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CROSS Safety Report

Steel portal frame erection

Report ID: 20 Published: 1 March 2006 Region: CROSS-UK

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Overview

A reporter describes how a steel portal frame collapsed during its erection.

Key Learning Outcomes

For construction professionals:

  • Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design

  • Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works are carefully considered and planned

  • Verification of temporary works erection by a competent person who can oversee and coordinate the whole process can also ensure the works are installed correctly

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The project described by the reporter involved the construction of a large span steel framed portal frame building. Longitudinal stability was provided by triangulated vertical side bracings and triangulated bracings in the plane of the rafters. The erection method statement had been based upon the principle that the first bay to be erected would be the bay containing the side and rafter bracings.

There would be temporary wire rope guys until the permanent bracings had been installed and fully bolted. A delay occurred in the delivery of the permanent bracings to the site but nevertheless it was decided to press ahead with construction and use the temporary wire rope guys to achieve longitudinal stability.

After erection of several bays of the structure had been completed a large quantity of purlins were lifted onto the roof. The eccentric load was sufficient to overcome the resistance to longitudinal de-stabilising forces provided by inadequately anchored guy ropes. The structure collapsed.

The eccentric load was sufficient to overcome the resistance to longitudinal de-stabilising forces provided by inadequately anchored guy ropes. The structure collapsed.

This happened slowly enough for those personnel involved in construction to move to safety. It was a near miss. This clearly illustrates the need to adhere rigidly to method statements and which should only be changed with the agreement of the originator or at least an accepted figure of authority who has sufficient technical knowledge to make decisions.

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Another example of the importance of temporary stability. Stiffness not just strength is needed for stability and guy ropes, even if strong, might well be too flexible to do the job properly. It is also the case that a vertical load applied eccentrically will cause lateral sway.

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