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Temporary works propping

Report ID: cross146

Published: Newsletter No 17 - January 2010

Source: CROSS

Report Content

A self-employed structural engineer was working for a consulting firm on a design and build contract refurbishing a large office building which was anything from 50-80 years old. The structure consisted of hollow pot floors with concrete encased steel columns. During previous works a plant room had been added to the roof and no remedial work undertaken, so after a load assessment it was decided that a column needed strengthening over one storey. The work involved the installation of a stressed prop to provide temporary support to the structure.

 It was noticed by the reporter on site that the props installed were in two sections - this was to facilitate installing them through two floors. The junction between the upper and lower sections consisted of a 152UC mounted horizontally. Four short sections of prop were balanced on top of it and four long props supported it from beneath. The 152UC would effectively act as a hinge in the middle of the props as there was no effective bracing. The Reporter’s responsibility was for the permanent works and he had not designed the temporary works. Notwithstanding, he passed on his concerns, which, eventually resulted in changes being made. The Reporter was not sure what system might have prevented this happening. He was not on site to inspect the works so it was almost by accident that the issue was spotted.

Comments

The key learning point in this example is that the initial design resulted in the creation of ‘pin’ in the middle of an un-braced prop. Toooften the design emphasis is on limiting stress, without sufficient regard to stability. Temporary works are a vital, and often safety-critical, part of many new build and refurbishment projects. Appropriate design is essential. In such cases there should also be a Temporary Works Co-ordinator appointed in accordance with BS5975:2008. Indeed contracts for such works are meant to ensure that temporary works are designed, checked and installed correctly. It would be expected that this action (or similar control measures) would flow from discharge of the designer’s (both permanent and temporary works) CDM duties. There is a trend to exclude site visits by engineers although enough anecdotal evidence exists to suggest that an experienced eye cast over site works is of extreme value. Most severe failures are not due to overstressing but to instability. However as this report shows, nobody should be afraid to raise concerns and no one, on receipt of those concerns, should try to avoid the issues.

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