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CROSS Safety Report

Car park staircase collapse at construction joint

Report ID: 106 Published: 1 July 2008 Region: CROSS-UK

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Overview

The top flight of a staircase to a multi-storey concrete car park collapsed approximately 10 years after construction.

Key Learning Outcomes

For car park owners and managers:

  • Regular inspection and maintenance regimes can help to keep a structure safe and help to identify any obvious safety issues that may need to be addressed

For civil and structural design engineers:

  • Take account of the durability requirements of car parks.  Three helpful publications are recommended in the Expert Panel Comments.

Full Report

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The top flight of a staircase to a multi-storey concrete car park collapsed approximately 10 years after construction. The stair was constructed in-situ and cast against the existing slab which had protruding continuity reinforcement. The failure occurred at the construction joint between the slab and the stair landing.

Corrosion issues

An examination indicated that the failure was due to corrosion and consequent failure of the bars across the joint. The reporter believes there to be a number of issues contributing to this premature failure. The car park was of a split level type construction and the staircase went between two ‘half’ levels. The stability of the car park was provided through moment frame action and the two split levels that the stair connected were separated in every other way by a movement joint.

This meant that any relative movement of the two slabs due to seasonal thermal, shrinkage or other action, was restrained by the staircase alone. The weakest point of the stair was likely to have been the construction joint and it is likely that this would open and close due to the varying imposed movements.

Poor construction

The joint was provided with a rebate to be filled with sealant. Whilst this should have sealed the joint it was dependent on future maintenance. There was evidence from other areas of the same car park to suggest that, in some places, the sealant was either missing or not performing satisfactorily. Evidence from the failed stair indicated that the construction joint had been poorly prepared. This means that there would have been little aggregate interlock between the slab and stair concrete.

Greater aggregate interlock would have made the water path more tortuous and provided a load transfer mechanisms across open cracks. Without aggregate interlock the only method of load transfer would have been dowel action in the reinforcement crossing the crack. The car park falls were such that the joint was placed at a low point, thus water tended to gather over the joint, leading to ponding and an increased flow of water through the joint.

Use of de-icing salts

As this was the top floor of the car park de-icing salts were used during cold weather Corrosion due to chlorides is often very local and pitting in nature, this means that significant section loss can occur without evidence of significant rust staining. Inspection of the failed staircase did not reveal such staining.

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

In addition to design issues the standards of construction and maintenance are important and with car parks these are more critical than with buildings generally due to the exposed and often harsh environment. The client should have been aware of the need for inspection and maintenance and with a modern structure this should be highlighted in the CDM Health and Safety file. The wisdom of a design detail that relies on the performance of a sealant for structural integrity must be questioned.

However, the most critical issue is whether the danger of tying together two large structures was appreciated, and if the stair had been supported by only one of these structures there would probably not have been a collapse. Designers should take account of the durability requirements of car parks and there are three helpful publications.

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