Multi-storey car parks
Lift slab floors
Lift slab construction is a method of constructing buildings with repetitive floor layouts by casting them on the ground and jacking them into place. There are several different methods but commonly a series of jacks are fitted to the top of precast columns, connected to threaded lifting rods and lifting collars cast within the floor slabs. After reaching a satisfactory strength the floors are jacked up into position and then fixed with wedges attached to the columns. The void between the floor and the column is then filled with fine aggregate concrete.
The system is fairly popular in the US but there have been several cases of collapse during construction due to insufficient lateral bracing during the hoisting operations. One characteristic of these types of floor is that they provide little by way of lateral bracing to a building and this has to be compensated by the construction of reinforced shear walls or braced frames with steel diagonal members.
In the UK, lift slab construction achieved notoriety on 20 March 1997, when a 120 tonne section of the top deck at the Pipers Row multi-storey car park in Wolverhampton collapsed. Although open, the car park was unoccupied and no-one was injured. (While Pipers Row is often cited, a further collapse of 3 floors occurred in Coventry in 1988 when another lift slab car park was being demolished.)
Following detailed analysis as to the cause of the failure, HSE published their findings in Pipers Row Car Park,Wolverhampton: Quantitative Study of the Causes of the Partial Collapse on 20th March 1997. HSE concluded, among other things, that if it had been properly maintained, the car park ought to have performed satisfactorily in service. However, while the design provided good falls for drainage, the construction tolerances, deflection and creep of the slab created areas of ponding on the surface. Waterproofing leakage and ponding saturated the slab and frost damage developed to fissure and microcrack the concrete, particularly in local areas of high porosity and low strength.
Deep carbonation developed into the microcrack system and zones of high porosity, possibly aggravated by carbon dioxide released from the breakdown of urea used for de-icing during the previous decade. When microcracking reached the top reinforcement, the degradation, sufficient to destroy most of the bond from steel to concrete, had developed for most of the upper top layer of reinforcement. This degradation, and the depth to which it penetrated, lead to punching shear around the column heads and, of course, the removal of support to the slab, which then collapsed.
A brief description of moment connections and shearShear can be described as a 'slicing' force. It is an internal force that can act at any point along the length of a structural member. A short beam with a heavy load close to one of the supports would be subjected to a slicing type force; this could be significantly greater than the forces generated in bending. A simple tent frame constructed of 2 members with a pivot joint is unstable until the ends of the frame are securely located or tied. If the 2 ends were to be joined by a third member, the frame would be rigid. Similarly, a square frame could easily be distorted into a parallelogram if a horizontal load were applied to one corner. If a diagonal member was inserted, the square would be rigid - in effect, inserting a single diagonal 'brace' creates 2 triangular structures. A framed building behaves in exactly the same way. Unless it can be suitably braced it would be free to move under lateral loads such as wind pressure. By putting diagonal structural members in place, the frame can be given rigidity. However, diagonal braces are not always satisfactory; they can intrude on the internal space and may detract from the architectural appearance of the building. An alternative to diagonal bracing is the provision of a shear wall - a stiff vertical cantilever that is able to resist lateral loads. Constructing a rigid reinforced concrete core for the lifts and stairs is a similar solution. However, both shear walls and diagonal bracing have disadvantages and if a large uninterrupted space is needed internally, the option is to create very rigid connections within the frame and floors that are able to provide sufficient stiffness to the frame to prevent buckling. Such connections are termed moment connections - they permit no relative rotation between the columns and slabs and their connections. A form of failure related to lift slabs is punching shear. Punching shear is a slicing action, acting in the same way as a pen being pushed through a sheet of card. When the force of the pen exceeds the shear strength of the card, it pushes through to the other side. |
In Pipers Row, the lack of maintenance to the waterproofing layer led to deterioration of the concrete but the failure generated further concerns over the design codes used for the structure. The structural integrity of this form of lift slab structure requires cautious engineering judgment. Interpretation of the unusual characteristics of the column/slab connection using current design codes can result in an under-estimation of load acting on the structure, and an over-estimation of strength.
Once deterioration, corrosion or cutting out exposes reinforcement, it must be considered as a potential structural problem requiring a check by a structural engineer to establish if a structural repair, as distinct from a corrosion control, infill or cosmetic repair, is required. For structural repairs a detailed method statement with reinforcement drawings marked up with limits of cutting out and support requirements should be prepared by a structural engineer.
Punching shear anchorage zones are particularly difficult to patch repair reliably and if effective anchorage has been damaged it may be better to bolt completely through the slab or recast to full depth.
Identification
Flat slabs without thickening around the column heads, together with evidence of infill around the column head/floor slab, should alert the surveyor to the possibility of lift slab construction. Early forms adopted columns at 4m centres across a bay width of 16m with a typical floor slab depth of 225mm.
By the late 1960s the method had changed to columns at 9.8m centres with cantilevers of 3.1m each side. Columns would support a slab area of 50m2 and a slab of 280mm thickness.
In the early 1980s the same structural configuration was used, but with post-tensioned slabs of 250mm thickness. Car parks constructed of this method may not have waterproofing layers on the top level.