multy storey carparks

A guide to design and

Contents

Contents

I

I

n

n

t

t

r

r

o

o

d

d

u

u

c

c

t

t

i

i

o

o

n

n

3

3

D

D

e

e

s

s

i

i

g

g

n

n

c

c

o

o

n

n

s

s

i

i

d

d

e

e

r

r

a

a

t

t

i

i

o

o

n

n

s

s

4

4

L

L

a

a

y

y

o

o

u

u

t

t

s

s

5

5

C

C

o

o

n

n

c

c

r

r

e

e

t

t

e

e

b

b

e

e

n

n

e

e

f

f

t

t

s

s

6

6

C

C

o

o

n

n

c

c

r

r

e

e

t

t

e

e

o

o

p

p

t

t

i

i

o

o

n

n

s

s

8

8

E

E

d

d

g

g

e

e

p

p

r

r

o

o

t

t

e

e

c

c

t

t

i

i

o

o

n

n

1

1

0

0

S

S

t

t

r

r

u

u

c

c

t

t

u

u

r

r

a

a

l

l

d

d

e

e

s

s

i

i

g

g

n

n

1

1

1

1

C

C

a

a

s

s

e

e

s

s

t

t

u

u

d

d

i

i

e

e

s

s

1

1

7

7

R

R

e

e





e

e

r

r

e

e

n

n

c

c

e

e

s

s

1

1

9

9

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.

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.

More recently, designers have recognised the need to improve saety 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 successully to create new car parks or a variety o uses.

Designconsiderations

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.

Carparkuserrequirements

Car park users have particular requirements aecting the layout and design o car parks. Typical user requirements include:

 Secure parking environment.

 Clear site lines.

 Ease o quickly fnding a parking place.

 Easy manoeuvrability.

 Minimum queuing.  Space to open car doors.

 Sae pedestrian routes through car park.

 Good way-fnding.

Clientrequirements

Clients or developers will have their own preerences, which will generally be aligned to user requirements; particularly i income is reliant on users returning to the car park regularly. Client requirements potentially aecting the structure include:

 Commercial viability based on initial and whole-lie costs.

 Durability, with low maintenance costs.

 Adaptability or uture changes in car park use and car design.

 Sustainability.

Carparkuse

Car parks are provided or users o dierent types o acilities such as hospitals, retail premises, oces 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.

Eectonthestructure

Long 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 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.

Headroom

 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 multi-storey 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.

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

-Layouts

While there are over 100 dierent 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.

 The relative merits o all the options are presented in the Car Park  Designers’ Handbook [3]. Generally one-way ow circulation is preerred or simplicity and eciency. Four layouts are shown to illustrate the variations.

Whichever option is chosen, the layout o the parking bays will be similar, with bays located either side o aisles carrying one-way trac. While this is an ecient 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.

Down Up Up B A 4.8m 6.0m 4.8m 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 2.4m bays *     3     b     i   n   s    r    e    c    o    m    m    e    n     d   e     d   m     i   n     i   m   u    m 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 at deck car park layout

Economics

Whether precast concrete or in-situ concrete is used or car park  construction, they both oer 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-lie costs should also be considered. A car park should have a design service lie o 50 years beore signifcant maintenance and repair is required.

Programme

Concrete solutions can be erected quickly and saely. 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, oer an early start on-site.

Design

Finishes

 The structure in car parks is usually let 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.

Long clear spans

Concrete can be used in a number o dierent 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.

Perormance

Fire

Concrete has inherent fre resistance, which is present during all construction phases. It is achieved without the application o additional treatments and is thereore maintenance-ree. Concrete has the best European fre rating possible because it does not burn and has low heat conductance. Further inormation can be ound in Concrete and Fire Safety [4] by The Concrete Centre.

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 sucient mass and stiness to satisy these criteria, even or longer span options.

Concretebenefts

Concrete’s unique exibility provides a wide range of framing options and design/construction solutions to suit the

exact needs of specic projects.

Durability

A well designed, detailed and constructed concrete car park should achieve a design service lie 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 lie beyond 50 years.

Some existing structures perorm poorly. To avoid poor perormance the ollowing should be ensured:

 Use good quality concrete and construction.

 Reinorcement 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.

I current knowledge and good practice is adopted, concrete will perorm more than adequately.

Robustness/vandal resistance

Concrete is, by its nature, very robust and capable o 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 fre. Properly designed and constructed concrete is relatively maintenance-ree over its design service lie.

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 reinorcement is produced rom UK scrap steel.

Reduced use of materials

 The long span options oten required or a car park need materials to be used eciently. In all the common concrete solutions, the sel-weight o the structure is minimised; use o materials is minimised and consequently transportation requirements are also reduced.

Concrete mix

Modern concretes generally contain cement replacements which lower the embodied CO₂and 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.

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:

Concreteoptions

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-eective and meet client and user requirements. These designs are ecient 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.

Precasthollowcoreunits

 These 1.2m-wide precast concrete units utilise prestressing and voids ormed within units to orm an ecient 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.

Benets:

 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.

Precastconcretedouble‘T’units

 These precast concrete units utilise prestressed concrete and a structurally ecient 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.

Benets:

 Low sel-weight –

minimises supporting structure.

 Standard or bespoke units available.

 Simple, ast erection.

 Cranked ramp units available.

 Good visual appearance.

Structural sizes:

 600mm deep unit.

 75mm thick screed.

Post-tensionedbandbeams

 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.

Precastcombinedbeamandcolumnrame

 This proprietary system has evolved to give ast erection times and an ecient 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 liting. The headroom is slightly reduced between

some o the parking spaces. 200mm deep precast oor units span between the beams.

Benets:

 No ormwork is required on site.

 Maximises the beneft o multiple span oor plates.

 Easily adapted to suit dierent column spacings.

 Flat sot.  No screed required. Structural sizes:  600mm deep (multi-span).  650mm deep (single-span). Benets:

 Short lead-in times.

 Maximises the beneft o multiple span oor plates.

 Easily adapted to suit dierent 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.

Benets:

 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.

Voidedslab

 This orm o construction mixes in-situ and precast concrete. A thin precast concrete ‘biscuit’ is cast containing

reinorcement 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.

  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 suciently 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 deection 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 ater impact. An ultimate load actor o 1.5 is recommended or the fxing.

 As their service lie is generally shorter than the car park, they will require replacement during the lie o the car park.

  They can be integrated into a exible cladding system.

 In sizing the car park, due allowance should be made or deection 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 thereore more robust. They have the ollowing characteristics:

  They require minimal space.

  They rarely require replacement but should be inspected and repaired as necessary ater 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 surace water.

Columns may be subject to direct vehicle impact and thereore it i s preerable or the corners to be rounded or chamered to minimise damage to both column and vehicle.

Edgeprotection

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:

Designactions

Imposed loads

 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:

q_k = 2.5 kN/m2_{(uniormly distributed load)}

Q_k = 10 kN (concentrated load)

Wind and lateral loads

Wind loading inormation 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 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 inormation 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 orklits may be taken as 30% o the vertical axle loads Q_k ’. 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 uniormly 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 eect o impact on the decks.

Snow

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

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 dark-coloured thin-layer waterproo fnish is used. Temperature eects 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, dierential 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.

Structuraldesign

Car parks are often treated as a standard building design. There are many similarities with buildings but also some

notable dierences. 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 oor/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 oor provides clear signage.

Photo:

Lateral stability can be provided in the ollowing ways:

 Using the walls in stair and lit cores.

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

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

 Using rame action or low-rise car parks.

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 transer lateral loads).

 Internal walls other than those orming the stair and lit 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 comort. 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 satisy static loads will be required to achieve the required dynamic perormance. 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.

Fireresistance

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 satisy 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.

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 lietime 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.

 Eects 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 eects such as creep, drying shrinkage and temperature changes.

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

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

simplied calculation method given in A Design Guide for Footfall Induced  Vibration of Structures [8].

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 100me 300me

Drying shrinkage 300me 400me

 Total strain 400me 700me

Intermsofmovement 0.4mm/m 0.7mm/m

Shrinkageover50m 20mm 35mm

Additional strain due to post-tensioning (PT)

Elastic strain due to prestress 75me 100me

Creep strain due to prestress 150me 250me

 Total strain for a PT structure 625me 1050me

Intermsofmovement 0.6mm/m 1.1mm/m

Shrinkageover50m 30mm 55mm

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

Strain due to thermal effects 200me 400me

Intermsofmovement 0.2mm/m 0.4mm/m

Note:

me= microstrain (strain x 10-6)

Movement joints

Given the potential range o movements, and as car park plan dimensions are oten large, careul consideration should be given to whether movement joints should be provided and i deemed necessary, where they should be located. The oten 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 eect o restraint and the construction sequence should also be considered.

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 stiness (see Figure 3, on page 14). Control o the construction sequence is an important way o limiting early-age linear horizontal movements, particularly when post-tensioning 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 eects 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 reinorcement – let open until the early age eects have taken place.

Bearings

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.

 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].

Durabilityothestructure

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 thereore some interpretation is required. The recommendations or various exposure conditions are given in Table 5. These have been developed ater 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 lie o 50 years.

 Freezing o internal elements is unlikely to occur.

 Sots, 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 suraces can become exposed when the membrane is damaged or worn out which can signifcantly impact the service lie 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, sots 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.

a) Favourable layout of restraining walls (low restraint)

b) Unfavourable layout of restraining walls (high restraint) Figure 3: Typical oor layouts

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_{& XM1}b _XD3(XC3/4)a_{, XS1}c_{& XM1}b

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

XF2 & XD1(XC3/4)a_{Optional - XM1}b _{XF2, XS1(XC3/4)}a_{& XD1}d_{Optional - XM1}b

 Top surface of decks and ramps in other locations XD1 (XC3/4)a_{Optional - XM1}b _{XS1 (XC3/4)}a_{& XD1}d_{Optional - XM1}b

Softs of decks and ramps XC3/4 XSI (XC3/4)a

Verticalelements 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 sacricial 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.

Waterresistance

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 dicult to ensure the decks are watertight without the application o a waterproo membrane. Water resistant concrete is thereore more suitable or use in specifc areas o a modest size such as control rooms and lit pits.

Membranes

A membrane should be selected with care to ensure it meets perormance requirements. Movement o 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 ater waterproofng and may be subject to rapid cyclic movement.

Design Recommendations for Multi-storey and Underground Car Parks has inormation on dierent types o membrane available including

spray-applied and thin membranes, as well as traditional mastic asphalt. Membranes are available in dierent light-stable colours to dierentiate between parking bays and trac 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.

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

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  

Drainage

An assessment should be made o the quantity o water likely to be deposited on a particular deck. Roo decks should be designed or local rainall conditions and appropriate drainage provided.

For other decks the quantity o water will depend on:

 Quantity o rainall 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 suraces. The recommended minimum all or drainage is 1 in 60 and, or user comort, a all greater than 1 in 20 should generally be avoided.

 The long-term deection o the structure should be considered to ensure that ponding does not occur under sustained loads.

Drainage outlets should be recessed below the surace o the concrete to ensure eective drainage o the decks.

Concretefnishes

All parts o the car park should be suitable or both vehicles and pedestrian use.

A smooth surace 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 ater oating produces a dense, smooth hardwearing surace with negligible ‘ripple’ marks. However, although it has become more popular, power trowelling is not really suitable or the reasons outlined above and thereore a uniorm lightly brushed surace is preerred 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 surace 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.

Surace 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 surace 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 suraces are considered appropriate.

Casestudies

StPauls,Shefeld

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  Sheeld 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.

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 shats.

 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.

Project team

Client: CTP ST James

Architect: Allies and Morrison

Structural engineer: Capita Symonds Structures Principal contractor: JF Finnegan

Specialist contractor: Tarmac

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 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

OceanVillage,

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 saety. Coloured membranes were used to improve way fnding and to reect 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.

Project team

Client: Marina Developments Ltd Architect: Tiger Stripe Architects Structural engineer: Price and Myers Principal contractor: Dean and Dyball Specialist contractor: Tarmac

SalordQuaysMedia

Centre

Project description

 This 2,000-space car park was built to serve the frst purpose-built media centre in Salord 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 inormation see page 9), modifed to suit the curved building shape.

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

Project team

Client: MediaCityUK  Architect: Chapman Taylor Contractor: SCC Design Build

Reerences

1 Design Recommendations for Multi-storey and Underground Car Parks ( Fourth Edition), The Institution o Structural Engineers, 2011 2 BS 8300: 2009, Design of buildings and their approaches to meet the needs of disabled people, British Standards Institution, 2009 3 Hill J, Car Park Designer’s Handbook , Thomas Telord Ltd, 2005

4 Concrete and Fire Safety , The Concrete Centre, 2008.

5 Recommendations for the Inspection, Maintenance and Management of Car Parks, Institution o Civil Engineers, 2010

6 BS EN 1991-1-1, Eurocode 1: Actions on structures: General actions – Densities, self-weight, imposed loads for building, British Standards Institution, 2002 7 BS EN 1991-1-5, Eurocode 1: Actions on structure: General actions – Thermal actions. British Standards Institution, 2003

8 Wilord, M & Young, P, A Design Guide for Footfall-induced Vibration of Structures, The Concrete Centre, 2006 9 Brooker, O et al, How to Design Concrete Structures using Eurocode 2, The Concrete Centre, 2006

10 Whittle, R & TAYLOR, H, Design of Hybrid Concrete Buildings, The Concrete Centre, 2009

11 BS EN 1992-1-1, Eurocode 2: Design of concrete structures. General rules and rules for buildings, British Standards Institution, 2002 12 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 reinorced concrete clad with precast concrete fns and

panels, the latter having an exposed aggregate fnish.

Photo:

www.concretecentre.com

Re. TCC/03/34 ISBN 978-1-908257-02-4

First published 2012 © MPA - The Concrete Centre 2012  The Concrete Centre is part o the Mineral Products Association, the trade association or the aggregates, asphalt, cement, concrete, lime, mortar

and silica sand industries. www.mineralproducts.org