Car parks may not be the most glamorous of construction projects, but designing them is more complex than you might think. Simon Rawlinson of Davis Langdon outlines the issues that must be thought through and provides a fully comprehensive cost list
Car parks are one of the most utilitarian of building types, but their impact on a user’s experience of their journey to work, a shopping trip, or special night out, can be considerable. There are more than 6,000 multistorey car parks in the UK, many of which date from the sixties and seventies. These were often built within tight constraints on difficult town-centre sites and can suffer from problems related to security and poor environment, poor layout and build quality, low durability and inadequate maintenance.
Although all car park design in the UK is informed by a set of standard dimensions, the variation in space standards, resulting from efficiency considerations, site constraints, or lack of experience on the part of the designer, is considerable.
Many cars can boast scratched paint, or dented bodywork, partly as a consequence of compromises between optimum geometry and static capacity determined during a car park’s design process. Positive steps taken to enhance a user’s experience of a car park, such as the design of an open structure and high-quality public areas, will reflect positively on the wider development.
With the shift of large-scale retail back into city-centre sites, the provision of parking standards as generous as those enjoyed by out-of-town developments, such as the Trafford Centre, or Bluewater, has become a real challenge. Land is more expensive, sites are more constrained and the impact of traffic flows in the wider street network on the functioning of the car park are more complex.
Drivers are prepared to pay for their short or medium-term parking in town centres, but want a better service in return than has previously been provided.
Planners also expect more of the design of car parks, seeking a greater contribution to streetscape and civic amenity from structures which, in many ways, function more like bridges than buildings.
Designers also recognise that pedestrians have as much of a place on the car park deck as cars, requiring greater consideration of circulation routes and personal security. As a result, car parks are being designed with more care and attention than might have been the case in the bad old days of the “Get Carter”-style concrete monolith.
02 Why car parks are important
Car parks have always been many customers’ first point of contact with a retail and leisure destination, but it is only relatively recently that developers have made the connection between the quality of the car park experience and the competitive differentiators of their development – as shown by the introduction of the Park Mark scheme.
The poor experience of a visitor in a car park, due to congestion, or perception of a low level of security, could affect the likelihood of repeat visits. At a more immediate level, centre managers are keen to avoid incidents of “car park rage”, when an individual’s frustration at delay could potentially affect a wider range of car park users. There are many aspects of a visitor’s use of a car park, and key determinants of the experience include:
- Personal safety. Good-quality lighting and visibility across the parking deck and into lobbies, with additional features, such as high-profile CCTV monitoring and a manned control centre
- Simple layouts. Straightforward and logical layouts which make searching for spaces as straightforward as possible, avoiding unnecessary queuing
- Easy parking. Generously dimensioned bays and ramps which do not require particularly high levels of driving skill
- Ease of entry and exit. Layouts designed to avoid bottlenecks at entry or exit, making best use of available parking capacity at all points in the car park
- Ready access from the car park to the destination itself.
With spaces in a multistorey car park costing around £9,000-17,000, getting the right balance between developmental and operational efficiency requires a careful consideration of factors such as the typical length of stay, or the familiarity of the user with the facility, together with briefing issues, such as the layout and the size of the site.
For example, for a major shopping centre, users prefer the look and feel of a surface car park and potentially also begin to get frustrated after five minutes of searching for a space. Parking associated with locations where drivers are chasing deadlines, such as airports or train stations, in particular need to facilitate fast and stress-free entry. By contrast, users of workplace car parks will tolerate smaller bays and slower access associated with “tidal flows” of workers at the start and end of the working day.
Issues of the quality of user experience can have a direct impact on the financial performance of a car park. Obviously, location is the most significant determinant of demand, as well as the price users are prepared to pay. Thereafter, visibility, personal safety and usability will determine revenues. If an aspect of design either puts visitors off, or creates a bottleneck which prevents capacity from being utilised, then visitor numbers, car park revenues and retail sales may all potentially suffer.
03 Key components of car park design
There are dozens of configurations of multi-storey car parks featuring different arrangements of deck and ramp.
The final selection of the configuration will be determined by the overall size of the car park, the shape of the site and the use for which the car park is intended. The main components of design are inter-related, so changes in one will have a knock-on effect on others. There are also absolute physical constraints, mainly related to car dimensions and turning circles, which have to be accommodated in the design.
• Planning dimensions. The standard planning module for a parking bay is 4.8m x 2.4m. This dimension accommodates 95% of all passenger vehicles licensed in the UK. Larger vehicles, such as SUVs, can be parked within the standard dimensions, although extra care has to be taken to avoid collision. Some anchor retailers ask for a larger grid (5.0 × 2.5m) to be used for planning so that more generous parking pads are provided, facilitating easier, faster parking for everyone.
• Bay width. The width of a typical “parking pad” – the footprint of a car is around 1.9m. The rest of the bay, between 400 and 600mm a vehicle, is an allowance for manoeuvring and door clearance. Bay width can vary from the planning module if, for example, structural columns interrupt the parking bays.
Variation in bay widths has a disproportionate effect on the ease of parking. For example, while a 100mm increase in bay width represents only a 4% change in the overall width, it results in a 20% increase in the size of the gap between two parked cars. Wider bays up to 2.5m across are particularly important for short-stay parking, such as that serving supermarkets, where there is a constant turnover of visitors. Car parks for town centres and shopping centres can be designed with bay widths of 2.4m. The absolute minimum for public parking is 2.3m.
In reality, from the perspective of the user, bay width is determined by the white lines painted on the concrete rather than the dimensions shown on a drawing. If columns encroach into the parking bay, then contractors have to be instructed very clearly as to how bays should be marked out, lest they use the edges of columns, potentially resulting in narrower bays throughout a scheme.
• Aisle width. Aisles separate parking bays and are used by both cars and pedestrians. Car parks are generally planned with a one-way circulation plan. On this basis, for parking arranged on a 90° pitch, a 6m-wide aisle provides space for car and pedestrian circulation, as well as space for the turning circle of a car manoeuvring into and out of a parking bay. Angled bays permit the design of narrower aisles, although ultimately the aisle dimension is determined by the minimum 180° turning circle required to pass from one adjacent aisle to the other.
• Ramp dimensions. Vertical circulation is the most problematic aspect of car park design and usage. Drivers typically don’t like using ramps, but due to their size and the way in which they encroach on both aisles and parking, designers operate within particular constraints with respect to layout and dimensions.
Widths of vehicle ramps range from 5m to 3.6m, although good practice suggests the minimum width should be 4.2m. Vehicle ramp length is determined by the maximum gradient of 10% which, with a typical clear height to the underside of the slab of 2.1m, means that straight ramps can be around 30m long on a flat slab scheme.
Pedestrian ramp gradients are set by BS 8300, a workplace standard, and are shallower and therefore longer – resulting in further pressure on parking spaces. The space taken up by ramps can be reduced by minimising the width of a ramp and by using a layout whereby the ramp projects into an aisle.
Both options reduce the space available to drivers for manoeuvring, breaking the design principle that there should be no intrusions into either the parking, or circulation zone.
Compromises such as these, often driven by site constraint, can result in circulation pinch points if, for example, drivers misjudge turning in ramps. Wider ramps make for more flexible car parks, particularly as cars get larger, and the consequence of poor ramp geometry is the potential for impact damage to vehicles, which will recur throughout the 60-year life of the building.
Where there is space for external ramps outboard of the main car park deck, these can often be sized to make manoeuvring easier, which lessens risk to both bodywork and structure.
• Planning grid. Based on the standard planning grid and aisle dimension, the ideal long-span dimension for a multistorey car park is 16.2m. The short-span module will be at least the width of three car bays. The minimum long span that is achievable before structure impedes car manoeuvring and getting in and out of vehicles is 15.6m.
Columns are also used to run services such as rainwater downpipes, dry risers and air ducts through the structure. These services and their impact protection should not be placed within the dimensions of the parking bay – otherwise the grid will be further constrained. Due to space constraints, a strong case can be made for the use of steel columns in car parks because of their smaller section size.
However, due to issues of robustness, fire protection and so on, most car parks continue to be built in insitu and precast concrete.
Other important dimensional considerations include:
- Alignment paths to exit barriers – these should be typically at least two car lengths, giving enough space to drivers to correct their positioning
- Means of escape distances, potentially requiring protected corridors on particularly deep plan car parks
- Travel distances from the car to the destination – 180m is considered to be an acceptable maximum.
Access and accessibility provisions. There are at least as many people movements on a car park deck as car movements. As the overall experience of the visitor contributes to the long term attraction of the facility the car park serves, investment in public areas and accessibility features is important. Areas where design and specification can make a difference include:
- Round the clock security. Many car parks are in use 18 or more hours a day, so a visible CCTV, help point and security presence will give users confidence during extended hours of operation
- Visibility. All public access routes need to be bright, well lit and free of blind spots so that users feel secure and environmental quality is maintained
- Space allowances. Lobbies that combine lift access, pay points and through traffic can become very congested. Spaces need to reflect anticipated levels of foot traffic
- Lift provision. 13-person lifts are the minimum acceptable size, providing adequate space for buggies and so on.
A key aspect of accessibility is disabled provision, as 6% of spaces need to be sized for disabled use. The ideal arrangement from a point of view of user friendliness is for these spaces to be clustered. However, in busy periods, the sight of a cluster of unoccupied spaces in a prominent position can lead to irritation, and at worst, misuse of the spaces. While it is important that disabled spaces are easy to find and are close to exits, an approach based the even distribution of disabled parking over a number of levels is probably the most effective with regard to ensuring that spaces are available when needed.
04 Car park efficiency
The optimum use of car park space is determined as much by drivers being able to reach and locate spaces as by the absolute number of spaces themselves. Unlike offices, where the net:gross floor ratio is a universal metric, there are necessarily multiple measures of car park performance. Key metrics include:
• Static efficiency: This is the closest equivalent to net to gross and measures the proportion of space taken up by “parking pads” – the actual area devoted to parking, as opposed to circulation etc. A large component of the non-parking element relates to vertical circulation – clearly favouring larger car parks, which require proportionally fewer vertical elements. Static efficiency metrics follow a stepped pattern, particularly on larger car parks where traffic volumes may dictate requirements for additional “by-pass” ramps, which direct traffic to upper or lower floors away from on-floor circulation, speeding up traffic flows. Another measure of static efficiency is area per space, which can vary from between 20m2 and 28m2 on a multi-storey development, depending on the area and number of cars on each deck.
• Dynamic capacity: This is a simple measure of flow rates of cars at particular points in a car park. Dynamic capacity is used to identify bottlenecks in primary circulation routes, which may prevent all car-parking spaces being readily accessed. Measures taken to increase dynamic capacity, such as the design of wider aisles and bays, or additional ramps or exits, will all reduce static efficiency.
Dynamic efficiency is important in the filling and emptying phases of car-park use. The performance standard which designers aim to meet is that 25% of total capacity should be able to enter, or exit, within 15 minutes.
Clearly the static and dynamic metrics are inter-related and over-emphasis on one tends to be at the expense of the overall effectiveness of the design.
Other important measures which affect performance include:
- Circulation speed: Ideally as close to 10mph as possible, and influences mainly by circulation space standards.
- Parking bay entry and exit times: Determined by bay and aisle width and bay alignment.
- Search time: Affected by the efficiency of layout including having to travel down an aisle more than once to complete a search pattern on a deck, or the availability of directional signage.
- Occupancy levels: Peak and average: Reflecting the usage of the facility and the effectiveness of circulation. Car parks which suffer entry congestion are unlikely to achieve sustained high levels of occupancy. A typical target over the full operating period is 50 to 65%.
- Visitor turnover per day: Helps to determine the number of vehicle movements and how important bay-width allowances are to the performance of the car park.
05 Typical car park layouts
There are more than 20 alternative arrangements for multistorey car parks used in the UK. These can be divided into three main families distinguished primarily by the slab arrangement. The main reason for the large number of variations is the range of vertical circulation options. The principal families are:
• Flat slab: This arrangement is preferred by developers and users as it is structurally and navigationally simple. Other benefits include good visibility, easy pedestrian access and also fewer vehicle conflicts owing to the simple circulation routes. A further advantage of flat slab car parks is that they have regular elevations on all four sides – potentially important for the design of an attractive external elevation. Flat slabs have few disadvantages but due to their effectiveness and high dynamic capacity they suffer from the potential for congestion on lower levels during peak periods. By-pass ramps and high-efficiency external D-ramps can be used to increase throughput, although in city-centre sites, these features may just transfer the congestion to the streets outside.
• Split level: Split level car parks use intermediate internal ramps and are compact on plan. This is the most common arrangement in the UK. The more complex ramped circulation of split level car parks reduces dynamic capacity. Tight allowances for aisle and bay widths, ramp penetrations and so on can potentially adversely affect the quality of the user experience. Pedestrian access to intermediate levels has often been routinely ignored by developers, with pedestrians being required to use vehicle ramps. Where dedicated stairs and ramps are provided, the space taken can be considerable. From a user perspective, complex vertical circulation is not popular and the absence of clear sight lines across a deck presents a security challenge.
• Ramped decks: Continuous ribbons of parking minimise the need for vertical circulation and as a result have a high static efficiency. Ramped decks will typically have a single flat section on each level where pedestrian access, disabled parking, and so on, can be provided. Ramped decks are particularly appropriate for narrow sites where there is insufficient space for a conventional cross ramp between split-level desks. Disadvantages of the arrangement include constraints on the planning of pedestrian access, together with risk of car damage, caused by the unexpected effects of gravity on car doors, and so on.
The cost model is based on a six- level, flat slab car park constructed in post-tensioned concrete. The scheme, with a gross floor area of 17,370 m2 has spaces for 580 cars. It features external D ramps, providing efficient by-pass entry and exit circulation to all levels.
The car park, associated with a retail development, is constructed on piled foundations. Shear walls have an aluminium cladding and other elevations have a protective barrier with mesh infill.
Costs are based on prices current in fourth quarter 2007 based on a location in outer London. Rates are appropriate for a project let on the basis of a competitively tendered lump sum design and build contract.
Costs exclude the following: demolition and site clearance, external services, public realm works, link buildings and highway works beyond the perimeter of the building.
Professional fees and VAT are also excluded. Costs can be adjusted using the location factors provided. Consideration should also be given to factors including, site conditions and constraints, project size, programme and procurement route.
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