Hybrid Concrete Construction.pdf

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

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Hybrid Concrete Construction

Transcript of Hybrid Concrete Construction.pdf

Page 1: Hybrid Concrete Construction.pdf

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

Hybrid Concrete Construction

Page 2: Hybrid Concrete Construction.pdf

ContentsBenefits 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

IntroductionHybrid 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 in-

situ concrete. Many of these are listed in Table 1 and described in the

Benefits of Hybrid Concrete Construction section (page 3).

Table 1: Benefits of hybrid concrete construction

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

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.

Cover imagesMain: Ideas Store, London, courtesy of Hanson

Top inset: Homer Road courtesy of Foggo AssociatesBottom inset: West Quay Car Park, Southampton

This page: Ideas Store, courtesy of Adjaye Associates

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Hybrid Concrete Construction

Benefits of hybrid concrete constructionHybrid 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

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.

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.

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.

Speed

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.

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.

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

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.

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.

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].

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

Day

Internal temperaturewith high thermal mass

Internal temperaturewith low thermal mass

Peak temperaturedelayed by up to six hours

Up to 6-8oC differencebetween peak externaland internal temperature

External temperature

30oC

15oC

Night Day

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

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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.

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 cost-

driven. 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.

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Benefits of precast concrete

Precast concrete

Economic for repetitive elements

Long clear spans

Speed of erection

Buildability

High-quality finishes and consistency of colour

Accuracy

Reduced propping on site

Reduced skilled labour 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.

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:

• Thebuildinglayoutshouldbedesignedtomaximiserepetition

of precast units

• Constructiondetailsshouldbedesignedtomaximisethenumber

of standardised components.

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.

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:

• Surfaceretardingandwash-off

• Rubbing

• Abrasiveblasting

• Bushhammering

• Mechanicalgrindingandpolishing

• Acidetching.

More information on architectural finishes can be found in Precast

Concrete in Buildings [5].

Accuracy

Precast elements are cast to close tolerances, and checked in the factory

before delivery to site.

Reduced propping on site

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.

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

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

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Benefits of in-situ concrete

In-situ concrete

Economic for bespoke areas

Continuity (structural efficiency)

Inherent robustness

Flexibility

Services coordination later in programme

Locally sourced materials

Short lead-in times

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.

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.

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.

Services coordination later in programme

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 optionsThe 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.

Ease of services distribution

Minimises storey height

Suitability for holes

Clear spans Deflection control

Minimise materials

Soffit can be exposed

Maximises off-site construction

Temporary works minimised

Type 1

Type 2

Type 3

Type 4

Type 5

Type 6

Excellent Good Can be used

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

HybridFinal versionOwen Brooker27.11.08

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

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Hybrid Concrete Construction

Precast twin wall and lattice girder slab with in-situ concreteHybrid 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 in-

situ 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:

• Cellulartypestructuresforresidentialuse

• Wallscarryingverticalloadsonly

• Shearandcorewalls;thishassignificantimplicationsforthedesign

• Retainingwalls;thishassignificantimplicationsforthedesign

• ‘Singlesided’formworksituations,wherethereisnoaccessto

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:

• Qualityfinishforwallsandsoffitsenablinguseofthermalmass

• Noformworkforverticalstructureandhorizontalstructure

when lattice girder slabs are used

• Structuralconnectionbetweenwallandslabsreliesonin-situ

reinforced concrete detail and is inherently robust

• Reducedpropping

Disadvantages:

• Proppingoflatticegirderslabsisrequiredpriortosufficient

strength gain of in-situ concrete

• Thesmallerdimensionoftheprecastunitsistypicallyamaximum

of 3.6m, so joints in walls and soffits must be dealt with (expressed

or concealed)

• Reducedflexibilityoflayoutasthisoptionrequireswallsrather

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 slabThe 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.

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:

• Columnscanbeerectedquickly

• Qualityfinishforcolumns

• Precastedgebeamcontainspost-tensioninganchorages

(if required), slab edge reinforcement and cladding fixings,

and avoids need for slab edge shuttering

• Canbeusedwithavarietyofin-situslabs,selectedtosuit

individual project requirements

• Moreflexibleforlatechanges

Disadvantages:

• In-situslabrequiresfalsework,formworkandcuringtime

Precast column and floor units with in-situ beamsThis 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:

• Verticalstructurecanbeerectedquickly;noformworkrequired

• Precastfloorstructurecanbeerectedquickly;no

formwork required

• Qualityfinishforcolumnsandsoffits(althoughthisisnotalways

possible with hollowcore units)

• Structuralconnectionbetweenprecastelementsisviastandard

reinforced or post-tensioned concrete

Disadvantages:

• Precastflooringmustbetemporarilypropped

• Sealingbetweenprecastunitsisrequired

In-situ columns or walls and beams with precast floor unitsA 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:

• Precastfloorstructurecanbeerectedquickly;noformworkrequired.

• Qualityfinishforsoffits(althoughthisisnotalwayspossiblewith

hollowcore units)

• Shortleadtimeforstandardprecastproduct

Disadvantages:

• Precastflooringmustbetemporarilypropped

• Sealingbetweenprecastunitsisrequired

In-situ column and structural topping with precast beams and floor unitsIn 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:

• Precastfloorstructurecanbeerectedquickly

• Precastbeamssupportprecastfloorunits,minimisingfloor

propping

• Precastqualityfinishforsoffits(althoughthisisnotalways

possible with hollowcore units)

• Formworkforin-situcolumnscanbeusedtoprop

precast beams

• Structuralconnectionbetweenprecastelementsisvia

standard reinforced concrete

• In-situstructuraltoppingtobeampermitsbeamstobe

continuous over columns

Disadvantages:

• Downstandbeamsneedtobecoordinatedwiththeservices

distribution

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Hybrid Concrete Construction

Spherical void formers

In-situ columns with lattice girder slabs with optional spherical void formersThe 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.

Advantages:

• Precastfloorstructurecanbeerectedquickly;noformwork

required

• Structuralconnectionbetweenprecastelementsisvia

standard reinforced concrete

• Qualityfinishforsoffits

• Moreflexibleforlatechanges

Disadvantages:

• Precastflooringmustbetemporarilypropped

INSTALL FORMWORK & PRECAST BEAM

Lifting with the crane

Precast beamSafety Safety

Level +1

Moving formwork

Steelformwork

Back-propping (if necessary)

2

1

AA

POURING COLUMNS

Precast beamSafety

Reinforcement

Propping

Level +2

Concreting (column & stitch)

Level +1

Steelformwork

3

4

PLACING HOLLOWCORE PLANKS

Level +1

Level +2

Lifting

Propping

Hollowcore

4

5

Props

POURING TOPPING

Level +1

Level +2

ToppingFinishing

Concreting

Slab Reinforcement

FREE AREA

67

Stage 1:

Column formwork erected to provide temporary support for the precast beams. Precast beams positioned on the column formwork

with beam rebars projecting into the column stitch.

INSTALL FORMWORK & PRECAST BEAM

Lifting with the crane

Precast beamSafety Safety

Level +1

Moving formwork

Steelformwork

Back-propping (if necessary)

2

1

AA

POURING COLUMNS

Precast beamSafety

Reinforcement

Propping

Level +2

Concreting (column & stitch)

Level +1

Steelformwork

3

4

PLACING HOLLOWCORE PLANKS

Level +1

Level +2

Lifting

Propping

Hollowcore

4

5

Props

POURING TOPPING

Level +1

Level +2

ToppingFinishing

Concreting

Slab Reinforcement

FREE AREA

67

Stage 3:

Hollowcore slabs placed between the beams.

INSTALL FORMWORK & PRECAST BEAM

Lifting with the crane

Precast beamSafety Safety

Level +1

Moving formwork

Steelformwork

Back-propping (if necessary)

2

1

AA

POURING COLUMNS

Precast beamSafety

Reinforcement

Propping

Level +2

Concreting (column & stitch)

Level +1

Steelformwork

3

4

PLACING HOLLOWCORE PLANKS

Level +1

Level +2

Lifting

Propping

Hollowcore

4

5

Props

POURING TOPPING

Level +1

Level +2

ToppingFinishing

Concreting

Slab Reinforcement

FREE AREA

67

Stage 4:

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

INSTALL FORMWORK & PRECAST BEAM

Lifting with the crane

Precast beamSafety Safety

Level +1

Moving formwork

Steelformwork

Back-propping (if necessary)

2

1

AA

POURING COLUMNS

Precast beamSafety

Reinforcement

Propping

Level +2

Concreting (column & stitch)

Level +1

Steelformwork

3

4

PLACING HOLLOWCORE PLANKS

Level +1

Level +2

Lifting

Propping

Hollowcore

4

5

Props

POURING TOPPING

Level +1

Level +2

ToppingFinishing

Concreting

Slab Reinforcement

FREE AREA

67

Stage 2:

Cast in situ columns poured to the top of the precast beams: stitching together the beam/column joint.

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

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Hybrid Concrete Construction

Design and procurementDesign

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.

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.

Procurement

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 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:

• Earlyinvolvementofspecialistcontractors

• Usingaleadframecontractor

• Usingbestvaluephilosophy

• Holdingplannedworkshops

• Measuringperformance

• Trust

• Closecooperation–withanemphasisonpartnering.

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 realbuildability.Image:MartineHamilton-Knight/BuiltVision.

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Hybrid Concrete Construction

Case study 1: Jubilee LibraryThe 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

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

Theuseofhybridconcretegaveafastconstructionprogramme–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.

The West Quay car park is one of the largest multi-storey car parks in the

UK.Thestructureis95mlong,95mwideand20mhigh–eightstoreys

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 beam-

and-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.

Case study 2: Hilton Hotel, Tower Bridge

Case study 3: West Quay car park

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

Project team

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

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Hybrid Concrete Construction

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.

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.

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)

Case study 4: Homer Road

References1. THE CONCRETE CENTRE. Concrete and Fire Safety. The Concrete Centre, 2008.

2. BRITISH STANDARDS INSTITUTION. BS 8500 Concrete – Complementary British Standard to BS EN 206-1. BSI, 2006.

3. THE CONCRETE CENTRE, Utilisation of Thermal Mass in Non-Residential Buildings, TCC, 2007

4. ARUP. Hospital floor vibration study. Comparison of possible floor structures with respect to NHS vibration criteria . Research Report, Arup, 2004.

5. THE CONCRETE CENTRE. Precast Concrete in Buildings. The Concrete Centre, 2007.

6. BROOKER, O. How to Design Concrete Buildings to Satisfy Disproportionate Collapse Requirements. The Concrete Centre, 2009.

7. WHITTLE, R & TAYLOR, H. Design of Hybrid Concrete Buildings. The Concrete Centre, 2009.

8. GOODCHILD, C H, WEBSTER & R M, ELLIOTT, K S. Economic Concrete Frame Elements. The Concrete Centre, 2009.

9. BROOKER, O. Concrete Buildings Scheme Design Manual. The Concrete Centre, 2009.

10. GOODCHILD, C H & GLASS, J. Best Practice Guidance for Hybrid Concrete Construction, The Concrete Centre 2004.

Page 16: Hybrid Concrete Construction.pdf

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