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915 Crane outrigger loads underestimated due to misuse of software

Report ID: 915

Published: Newsletter 58 - April 2020

Report Overview

The crane boom was not in the worst-case position when crane outrigger loads were calculated for a particular lift.

Report Content

A reporter would like to raise awareness of an issue they have experienced with incorrect outrigger loads for mobile cranes. They work as an in-house temporary works design engineer for a main contractor and regularly deal with designing the foundations for crane outriggers. This requires knowing the accurate loads in the outriggers to check the ground and any surrounding structures or slopes.

For a particular crane lift, computer calculations had been provided to give the outrigger loads but these were not for the crane boom in the worst-case position and had to be corrected. With large mobile cranes with different rig configurations, it may not be possible to adequately check the outrigger loads without in-depth working knowledge of the crane.

This is not an isolated case, continues the reporter, and due to incorrect use of software, it seems to be becoming more common. The consequence of being supplied with the incorrect outrigger loads can be severe. Lifts regularly take place close to retaining walls, tunnels and underground services. The reporter feels that this issue needs addressing before it becomes a contributing factor in a serious incident.



There are some similarities between this report and report 904. In both cases, there are significant structural engineering issues in design areas that will be unfamiliar to many engineers (no matter how much general experience they have). Not everyone will be familiar enough with the operation of mobile cranes to be able to identify configurations that give worst case outrigger loads.

A lesson is to beware when taking on anything novel and indeed this is integral with the ethics codes of professional institutions and the Royal Academy of Engineering’s Statement of Ethical Principles, one of which is ”perform services only in areas of current competence”.

The risk associated with a crane collapse may be to adjacent infrastructure with disproportionate consequences, such as equipment falling onto a railway track in the path of a train. In the UK, this was examined in the 2011 HSE report Preventing catastrophic events in construction. In December 2018, the Crane Interest Group (CIG) published CPA 1801 Good Practice Guide - Requirements for Mobile Cranes Alongside Railways Controlled by Network Rail.

It is sensible for all crane bases to be checked and in the first instance basic hand calculations and rule-of thumb methods will give an indication of foundation loads, including outrigger loads. As always, key operations on site need to be under the control of qualified competent staff who work to procedures provided by crane suppliers. Correct procedures for erection and dismantling must be followed.


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