mb carparks final
TRANSCRIPT
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A guide to design and construction
Multi-storey Concrete Car Parks
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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.
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Multi-storey concrete car parks
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.
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Multi-storey concrete car parks
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 -
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Multi-storey concrete car parks
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
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Multi-storey concrete car parks
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.
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Multi-storey concrete car parks
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.
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Multi-storey concrete car parks
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.
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Multi-storey concrete car parks
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.
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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:
Multi-storey concrete car parks
10
St Pauls car park, Sheffield. For more information see page 17.
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Multi-storey concrete car parks
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
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Multi-storey concrete car parks
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].
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Table 4: Indicative strains and movements for typical design situations
Phenomenon Minimum Maximum
Typical strains for an internal reinforced concrete structure
Early thermal shrinkage strain 100 me 300 me
Drying shrinkage 300 me 400 me
Total strain 400 me 700 me
In terms of movement 0.4mm/m 0.7mm/m
Shrinkage over 50m 20mm 35mm
Additional strain due to post-tensioning (PT)
Elastic strain due to prestress 75 me 100 me
Creep strain due to prestress 150 me 250 me
Total strain for a PT structure 625 me 1050 me
In terms of movement 0.6mm/m 1.1mm/m
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
In terms of movement 0.2mm/m 0.4mm/m
Note:me= microstrain (strain x 10 -6)
Movement joints
Given the potential range of movements, and as car park plan
dimensions are often large, careful consideration should be given to
whether movement joints should be provided and if deemed necessary,
where they should be located. The often used rule that a 25mm
movement joint should be provided every 50m is too simplistic for a car
park situation. As well as potential movement, the effect of restraint and
the construction sequence should also be considered.
Restraining the free movement of the slab deck will cause stresses
that can lead to cracking. To reduce restraint to movement, it is best
if the stability bracing system is near the centre of the plan or at least
symmetrical in location and stiffness (see Figure 3, on page 14).
Control of the construction sequence is an important way of limiting
early-age linear horizontal movements, particularly when post-
tensioning is used. Pours should generally be isolated from any fixed
structure such as ramps or cores for as long as possible to allow the
early-age effects to pass without locking in any movements or restraints.
The sequence of connected pours should be planned to minimise the
movement at the free edges; for instance, if three pours are cast in the
sequence 2-1-3 - as opposed to 1-2-3 - this may significantly reduce the
slab movement. If this is inconvenient, pours can be separated by pour
strips gaps with discontinuous but overlapping reinforcement left
open until the early age effects have taken place.
Bearings
At the support positions of precast concrete slabs, horizontal forces
caused by movements can cause the supporting member and slab
to split or shear. This will reduce the load carrying capacity of the
connection.
This movement should be dealt with in one of two ways:
Allow movement to occur and ensure there is no restraint to
movement. Precast concrete units with spans in excess of 8.0m
should be bedded on a suitable flexible bedding material such as
neoprene; or
Design the joint to be monolithic in the permanent situation.
Whichever option is chosen, and the latter is favoured, the implications
should be considered throughout the design.
The design of bearings and all the considerations to take into account
are explained in Design of Hybrid Concrete Buildings [10].
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Multi-storey concrete car parks
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
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Table 5: Proposed exposure classes for car parks
Element type and location Recommended exposure class Recommended exposure class in coastal areas
Top surface of decks and ramps at the entry level of car park XD3 (XC3/4)a& XM1b XD3(XC3/4)a, XS1c& XM1b
Top surface of decks and ramps exposed to freezing e.g.
roof level
XF2 & XD1(XC3/4)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
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DrainageAn assessment should be made of the quantity of water likely to be
deposited on a particular deck. Roof decks should be designed for local
rainfall conditions and appropriate drainage provided.
For other decks the quantity of water will depend on:
Quantity of rainfall penetrating the cladding.
Quantity of water brought in on vehicles.
Overspill water from car washing facilities. The facility should
incorporate a water recycling system.
Washing down of decks.
Facilities for extinguishing car fires.
Decks and ramps should be laid to falls to prevent ponding and ensure
water containing de-icing salt drains away quickly and so reduces
the opportunity for chloride ions to penetrate concrete surfaces. The
recommended minimum fall for drainage is 1 in 60 and, for user comfort,
a fall greater than 1 in 20 should generally be avoided.
The long-term deflection of the structure should be considered to
ensure that ponding does not occur under sustained loads.
Drainage outlets should be recessed below the surface of the concrete
to ensure effective drainage of the decks.
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.
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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.
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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
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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.
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