Car park collapses: A weighty problem

The overloading of older multi-storey and underground car parks is a cause of concern, says structural and fire safety reporting group CROSS-UK

The growing weight of cars presents a potential problem of overloading of car parks, a report by a structural and fire safety group has warned. The report was published by Collaborative Reporting for Safer Structures UK (CROSS-UK), an initiative of the Institution of Structural Engineers (IStructE), Institution of Civil Engineers (ICE) and Institution of Fire Engineers.

CROSS-UK seeks to help professionals to make structures safer by publishing safety information based on the reports they receive and information in the public domain. The initiative shares information and concerns produced by reporters, sector experts whose anonymity is protected.

Older car parks

A CROSS reporter notes that for over 80 years multi-storey car parks (MSCPs) 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/m², as outlined in 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.

The reporter also 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.

Older car parks, and particularly those of the 1960s and 1970s, were designed and constructed to design standards that have since been found inadequate. For example punching shear in flat slabs was designed to BS CP 114: Part 2 1969  – The structural use of reinforced concrete in buildings.

During the 1960s and 1970s 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.

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.

The reporter also recommends that 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 builds

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.

CROSS Expert Panel comments

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 kerbside weight and thereby exclude vehicles above a given weight. Drivers of heavy vehicles would have to get 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.

To find out more about CROSS-UK visit: www.cross-safety.org/uk/about-cross-uk

Key learning outcomes

The CROSS-UK Expert Panel offers the following advice for car park owners, designers and engineers.

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, for example, by limiting their spacing or access before adopting strengthening of the structure.

Essential reading: CROSS-UK reports

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

• Safety Report 119 – Multi-storey car park defects
• Safety Report 224 – Structural assessments of multi-storey car parks
• Safety Report 265 – Multi-storey car parks – Structural condition concerns
• Safety Report 267 – Multi-storey car parks demolished due to structural defects.

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

• Safety Report 857 – Fire resistance of multi-storey car parks
• Safety Report 940 – Fire risks in multi-storey car parks
• Safety Report 1007 – Fire protection to car park steel frame
• Safety Alert – Fire in multi-storey car parks.