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CROSS Safety Report

Collapse of tank falsework

Report ID: 150 Published: 1 October 2009 Region: CROSS-UK

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Overview

This report relates to the collapse of falsework erected to construct an in situ, reinforced concrete, circular water retaining structure.

Key Learning Outcomes

For the construction team:

  • 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

Full Report

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The issue relates to the collapse of falsework erected to construct an in-situ, reinforced concrete, circular water retaining structure. It was to be about 20m diameter by 15m high, with a cupola roof – a typical digester tank on a sewage treatment works. The company that employed the reporter was the design and build contractor for a number of tanks on the site.

Proprietary formwork/falsework

The formwork/falsework system was a proprietary system, not commonly available in the UK. It comprised an outer formwork skin designed to go into ring tension, and an inner formwork skin designed to go into ring compression. Because the latter would be subject to buckling action, it was supported by backing falsework, which consisted of a circular truss made up from adjustable sections.
Sections were fixed together by dowelling pins. There was thus a high shear force acting on the pins when the falsework was subjected to wet concrete pressures. The tank wall was on the second lift, so at about +6m above ground level. A working platform was fixed at top of shutter level, incorporating a concrete hopper travelling on rails to deposit wet concrete into the formwork – hence a further local vertical load.

Falsework failure

The falsework failed during concrete placement and a partial collapse of the system occurred. Fortunately, nobody was seriously injured. Collapse appeared to have occurred because of shear failure of the pins, which exhibited crystalline fracture surfaces indicating brittle failure. The reporter had several sent away to a testing lab, which reported yield strengths in the range of about 250 N/mm2. In other words, mild steel.

Collapse appeared to have occurred because of shear failure of the pins, which exhibited crystalline fracture surfaces indicating brittle failure.

As part of the investigation, the reporter reviewed the only available design calculations for the temporary works. These had been translated from the original documents by a non-technical person, and the following points were evident:

  • The calculations were for a different diameter tank and less onerous loads
  • The limits for the calculations had been exceeded
  • The pin in the model calculations was about 7mm diameter, with a UTS of about 700 N/mm2. whereas the pin that failed was about 6mm diameter, with a yield strength of about 250 N/mm2.

The technical reason for failure was under-strength pins which failed in shear, in a temporary structure with little or no redundancy. Beneath the obvious technical shortcomings, there were significant managerial and cultural issues in this case. It was these underlying problems that had ultimately permitted the gross technical errors to occur.

Expert Panel Comments

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There was obviously a lack of supervision and of the appreciation of the importance of temporary works. The Final Report of the Advisory Committee on Falsework, HMSO, June 1975, often referred to as the Bragg Report drew attention to the fundamental importance of the role and functions of the Temporary Works co-ordinator. There is always a danger in using standard designs, particularly those which may have been translated from another language and which may have used different Codes.

There is always a danger in using standard designs, particularly those which may have been translated from another language and which may have used different Codes.

Information about the responsibilities associated with temporary works can be found in the Institution of Civil Engineers Conditions of Contract. BS 5975:2008 Code of practice for temporary works procedures and the permissible stress design of falsework requires a careful approach to falsework with design and review undertaken by competent persons and the appointment of a Temporary Works Co-ordinator to oversee the process.

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