Car Parks - Cost model, December 1993

Introduction

Quality has improved so dramatically in recent years that schemes such as the Avenue de Chartres multi-storey car park in Chichester, desinged by Birds Portchmouth and Russum, have won architectural awards.

Among the current design trends to improve safety and make car parks look good and easier to use are:

• no internal columns in parking bay areas, to provide the largest possible clear spans and bigger aisles and bays to makr parking easier
• controlled pedestrian access at ground-floor level and bridge links to adjoining shopping centres
• spacious lift and lobby area, well finished and easily cleanable
• TV cameras that photograph drivers and registration numbers as cars leave brick or metal cladding to external elevations and brick parapets to terraces
• lightweight roofs to enhance the roofscape and provide protection to the top parking level because waterproof finishes to slabs generally need replacing after 10 years.

The increase in muggings and car thefts has given rise to improved car park safety with useful guidelines published by the Association of Chief Police Officers. These recommendations include:

• installing high levels of illumination (up to 200 lux) to BS 5489 Part 9 (1990);
• painting surfaces white in areas where shadows ands pools of darkness may occur - this limits the number of light fittings;
• minimising the number of columns, avoiding recesses and arranging parking spaces in straight rows without blind spots;
• fitting grilles over external openings at ground level and lockable entrances and exits providing at least two adjoining spacious lifts for two levels or more, preferably with lgazed doors on to landings. All doors to stairwells to be glazed
• closed circuit television or manned security patrol

There are no national standards governing local planning requirements for car parking. But most authorities set their own local standards which are often, but not always, similar to those shown in table 1:

When local authority car parking “standards” cannot be met, a “commuted payments policy” is sometimes introduced where penalty costs are imposed on developers.

For design, the following legislation should be consulted:

• Building Regulations 1991;
• London Building Acts (applies only to inner London boroughs);
• local by-laws and regulations. If multi-storey car parking is open sided, half-hour fire rating is acceptable where ventilation is adequate, but most engineers design to a minimum of one hour. Underground car parks require a fire rating of two hours;
• the Petroleum (Consolidation) Act 1928.

Types of Car Park

Pay-and-Display
Often used by local authorities but not popular with developers or retailers as customers are “penalises” if they are away longer than envisaged and the ticket is an advertisement for the length of time the car will be unattended. Research has shown that underpayments can be as high as 15-20% where there are not enough staff to check tickets.

Pay-on-foot
Customers obtain a ticket on entering and pay at a machine before leaving the car park. Exit rates can be double those on pay-on-exit schemes.

Pay-on-exit
This is the most popular because it ensures that all potential revenue is realised and greater control is exercised over spaces available.

Finance

Finance is generally raised in a traditional manner. The appraisal in Table 2 shows the approach adopted by car parking developers. A first-year tariff of 75p-95p per parking space per hour is generally regarded as necessary for schemes to be viable. More long-term use may make schemes with lower tariffs viable also. The rule of thumb for testing the viability of a multi-storey car park development is an hourly tariff of at least 10p-12p per £1000 of construction costs per car space.

Layouts

Split Level
The slab is split longitudinally, each part raised or lowered by half a storey. Slabs are joined by one-way or two-way ramps, usually with a 1:7 slope. It is the most economical design but full use of the site is not made at ground-floor level. Users also have difficulties trying to locate their cars on adjoining split levels. Mixed flow systems can allow rapid egress from the centre of the car park.

Horizontal slab
All floors are level and lone full storey apart. Long ramps and exit times can result unless external spiral ramps are introduced, adding to the cost.

Parking ramps
One end of a floor is half a storey higher or lower than the other end. The slab is split centrally along its length and each half-width is ramped up and down to meet the adjoining floors. The design results in wasted area at ground-floor level and long exit journeys unless external spirals ate introduced. Few are built in the UK because they require a minimum length of 72m with a one-way flow system.

Warped Slab
Peripheral areas of each floor are level for the outer parking bay and the adjoining aisle. The remainder of the slab is is split centrally across its width and the centre of each split edge is then warped up and down by half a storey to connect to the next floor. It leads to slopes as steep as 1 in 12, which create difficulties for pedestrians with trolleys.

Construction Methods

Insitu reinforced concrete construction is the most popular because of its economy and flexibility. A recent comparative exercise by DLE indicated that precast concrete and steel solutions are some £2-5/m(2) more expensive than equivalent insitu concrete solutions. Specialists in precast concrete and steel argue that construction time can be cut by as much as 25% compared with insitu solutions, and that the car park thus starts earning revenue faster.

The cost model is for a six-storey pay-on-foot 15 000m2 car park with 600 spaces (25 m2 per space) at £4400 per space. It is based on a design and build scheme under construction by Norwest Holst to a design by parking consultant Hill Cannon Partnership and architect Gibson Hamilton Partnership.

Hill Cannon’s design comprises internal cross deck traffic ramps with short lengths of parking deck at an incline of 1 in 20 for vehicle circulation. The design virtually elminates split levels.

The cost model assumes a level site requiring piled foundations with good access. The price level (DL & E tender price index 235) is based on a competitive tender in December 1993 for a fixed price contract in the South East. Adjustments for other schemes should take account of regional factors (see above), alternative procurement routes, specification standards, site constraints and local conditions.

External works can add from as little as 1% on small sites with limited access road, to 5-10% for larger sites with longer access roads.

Planning and Programming

Design and build is the most common procurement route. Where car parks are part of the larger development, the procurement route usually follows that of the main building work.

The model assumes a lead-in period of 12 weeks and application for planning approval eight weeks from appointment. In practice, these periods could be shorter. Architectural and engineering fees for this type of work vary from 4%-9%. A construction period of 52 weeks is assumed.

As multi storey car parks are a stack of repetitive open floors connected by ramps and enclosed by a decorative shell, speed of construction of the floors will determine the critical path. The floors are built generally to falls which slow the rate of construction.

In the model, the upper floors are a form of waffle slab construction, comprising a grid of reinforced concrete columns and beams with a trough floor constructed by using proprietary formwork moulds.

Alternative Designs

The model programme could be shortened by three or four weeks if the ground’s bearing capacity allowed concrete pad foundations to be used instead of piles.

A precast concrete frame offers an 8-10 week saving over insitu concrete but requires the early involvement of the supplier in the design process and a lead time of 12 weeks from placing an order to delivery to site. The erection sequence for a precasrt frame is also different in that the frame is usually erectred to full height for between 25% - 33% of the length of the car park. Brick cladding would commence upon completion of each section.

A steel frame would require a similar lead-in period to that of precast and would also cut the construction programme by 8-10 weeks.

Steel-framed car parks have an inherent fire rating of only 30 minutes but this is generally acceptable provided the structure’s overall height does not exceed 15.9m.