multy storey carparks

A guide to design and construction

Multi-storey Concrete Car Parks

Multi-storey concrete car parks

ContentsIntroduction 3

Designconsiderations 4

Layouts 5

Concretebenefits 6

Concreteoptions 8

Edgeprotection 10

Structuraldesign 11

Casestudies 17

References 19

Nottingham Railway Station’s car park was redeveloped to increase its capacity from 500 to 950 spaces. The five storey car park was reopened in 2012, with 2,107 coloured copper panels now fixed to the precast concrete structure’s outside.Architect: BDP.Photo: courtesy of Martine Hamilton Knight.

Cover image: Ocean Village, Southampton. See page 18 for more information.

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IntroductionMulti-storey car parks are a common feature in the UK’s towns and cities. In the past they tended to be utilitarian structures, often designed to be functional without an appreciation of the perceptions of users.

Glossary of terms

Access-way Carriageway not adjoining bays and used solely for the movement of vehicles.

Aisle A carriageway serving adjoining bays.

Bay or stall A parking space allocated to one car.

Bin Two rows of bays with the access aisle running between them.

Clear span construction All columns are located at the perimeter of parking bins.

Deck A slab at any level of the car park.

Dynamic capacity The maximum flow per hour of cars which the car park, or part thereof, can accommodate.

Parking angle The angle between the longitudinal centreline of a bay and the aisle from which it is served.

Ramp An access-way or aisle connecting parking areas at different levels.

Static capacity The total number of bays in a car park.

More recently, designers have recognised the need to improve safety and security through providing long clear spans by removing columns from the parking spaces. This has led to a series of solutions using spans of up to 16m.

This guide presents a variety of solutions using concrete; either precast in a factory or placed on-site. It also explains the design requirements for car parks in more detail, and presents typical car park layouts.

Concrete has many benefits which can be utilised for a car park, including edge protection. Using the latest developments in concrete durability, the corrosion problems seen in older car parks can be designed out and this guide explains how this can be achieved.

The final design and detailing of a concrete car park is important, and this publication also presents some guidance for areas such as stability, fire resistance, movement, drainage and waterproofing.

A number of case studies illustrate how concrete has been used successfully to create new car parks for a variety of uses.

Long spans provided by precast concrete beams.

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DesignconsiderationsAs with any other building type, there are a number of issues to consider in the design of car parks. This guide is not intended to replace other publications, for example Design Recommendations for Multi-storey and Underground Car Parks [1], which cover design considerations and development of the design brief in detail. Instead, this guide focuses on key issues of importance in the design and construction of concrete frames for car parks.

CarparkuserrequirementsCar park users have particular requirements affecting the layout and design of car parks. Typical user requirements include:

� Secure parking environment.

� Clear site lines.

� Ease of quickly finding a parking place.

� Easy manoeuvrability.

� Minimum queuing.

� Space to open car doors.

� Safe pedestrian routes through car park.

� Good way-finding.

ClientrequirementsClients or developers will have their own preferences, which will generally be aligned to user requirements; particularly if income is reliant on users returning to the car park regularly. Client requirements potentially affecting the structure include:

� Commercial viability based on initial and whole-life costs.

� Durability, with low maintenance costs.

� Adaptability for future changes in car park use and car design.

� Sustainability.

CarparkuseCar parks are provided for users of different types of facilities such as hospitals, retail premises, offices and short or long-stay transport interchange sites. Recommended bay sizes vary according to the length of stay and are provided in Table 1. Short stay and high usage car parks should be provided with larger parking bays and access route widths allowing users easily to manoeuvre their vehicle around the car park. Consideration should also be given to the size of vehicles likely to use the car park. Where larger than normal vehicles are expected, bay sizes and headroom may need to be increased.

EffectonthestructureLong clear spans

Typically, end user requirements translate into car parks which are airy, well lit, have clear sight lines, are well signed, and are easy to manoeuvre around.

Structurally, large clear spans of up to 16m make manoeuvring easier and give better sight lines. Parking bays clear of columns to allow unrestricted door opening are usually considered the best option.

Headroom

The minimum clear headroom for vehicles given in Design recommendations for multi-storey and underground car parks is 2.10m. However, BS 8300 Design of buildings and their approaches to meet the needs of disabled people – Code of practice [2] advises provision of a minimum height of 2.6m from the entrance of the car park to (and including) designated parking spaces and exits from those spaces. This additional headroom requirement is not usually achievable in multi-storey car parks owing to the need to maintain ramps at an acceptable gradient and, under such circumstances, provision for taller vehicles is generally made outside the car park.

Table 1: Recommended bay size

Type of Parking Length (m) Width (m) Comment

Mixed use 4.8 2.4 Mixed occupancy

Short stay 4.8 2.5 < 2 hours

Long stay 4.8 2.3 One movement per day

Disabled user 4.8 3.6 Refer to text on headroom

Parent/child 4.8 3.2 -

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LayoutsWhile there are over 100 different options for laying out a car park, in practice three layouts with 90° parking angle are most commonly used. These are:

� Ramped deck.

� Flat deck.

� Split level.

The relative merits of all the options are presented in the Car Park Designers’ Handbook [3]. Generally one-way flow circulation is preferred for simplicity and efficiency. Four layouts are shown to illustrate the variations.

Whichever option is chosen, the layout of the parking bays will be similar, with bays located either side of aisles carrying one-way traffic. While this is an efficient layout, the constraints it imposes on the structure are shown in Figure 1. To meet the requirement for clear spans, without any interbin supports, it is usually necessary to span 15.6m across the aisle and adjacent parking bays. The structural grid for many car parks is then 15.6 x 7.2m.

Down

Up

Up

BA

4.8m 4.8m6.0m

Bin width

Interbin support zone

AISLE

A: 0.46m minimum 0.8m to 1.0m preferred range

B: 3.3m minimum 3.6m desirable

3 x 2.4mbays *3

bins

reco

mm

ende

d m

inim

um

BAY

BAY

Acceptable support positions

* Typical bay dimensions

Figure 1: Typical car park layout for mixed use

Figure 2: Examples of layout options

Examples of ramped deck car park layout Example of split level car park layout

Example of flat deck car park layout

Up

Down

Up

Up

Down

Down

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EconomicsWhether precast concrete or in-situ concrete is used for car park construction, they both offer economic overall solutions. An important conclusion from a series of cost model studies undertaken on behalf of The Concrete Centre found that the cost of the structural frame should include the cost of edge protection. The whole-life costs should also be considered. A car park should have a design service life of 50 years before significant maintenance and repair is required.

ProgrammeConcrete solutions can be erected quickly and safely. Precast concrete frames are designed and detailed to be highly buildable with short erection periods. In-situ concrete frames with proprietary formwork systems are also quick to erect and, with their short lead-in times, offer an early start on-site.

DesignFinishes

The structure in car parks is usually left exposed. With attention to detail during specification, and particularly during construction, concrete can have a good visual finish. Precast concrete in particular usually has a high quality finish due to the quality of the moulds used and greater control of the production of the concrete.

Long clear spans

Concrete can be used in a number of different options to economically achieve a long clear span. Clear spans are now regularly used in car parks to improve visibility and manoeuvrability.

The long clear spans are achieved without compromising floor-to-floor heights. The solutions available typically range in floor depth from 475 to 650mm, although 400mm floor depth solutions are available. The thinnest solutions take advantage of spans being continuous over more than one bay.

PerformanceFire

Concrete has inherent fire resistance, which is present during all construction phases. It is achieved without the application of additional treatments and is therefore 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 [4] by The Concrete Centre.

Vibration control

It is usually recommended that the natural frequency of the floor and frame, when designed as simply supported and free of live load, should exceed 5 Hz. Most concrete car park structures have sufficient mass and stiffness to satisfy these criteria, even for longer span options.

ConcretebenefitsConcrete’s unique flexibility provides a wide range of framing options and design/construction solutions to suit the exact needs of specific projects.

An in-situ concrete car park in construction. Sainsbury’s, Penrith. Photo: courtesy of Northfield Construction Ltd.

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Durability

A well designed, detailed and constructed concrete car park should achieve a design service life of 50 years without the need for significant maintenance or repair. If subject to a proper inspection and maintenance regime in accordance with ICE Recommendations for inspection, maintenance and management of car park structures [5], it should be possible to extend the service life beyond 50 years.

Some existing structures perform poorly. To avoid poor performance the following should be ensured:

� Use good quality concrete and construction.

� Reinforcement fixed to provide the designed-for cover.

� Use concrete designed to resist chlorides.

� The actual floors of the car park are not ‘salted’ by maintenance staff.

If current knowledge and good practice is adopted, concrete will perform more than adequately.

Robustness/vandal resistance

Concrete is, by its nature, very robust and capable of resisting accidental damage and vandalism.

Minimum maintenance

Unlike other materials, concrete does not need any toxic coatings or paint to protect it against deterioration or fire. Properly designed and constructed concrete is relatively maintenance-free over its design service life.

SustainabilityLocally sourced

The constituent parts of concrete (water, cement and aggregate) are all readily and locally available to any construction site, minimising the impact of transporting raw materials.

It is worth noting:

� 99.9% of aggregates used in the UK are sourced in the UK (80% are used within 30 miles of extraction).

� 90% of Ordinary Portland Cement is produced in the UK and there are cement kilns throughout the UK.

� 100% of UK-sourced reinforcement is produced from UK scrap steel.

Reduced use of materials

The long span options often required for a car park need materials to be used efficiently. In all the common concrete solutions, the self-weight of the structure is minimised; use of materials is minimised and consequently transportation requirements are also reduced.

Concrete mix

Modern concretes generally contain cement replacements which lower the embodied CO2 and use by-products from other industries. Care should be exercised to balance the environmental benefits of cement replacements with their slower strength gain, which delays the initial prestress and stripping of formwork or moulds.

Visit www.sustainableconcrete.org.uk to compare alternative mix constituents.

Precast concrete ‘T’ units give a low span-to-weight ratio. Avenue de Chartres car park, Chichester. Architect: Birds Portchmouth Russum.Photo: courtesy of Nick Kane of Arcaid.

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ConcreteoptionsFor a typical 15.6 x 7.2m grid, a number of concrete options are available. Five are presented here, all of which have proved to be cost-effective and meet client and user requirements. These designs are efficient because they use prestressing, are designed to be lightweight or are a combination of the two. They can all be adapted to suit ramped, flat deck and split-level car park layouts.

PrecasthollowcoreunitsThese 1.2m-wide precast concrete units utilise prestressing and voids formed within units to form an efficient structural element with a low span-to-weight ratio. While the units can be supported with a variety of beam types, the units have to be supported from below.

Benefits:

� Standard units.

� Simple, fast erection.

� Small overall depth for single span situations.

Structural sizes:

� 400mm deep unit.

� 75mm thick screed.

� 475mm overall structural depth above parking areas.

� 675mm depth along beam lines on short span.

Precastconcretedouble‘T ’unitsThese precast concrete units utilise prestressed concrete and a structurally efficient shape to give a low span-to-weight ratio. The standard width for these units is 2.4m. While they can be supported with a variety of beams types, a common approach is an L-shaped beam with a notched end to the units to give a constant structural depth.

Benefits:

� Low self-weight – minimises supporting structure.

� Standard or bespoke units available.


� Cranked ramp units available.

� Good visual appearance.

Structural sizes:

� 600mm deep unit.

� 75mm thick screed.

� 675mm overall structural depth.

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Post-tensionedbandbeamsThis in-situ concrete option uses prestressing in the form of post-tensioning to minimise the structural depth. A shallow slab spans onto integral beams. The formwork for this option is relatively simple.

PrecastcombinedbeamandcolumnframeThis proprietary system has evolved to give fast erection times and an efficient structure. The main feature is the precast combined beam and columns units which are designed to minimise the structural depth at mid-span by using moment connections at the beam/column joint. Void formers are used in the units to reduce self-weight for lifting. The headroom is slightly reduced between some of the parking spaces. 200mm deep precast floor units span betweenthe beams.

Benefits:

� No formwork is required on site.

� Maximises the benefit of multiple span floor plates.

� Easily adapted to suit different column spacings.

� Flat soffit.

� No screed required.

Structural sizes:

� 600mm deep (multi-span).

� 650mm deep (single-span).

Benefits:

� Short lead-in times.

� Maximises the benefit of multiple span floor plates.

� Easily adapted to suit different column spacings or geometry.

� No beam required in short span direction.

� No screed required.

Structural sizes:

� 150mm thick slab.

� 550mm deep beam (multi-span).

� 650mm deep beam (single-span)

� 550-650mm overall structural depth.

Benefits:

� System developed specifically for car parks.


� No formwork required.

Structural sizes:

� 200mm thick slab.

� 600mm deep beam (mid-span).

� 600mm overall structural depth.

VoidedslabThis form of construction mixes in-situ and precast concrete. A thin precast concrete ‘biscuit’ is cast containing reinforcement lattice girders. The units are up to 3.6m wide and are positioned and propped on site, where in-situ concrete is placed to complete the structure. Recycled plastic or polystyrene void formers are used to reduce the self-weight of the structure. This can also be 100% in-situ or fully precast on in-situ beams.

� Type A -Spanning horizontally between the columns.

� Type B - Bolted to the deck and cantilevering up from it.

� Type C - Monolithic with the deck.

Concrete barriers are usually type A or C or a combination of the two. For type B to be an option, the deck must be sufficiently strong to resist the bending moment and shear forces from the cantilever barrier.

The barriers are designed to resist the impact load either by absorbing the impact energy through deflection of the barrier, or by relying on the rigidity and mass of the barrier to distribute impact energy through much of the structure, absorbing it by elastic strain.

Energy absorbing barriers tend to be of steel construction and have the following characteristics:

� They can be damaged by impact, and should be inspected regularly and replaced as necessary.

� They rely on fixings into the deck, which should be designed to minimise replacement after impact. An ultimate load factor of 1.5 is recommended for the fixing.

� As their service life is generally shorter than the car park, they will require replacement during the life of the car park.

� They can be integrated into a flexible cladding system.

� In sizing the car park, due allowance should be made for deflection of the barrier under impact; particularly if the cladding is fragile.

Concrete barriers tend to rely on their mass to resist impact forces, and are therefore more robust. They have the following characteristics:

� They require minimal space.

� They rarely require replacement but should be inspected and repaired as necessary after impact.

� They can be cast monolithically with the structure.

� They can form the load bearing structure or cladding or both, reducing the overall building cost.

� They form an upstand to the edge of the deck which helps to control surface water.

Columns may be subject to direct vehicle impact and therefore it is preferable for the corners to be rounded or chamfered to minimise damage to both column and vehicle.

EdgeprotectionEdge protection is an important consideration in the design of car parks. Barriers are provided to prevent pedestrians or cars from falling from upper levels. Barriers can be divided into three types:


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St Paul’s car park, Sheffield. For more information see page 17.

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DesignactionsImposed loads

The imposed loads applicable to decks and ramps are in Category F of the UK National Annex to BS EN 1991-1-1:2002 [6]. For a maximum gross vehicle weight under 3000kg, the characteristic loads are:

qk = 2.5 kN/m2 (uniformly distributed load) Qk = 10 kN (concentrated load)

Wind and lateral loads

Wind loading information applicable to car parks is given in BS EN 1991-1-4:2005 [7] and its UK National Annex. Design recommendations for multi-storey and underground car parks recommends the wind loading be taken as acting over the entire elevation area of the structure with no reduction for openings.

Lateral loads also arise when vehicles change direction or speed. Clause 6.3.2.4 (3) in EN 1991-1-1:2002 states that the ‘horizontal wheel loads should be determined for the specific case’. No information is given to determine the horizontal wheel loads for cars in a car park. However, as a guide clause 6.3.2.3 (7) states that ‘horizontal loads due to acceleration or deceleration of forklifts may be taken as 30% of the vertical axle loads Qk’. Judgement is needed to determine how many cars may be accelerating or braking in the same direction in a car park.

Vehicle impact and edge protection

Car park structures should be designed to withstand vehicle impact loads. The design loads are given in Annex B to BS EN 1991-1-1:2002. For car parks designed for vehicles up to 2500 kg gross mass, the horizontal characteristic force, F (in kN) - normal to and uniformly distributed over any length of 1.5m of a rigid barrier - are given in Table 2.

Where speed retarders in the form of speed humps are used to decelerate cars on long straights, consideration should be given to the effect of impact on the decks.

Snow

Design Recommendations for Multi-storey and Underground Car Parks [1] states that snow loading on roofs need not normally be considered in combination with vehicle loading. Possible exceptions are long-stay car parks and those in areas with high snowfall.

Thermal actions

Multi-storey car parks are open to the climate year-round and are thus subjected to a large range of temperatures and humidity. In addition, the top deck is heated by solar radiation which is made worse if a dark-coloured thin-layer waterproof finish is used. Temperature effects for car parks are thus significant by comparison with other building structures.

The relatively large temperature range in a car park deck leads to significant horizontal movements or forces which must be allowed for in the design of the frame: both elements and joints. Further guidance is given on page 13.

When the roof deck is subject to solar gain during the day or heat loss during the night, differential strains are induced across the thickness of the concrete which causes bowing and/or reverse bending. These additional bending forces can add significantly to the bending moments and shears generated by normal loadings. The method of calculation is given in BS EN 1991-1-5.

StructuraldesignCar parks are often treated as a standard building design. There are many similarities with buildings but also some notable differences. This section provides useful information for the design of car parks to Eurocodes and highlights some important areas for further consideration.

Table 2: Horizontal forces on edge barriers

Horizontal force over a 1.5m length of rigid barrier

Horizontal force in kN

Height above floor/ramp in mm

Edge barrier to deck 150 375

Edge barrier to ramps 75 610

Bottom end of straight ramp over 20m long

300 610

Colouring the floor provides clear signage.Photo: courtesy of Dunne Group

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Lateral stability

Lateral stability can be provided in the following ways:

� Using the walls in stair and lift cores.

� Using the skeletal bracing adjacent to ramps between car decks.

� Using the ramps as scissor bracing (subject to circulation layout).

� Using frame action for low-rise car parks.

Other issues to consider for lateral stability include:

� Core walls located at the ends of the building act as restraints to shrinkage – see page 11 for more guidance.

� Split level decks require lateral stability to both sets of decks (alternatively the ramps should be designed to transfer lateral loads).

� Internal walls other than those forming the stair and lift cores for stability should be avoided within the parking areas or adjacent to the ramps as they restrict visibility and increase crime.

� The decks are usually considered to be stiff plates which can carry horizontal forces to the stability system but where there is no structural topping to precast elements, this should be justified.

Vibration

Modern car parks are now commonly designed for clear spans of at least 15.6m and their dynamic response should be checked to ensure user comfort. A Design Guide for Footfall Induced Vibration of Structures [8] gives a methodology for predicting vertical vibrations in structures.

For most concrete car parks, no increase in member sizes over that needed to satisfy static loads will be required to achieve the required dynamic performance. Design Recommendations for Multi-storey and Underground Car Parks recommends a minimum natural frequency of 5 Hz, Table 3 shows guideline values for the options presented in this guide.

FireresistanceFor open-sided car parks up to 30m in height, the required fire resistance period is 15 minutes in England and Wales and 30 minutes in Scotland. For elements protecting a means of escape, it is 30 minutes (England and Wales) and 60 minutes (Scotland) for compartment walls separating buildings.

The fire resistance of slabs, beams and columns can simply be checked in most cases by using the tabular method in BS EN 1992-1-2. The method is based on the nominal axis distance. A fire resistance of at least 60 minutes can usually be achieved without increasing the minimum cover required to satisfy durability requirements. The Concrete Centre’s How to Design Concrete Structures using Eurocode 2 [9] provides tables to quickly check the fire resistance of concrete elements.

RobustnessAs the structural frame can be subject to direct impact from a vehicle, both inside and outside the car park, it should be designed to prevent disproportionate collapse based upon the number of storeys in accordance with BS EN 1991-1-7.

Design for movements

In concrete structures, a number of movements potentially occur throughout the lifetime of the structure and should be considered during the design development.

The principal movements include:

� Early age thermal contraction (due to cooling of the concrete following the heating generated by the cement hydration process).

� Elastic shortening; particularly for post-tensioned members.

� Effects of creep (increase in strain under constant stress).

� Long-term drying shrinkage.

� Temperature induced movements or bending.

� Autogenous shrinkage (induced by cement hydration, in concrete with very low water cement ratios).

Movements are generally considered in two stages:

� Early age contractions due to early age thermal contraction, autogenous shrinkage and elastic shortening.

� Long-term effects such as creep, drying shrinkage and temperature changes.

An indication of the range of strains, and hence movement, is shown in Table 4.

Table 3: Guideline natural frequencies for concrete car park options

Structural system Guideline natural frequency (Hz)

Precast concrete double ‘T’ units 5.6

Post-tensioned band beams 5.4

Precast hollowcore units 8.7

Biaxial voided slabs 10.9

Precast combined beam and column frame

5.3

Note:The natural frequencies stated are for 15.6m spans based on the simplified calculation method given in A Design Guide for Footfall Induced Vibration of Structures [8].

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Table 4: Indicative strains and movements for typical design situations

Phenomenon Minimum Maximum

Typical strains for an internal reinforced concrete structure

Early thermal shrinkage strain 100 me 300 me

Drying shrinkage 300 me 400 me

Total strain 400 me 700 me

In terms of movement 0.4mm/m 0.7mm/m

Shrinkage over 50m 20mm 35mm

Additional strain due to post-tensioning (PT)

Elastic strain due to prestress 75 me 100 me

Creep strain due to prestress 150 me 250 me

Total strain for a PT structure 625 me 1050 me


Shrinkage over 50m 30mm 55mm

Additional strain due to exposure of top deck of a car park

Strain due to thermal effects 200 me 400 me


Note:me = microstrain (strain x 10-6)

Movement joints

Given the potential range of movements, and as car park plan dimensions are often large, careful consideration should be given to whether movement joints should be provided and if deemed necessary, where they should be located. The often used rule that a 25mm movement joint should be provided every 50m is too simplistic for a car park situation. As well as potential movement, the effect of restraint and the construction sequence should also be considered.

Restraining the free movement of the slab deck will cause stresses that can lead to cracking. To reduce restraint to movement, it is best if the stability bracing system is near the centre of the plan or at least symmetrical in location and stiffness (see Figure 3, on page 14).

Control of the construction sequence is an important way of limiting early-age linear horizontal movements, particularly when post-tensioning is used. Pours should generally be isolated from any fixed structure such as ramps or cores for as long as possible to allow the early-age effects to pass without locking in any movements or restraints. The sequence of connected pours should be planned to minimise the movement at the free edges; for instance, if three pours are cast in the sequence 2-1-3 - as opposed to 1-2-3 - this may significantly reduce the slab movement. If this is inconvenient, pours can be separated by ‘pour strips’ – gaps with discontinuous but overlapping reinforcement – left open until the early age effects have taken place.

Bearings

At the support positions of precast concrete slabs, horizontal forces caused by movements can cause the supporting member and slab to split or shear. This will reduce the load carrying capacity of the connection.

This movement should be dealt with in one of two ways:

� Allow movement to occur and ensure there is no restraint to movement. Precast concrete units with spans in excess of 8.0m should be bedded on a suitable flexible bedding material such as neoprene; or

� Design the joint to be monolithic in the permanent situation.

Whichever option is chosen, and the latter is favoured, the implications should be considered throughout the design.

The design of bearings and all the considerations to take into account are explained in Design of Hybrid Concrete Buildings [10].

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DurabilityofthestructureExposure conditions

While car parks are subject to de-icing salts, the quantity of exposure to these salts is significantly lower than for highway structures. Although the durability requirements for concrete car parks should be determined from BS 8500, this standard does not address car parks specifically and therefore some interpretation is required. The recommendations for various exposure conditions are given in Table 5. These have been developed after consultation with industry experts and assume the following:

� De-icing salts will not be applied directly to the elements as part of a maintenance regime.

� The car park will be well-drained.

� The car park will have good ventilation.

� The car park is located in the UK.

� Design service life of 50 years.

� Freezing of internal elements is unlikely to occur.

� Soffits, columns, and walls are rarely exposed to spray from de-icing salts.

Elements immediately adjacent to a highway are not included.

It is recommended that the concrete class should be C32/40 or greater. There is little guidance on how to deal with abrasion but BS EN1992-1-1 cl 4.4.1.2 (13) [11] does advise that for abrasion class XM1 (moderate), a sacrificial layer of 5mm of concrete may be used. This is appropriate for use at the entry level to the car park, which will be subject to the most severe conditions.

Car parks protected with waterproofing may have reduced exposure conditions but consideration should be given to the maintenance regime. Concrete surfaces can become exposed when the membrane is damaged or worn out which can significantly impact the service life of the structure.

Chlorides and prestressed concrete

Table NA.4 of the UK NA to BS 1992-1-1 [12] requires bonded prestressing steel within concrete of exposure classes XD1, XD2, XD3, XS1 and XS3 to be in an area of decompression under frequent load combinations. This ‘decompression’ requirement stipulates that all parts of the bonded tendons or duct lie at least 25mm within concrete in compression.

Apart from coastal locations where exposure class XS1 (airborne chlorides originating from sea water) should be applied, soffits may be regarded as being ‘not subject to chlorides’ and decompression is not considered to be an issue for prestressing steel at the bottom of concrete members.

Post-tensioned bonded tendons near the top surface should satisfy the ‘decompression’ requirement; alternatively, the use of plastic ducts may be considered.

a) Favourable layout of restraining walls (low restraint)

b) Unfavourable layout of restraining walls (high restraint)

Figure 3: Typical floor layouts

fritz

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fritz

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fritz

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Table 5: Proposed exposure classes for car parks

Element type and location Recommended exposure class Recommended exposure class in coastal areas

Top surface of decks and ramps at the entry level of car park XD3 (XC3/4)a & XM1b XD3(XC3/4)a, XS1c & XM1b

Top surface of decks and ramps exposed to freezing e.g. roof level

XF2 & XD1(XC3/4)a Optional - XM1b XF2, XS1(XC3/4)a & XD1d Optional - XM1b

Top surface of decks and ramps in other locations XD1 (XC3/4)a Optional - XM1b XS1 (XC3/4)a & XD1d Optional - XM1b

Soffits of decks and ramps XC3/4 XSI (XC3/4)a

Vertical elements XC3/4 XSI (XC3/4)a

Vertical elements exposed to freezing XC3/4 XFI XSI (XC3/4)a XFI

Elements protected from rainfall e.g. internal area such as stair enclosures

XCI XCI

Key:a Exposure classes given in brackets denote classes which are less critical and assumed in BS 8500 to occur simultaneously with the main exposure class.b BS EN1992-1-1 Cl 4.4.1.2(13) advises that for abrasion class XM1 (moderate) a sacrificial layer of 5mm of concrete may be used. This is appropriate for use at the entry level to the car park, which will be subject to the most severe conditions and may also be adopted for other situations.c XD3 condition is more critical. d XSI condition is more critical.

WaterresistanceDecks required to be water resistant should be coated with a waterproof membrane capable of crack bridging. Alternatively, water resistant concrete can be used but as car parks are large open structures subject to movement and vibration, it is difficult to ensure the decks are watertight without the application of a waterproof membrane. Water resistant concrete is therefore more suitable for use in specific areas of a modest size such as control rooms and lift pits.

Membranes

A membrane should be selected with care to ensure it meets performance requirements. Movement of the structure is a particular issue and the membrane may be required to accommodate:

� Passive non-structural cracks opening and closing slowly in response to temperature changes; typically 0.5 to 1.0mm wide.

� Live structural cracks which open up after waterproofing and may be subject to rapid cyclic movement.

Design Recommendations for Multi-storey and Underground Car Parks has information on different types of membrane available including spray-applied and thin membranes, as well as traditional mastic asphalt. Membranes are available in different light-stable colours to differentiate between parking bays and traffic aisles.

It should be noted that regular inspection is important to ensure waterproofing is fulfilling its requirements, and repairs are carried out when needed. Particular areas to focus on are the turning areas adjacent to the ramps, where the membrane can wear significantly.

Water resistant concrete

If concrete is to be designed to resist water, Table 6 gives guidance on the approach to the control of cracking; based on BS EN 1992-3. This guidance is specifically for concrete structures under sustained water pressure. Wherever possible car parks should be designed to have minimum water leakage but some staining may be acceptable, but where they are part of a mixed use or habitable development then more stringent conditions may be required.

Table 6: Recommendations for water resistant concrete

Tightness class

Requirements for leakage

Recommendations for liquid retaining structures

0 Some degree of leakage acceptable, or leakage of liquids irrelevant

Design to BS EN 1992-1-1e.g. 0.3 mm crack width

1 Leakage to be limited to a small amount

Some surface staining or damp patches acceptable

Design for 0.2 mm crack width using BS EN 1992-1-1

2 Leakage to be minimal.Appearance not to be impaired by staining

Ensure no cracks through full deck thickness or provide a waterproof deck membrane

3 No leakage permitted Provide a waterproof deck membrane

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DrainageAn assessment should be made of the quantity of water likely to be deposited on a particular deck. Roof decks should be designed for local rainfall conditions and appropriate drainage provided.

For other decks the quantity of water will depend on:

� Quantity of rainfall penetrating the cladding.

� Quantity of water brought in on vehicles.

� Overspill water from car washing facilities. The facility should incorporate a water recycling system.

� Washing down of decks.

� Facilities for extinguishing car fires.

Decks and ramps should be laid to falls to prevent ponding and ensure water containing de-icing salt drains away quickly and so reduces the opportunity for chloride ions to penetrate concrete surfaces. The recommended minimum fall for drainage is 1 in 60 and, for user comfort, a fall greater than 1 in 20 should generally be avoided.

The long-term deflection of the structure should be considered to ensure that ponding does not occur under sustained loads.

Drainage outlets should be recessed below the surface of the concrete to ensure effective drainage of the decks.

ConcretefinishesAll parts of the car park should be suitable for both vehicles and pedestrian use.

A smooth surface is generally required only in areas where waterproofing is to be applied as smooth surfaces have less skid resistance. However, they increase the levels of tyre noise in turning areas and where vehicle speeds are low, even in the wet, skid resistance may not be critical.

Power trowelling after floating produces a dense, smooth hardwearing surface with negligible ‘ripple’ marks. However, although it has become more popular, power trowelling is not really suitable for the reasons outlined above and therefore a uniform lightly brushed surface is preferred for the finish to the decks.

A tamped finish is produced by raising and lowering the compacting beam in its final pass to produce a surface with ridges at a fairly regular spacing of 20 - 30mm and up to 5mm high. Generally, the grooves should be in the direction of drainage falls and, on ramps, should follow a chevron pattern. Due to the lack of compaction in ridges, this finish can be dusty.

Surface texture may be applied by roller or by stiff brush. Brush worked finishes are produced with a stiff wire or bristle brush.

A lightly tamped surface is recommended where ramps are steeper than 1 in 10. Where slopes are less than 1 in 10, power floating followed by brushed or lightly tamped surfaces are considered appropriate.

Painted concrete produces a reflective surface to increase light levels.

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Casestudies

StPauls,SheffieldProject description

The 10-storey car park, with two retail floors below, forms part of phase two of the 1.6 ha masterplan for the regeneration of Sheffield city centre in 2002. The brief was to provide an inner city car park incorporating 520 spaces completing the public realm to St Paul’s Place.

Construction

The car park is of a split-level layout using precast double ‘T’ units and a precast concrete frame. Piled foundations support the basement, ground floor and first floor, above which sits the car park. The prestressed double ‘T’ floor units span 16m and are 600mm deep to provide a clear internal parking area. Structural stability is provided by precast concrete core walls around the stair towers and service shafts.

To avoid increasing floor-to-floor height, 200mm deep×500mm long scarf cut-outs were introduced to the ends of the double-Ts to allow services to run parallel to edge beams. Holes through double ‘T’ ribs were also introduced for lighting cables.

On-site erection was complete in 14 weeks and, at its peak, the concrete supplier was delivering 20 loads every day.

Project team

Client: CTP ST JamesArchitect: Allies and MorrisonStructural engineer: Capita Symonds StructuresPrincipal contractor: JF FinneganSpecialist contractor: Tarmac

Broadmead,BristolProject description

Broadmead multi-storey car park formed part of the £500m Cabot Circus scheme in Bristol, which saw extensive demolition to the existing retail buildings, and restructuring of the roads in order to extend the existing facilities and regenerate land to the north east of the site.

Construction

The car park decks consisted of 650mm deep by 1200/1800mm wide post-tensioned (PT) beams spanning 16m with 175mm thick PT slabs between. The total suspended floor area of the eight-storey structure was 54,000m2.

Project team

Client: Bristol Alliance Structural engineer: Waterman Principal contractor: Norwest Holst Frame contractor: Febrey Ltd Specialist PT contractor: Freyssinet

Photo: courtesy of Tarmac Ltd.

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OceanV illage,SouthamptonProject description

This five-storey car park has been provided for users of the Ocean Village marina in Southampton. From the outset, it was decided to use long clear spans and high ceilings to improve visibility and create a sense of space and safety. Coloured membranes were used to improve way finding and to reflect light, minimising the lighting requirements.

Construction

The car park has a 15.6 x 7.2m typical grid, so that no columns are located within parking spaces. The floor consists of 400mm deep precast hollowcore concrete units, finished with an 80mm-thick structural topping. The hollowcore units are supported on precast concrete edge beams, which in turn are supported by precast concrete columns. Precast concrete shear walls are located towards the ends of the rear façade and in the centre adjacent to the movement joint.

Stability along the front is provided by the walls of the escape stair towers.

Project team

Client: Marina Developments LtdArchitect: Tiger Stripe ArchitectsStructural engineer: Price and MyersPrincipal contractor: Dean and DyballSpecialist contractor: Tarmac

SalfordQuaysMediaCentreProject description

This 2,000-space car park was built to serve the first purpose-built media centre in Salford Quays. The car park was built over a two–storey area, which forms the hub of the development and provides a further nine storeys of parking.

A key feature of the building is its curved plan area.

Construction

The car park uses a proprietary combined beam and column frame (for more information see page 9), modified to suit the curved building shape.

Early design, detailing and prefabrication enabled the on-site construction period to be reduced.

Project team

Client: MediaCityUKArchitect: Chapman TaylorContractor: SCC Design Build

Photo: courtesy of Ben Ghibaldan

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References1 Design Recommendations for Multi-storey and Underground Car Parks (Fourth Edition), The Institution of Structural Engineers, 20112 BS 8300: 2009, Design of buildings and their approaches to meet the needs of disabled people, British Standards Institution, 20093 Hill J, Car Park Designer’s Handbook, Thomas Telford Ltd, 20054 Concrete and Fire Safety, The Concrete Centre, 2008.5 Recommendations for the Inspection, Maintenance and Management of Car Parks, Institution of Civil Engineers, 20106 BS EN 1991-1-1, Eurocode 1: Actions on structures: General actions – Densities, self-weight, imposed loads for building, British Standards Institution, 20027 BS EN 1991-1-5, Eurocode 1: Actions on structure: General actions – Thermal actions. British Standards Institution, 20038 Wilford, M & Young, P, A Design Guide for Footfall-induced Vibration of Structures, The Concrete Centre, 20069 Brooker, O et al, How to Design Concrete Structures using Eurocode 2, The Concrete Centre, 200610 Whittle, R & TAYLOR, H, Design of Hybrid Concrete Buildings, The Concrete Centre, 200911 BS EN 1992-1-1, Eurocode 2: Design of concrete structures. General rules and rules for buildings, British Standards Institution, 200212 UK National Annex to Eurocode 2: Design of concrete structures. General rules and rules for buildings, British Standards Institution

Queen Anne Terrace Car Park, Cambridge. Built in 1971, the main structure is reinforced concrete clad with precast concrete fins and panels, the latter having an exposed aggregate finish.Photo: © Nick Stone, All Rights Reserved.

Back cover image: Parc des Celestins, Lyon. This underground car park is a circular structure thats takes users 22m below the city. Photo: courtesy of Guillaume Perret.

All advice or information from MPA -The Concrete Centre is intended only for use in the UK by those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted by Mineral Products Association or its subcontractors, suppliers or advisors. Readers should note that the publications from MPA - The Concrete Centre are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

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