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CROSS Safety Report

Falsework support to a bridge – a near miss

Report ID: 333 Published: 1 October 2012 Region: CROSS-UK

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Overview

This report follows the near collapse of a birdcage scaffold falsework structure during an 800 cubic metre concrete pour on a motorway bridge.

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

  • 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

  • Structural alterations should not be made without approval from the design engineer

  • Regular toolbox talks are a good way of engaging with work crews and highlighting any risks associated with work activities

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This report follows the near collapse of a birdcage scaffold falsework structure during an 800 cubic metre concrete pour on a motorway bridge. Site based temporary works coordinators were appointed by the contractor’s joint venture (JV) to oversee specified portions of the project and there was a design JV for temporary works.

Temporary works manager

A site based temporary works manager allocated both design and design checking resources. Temporary works drawings were prepared showing a birdcage scaffold structure with proprietary connectors. Adjustable diagonal braces were used throughout and in the transverse direction there was a diagonal brace shown at each lift of each birdcage. In the longitudinal direction diagonal braces were shown at each lift.

The Structural Concrete Checklist was signed off by members of the construction JV and the Design JV. A temporary works permit to load was issued, noting the specialist’s drawings and referring to email correspondence which included a marked up copy of one of the specialist’s drawings. Shortly after the pour there was evidence of buckled standards in the falsework (Figure 1).

Image
Figure 1: sketch showing buckled standards in the falsework

Deviations to scaffold construction

An inspection revealed numerous deviations from the scaffold arrangement shown on the certified design drawings, primarily that transverse diagonal bracing was fitted to every 4th bay and not to every bay. Collapse was prevented by bowing distortion of the standards caused the proprietary ledgers to become wedged and thus provided some moment capacity at each joint between the standards and the ledgers.

Furthermore, the decking was locked between reinforced concrete (RC) walls, thus preventing sway at the top of the falsework. It is likely that further distortion would have resulted in local collapse, as the diaphragm effect of the decking would be unlikely to extend to seven bays. Local collapse of the structure could have precipitated a more comprehensive progressive collapse.

How a lack of competence can compromise safety

This near miss, continues the reporter, exposed several lessons to be learnt by those involved and by the construction industry as a whole. They say that the industry relies on competent people to undertake operations where safety is a consideration. A hazard occurs in situations where individuals or groups either consider themselves, or are considered by others, as competent in specific areas of knowledge but are actually unaware of their lack of competence.

In this case a copy of the specialist’s drawing, marked up by site in an uncontrolled manner and apparently showing bracing every fourth bay rather than on every bay, was not distributed to all relevant parties. Good practice, says the reporter, would be to arrange an inspection of any falsework of significance by the designer, to confirm that it is in accordance with the design. It is noteworthy that the specialist supplier was not invited by site to inspect the structure pre-pour. The incident could, continues the reporter, have resulted in fatalities, extremely high costs and damage to the reputation of all companies of the JV. The following recommendations are made by the reporter to minimizing the risk of recurrence.
•    Vetting of sub-contractors
•    Adequacy of Management Procedures
•    Training on Management Procedures
•    Training and recognition of Temporary Works Co-ordinator role

Expert Panel Comments

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Lack of appreciation of basic stability is a vital issue and the competency of the individuals making the decisions on site is part of the problem. It can and does lead to fatalities. Routing the inspection back to the designer of the temporary works is important as it emphasises that these are designed systems; the cost of an inspection is trivial. The general management issues recommended above could be extended to training of the people installing the equipment – tool box talks on the consequences of omitting parts of the design.

The person signing the installation checks should be carrying out regular checks whilst an important structure like this is installed. It is very difficult to check on completion and much more time consuming and expensive to correct.  A ‘scaff tag’ type system should be used to ensure the installer is certifying their work for use. Record photos and even video clips can be used to help check the installation against the drawings. Whilst experienced old hands on site may know as much or more than the designer this cannot be relied upon. In all circumstances BS5975 should be followed. (BS 5975:2008+A1:2011Code of practice for temporary works procedures and the permissible stress design of falsework).

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