Multi-storey car parks

All buildings move; they are not static structures. The Institute of Structural Engineers considers car park construction to be similar in many respects to bridge construction because of, among other things, the large unobstructed spans (frequently 16m) and also the transient loadings of vehicles parking and moving about. Further, the structures are exposed to the elements; not only are the top decks usually large areas exposed to solar radiation and resultant thermal movements, but also the ramps, parking zones and main traffic routes are subject to chemical attack from salts - usually calcium chloride or urea - brought into the building by vehicles. Being in the main open-sided and unheated structures, car parks are also subject to frequent wetting and drying, with the attendant risks of frost action and concentration of aggressive chemicals.

The Standing Committee on Structural Safety (SCOSS) in its 10th annual report, warned of a threat to structural safety as a result of neglected corrosion, especially of bonded or unbonded prestressing tendons or of reinforcement to cantilevers. SCOSS went on to draw attention to the potential lack of precautions against progressive collapse as this subject was not initially addressed by Building Regulations in buildings of less than 5 storeys in height. The Committee concluded that:

'... the premature deterioration of multi-storey car parks if ignored, could lead to serious structural distress, and even collapse, in some cases. Attention should be paid to zones likely to be affected by drainage water contaminated by road salt. This could be particularly damaging at half-joints or bearings. Corrosion may, in some cases, be advanced without conspicuous cracking and in other cases cosmetic repairs may have masked signs of deterioration'.

Movements in the building due to loading and thermal expansion can cause a number of effects. Bowing of a large tee-ribbed deck will create the risk of cracking of surface finishes around the perimeter. Water can then collect at the perimeter and penetrate cracks. Water entering at a bearing joint is unsatisfactory since this is where shear reinforcement is located: the resultant spalling or deterioration in the bearing could cause unwelcome structural effects.

Similarly, the effects of insufficient falls on the decks, or actions such as regular car washing, create pools of water that can gradually percolate the structure to affect reinforcement.

So deterioration in car parks is likely to be the result of a combination of factors each leading to a further problem. For example:

• expansion causing bowing;
• bowing causing ponding;
• ponding causing water ingress; and
• water ingress permitting deterioration in the concrete.

Water ingress through a defective perimeter joint has caused corrosion of steel reinforcement and subsequent spalling of the bearing to this column head.

It is easy to underestimate the effects of water ingress into a structure. However, a brief reflection reveals that the areas that are most likely to leak are those either at mid-span or at or close to the perimeter. At the perimeter will be found bearing conditions for precast structures, prestressing anchorages, reinforcement for edge beam and vehicle protection balustrades, structural joints and so on - precisely the details that need to be protected against corrosion.

An MSCP is not generally a controlled temperature environment. In particular the top deck can suffer from the effects of thermal expansion. BS 5400 suggests that the top deck of a car park structure can be expected to endure a temperature range of circa 45°C (within the UK). In general, steel and concrete frames have a similar coefficient of expansion, which may be taken as 12 x 10-6/°C. So, for a 75m length deck, movement due to temperature effects could be as much as 40mm; this must be accommodated by movement joints. If the ends of the slab are restrained by elements of structure (such as shear walls, cores or ramps), load reversal can take place whereby elements that are not designed to take high internal stresses become damaged.

Similarly, the restraint of the perimeter of the deck will result in the bowing of the deck in a way that could reverse drainage falls and gradually damage movement joints and bearings. A method of estimating the bow of a deck due to the restraining influence of the structure is given in the IStructE Report Design recommendations for multi-storey and underground car parks (3rd edition), as follows:

Coefficient of thermal expansion x temperature difference x length2

8 x thickness

For example: a 200mm thick deck spans 16.5m and is finished with 50mm asphalt. In summer, the temperature differential between the top surface and the underside of the deck will subjected to a temperature differential of 13°C. Taking the same coefficient of thermal expansion as before gives:

12 x 10-6 x 13 x 165002 = 26.5mm

8 x 200

A similar calculation this time using a double tee beam 600mm deep finished with a thin, dark coloured waterproofing coating produces the following result:

12 x 10-6 x 23°C x 165002 = 15.6mm

8 x 600

However, bowing effects caused by temperature are not the only problem since the effects of long-term creep must be considered. Creep is generally more significant (and severe) in car parks than in enclosed buildings because the ratio of dead load to total load is higher. Creep modifies the elastic modulus of concrete beams over time such that over a period of say 25 years, the amount of deflection in a beam could increase by 25-50% (Design recommendations for multi-storey and underground car parks). Such deflections mean that drainage falls and outlet positions that worked perfectly well when the building was constructed may no longer be effective, giving rise to ponding.

Ponding is undesirable in car parks, as it can create conditions in which contaminated water can percolate into the structure.

Deflections in top deck giving rise to poor falls and ponding remote from rainwater outlets

A span of 16m (which is the usual design module) leads to a high dead/live load ratio by comparison with other structures together with a greater live load being experienced for longer periods.

As an alternative, some early designs reduced the span between columns to around 9m, with cantilevered portions extending to approximately 3m each side. The performance of the cantilever is therefore critical, particularly as it will in all probability carry the heavy dead loads of a robust balustrade or edge beam system.

From a structural point of view, deterioration of the concrete 3m back from the edge of a slab could be serious, as this is where shear between the column heads and the slab will be highest, and where the bending moments for the cantilever will be concentrated.