mb carparks final

8/13/2019 Mb Carparks FINAL

1/20

A guide to design and construction

Multi-storey Concrete Car Parks


2/20

Multi-storey concrete car parks

Contents

Introduction 3

Design considerations 4

Layouts 5

Concrete benefits 6

Concrete options 8

Edge protection 10

Structural design 11

Case studies 17References 19

Nottingham Railway Stations car park was redeveloped to increase its capacity from 500 to 950 spaces. The five storey car park wasreopened in 2012, with 2,107 coloured copper panels now fixed to the precast concrete structures outside.

Architect: BDP.

Photo: courtesy of Martine Hamilton Knight.

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


3/20

3


IntroductionMulti-storey car parks are a common feature in the UKs 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 ofparking 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 capacit y 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 bedesigned 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.


4/20

4


Design considerationsAs 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 exampleDesign Recommendations for Multi-storey and Underground CarParks[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 requirementsCar 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.

Client requirementsClients 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 requirementspotentially 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.

Car park useCar parks are provided for users of different types of facilities such

as hospitals, retail premises, offices and short or long-stay transportinterchange 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.

Effect on the structureLong 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 easierand 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 inDesign

recommendations for multi-storey and underground car parksis 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

ParkingLength (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 -


5/20

5


LayoutsWhile there are over 100 different options for laying out a car park,

in practice three layouts with 90 parking angle are most commonlyused. 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

recommendedminim

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


6/20

6


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.

Concrete benefitsConcretes 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. Sainsburys, Penrith. Photo: courtesy of Northfield Construction Ltd.


7/20

7


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], itshould 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 andconsequently transportation requirements are also reduced.

Concrete mix

Modern concretes generally contain cement replacements which lower

the embodied CO2and 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.ukto 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.


8/20

8


Concrete optionsFor 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 useprestressing, 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.

Precast hollowcore unitsThese 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.

Precast concrete double 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.

Simple, fast erection. Cranked ramp units available.

Good visual appearance.

Structural sizes:

600mm deep unit.

75mm thick screed.

675mm overall structural depth.


9/20

9


Post-tensioned band beamsThis 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.

Precast combined beam and column frameThis 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 between

the 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.

Simple, fast erection.

No formwork required.

Structural sizes:

200mm thick slab.

600mm deep beam (mid-span).

600mm overall structural depth.

Voided slabThis 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.


10/20

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.

Edge protectionEdge 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:


10

St Pauls car park, Sheffield. For more information see page 17.


11/20

11


Design actionsImposed 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 parksrecommends 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 isgiven 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.

Structural designCar 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 highlightssome 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


12/20

12


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 Parksrecommends a minimum natural frequency of5 Hz, Table 3 shows guideline values for the options presented in this

guide.

Fire resistanceFor 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 checkedin 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 Centres

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 systemGuideline 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 inA Design Guide for Footfall Induced

Vibration of Structures [8].


13/20

13


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].


14/20

14


Durability of the structureExposure 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 foruse 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


15/20

15


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)aOptional - XM1b XF2, XS1(XC3/4)a& XD1dOptional - XM1b

Top surface of decks and ramps in other locations XD1 (XC3/4)aOptional - XM1b XS1 (XC3/4)a& XD1dOptional - 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)aXFI

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.

Water resistanceDecks 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 Parkshas

information on different types of membrane available includingspray-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 1992-1-1

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

membrane


16/20

16


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.

Concrete finishesAll parts of the car park should be suitable for both vehicles and

pedestrian use.

A smooth surface is generally required only in areas wherewaterproofing 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.


17/20

17


Case studies

St Pauls, 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 Pauls 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 deep500mm

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 James

Architect: Allies and Morrison

Structural engineer: Capita Symonds Structures

Principal contractor: JF Finnegan

Specialist 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.


18/20

18


Ocean Village,

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 faade 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 Ltd

Architect: Tiger Stripe Architects

Structural engineer: Price and Myers

Principal contractor: Dean and Dyball

Specialist contractor: Tarmac

Salford Quays Media

CentreProject 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: MediaCityUK

Architect: Chapman Taylor

Contractor: SCC Design BuildPhoto: courtesy of Ben Ghibaldan


19/20

19


References1 Design Recommendations for Multi-storey and Underground Car Parks (Fourth Edition), The Institution of 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 Designers Handbook, Thomas Telford Ltd, 2005

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

5 Recommendations for the Inspection, Maintenance and Management of Car Parks, Institution of 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 Wilford, 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 reinforced concrete clad with precast concrete fins andpanels, 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.


20/20

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.

www.concretecentre.com

The Concrete Centre,

Riverside House,

4 Meadows Business Park,

Station Approach, Blackwater,

Camberley, Surrey GU17 9AB

Ref. TCC/03/34

ISBN 978-1-908257-02-4

First published 2012

MPA - The Concrete Centre 2012

The Concrete Centre is part of the Mineral

Products Association, the trade association for the

aggregates, asphalt, cement, concrete, lime, mortar

and silica sand industries.

www.mineralproducts.org

mb carparks final

Documents