Skip to main content

CROSS Safety Report

Overloading of older multi storey and underground car parks

Report ID: 1257 Published: 17 May 2024 Region: CROSS-UK


Overview

A reporter notes that for over 80 years multi storey car parks (MSCP) have been designed for vehicles with a maximum gross weight of 2,500kg, which equates to a uniformly distributed load of 2.5kN/m2. They add that the maximum gross weight has now increased to over 3,000kg, necessitating the design loading to be increased to 3.0kN/min the IStructE's recently published Car Park Design guidance.

The average gross weight of vehicles has also increased from 1,500kg to 2,000kg requiring the impact force on rigid vehicle restraint barriers to be increased from 150kN to 200kN.  The reporter is concerned that the significant increase in size and gross weight of vehicles may impact the safety of some older car parks.

Key Learning Outcomes

For car park owners:

  • Consider the suitability of car parks for the size and weight of modern vehicles
  • Seek advice from suitably qualified and experienced engineers with regard to any remedial measures that may need to be taken
  • Consider entry control by electronic means for example automatic number plate recognition
  • Consider parking heavy vehicles on non suspended levels

For designers and structural engineers:

  • Consider the effects of modern vehicles on older structures and how these can be accommodated e.g. by limiting their spacing or access before adopting strengthening of the structure

Full Report

Find out more about the Full Report

This section contains the Full Report submitted to CROSS and describes the reporter’s concerns or experiences. However, the text has been edited for clarity, and identifiable details have been removed to ensure anonymity and confidentiality. If you would like to know more about our secure reporting process, or submit a report yourself, please visit the reporting to CROSS-UK page.

 

The reporter notes that over the last 20 years there has been a significant increase in size and gross weight of vehicles that are able to use MSCPs, leading to fears over the safety of some older car parks.

The reporter explains that, for over 80 years, structured car parks have been designed for vehicles with a maximum gross weight of 2,500kg, which equates to a uniformly distributed load of 2.5kN/m2. The maximum gross weight has now increased to over 3,000kg, necessitating the design loading to be increased to 3.0kN/min the IStructE's recently published Car Park Design guidance. 

The average gross weight of vehicles has increased from 1,500kg to 2,000kg, requiring the impact force on a rigid restraint barrier to be increased from 150kN to 200kN.  The gross weight includes passengers and luggage which can add around 400kg to the kerb weight of the vehicle.

Older car parks, and particularly those of the 1960’s and 1970’s, were designed and constructed to design standards that have since been found inadequate, for example punching shear in flat slabs when designed to BS CP 114: Part 2 1969 - The structural use of reinforced concrete in buildings. During the 1960's and 1970's there was a construction boom resulting in a shortage of materials and labour, and a fall in the quality of the finished product.

The reporter shares that many, but not all, of these car parks were demolished due to structural fears around premature corrosion and loss of strength. Some have been subject to repairs, but these may have been cosmetic patch repairs with incompatible materials and may not have restored the structural integrity. The reporter highlights that repairs to cantilevers are particularly vulnerable, as the reinforcement is in the top of the deck where the risk of corrosion from penetration of chlorides is a maximum.

There are still a number of specialist system build car parks in service which are sensitive to structural deterioration and the manner in which repairs were undertaken. The reporter believes these car parks need to be assessed for modern vehicles.

To build bigger and faster, construction has moved in the direction of off site manufacture

Outlining common areas of concern, the reporter believes drainage should be an integral part of the design but notes it can be installed after coring the structure (sometimes in structurally sensitive areas such as punching shear zones) thus affecting the continuity of the reinforcement.  They add that some car park decks have no falls or internal drainage, which leads to increased chloride penetration in areas where ponding is taking place.

Having highlighted common areas of concern, the reporter adds there are many others and advises engineers to study the structural morphology and interrogate the history of a car park before carrying out any strength assessment.

They also recommend older car parks be assessed in accordance with the ICE's Recommendations for Inspection, Maintenance and Management of Car Park Structures using the loads recommended in the IStructE's Car Park Design guidance. The reporter concludes that strengthening may have to be carried out should the structure be incapable of withstanding the increased loading or, if this is prohibitively expensive, a weight limit should be imposed to restrict entry to the suspended levels.

New construction

To build bigger and faster, construction has moved in the direction of off site manufacture and design has adopted Lean Design principles where the use of materials has been minimised. The reporter highlights that the curvature of long-span precast deck units can impose high bearing stresses on the seating and that, if the seating is concrete, high bearing stresses can cause spalling and reduce the bearing area - especially where the interface has no slip membrane.

Overloading can cause instability of steel frames and bolted connections especially where the structure has been designed to maximise the efficiency of the components. The performance and longevity of permanent metal formwork is dependent on the ability of the deck membrane to prevent water ingress and internal corrosion at the interface with the concrete. The reporter notes that when permanent metal formwork is used structurally and the membrane has not been regularly maintained, the strength of the deck can be reduced in proportion to the magnitude of the corrosion. Decks that have bolted units or constructed from Glass Reinforced Plastic (GRP) components often suffer from the bolts working loose and falling out under repeated loading.

The reporter also shares that the ability of a restraint barrier to withstand impact loading can depend solely upon the performance of the holding down bolts, some of which are incapable of withstanding repeated loading from vehicles nudging the barrier.

The reporter concludes by highlighting that Eurocode and National Annex for the UK implies the design loadings are for vehicles of maximum gross weight up to 3,000kg. However, this figure defines only the category in which the vehicle sits and the loadings are applicable for vehicles of maximum gross weight of 2,500kg and average gross weight of 1,500kg which is no longer compatible with the IStructE’s recently published Car Park Design guidance.

Expert Panel Comments

Find out more about the Expert Panels

Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

This is a timely warning for the industry, as vehicles get heavier and particularly with the increase in popularity of electric vehicles.  The Panel notes that flammability and fire load are increasing concerns as much as structural load.

Picking up on the reporter’s last point about weight limits, digital methodologies could be part of a solution. Structures can already be automatically monitored for the development of structural distress, perhaps digital number plate recognition could link to vehicle curb side weight and thereby exclude vehicles above a given weight. Drivers of heavy vehicles would have to get to used to being excluded from some car parks, just as drivers of high ones are now or be restricted to ground supported floors.  This approach may be a cost effective alternative to strengthening.

Automatic Number Plate Recognition (ANPR) could also provide an excellent opportunity to gather data on the distribution of kerb weights of vehicles using a car park.  The technology is already in use, as one's number plate is often read on entry and again on exit to decide whether one has paid and the barrier can be lifted.  A statistical analysis similar to those routinely used for Bridge Specific Assessment Live Load (BSALL) studies on long span bridges is possible to define a suitable level of live load, and how it might change with the population of vehicles.

CROSS has previously published Safety Reports concerning potential structural issues with car parks:

More recently, CROSS has issued safety information concerning fire risks:

Submit a report

Your report will make a difference. It will help to create positive change and improve safety.

Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others.