Concrete can also be used for curved beams, unusual plan shapes and shell structures. The layout of the vertical structure can be arranged to suit the use of the building rather than having rigidly to follow a structural grid. V ibration control For some types of buildings the control of vibrations induced by people walking across the floor plate are important. This is particularly the case for hospitals and laboratories containing sensitive equipment, but even in offices long slender spans can vibrate excessively. The inherent mass of concrete means that concrete floors generally meet vibration criteria at no extra cost as they do not require additional stiffening. For more stringent criteria, such as for laboratories or hospital operating theatres, the additional cost to meet vibration criteria is small compared with other structural materials. An independent study [4] into the vibration performance of different structural forms in hospitals has confirmed that concrete can normally be readily designed for the most complete control of vibration over whole areas, without the need for significantly thicker floor slabs than those used for a basic ‘office’ structure. This gives great flexibility for change in use and avoids the cost penalties of providing this extra mass and stiffness.

Homer Road speculative office development showing the tapered edge of the precast concrete perimeter unit. Courtesy of Foggo Associates. For the full case study, see page 15.

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Hybrid Concrete Construction

Benefits of precast concrete Precast concrete Economic for repetitive elements Long clear spans Speed of erection Buildability

Acceptability of finishes and consistency of tone can be confirmed prior to leaving the factory. A wide choice of precast concrete cladding finishes and facings is available, including: • • • • • •

Surface retarding and wash-off Rubbing Abrasive blasting Bush hammering Mechanical grinding and polishing Acid etching.

High-quality finishes and consistency of colour

More information on architectural finishes can be found in Precast Concrete in Buildings [5].

Accuracy

Accuracy

Reduced propping on site

Precast elements are cast to close tolerances, and checked in the factory before delivery to site.

Reduced skilled labour on site Reduced propping on site Economic for repetitive elements Using precast elements reduces requirements for falsework; this saves cost through reduced resources and by shortening the programme. There is also less reliance on wet trades, which can be delayed by unfavourable weather conditions. However, to maximise economy the mould created to cast the concrete should be re-used as much as possible, thus precast concrete is most economic where repetition is maximised. Repetition does not mean the finished building will be uninspiring; designers can produce aesthetically pleasing designs by innovative use of repeat elements.

Depending on the type of element used, there may be no temporary propping or minimal propping required. This increases productivity and reduces the temporary works. Reduced skilled labour on site The production of precast concrete takes place in a factory environment, removing labour requirements from site. The factory work is carried out in an internal environment at safe working heights.

Long clear spans Reducing the number of columns is often important in developments such as offices, sports stadia and car parks. Prestressing the concrete can deliver these longer spans or shallower construction depths. Speed of erection Speed of erection and tight construction programmes are primary considerations in many building projects. To maximise the speed of construction with precast elements two critical factors should be taken into consideration: • •

The building layout should be designed to maximise repetition of precast units Construction details should be designed to maximise the number of standardised components.

Toyota UK Headquarters is an exposed precast and hidden in-situ reinforced concrete hybrid building. Courtesy of Trent Concrete.

Buildability Precast elements are designed by specialist precast concrete designers. Within their design they consider the erection sequence and process so that the elements are engineered to be constructed easily. This planning makes the frame highly buildable. High-quality finishes and consistent colour High-quality consistent finishes are generally achieved through the use of robust, purpose-made formwork and dedicated concrete mix designs in a factory environment. Sample finishes can be approved by the client as a benchmark for the project requirements. For visual concrete that is to be exposed to exploit the thermal mass of concrete construction, consistency of tone and texture is important. Precast factories have dedicated concrete supplies ensuring consistency of supply and giving greater control of the constituent materials used.

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Precast glazed insulated panels. These site-ready panels can reduce programme time on site. Courtesy of Roger Bullivant Ltd.

Hybrid Concrete Construction

Benefits of in-situ concrete In-situ concrete Economic for bespoke areas Continuity (structural efficiency)

Inherent robustness An in-situ concrete frame is generally very robust because of its monolithic nature. Usually the tying requirements of the Building Regulations to avoid disproportionate collapse are met with normal detailing of concrete. In-situ concrete areas can be used with precast concrete elements to provide the necessary tying without having to introduce ties specifically for this role. How to Design Concrete Buildings to Satisfy Disproportionate Collapse Requirements [6] is available at www.concretecentre.com/publications.

Inherent robustness Flexibility Services coordination later in programme Locally sourced materials

Flexibility In-situ concrete is a flexible material to use; it can be cast into an infinite number of shapes, and can be varied from floor to floor. It is available throughout the UK from concrete suppliers and placed by experienced contractors.

Short lead-in times Services coordination later in programme Economic for bespoke areas In-situ concrete can be cost-effective for bespoke areas and can therefore be combined effectively with precast concrete for more unusual areas or elements of a building. Continuity (structural efficiency) In-situ concrete is generally designed to maximise the benefit of the monolithic structure, by use of structural continuity which increases spans and stiffness and reduces construction depths.

With in-situ concrete the location of services penetrations can occur later in the programme. This is because the final design of the concrete elements can occur later in the overall programme than for elements fabricated off-site. Locally sourced materials In-situ concrete is available close to project sites, wherever they are in the UK. Many ready-mix plants are located where the aggregate is extracted or, where this is not possible, aggregate is often transported by rail or water. Short lead-in times The lead-in time for in-situ concrete can be considerably shorter than other materials, this is because the materials are readily available and assembled in position. This can result in in-situ concrete delivering quickest overall construction times.

The average distance from a concrete plant to any building site in the UK is 8km, providing a sustainable solution to transportation.

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Hybrid Concrete Construction

Hybrid options The ideal combination of precast and in-situ concrete is influenced by project requirements. There is a wide range of possible options, a selection of which is presented here as representative of current UK practice. It is not intended to be an exhaustive list.

Type 1 Precast twin wall and lattice girder slab with in-situ concrete

Type 2 Precast column and edge beam with in-situ floor slab

Type 3 Precast column and floor units with in-situ beams

Type 4 In-situ columns or walls and beams with precast floor units

Type 5 In-situ column and structural topping with precast beams and floor units

Type 6 In-situ columns with lattice girder slabs with optional spherical void formers

Ease of services distribution

Minimises storey height

Suitability for holes

Clear spans

Deflection control

Minimise materials

Type 1 Type 2 Type 3 Type 4 Type 5 Type 6

Excellent

8

Good

Soffit can be exposed

Hybrid Final version Owen Brooker 27.11.08

Can be used

Maximises off-site construction

Temporary works minimised

Hybrid Concrete Construction

Precast twin wall and lattice girder slab with in-situ concrete Hybrid concrete wall panels are increasingly being specified on projects throughout the UK and are often known as ‘twin wall’. They comprise two skins of precast concrete connected by steel lattices, which are filled with in-situ concrete on site. The external skins of the twin wall system are factory made, typically using steel moulds. This results in a high-quality finish. The panel surface quality is suitable to receive a plaster finish or wallpaper. The panel surface is not normally appropriate for visual concrete. Joints either have to be expressed as a feature of the finish, or concealed. This type of HCC offers advantages to the contractor in terms of speed of construction, as well as reducing the number of skilled site staff required to construct walls. Often the twin wall system is combined with the use of lattice girder precast soffit slabs, with or without spherical void formers (Type 6, shown on page 8). These provide permanent shuttering for an insitu slab that can be relatively easily combined with the wall system. Spans of up to 8m are common and spans up to 14m are possible. (The manufacturer should be consulted early on to ensure the longer spans are viable.)

Potential structural uses of the twin wall system include: • • • • •

Cellular type structures for residential use Walls carrying vertical loads only Shear and core walls; this has significant implications for the design Retaining walls; this has significant implications for the design ‘Single sided’ formwork situations, where there is no access to one side of the wall to erect formwork, for example wall construction on a party wall line against neighbouring buildings.

The major advantage is that it is an ‘in-situ structure’, fully continuous and tied together, but without the need for shuttering on site. Twin wall can also be cast with fully trimmed openings and with ducts for cables and other services. Advantages: • • • •

Quality finish for walls and soffits enabling use of thermal mass No formwork for vertical structure and horizontal structure when lattice girder slabs are used Structural connection between wall and slabs relies on in-situ reinforced concrete detail and is inherently robust Reduced propping

Disadvantages: • • •

Propping of lattice girder slabs is required prior to sufficient strength gain of in-situ concrete The smaller dimension of the precast units is typically a maximum of 3.6m, so joints in walls and soffits must be dealt with (expressed or concealed) Reduced flexibility of layout as this option requires walls rather than columns.

One Coleman Street, London. Inset: Off loading twin wall units. Courtesy of John Doyle Construction.

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Hybrid Concrete Construction

Precast column with in-situ floor slab The combination of an in-situ slab, e.g. post-tensioned flat slab, with precast columns can provide an economic and fast construction system. Precast concrete edge beams may also be used to avoid edge shutters on site and to allow perimeter reinforcement, cladding fixings or prestressing anchorages to be cast in. This reduces the time required for reinforcement fixing and erecting the formwork.

In-situ columns or walls and beams with precast floor units A variety of precast floor products could be used with this type of construction, including hollowcore units, double tees, lattice girder slabs (with or without spherical void formers) or bespoke coffered floor units. Advantages:

The maximum span for this form of construction depends largely on whether the in-situ slab is post-tensioned. For flat slabs with spans greater than 10m punching shear is likely to be a critical design issue. This form of construction relies on the structure being braced. This is achieved by the lift core(s) or separate shear walls. Advantages: • • • • •

Columns can be erected quickly Quality finish for columns Precast edge beam contains post-tensioning anchorages (if required), slab edge reinforcement and cladding fixings, and avoids need for slab edge shuttering Can be used with a variety of in-situ slabs, selected to suit individual project requirements More flexible for late changes

Disadvantages: •

In-situ slab requires falsework, formwork and curing time

Precast column and floor units with in-situ beams This form of construction allows a high proportion of the structure to be manufactured in quality controlled factory conditions off site leading to fast construction on site. A variety of precast floor products could be used with this type of construction, including hollowcore units, double tees, lattice girder slabs (with or without spherical void formers) or bespoke coffered floor units. The latter have successfully been used in high quality buildings designed for energy efficiency, where the lighting, architectural features and cooling systems have all been incorporated into the unit. Advantages: • • • •

Vertical structure can be erected quickly; no formwork required Precast floor structure can be erected quickly; no formwork required Quality finish for columns and soffits (although this is not always possible with hollowcore units) Structural connection between precast elements is via standard reinforced or post-tensioned concrete

Disadvantages: • •

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Precast flooring must be temporarily propped Sealing between precast units is required

• • •

Precast floor structure can be erected quickly; no formwork required. Quality finish for soffits (although this is not always possible with hollowcore units) Short lead time for standard precast product

Disadvantages: • •

Precast flooring must be temporarily propped Sealing between precast units is required

In-situ column and structural topping with precast beams and floor units In this form of construction the floor consists entirely of precast elements, which are tied together with an in-situ structural topping. The column formwork can be designed as a temporary support for the precast beams and slabs to reduce the requirement for propping of the precast floor. The joint between the beam and columns and any structural screed is concreted with the columns to form a monolithic, robust structure. This system requires particular attention to the connection details between the precast beam and floor units. It should be ensured that adequate structural ties are provided to achieve a robust structure. Advantages: • • • • • •

Precast floor structure can be erected quickly Precast beams support precast floor units, minimising floor propping Precast quality finish for soffits (although this is not always possible with hollowcore units) Formwork for in-situ columns can be used to prop precast beams Structural connection between precast elements is via standard reinforced concrete In-situ structural topping to beam permits beams to be continuous over columns

Disadvantages: •

Downstand beams need to be coordinated with the services distribution

Hybrid Concrete Construction INSTALL FORMWORK & PRECAST BEAM

• • • •

Precast floor structure can be erected quickly; no formwork required Structural connection between precast elements is via standard reinforced concrete Quality finish for soffits More flexible for late changes

Disadvantages: •

Precast flooring must be temporarily propped

Lifting with 2 the crane Precast Lifting with beam the crane Precast beam

Moving 1 formwork Safety Safety

Safety Steel Safety formwork Safety

Precast beam

Moving Safety formwork Level +11 Moving formwork 1 Moving formwork 1 Level +1

Steel formwork Steel formwork Steel formwork

Back-propping (if necessary)

Level +1 Back-propping (if necessary)

Level +1

Back-propping (if necessary) Back-propping (if necessary)

Stage 1: Column formwork erected to provide temporary support for the POURING COLUMNS precast beams. Precast beams positioned on the column formwork with beam rebars projecting into the column stitch.

A

POURING COLUMNS POURING COLUMNS Concreting (column & stitch) 3 POURING COLUMNS

Reinforcement Safety

Level +2

Concreting (column & stitch) Precast beam

Level +2

3 Propping& stitch) Concreting (column Precast beam 4 3

Safety

Reinforcement Reinforcement

Precast beam Steel formwork Propping Precast beam

Safety Safety

3

Reinforcement

Concreting (column & stitch)

A A A

Advantages:

Safety

Level +2 Level +2 Level +1

4

Propping Steel 4 Propping formwork 4 Steel formwork Steel formwork

A

The main feature of this system is the use of the lattice girder panels to act as permanent formwork for a flat slab. A variation is to include spherical void formers. These reduce the self-weight of the slab for only a small reduction in flexural strength and stiffness. Lattice girders and void former cages are cast into concrete panels containing reinforcement in two directions, providing a precast panel that acts as the permanent formwork. If the spherical void formers are used, they are removed in areas of high shear where a solid section provides greater shear resistance. The slab may be designed as a flat slab to reduce the overall floor zone of the building and to simplify installation of services. Propping of the panels will be required. The quality of the factory produced soffits provides the opportunity to take advantage of the thermal mass properties of the concrete slab by exposing them.

2 beam Precast Lifting with the crane 2

Safety

Level +1 Level +1

A A A

In-situ columns with lattice girder slabs with optional spherical void formers

INSTALL FORMWORK & PRECAST BEAM 2 INSTALL FORMWORK & PRECAST Lifting BEAM with the crane INSTALL FORMWORK & PRECAST BEAM

Level +1

Stage 2: Cast in situ columns poured to the top of the precast beams: stitching together the beam/column joint. PLACING HOLLOWCORE PLANKS PLACING HOLLOWCORE PLANKS Lifting PLACING HOLLOWCORE PLANKS PLACING HOLLOWCORE PLANKS Level +2

Level +2

Lifting Props

4 Propping

Lifting Lifting

5

Hollowcore 5

Hollowcore 5 Hollowcore 5

Level +2 Level +2 Level +1

Hollowcore

Props Props Props

4 Propping 4 Propping 4 Propping

Level +1 Level +1 Level +1

Stage 3: Hollowcore slabs placed between the beams.

POURING TOPPING Concreting

POURING TOPPING Topping Finishing POURING TOPPING 7 POURING TOPPING Level +2Finishing

Topping

Finishing

7 Topping

Finishing Level +2

7 Topping 7

Concreting

6 Slab Reinforcement

Concreting Concreting

6 Slab Reinforcement 6 Slab Reinforcement

FREE AREA

6 Slab Reinforcement

Level +2 Level +2 Level +1

FREE AREA FREE AREA

Level +1 Level +1

FREE AREA

Stage 4:

Level +1

Slabs topped with 50mm cast in situ concrete to achieve a monolithic structural unit.

Spherical void formers

The Home Office headquarters hybrid concrete structure was constructed using the above four stage sequence.

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Hybrid Concrete Construction

Design and procurement Design

Procurement

Hybrid concrete construction can be designed as a normal reinforced concrete building, with full composite action between in-situ and precast elements. The design should also consider the construction phase, as one of the load cases is normally precast concrete elements supporting the weight of wet in-situ concrete. An additional stage may be considered if de-propping happens before the in-situ concrete reaches its design strength.

Many UK engineers are experienced in using in-situ concrete, but may feel less confident specifying precast concrete. To obtain the maximum benefit, it is advisable to involve the precast concrete manufacturer at the earliest opportunity. The precast industry is able to give initial advice.

The interface between precast and in-situ concrete elements should be considered in the design process and a detailed guide, Design of Hybrid Concrete Buildings [7] is available from www.concretecentre.com/publications. This gives essential guidance on the key considerations. Initial sizing The initial sizing of the elements for HCC can be carried out using normal methods, for example The Concrete Centre publications Economic Concrete Frame Elements [8] and Concrete Buildings Scheme Design Manual [9] both give guidance on sizing concrete frames.

The publication Best Practice Guidance for Hybrid Concrete Construction [10] looks at the procurement process from concept stage through to design and construction, suggesting processes that allow the capture of best practice. It is supported by a number of case studies. The guidance explains the benefits that result from: • • • • • • •

Early involvement of specialist contractors Using a lead frame contractor Using best value philosophy Holding planned workshops Measuring performance Trust Close cooperation – with an emphasis on partnering.

It is recommended that this guidance is used to maximise the advantages of using HCC.

Inland Revenue, Nottingham, interior of building. The design fully exploited the potential of precast concrete and prefabrication of other major structural elements to achieve real buildability. Image: Martine Hamilton-Knight/Built Vision.

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Hybrid Concrete Construction

Case study 1: Jubilee Library The Jubilee Library, Brighton has been lauded for its design values and sustainability performance. It has won numerous accolades and achieved a BREEAM rating of ‘Excellent’. A mixture of precast and in-situ concrete was used to meet these high standards. Construction The building consists of four storeys, with reading rooms, meeting rooms and staff accommodation situated either side of a central, double-height atrium, itself built on two floors. The central space was constructed using an in-situ concrete slab supported by a series of eight tree-like in-situ concrete columns with fins. The thermal mass of the concrete assists with moderating the temperature fluctuations within the building. Elsewhere, 260mm thick precast hollowcore units have been used as part of a Termodeck system. Air is pumped through the cores in the units to heat or cool the building as necessary; again the thermal mass of the concrete is used to minimise the energy required for heating and cooling. What HCC brought to the project Concrete was a key component of the buildings heating and cooling systems. A variety of concrete elements were used to suit specific situations. The hollowcore units provided the ducts for the air flow. Precast was also used where a high quality finish was required. In-situ concrete was used for larger floor areas to avoid visible joints, and for the feature fins. Project team: Architect: Bennetts Associates with Lomax, Cassidy and Edwards Architects Structural engineer: SKM Anthony Hunt Contractor: Llewellyn Concrete frame contractor: Gallaghers

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Hybrid Concrete Construction

Case study 2: Hilton Hotel, Tower Bridge The Hilton Hotel, Tower Bridge is located on the south bank overlooking the river Thames. It is 13-storeys high and contains 255 bedrooms. The lower three storeys contain public spaces and a 500-seat conference centre. Why hybrid concrete construction was chosen The twin wall solution, with lattice girder slabs was proposed as an alternative to fully cast in situ walls and slabs. This proposal allowed the contractor to reduce the frame construction programme enabling earlier opening of the hotel. Construction The building has a double storey height basement over part of the area with a conventional concrete frame for the lower storeys. Above the public spaces the vertical structure consists of twin wall precast units and floors that use lattice girder slabs. The lattice girder slabs were lifted into position with the edge protection already in place. What HCC brought to the project The use of hybrid concrete gave a fast construction programme – each floor was completed in just five days, including placing the bathroom pods. The precast walls, which were used for all the dividing walls and soffits, gave a high-quality, accurate finish and minimised following trades. The use of precast lattice girder slabs gave a safe working platform for fixing reinforcement and pouring the topping concrete. The lattice girder slabs also reduced the falsework and propping requirements allowing the bathroom pods to be lifted into position before placing the floor above. Overall, compared to other construction methods, the site was cleaner and there was less construction noise.

Case study 3: West Quay car park The West Quay car park is one of the largest multi-storey car parks in the UK. The structure is 95m long, 95m wide and 20m high – eight storeys comprising 15 split levels with a 2m clear headroom throughout. Access to the car park is by means of seven staircases and two double lifts. Why hybrid concrete construction was chosen At scheme design stage the design team considered various options for the structural frame, before selecting a HCC structure based on precast concrete double-tee floor slabs on to cast in situ concrete beamand-column frames. The decision to use HCC was based on a ‘value engineering’ exercise. By combining the cost advantages of cast in situ concrete with the speed of assembly of precast, meant that the structure could be completed on time and within budget. Construction The precast concrete double-tee floor slabs span 15.8m and are 2.4m wide, matching the width of a standard car parking bay and fitting neatly into the 7.2m grid in the east-west direction. The cast in situ concrete beams were cast with nibs projecting at both sides and the ends of the slabs were cast with extended scarf joints; they rest on the nibs and create a 300mm wide channel for service trunking. The east wall of the car park takes the form of a sloping buttress clad with precast concrete panels with a reconstructed stone mix and knapped flint aggregate inserts. At upper levels the car park is clad with precast spandrel panels of reconstructed stone. The panels were doweled to the cast in situ concrete structure with cast-in sockets. What HCC brought to the project The use of HCC allowed the project to be completed on time and within budget, with a remarkable lack of interface problems. In particular the advantages of precast concrete double-tee floor slabs were fully realised; they proved to be a positive way to create large areas of floor very quickly, whilst maintaining a high quality finish.

Project team Project team Architect: Jestico & Whites Structural engineer: Adams Kara Taylor Cost consultant: EC Harris Construction manager: Bovis Lend Lease Concrete frame contractor: John Doyle Construction

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Architect: BDP Structural engineer: Pell Frischmann D & B contractor: Sir Robert McAlpine D & B engineer: Sir Robert McAlpine Design Group Precast floors: Tarmac Precast concrete cladding: The Marble Mosaic Company

Hybrid Concrete Construction

Case study 4: Homer Road Why hybrid concrete construction was chosen With the Homer Road office building, hybrid concrete construction was used to create a structure which allows full continuity to occur between the vertical and horizontal structural elements, thus providing a stiff sway framework. The combination of elements allowed the whole frame to act as a composite structure without relying on expensive mechanical fixings. This method of construction produces a rigid frame which is inherently stable without the need for shear walls or bracing. HCC was the natural choice of material. It fulfilled the design criteria for a visible expression of the structure; behind the delicate glazed facades the precast column and beam structure is clearly visible, needing no further treatment such as cladding for fire protection. In addition, by exposing the painted soffits of the concrete floor slabs in the offices, the temperature and ventilation strategy could exploit the thermal mass potential of the concrete.

At the perimeter the same principle was used with a slightly different detail. The spine edge beam was cast between the final row of end plates (which ran up to the inner side of each perimeter column) on one side and a special precast perimeter unit on the other side, which creates a tapered edge to the ceiling soffit. The perimeter unit has a row of precast holes which allows warm buoyant air rising up the facade to be effectively captured and cooled by the passive chilled beam elements above the ceiling panels. Similar precast holes connect each trough and provide return air paths to the central atrium. The precast perimeter units were cast with a sculpted feature where they meet the column heads. They were also used at the atrium and core perimeters, cast in the same moulds with minor adaptations. Project team Architect, engineer and cost consultant: Foggo Associates Construction manager: Bovis Lend Lease Precaster: SCC (Structural Concrete Contractors)

Construction The hybrid concrete structure consists of 430mm diameter precast columns and precast floor units connected together by means of cast in situ concrete spine beams. Each floor unit takes the form of a double tee-section with end plates to each trough. At each column connection the end plates are cast with a curved ‘cut-out’ to follow part of the column profile. Once the precast columns were fixed on site, the double tee-section floor units were connected to them, positioned so that the curved edge profiles trimmed the outer edge of the columns. The cast in situ concrete spine beam was then cast between two rows of end plates, stitching lower and upper columns and adjacent units together. Between the longitudinal joints, loop connectors were cast into the units and a continuous cast in situ beam joined the units together. The floor units are self-finished and no screed or topping was required.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

THE CONCRETE CENTRE. Concrete and Fire Safety. The Concrete Centre, 2008. BRITISH STANDARDS INSTITUTION. BS 8500 Concrete – Complementary British Standard to BS EN 206-1. BSI, 2006. THE CONCRETE CENTRE, Utilisation of Thermal Mass in Non-Residential Buildings, TCC, 2007 ARUP. Hospital floor vibration study. Comparison of possible floor structures with respect to NHS vibration criteria . Research Report, Arup, 2004. THE CONCRETE CENTRE. Precast Concrete in Buildings. The Concrete Centre, 2007. BROOKER, O. How to Design Concrete Buildings to Satisfy Disproportionate Collapse Requirements. The Concrete Centre, 2009. WHITTLE, R & TAYLOR, H. Design of Hybrid Concrete Buildings. The Concrete Centre, 2009. GOODCHILD, C H, WEBSTER & R M, ELLIOTT, K S. Economic Concrete Frame Elements. The Concrete Centre, 2009. BROOKER, O. Concrete Buildings Scheme Design Manual. The Concrete Centre, 2009. GOODCHILD, C H & GLASS, J. Best Practice Guidance for Hybrid Concrete Construction, The Concrete Centre 2004.

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