Multi-storey Concrete Car Parks
A guide to design and construction
Multi-storey concrete car parks
Contents Introduction
3
Design considerations
4
Layouts
5
Concrete benefts
6
Concrete options
8
Edge protection
10
Structural design
11
Case studies
17
Reerences
19
Nottingham Railway Station’s car park was redeveloped to increase its capacity rom 500 to 950 spaces. The fve storey car park was reopened in 2012, with 2,107 coloured copper panels now fxed to the precast concrete structure’s outside. Architect: BDP. Photo: courtesy o Martine Hamilton Knight. Cover image: Ocean Village, Southampton. See page 18 or more inormation.
Multi-storey concrete car parks
Introduction Multi-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. More recently, designers have recognised the need to improve saety and security through providing long clear spans by removing columns rom the parking spaces. This has led to a series o solutions using spans o up to 16m. This guide presents a variety o solutions using concrete; either precast in a actory or placed on-site. It also explains the design requirements or car parks in more detail, and presents typical car park layouts. Concrete has many benefts which can be utilised or 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 fnal design and detailing o a concrete car park is important, and this publication also presents some guidance or areas such as stability, fre resistance, movement, drainage and waterproofng. A number o case studies illustrate how concrete has been used successully to create new car parks or a variety o uses.
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 ow per hour of cars which the car park, or part thereof, can accommodate.
Parking angl e
The angl e between the longitudinal centreli ne of a bay and the aisle from which it is served.
Ramp
An access-way or aisle connecting parking areas at different levels.
Static capacit y
The total number of bays in a car park.
Long spans provided by precast concrete beams.
3
Multi-storey concrete car parks
Designconsiderations As 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.
Carparkuserrequirements
Eectonthestructure
Car park users have particular requirements aecting the layout and design o car parks. Typical user requirements include:
Long clear spans
Secure parking environment.
Clear site lines.
Ease o quickly fnding a parking place.
Easy manoeuvrability.
Minimum queuing.
Space to open car doors.
Headroom
Sae pedestrian routes through car park.
Good way-fnding.
The minimum clear headroom or 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 o a minimum height o 2.6m rom the entrance o the car park to (and including) designated parking spaces and exits rom those spaces. This additional headroom requirement is not usually achievable in multistorey car parks owing to the need to maintain ramps at an acceptable gradient and, under such circumstances, provision or taller vehicles is generally made outside the car park.
Clientrequirements Clients or developers will have their own preerences, which will generally be aligned to user requirements; particularly i income is reliant on users returning to the car park regularly. Client requirements potentially aecting the structure include:
Commercial viability based on initial and whole-lie costs.
Durability, with low maintenance costs.
Adaptability or uture changes in car park use and car design.
Sustainability.
Carparkuse Car parks are provided or users o dierent types o acilities such as hospitals, retail premises, oces and short or long-stay transport interchange sites. Recommended bay sizes vary according to the length o 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 o vehicles likely to use the car park. Where larger than normal vehicles are expected, bay sizes and headroom may need to be increased.
4
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 o up to 16m make manoeuvring easier and give better sight lines. Parking bays clear o columns to allow unrestricted door opening are usually considered the best option.
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
-
Multi-storey concrete car parks
Layouts
B
While there are over 100 dierent options or 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.
m u m i n i s m d n i e b d 3 n e m m o c e r
3 x 2.4m bays *
Whichever option is chosen, the layout o the parking bays will be similar, with bays located either side o aisles carrying one-way trac. While this is an ecient layout, the constraints it imposes on the structure are shown in Figure 1. To meet the requirement or clear spans, without any interbin supports, it is usually necessary to span 15.6m across the aisle and adjacent parking bays. The structural grid or many car parks is then 15.6 x 7.2m.
AISLE
BAY
6.0m
4.8m
BAY 4.8m
The relative merits o all the options are presented in the Car Park Designers’ Handbook [3]. Generally one-way ow circulation is preerred or simplicity and eciency. Four layouts are shown to illustrate the variations.
Interbin support zone
A
Bin width
A: 0.46m minimum 0.8m to 1.0m preferred range
Acceptable support positions *
B: 3.3m minimum 3.6m desirable
Typical bay dimensions
Figure 1: Typical car park layout for mixed use
Up Up
Down
Example of at deck car park layout
Examples of ramped deck car park layout
Example of split level car park layout
Figure 2: Examples of layout options
5
Multi-storey concrete car parks
Concretebenefts Concrete’s unique exibility provides a wide range of framing options and design/construction solutions to suit the exact needs of specic projects.
Economics Whether precast concrete or in-situ concrete is used or car park construction, they both oer economic overall solutions. An important conclusion rom a series o cost model studies undertaken on behal o The Concrete Centre ound that the cost o the structural rame should include the cost o edge protection. The whole-lie costs should also be considered. A car park should have a design service lie o 50 years beore signifcant maintenance and repair is required.
Long clear spans Concrete can be used in a number o dierent 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 oor-to-oor heights. The solutions available typically range in oor depth rom 475 to 650mm, although 400mm oor depth solutions are available. The thinnest solutions take advantage o spans being continuous over more than one bay.
Programme Concrete solutions can be erected quickly and saely. Precast concrete rames are designed and detailed to be highly buildable with short erection periods. In-situ concrete rames with proprietary ormwork systems are also quick to erect and, with their short lead-in times, oer an early start on-site.
Design
Perormance Fire Concrete has inherent fre resistance, which is present during all construction phases. It is achieved without the application o additional treatments and is thereore maintenance-ree. Concrete has the best European fre rating possible because it does not burn and has low heat conductance. Further inormation can be ound in Concrete and Fire Safety [4] by The Concrete Centre.
Finishes The structure in car parks is usually let exposed. With attention to detail during specifcation, and particularly during construction, concrete can have a good visual fnish. Precast concrete in particular usually has a high quality fnish due to the quality o the moulds used and greater control o the production o the concrete.
Vibration control It is usually recommended that the natural requency o the oor and rame, when designed as simply supported and ree o live load, should exceed 5 Hz. Most concrete car park structures have sucient mass and stiness to satisy these criteria, even or longer span options.
An in-situ concrete car park in construction. Sainsbury’s, Penrith. Photo: courtesy o Northfeld Construction Ltd.
6
Multi-storey concrete car parks
Durability A well designed, detailed and constructed concrete car park should achieve a design service lie o 50 years without the need or signifcant maintenance or repair. I 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 ser vice lie beyond 50 years. Some existing structures perorm poorly. To avoid poor perormance the ollowing should be ensured:
Use good quality concrete and construction.
Reinorcement fxed to provide the designed-or cover.
Use concrete designed to resist chlorides.
The actual oors o the car park are not ‘salted’ by maintenance sta.
Sustainability Locally sourced The constituent parts o concrete (water, cement and aggregate) are all readily and locally available to any construction site, minimising the impact o transporting raw materials. It is worth noting:
99.9% o aggregates used in the UK are sourced in the UK (80% are used within 30 miles o extraction).
90% o Ordinary Portland Cement is produced in the UK and there are cement kilns throughout the UK.
100% o UK-sourced reinorcement is produced rom UK scrap steel.
Reduced use of materials
I current knowledge and good practice is adopted, concrete will perorm more than adequately.
The long span options oten required or a car park need materials to be used eciently. In all the common concrete solutions, the selweight o the structure is minimised; use o materials is minimised and consequently transportation requirements are also reduced.
Robustness/vandal resistance
Concrete mix
Concrete is, by its nature, very robust and capable o resisting accidental damage and vandalism.
Modern concretes generally contain cement replacements which lower the embodied CO2 and use by-products rom other industries. Care should be exercised to balance the environmental benefts o cement replacements with their slower strength gain, which delays the initial prestress and stripping o ormwork or moulds.
Minimum maintenance Unlike other materials, concrete does not need any toxic coatings or paint to protect it against deterioration or fre. Properly designed and constructed concrete is relatively maintenance-ree over its design service lie.
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 o Nick Kane o Arcaid.
7
Multi-storey concrete car parks
Concreteoptions For 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-eective and meet client and user requirements. These designs are ecient because they use prestressing, are designed to be lightweight or are a combination of the two. They can all be adapted to suit ramped, at deck and split-level car park layouts.
Precasthollowcoreunits These 1.2m-wide precast concrete units utilise prestressing and voids ormed within units to orm an ecient structural element with a low span-to-weight ratio. While the units can be supported with a variety o beam types, the units have to be supported rom below.
Precastconcretedouble‘T’units These precast concrete units utilise prestressed concrete and a structurally ecient shape to give a low span-to-weight ratio. The standard width or these units is 2.4m. While they can be supported with a variety o beams types, a common approach is an L-shaped beam with a notched end to the units to give a constant structural depth.
Benets:
Standard units.
Simple, ast erection.
Small overall depth or 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.
Benets:
Low sel-weight – minimises supporting structure.
Standard or bespoke units available.
Simple, ast erection.
Cranked ramp units available.
Good visual appearance.
Structural sizes:
8
600mm deep unit.
75mm thick screed.
675mm overall structural depth.
Multi-storey concrete car parks
Voidedslab This orm o construction mixes in-situ and precast concrete. A thin precast concrete ‘biscuit’ is cast containing reinorcement 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 ormers are used to reduce the sel-weight o the structure. This can also be 100% in-situ or ully precast on in-situ beams.
Post-tensionedbandbeams This in-situ concrete option uses prestressing in the orm o post-tensioning to minimise the structural depth. A shallow slab spans onto integral beams. The ormwork or this option is relatively simple.
Benets:
No ormwork is required on site.
Maximises the beneft o multiple span oor plates.
Easily adapted to suit dierent column spacings.
Flat sot.
No screed required.
Structural sizes:
600mm deep (multi-span).
650mm deep (single-span).
Benets:
Short lead-in times.
Maximises the beneft o multiple span oor plates.
Easily adapted to suit dierent 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.
Precastcombinedbeamandcolumnrame This proprietary system has evolved to give ast erection times and an ecient structure. The main eature 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 ormers are used in the units to reduce sel-weight or liting. The headroom is slightly reduced between some o the parking spaces. 200mm deep precast oor units span between the beams.
Benets:
System developed specifcally or car parks.
Simple, ast erection.
No ormwork required.
Structural sizes:
200mm thick slab.
600mm deep beam (mid-span).
600mm overall structural depth.
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Multi-storey concrete car parks
Edgeprotection Edge 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 t ypes:
Type A -Spanning horizontally between the columns.
Type B - Bolted to the deck and cantilevering up rom it.
Type C - Monolithic with the deck.
Concrete barriers are usually type A or C or a combination o the two. For type B to be an option, the deck must be suciently strong to resist the bending moment and shear orces rom the cantilever barrier. The barriers are designed to resist the impact load either by absorbing the impact energy through deection o the barrier, or by relying on the rigidity and mass o the barrier to distribute impact energy through much o the structure, absorbing it by elastic strain.
Energy absorbing barriers tend to be o steel construction and have the ollowing characteristics:
They can be damaged by impact, and should be inspected regularly and replaced as necessary.
They rely on fxings into the deck, which should be designed to minimise replacement ater impact. An ultimate load actor o 1.5 is recommended or the fxing.
As their service lie is generally shorter than the car park, they will require replacement during the lie o the car park. They can be integrated into a exible cladding system. In sizing the car park, due allowance should be made or deection o the barrier under impact; particularly i the cladding is ragile.
Concrete barriers tend to rely on their mass to resist impact orces, and are thereore more robust. They have the ollowing characteristics:
They require minimal space.
They rarely require replacement but should be inspected and repaired as necessary ater impact.
They can be cast monolithically with the structure.
They can orm the load bearing structure or cladding or both, reducing the overall building cost.
They orm an upstand to the edge o the deck which helps to control surace water.
Columns may be subject to direct vehicle impact and thereore it i s preerable or the corners to be rounded or chamered to minimise damage to both column and vehicle.
St Paul’s car park, Sheeld. For more inormation see page 17. 10
Multi-storey concrete car parks
Structuraldesign Car parks are often treated as a standard building design. There are many similarities with buildings but also some notable dierences. This section provides useful information for the design of car parks to Eurocodes and highlights some important areas for further consideration.
Designactions Imposed loads
Table 2: Horizontal forces on edge barriers
Horizontal force over a 1.5m length of rigid barrier
The imposed loads applicable to decks and ramps are in Category F o the UK National Annex to BS EN 1991-1-1:2002 [6]. For a maximum gross vehicle weight under 3000kg, the characteristic loads are:
Horizontal force in kN
Height above oor/ramp in mm
Edge barrier to deck
150
375
qk = 2.5 kN/m (uniormly distributed load) Qk = 10 kN (concentrated load)
Edge barrier to ramps
75
610
Bottom end of straight ramp over 20m long
300
610
Wind and lateral loads
2
Wind loading inormation applicable to car parks is given in BS EN 19911-4:2005 [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 o the structure with no reduction or 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 or the specifc case’. No inormation is given to determine the horizontal wheel loads or cars in a car park. However, as a guide clause 6.3.2.3 (7) states that ‘horizontal loads due to acceleration or deceleration o orklits may be taken as 30% o 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 or vehicles up to 2500 kg gross mass, the horizontal characteristic orce, F (in kN) - normal to and uniormly distributed over any length o 1.5m o a rigid barrier - are given in Table 2. Where speed retarders in the orm o speed humps are used to decelerate cars on long straights, consideration should be given to the eect o impact on the decks.
Thermal actions Multi-storey car parks are open to the climate year-round and are thus subjected to a large range o temperatures and humidity. In addition, the top deck is heated by solar radiation which is made worse i a darkcoloured thin-layer waterproo fnish is used. Temperature eects or car parks are thus signifcant by comparison with other building structures. The relatively large temperature range in a car park deck leads to signifcant horizontal movements or orces which must be allowed or in the design o the rame: both elements and joints. Further guidance is given on page 13. When the roo deck is subject to solar gain during the day or heat loss during the night, dierential strains are induced across the thickness o the concrete which causes bowing and/or reverse bending. These additional bending orces can add signifcantly to the bending moments and shears generated by normal loadings. The method o calculation is given in BS EN 1991-1-5.
Colouring the oor provides clear signage. Photo: courtesy o Dunne Group
Snow Design Recommendations for Multi-storey and Underground Car Parks [1] states that snow loading on roos need not normally be considered in combination with vehicle loading. Possible exceptions are long-stay car parks and those in areas with high snowall.
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Multi-storey concrete car parks
Lateral stability
Table 3: Guideline natural frequencies for concrete car park options
Lateral stability can be provided in the ollowing ways:
Guideline natural frequency (Hz)
Precast concrete double ‘T’ units
5.6
Using the walls in stair and lit cores.
Using the skeletal bracing adjacent to ramps between car decks.
Using the ramps as scissor bracing (subject to circulation layout).
Post-tensioned band beams
5.4
Using rame action or low-rise car parks.
Precast hollowcore units
8.7
Biaxial voided slabs
10.9
Precast combined beam and column frame
5.3
Other issues to consider or lateral stability include:
Core walls located at the ends o the building act as restraints to shrinkage – see page 11 or more guidance.
Split level decks require lateral stability to both sets o decks (alternatively the ramps should be designed to transer lateral loads).
Internal walls other than those orming the stair and lit cores or 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 sti plates which can carry horizontal orces to the stability system but where there is no structural topping to precast elements, this should be justifed.
Vibration Modern car parks are now commonly designed or clear spans o at least 15.6m and their dynamic response should be checked to ensure user comort. A Design Guide for Footfall Induced Vibration of Structures[8] gives a methodology or predicting vertical vibrations in structures. For most concrete car parks, no increase in member sizes over that needed to satisy static loads will be required to achieve the required dynamic perormance. Design Recommendations for Multi-storey and Underground Car Parks recommends a minimum natural requency o 5 Hz, Table 3 shows guideline values or the options presented in this guide.
Fireresistance For open-sided car parks up to 30m in height, the required fre resistance period is 15 minutes in England and Wales and 30 minutes in Scotland. For elements protecting a means o escape, it is 30 minutes (England and Wales) and 60 minutes (Scotland) or compartment walls separating buildings. The fre resistance o 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 fre resistance o at least 60 minutes can usually be achieved without increasing the minimum cover required to satisy durability requirements. The Concrete Centre’s How to Design Concrete Structures using Eurocode 2 [9] provides tables to quickly check the fre resistance o concrete elements.
12
Structural system
Note: The natural frequencies stated are for 15.6m spans based on the simplied calculation method given in A Design Guide for Footfall Induced Vibration of Structures [8].
Robustness As the structural rame can be subject to direct impact rom a vehicle, both inside and outside the car park, it should be designed to prevent disproportionate collapse based upon the number o storeys in accordance with BS EN 1991-1-7.
Design for movements In concrete structures, a number o movements potentially occur throughout the lietime o the structure and should be considered during the design development. The principal movements include:
Early age thermal contraction (due to cooling o the concrete ollowing the heating generated by the cement hydration process).
Elastic shortening; particularly or post-tensioned members.
Eects o 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 eects such as creep, drying shrinkage and temperature changes.
An indication o the range o strains, and hence movement, is shown in Table 4.
Multi-storey concrete car parks
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
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
In terms of movement
0.6mm/m
1.1mm/m
Shrinkage over 50m
30mm
55mm
Additional strain due to post-tensioning (PT)
Additional strain due to exposure of top deck of a car park Strain due to thermal effects
200 me
400 me
In terms of movement
0.2mm/m
0.4mm/m
Note: me = microstrain (strain x 10 -6)
Movement joints
Bearings
Given the potential range o movements, and as car park plan dimensions are oten large, careul consideration should be given to whether movement joints should be provided and i deemed necessary, where they should be located. The oten used rule that a 25mm movement joint should be provided every 50m is too simplistic or a car park situation. As well as potential movement, the eect o restraint and the construction sequence should also be considered.
At the support positions o precast concrete slabs, horizontal orces caused by movements can cause the supporting member and slab to split or shear. This will reduce the load carrying capacity o the connection.
Restraining the ree movement o the slab deck will cause stresses that can lead to cracking. To reduce restraint to movement, it is best i the stability bracing system is near the centre o the plan or at least symmetrical in location and stiness (see Figure 3, on page 14). Control o the construction sequence is an important way o limiting early-age linear horizontal movements, particularly when posttensioning is used. Pours should generally be isolated rom any fxed structure such as ramps or cores or as long as possible to allow the early-age eects to pass without locking in any movements or restraints. The sequence o connected pours should be planned to minimise the movement at the ree edges; or instance, i three pours are cast in the sequence 2-1-3 - as opposed to 1-2-3 - this may signifcantly reduce the slab movement. I this is inconvenient, pours can be separated by ‘pour strips’ – gaps with discontinuous but overlapping reinorcement – let open until the early age eects have taken place.
This movement should be dealt with in one o two ways:
Allow movement to occur and ensure there is no restraint to movement. Precast concrete units with spans in excess o 8.0m should be bedded on a suitable exible bedding material such as neoprene; or
Design the joint to be monolithic in the permanent situation.
Whichever option is chosen, and the latter is avoured, the implications should be considered throughout the design. The design o bearings and all the considerations to take into account are explained in Design of Hybrid Concrete Buildings [10].
13
Multi-storey concrete car parks
a) Favourable layout of restraining walls (low restraint)
b) Unfavourable layout of restraining walls (high restraint)
Figure 3: Typical oor layouts
Durabilityothestructure Exposure conditions While car parks are subject to de -icing salts, the quantity o exposure to these salts is si gnifcantly lower than or highway structures. Although the durability requirements or concrete car parks should be determined rom BS 8500, this standard does not address car parks specifcally and thereore some interpretation is required. The recommendations or various exposure conditions are given in Table 5. These have been developed ater consultation with industry experts and assume the ollowing:
De-icing salts will not be applied directly to the elements as part o 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 lie o 50 years.
Freezing o internal elements is unlikely to occur.
Sots, columns, and walls are rarely exposed to spray rom 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 or abrasion class XM1 (moderate), a sacrifcial layer o 5mm o concrete may be used. This is appropriate or use at the entry level to the car park, which will be subject to the most severe conditions. Car parks protected with waterproofng may have reduced exposure conditions but consideration should be given to the maintenance regime. Concrete suraces can become exposed when the membrane is damaged or worn out which can signifcantly impact the service lie o the structure.
Chlorides and prestressed concrete Table NA.4 o the UK NA to BS 1992-1-1 [12] requires bonded prestressing steel within concrete o exposure classes XD1, XD2, XD3, XS1 and XS3 to be in an area o decompression under requent load combinations. This ‘decompression’ requirement stipulates that all parts o the bonded tendons or duct lie at least 25mm within concrete in compression. Apart rom coastal locations where exposure class XS1 (airborne chlorides originating rom sea water) should be applied, sots may be regarded as being ‘not subject to chlorides’ and decompression is not considered to be an issue or prestressing steel at the bottom o concrete members. Post-tensioned bonded tendons near the top surace should satisy the ‘decompression’ requirement; alternatively, the use o plastic ducts may be considered.
14
Multi-storey concrete car parks
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 - XM1 b
Top surface of decks and ramps in other locations
XD1 (XC3/4)a Optional - XM1 b
XS1 (XC3/4)a & XD1d Optional - XM1b
Softs 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 b c d
Exposure classes given in brackets denote classes which are less critical and assumed in BS 8500 to occur simultaneously with the main exposure class. BS EN1992-1-1 Cl 4.4.1.2(13) advises that for abrasion class XM1 (moderate) a sacricial 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. XD3 condition is more critical. XSI condition is more critical.
Waterresistance Decks required to be water resistant should be coated with a waterproo membrane capable o crack bridging. Alternatively, water resistant concrete can be used but as car parks are large open structures subject to movement and vibration, it is dicult to ensure the decks are watertight without the application o a waterproo membrane. Water resistant concrete is thereore more suitable or use in specifc areas o a modest size such as control rooms and lit pits.
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 1992-1-1 e.g. 0.3 mm crack width
1
Leakage to be limited to a small amount
Design for 0.2 mm crack width using BS EN 1992-1-1
Membranes A membrane should be selected with care to ensure it meets perormance requirements. Movement o the structure is a particular issue and the membrane may be required to accommodate:
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
Passive non-structural cracks opening and closing slowly in response to temperature changes; typically 0.5 to 1.0mm wide.
Some surface staining or damp patches acceptable
Live structural cracks which open up ater waterproofng and may be subject to rapid cyclic movement.
Design Recommendations for Multi-storey and Underground Car Parks has inormation on dierent types o membrane available including spray-applied and thin membranes, as well as traditional mastic asphalt. Membranes are available in dierent light-stable colours to dierentiate between parking bays and trac aisles. It should be noted that regular inspection is important to ensure waterproofng is ulflling its requirements, and repairs are carried out when needed. Particular areas to ocus on are the turning areas adjacent to the ramps, where the membrane can wear signifcantly.
Water resistant concrete I concrete is to be designed to resist water, Table 6 gives guidance on the approach to the control o cracking; based on BS EN 1992-3. This guidance is specifcally or 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 o a mixed use or habitable development then more stringent conditions may be required.
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Multi-storey concrete car parks
Drainage
Concretefnishes
An assessment should be made o the quantity o water likely to be
All parts o the car park should be suitable or both vehicles and pedestrian use.
deposited on a particular deck. Roo decks should be designed or local rainall conditions and appropriate drainage provided. For other decks the quantity o water will depend on:
Quantity o rainall penetrating the cladding.
Quantity o water brought in on vehicles.
Overspill water rom car washing acilities. The acility should incorporate a water recycling system.
Washing down o decks.
Facilities or extinguishing car fres.
Decks and ramps should be laid to alls to prevent ponding and ensure water containing de-icing salt drains away quickly and so reduces the opportunity or chloride ions to penetrate concrete suraces. The recommended minimum all or drainage is 1 in 60 and, or user comort, a all greater than 1 in 20 should generally be avoided. The long-term deection o the structure should be considered to ensure that ponding does not occur under sustained loads. Drainage outlets should be recessed below the surace o the concrete to ensure eective drainage o the decks.
Painted concrete produces a reective surace to increase light levels.
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A smooth surace is generally required only in areas where waterproofng is to be applied as smooth sur aces have less skid resistance. However, they increase the levels o tyre noise in turning areas and where vehicle speeds are low, even in the wet, skid resistance may not be critical. Power trowelling ater oating produces a dense, smooth hardwearing surace with negligible ‘ripple’ marks. However, although it has become more popular, power trowelling is not really suitable or the reasons outlined above and thereore a uniorm lightly brushed surace is preerred or the fnish to the decks. A tamped fnish is produced by raising and lowering the compacting beam in its fnal pass to produce a surace with ridges at a airly regular spacing o 20 - 30mm and up to 5mm high. Generally, the grooves should be in the direction o drainage alls and, on ramps, should ollow a chevron pattern. Due to the lack o compaction in ridges, this fnish can be dusty. Surace texture may be applied by roller or by sti brush. Brush worked fnishes are produced with a sti wire or bristle brush. A lightly tamped surace is recommended where ramps are steeper than 1 in 10. Where slopes are less than 1 in 10, power oating ollowed by brushed or lightly tamped suraces are considered appropriate.
Multi-storey concrete car parks
Casestudies Broadmead,Bristol Project description Broadmead multi-storey car park ormed part o the £500m Cabot Circus scheme in Bristol, which saw extensive demolition to the existing retail buildings, and restructuring o the roads in order to extend the existing acilities and regenerate land to the north east o the site.
Construction The car park decks consisted o 650mm deep by 1200/1800mm wide post-tensioned (PT) beams spanning 16m with 175mm thick PT slabs between. The total suspended oor area o the eightstorey structure was 54,000m2.
Project team Client: Bristol Alliance Structural engineer: Waterman Principal contractor: Norwest Holst Frame contractor: Febrey Ltd Specialist PT contractor: Freyssinet
StPauls,Shefeld Project description The 10-storey car park, with two retail oors below, orms part o phase two o the 1.6 ha masterplan or the regeneration o Sheeld city centre in 2002. The brie was to provide an inner city car park incorporating 520 spaces completing the public realm to St Paul’s Place.
Project team Client: CTP ST James Architect: Allies and Morrison Structural engineer: Capita Symonds Structures Principal contractor: JF Finnegan Specialist contractor: Tarmac
Construction The car park is o a split-level layout using precast double ‘T’ units and a precast concrete rame. Piled oundations support the basement, ground oor and frst oor, above which sits the car park. The prestressed double ‘T’ oor 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 shats. To avoid increasing oor-to-oor height, 200mm deep×500mm long scar cut-outs were introduced to the ends o the double-Ts to allow services to run parallel to edge beams. Holes through double ‘T’ ribs were also introduced or lighting cables. On-site erection was complete in 14 weeks and, at its peak, the concrete supplier was delivering 20 loads every day.
Photo: courtesy o Tarmac Ltd.
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Multi-storey concrete car parks
SalordQuaysMedia Centre Project description This 2,000-space car park was built to serve the frst purpose-built media centre in Salord Quays. The car park was built over a two– storey area, which orms the hub o the development and provides a urther nine storeys o parking. A key eature o the building is its curved plan area.
Construction The car park uses a proprietary combined beam and column rame (or more inormation see page 9), modifed to suit the curved building shape. Early design, detailing and preabrication enabled the on-site construction period to be reduced.
Project team Client: MediaCityUK Architect: Chapman Taylor Contractor: SCC Design Build
Photo: courtesy o Ben Ghibaldan
OceanVillage, Southampton Project description This fve-storey car park has been provided or users o 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 o space and saety. Coloured membranes were used to improve way fnding and to reect 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 oor consists o 400mm deep precast hollowcore concrete units, fnished 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 o the rear açade and in the centre adjacent to the movement joint. Stability along the ront is provided by the walls o the escape stair towers.
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Project team Client: Marina Developments Ltd Architect: Tiger Stripe Architects Structural engineer: Price and Myers Principal contractor: Dean and Dyball Specialist contractor: Tarmac
Multi-storey concrete car parks
Reerences 1 2 3 4 5 6 7 8 9 10 11 12
Design Recommendations for Multi-storey and Underground Car Parks ( Fourth Edition), The Institution o Structural Engineers, 2011 BS 8300: 2009, Design of buildings and their approaches to meet the needs of disabled people, British Standards Institution, 2009 Hill J, Car Park Designer’s Handbook , Thomas Telord Ltd, 2005 Concrete and Fire Safety , The Concrete Centre, 2008. Recommendations for the Inspection, Maintenance and Management of Car Parks, Institution o Civil Engineers, 2010 BS EN 1991-1-1, Eurocode 1: Actions on structures: General actions – Densities, self-weight, imposed loads for building, British Standards Institution, 2002 BS EN 1991-1-5, Eurocode 1: Actions on structure: General actions – Thermal actions. British Standards Institution, 2003 Wilord, M & Young, P, A Design Guide for Footfall-induced Vibration of Structures, The Concrete Centre, 2006 Brooker, O et al, How to Design Concrete Structures using Eurocode 2, The Concrete Centre, 2006 Whittle, R & TAYLOR, H, Design of Hybrid Concrete Buildings, The Concrete Centre, 2009 BS EN 1992-1-1, Eurocode 2: Design of concrete structures. General rules and rules for buildings, British Standards Institution, 2002 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 reinorced concrete clad with precast concrete fns and panels, the latter having an exposed aggregate fnish. 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 o Guillaume Perret. 19
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