Safety concerns over significant increase in overloaded multi-storey car parks

The increased weight of cars has seen a safety warning issued over the capacity for older multi-storey and underground car parks.

Cross UK notes that for more than 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. The maximum gross weight has now increased to more than 3,000kg, necessitating the design loading to be increased to 3.0kN/m2 in 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. A report to Cross UK concerned the significant increase in size and gross weight of vehicles may impact the safety of some older car parks.

What does this mean for building engineers?

Those in building control should 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.

Older car parks, and particularly those built in the 1960s and 1970s, were designed and constructed to 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. Fifty years ago 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. 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 at a maximum.

There are still a number of specialist system built 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.

Building bigger and faster

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). This can affect the continuity of the reinforcement. Some car park decks have no falls or internal drainage, which leads to increased chloride penetration in areas where ponding is taking place.

The reporter adds that 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. 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.

In recent years, construction has moved in the direction of off-site manufacture, the reporter highlights, and design has adopted lean design principles where the use of materials has been minimised. The curvature of long-span, precast deck units can impose high-bearing stresses on the seating and, 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.

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.

What's the solution?

This is a timely warning for the industry, as vehicles get heavier and particularly with the increase in popularity of electric vehicles. Cross UK's Expert 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, said the Panel. "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," it added. "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 a number plate is often read on entry and again on exit to verify payment so 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, concluded the Panel.

Original report provided by Cross UK.