Ramps in Car Parks

The most frequent topic on my ‘Ask the Expert’ web pages is the gradient of vehicle ramps in car parks!

Where engineers and architects are trying to maximise the parking capacity of a ‘cube’ they inevitably explore the scope for short ramps and low floor to ceiling clearances. This leads to:

The question:             What is the maximum gradient for a car park ramp?

And the response:    It depends!

Ramp Types

To appreciate the complexity of ramps in car parks one has to recognise that there are many types of ramps used in many different situations.

Connecting ramps         ramps linking vertically separated parking decks as commonly found in multi-storey car parks

Parking ramps               ramps combining the functions of circulation and storage as occur in spiral or helical car parks

Whether a ramp is curved or straight has an important bearing on the issue of gradient. Spiral ramps used to connect different levels within a car park are curved and longer than straight ramps serving the same purpose.

Checklist

The following questions need to be answered before considering ramp designs queries:

·        Is the car park open to the public or used by only one group, e.g. staff?

·        Will the ramp be used both by pedestrians and vehicles?

·        Will shopping trolleys, buggies, bicycles, scooters, motorbikes use the ramp?

·        Does the car park have to cater for a broad spectrum of vehicle types or just a narrow group?

The ramp design limitations for a general purpose public car park at a shopping centre, for example, are very different to those for a car park built only to accommodate one type of vehicle, e.g. as might happen at a car manufacturing location.

Straight Ramp Slope

The recommended slope for straight ramps in situations such as:

·        Half deck car parks where the vertical separation between decks is less than 1.5m, is 1:6.  This relatively steep slope is only possible when using transition gradients top and bottom.

·        Where vertical differences are greater than 1.5m, is not less than 1:10.

·        Where ramps are curved, 1:10 or 1:12 depending on the separation.

Many modern cars have wheel bases that are …..  long and under-body clearances of less than… The effect of these specifications is that in any situation where a ramp of gradient steeper than 1:10 intersects with a flat slab that cars will bottom out on the transition line at the top of the ramp. Figure 1.

The other side of this phenomenon occurs at the bottom of ramps. Long tall vehicles moving off a ramp onto a flat deck effectively rise up within the car park structure and run the risk of striking the soffit or structural beams at the bottom of the ramp. Figure 2.

Gradient suitable for Pedestrians

In a car park where members of the public are likely to walk on ramps, then any gradient steeper than 1:10 is likely to be problematic.

·        A person wearing shoes with elevated heels finds steep slopes very uncomfortable and possibly dangerous.

·        People pushing shopping trolleys, buggies or even bicycles find steep ramps very uncomfortable and in many cases dangerous.

·        People with mobility challenges requiring aids such as walking sticks, crutches or wheelchairs experience severe difficulties on ramps steeper than 1:12.

Three-Stage Ramp Structures

To address some of these issues, engineers have developed three stage ramp structures:

·        The top and the bottom of the ramp are constructed to a gentle gradient say 1:16 or 1:20.

·        The central section of the ramp is built to a steeper slope – 1:8 or 1:10.

Three additional issues with car park ramps include the:

·        clear edge to edge width

·        the turning circle on approach routes to the bottom or from the top of ramps.

·        the location of ticket machines on ramps

Ramp Width

Many drivers find ramps too narrow and scrape their bumpers along walls at the top or bottom of ramps. The recommended minimum width for a one-way ramp is 3.0m with an additional 0.3m for side clearance to the structure. The recommended width of the entry section for a turning approach to a ramp is 3.5m. Bearing in mind that very few cars are more than 1.8m wide these recommendations allow for a broad range of driver behaviour and skills.

Turning Circles

Well-designed turning circles in car parks depend on the types of vehicles using them. Some modern cars have turning circles of radius 6.0m to 7.5m. If these have to be accommodated in a car park then it is prudent to design turning lanes on the basis of an outside kerb radius of 9.0m. It is highly desirable that there are no structural columns located at the turning pints onto or from ramps. They intimidate drivers and lead to damage to walls and vehicles.

Ticket Machines

It is undesirable to have ticket machines located on ramps, as drivers and vehicles experience difficulties. Practical experience suggests that ticket machines on down ramps can work well while those on up-ramps are generally unfriendly. On the down ramp the driver can see the barrier in front and can respond to any slippage. There is little risk of collision with another vehicle. On an up ramp the driver has very poor views of cars behind and will be nervous in case his car drifts back or the car behind gets too close. Handbrake starts on up ramps make many drivers nervous

Sight Lines

Finally in designing ramps in car parks, it is vital that clear sight lines are maintained at the top and bottom of the ramps. These are essential to the safety of pedestrians and vehicles alike. The driver of a car on a ramp cannot readily see directly in front because the bonnet of the car intrudes, and as a consequence must depend on being able to see to the side to ensure he/she can proceed safely. Curtain walls on ramp sides have been built with large holes to provide very good sight lines.

The Institution of Structural Engineers excellent publication “Design Recommendations for Multi-storey and Underground Car Parks” provides detailed information on many aspects of car park ramp gradients. I have used their recommendations throughout this paper. www.istructe.org.uk

Liam Keilthy is MD of Parking Consultants Ltd., a specialist car park consultancy practice based in Dublin. He provides ‘Ask the Expert’ assistance on his web site at www.parkingconsultantsltd.com

Gradient Examples:  A gradient of 1:10 represents a steeper gradient than 1:12:

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