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662 Tower crane foundation design error

Report ID: 662

Published: Newsletter No 51 - July 2018

Report Overview

The author was asked by a Principal Contractor to undertake an independent design check for two tower crane foundations for a project.

Report Content

The author was asked by a Principal Contractor to undertake an independent design check for two tower crane foundations for a project. Both cranes were to be founded off permanent pile caps that had been deepened to accommodate the mast fixing assemblies. These comprised of reinforced concrete caps each supported by four 600mm diameter symmetrically placed bored piles.

Naturally, it was the Contractor's intention to cast these caps ahead of the surrounding foundations so that the cranes could be erected early in the build programme. Consequently, the worst-case condition in terms of pile tension load was when the cranes were first installed. It quickly became apparent that the pile tension loads given on the structural engineer's pile load schedule fell well short of the potential tension created for both in-service and out-of-service load conditions. In the case of the latter, the tension generated using the approach recommended in CIRIA C654 was around 900kN (unfactored) compared to a value of around 300kN (unfactored) stated on the pile schedule.

Having alerted the client about this error, the author was then horrified to learn that the first crane had already been erected and had been working for a week! On reviewing the pile schedule issued by the engineer, it was apparent that the dead load for the piles had been combined with the crane load values. However, the dead load figure should have been ignored as this represented the full building's weight once completed.

This is the third occasion within the past 5 years that the reporter has experienced a similar tension shortfall on piled tower crane foundations. The reporter believes the root causes is that there is no overall design responsibility for the crane foundations.

Comments

This highlights the importance of a designer recognising that temporary load cases can be the determining factor for satisfactory structural safety. Did the Principal Designer only have responsibility for the permanent design with the temporary works design being carried out by another? Responsibility may have been divided without anyone having an overview of the processes. Or the designer(s) may have been inadequately experienced or trained.

Information across interfaces is a generic problem and hence the need for any designer to be absolutely clear as to what any given design information, such as pile loads, refers to. Did the incorrect pile loads affect the design of the pile cap too?

The problem was discovered during an independent check, which is good, although it may be that the check was not commissioned early enough in the process to identify the issue before construction started. A source of good advice on such matters is the Temporary Works Forum. The Temporary Works Coordinator (TWC) could also have had a role here in directing the design and sequencing of the temporary works - see the temporary works section on the HSE website.

 

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View other CROSS reports published in Newsletter No. 51


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