Multi-storey car parks

Multi-storey car parks (MSCPs) are unlikely to arouse much passion other than a general belief that most are not very pleasant places to visit. Many early versions were poorly built and continue to be poorly maintained; they are often rated as a low priority for expenditure and command only cursory inspection during sale or purchase.

There are in excess of 6,000 MSCPs in the UK, virtually all having been constructed since the 1940s and many free-standing types constructed during the 1960s. Many early car parks were procured as cheaply as possible, creating a legacy of poor construction and a lack of durability. Construction to the lowest cost per bay did not necessarily lead to good construction.

For surveyors, there are significant risks in underestimating the complexity of forces and agencies of deterioration that are at work. There are relatively few collapses, but MSCPs are large structures and repair bills can be high. The apparent simplicity of a car park structure often means that its inspection is not treated in the same light as, for example, the shopping centre or office building to which it is attached. However, the conditions that car parks have to endure are far more onerous than the adjoining unexposed buildings. If the modes of deterioration are not understood or appreciated there is a risk that something may be missed.

Following the collapse of a car park floor slab at Pipers Row, Wolverhampton in 1997, the design and condition of MSCPs was placed in the spotlight. The collapse spawned various reports and investigations, leading, in September 2002, to an OPDM-sponsored publication Enhancing the Whole Life Structural Performance of Multi-Storey Car Parks together with Recommendations for the inspection and maintenance of car park structures, published by the Institution of Civil Engineers in the same year.

Most MSCPs in the UK are concrete column and slab structures. The main types are:

• in-situ reinforced concrete column and slab;
• in-situ frame, precast prestressed deck;
• precast frame and deck;
• lift-slab construction;
• steel frame with in-situ or composite deck; and
• in-situ concrete with bonded post-tensioning.

All of the above types are vulnerable to deterioration due to chemical agencies, but the design of the car park will affect its durability to a large extent. For example, a prestressed deck will have a much lower tolerance to ingressed chlorides than one cast in-situ, simply because the tendons are of smaller diameter and the threshold chloride for prestressed concrete is much lower.

Early car parks are most likely to suffer the greatest levels of deterioration due to their age and exposure to the elements, but also as a result of poor construction or inadequate design.

Structural and design codes were less onerous in the 1960s, meaning that matters such as shear reinforcement, depth of cover, joint design, etc. were often inadequate, while higher water to cement ratios and lower strength concrete created more permeable structures that would be more vulnerable to chemical attack and carbonation.

Early design codes for flat slab construction are now regarded as being particularly suspect as evidenced by the Pipers Row collapse. The simplified rules set out in BS 8110 do not necessarily ensure sufficient robustness with all forms of flat slab construction.