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CROSS Safety Report

Failure of waling beam connected to raking prop

Report ID: 813 Published: 1 July 2019 Region: CROSS-UK

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Overview

A reporter discusses how they witnessed the failure of a temporary support for an excavation on site due to similar design issues discussed in CROSS report 298 (Props to large excavations).

Key Learning Outcomes

For construction professionals:

  • If the design cannot be built due to a sequencing issue, this should be raised with the temporary works designer or designer to allow for a revised solution

  • Consider appointing a competent Temporary Works Coordinator (TWC) on site who could recognise and coordinate any sequencing issues with the design team

For designers:

  • For complex designs where the sequencing of the works is critical, consider providing a construction sequencing statement on the drawings to highlight this to the contractor

  • Consider inspecting safety critical connection designs on site, such as for excavation supports, to ensure they have been fabricated and constructed in accordance with the design details

Full Report

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A reporter writes in response to report 298 (Props to large excavations) as they have witnessed a failure on site as a result of the design issues discussed in this report.

Figure 1 shows a detail where the design required a cantilever beam to resist the vertical reaction from a raking prop where it connected to the waling beam. The cantilever beam was welded to the sheet pile behind and the design required that a large fillet weld be applied around the entire beam.

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Figure 1: failure of waling beam connected to raked prop

However, the contractor installed the cantilever beam after the waling beam had been fixed and was therefore unable to fully weld around the bottom flange.

However, the contractor installed the cantilever beam after the waling beam had been fixed and was therefore unable to fully weld around the bottom flange

The partial weld to the bottom flange failed, allowing the beam to rotate upwards. The weld to the web of the beam seemed to remain intact, with the rotation causing local deformation of the sheet pile at the web.

The reporter adds that additional brackets as shown in Figure 2 were welded on as an emergency remedial measure.

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Figure 2: view from above waling beam that has not resisted moment (with emergency retrofitted supports highlighted in green)

Expert Panel Comments

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One feature that this failure highlights is that design is not just a matter of calculation. If it was impractical for the contractor to install the cantilever beams prior to the waling beam, then the temporary works designer could have realised that with the waling beam in place first, it would be difficult or impossible to weld or otherwise fix the bottom flange of the cantilever beam.

Ensuring buildability

Equally, if the design cannot be built, there should be an onus on the contractor to go back the temporary works coordinator to flag the issue and get a revised solution from the temporary works designer. A discussion on construction sequencing between the temporary works designer and the contractor should have resolved this issue.

From the description, the cantilever beam failed upwards, implying tension on the bottom flange. This is the exact location where the weld was omitted. Self-evidently, the resistance of the profile weld must have been significantly less than the design intent.

This incident shows a repeated theme from numerous CROSS reports that what was actually built did not match the design intent. As with CROSS report 844 (Defects in tapered thread reinforcement bars for coupling), site inspections are highly desirable.

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