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602 Padstones out of position leads to collapse

Report ID: 602

Published: Newsletter No 46 - April 2017

Report Overview

This report concerns the partial collapse of a terrace of four storey houses during construction. The roof was timber and steelwork. The superstructure precast concrete floors were supported on front and rear façade walls and internal steel beams.

Report Content

This report concerns the partial collapse of a terrace of four storey houses during construction. The main contractor employed a firm of consulting engineers known to the reporter to finalise and detail an outline scheme design by others. The roof was timber and steelwork. The superstructure precast concrete floors were supported on front and rear façade walls and internal steel beams. The internal steel beams were supported on dense concrete padstones on the compartment walls and on the inner leaf of the end walls which were of aerated concrete blockwork. At a late stage in construction it was found that the masonry subcontractor had positioned the padstones so as to maintain the blockwork bond irrespective of the positions of the steel beams. Consequently several beams had bearing close to the ends of the padstones. Instructions were given to rebuild the work so that the beams would be central on the padstones. Some days later one of the end walls collapsed together with the supported bay of roof and floors. Fortunately there were no casualties although the start of the collapse was witnessed as the face of the aircrete blocks peeled off beneath the padstones allowing the steel beams to fall. Subsequent investigation found that the remedial work had not been carried out and there was evidence of covering up with a plaster skim coat. It seems that some contractors cannot be trusted to follow traditional good practice in positioning padstones and it seems now necessary to give explicit instructions on drawings. Sadly other consulting engineers were only too willing to be employed by the main contractor to snag other unrelated aspects of the design resulting in a sharing of costs by the designers insurer despite the incident being caused by others.

Comments

The main feature here is that the padstone bearings were eccentric. It is surprising how quickly a small increase in eccentricity of loading can reduce the capacity of a wall panel.. A combination of adverse tolerances, positional eccentricities and, in some cases, a lack of appropriate torsional restraint (with fixings if needed) will soon lead to a heavily reduced margin of safety. The contractor’s desire to maintain bond rather than correctly locate padstones demonstrates the importance of adequate site supervision and inspection prior to covering up. To hide the ends of load-bearing padstones in order to disguise the inadequacy of their construction is extremely poor practice. Shades of the Edinburgh schools’ problems and the need for industry wide action. The report is also reminiscent of a number of investigations carried out by the HSE on this type of project where poor workmanship and poor supervision has resulted in collapse and ultimately in prosecution.   


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