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CROSS Safety Report

Contractor varies structural design intention

Report ID: 420 Published: 1 January 2015 Region: CROSS-UK

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Overview

A reporter was made aware by the building owner of an unauthorised structural alteration that had been made on site.

Key Learning Outcomes

For construction professionals:

  • Alterations or omissions should not be made without approval from the design engineer

  • It is good practice to carry out a risk assessment and method statement (RAMS) for all construction activities. This can ensure the sequencing of work activities are carefully considered and planned and any constraints identified.

For civil and structural design engineers:

  • It is important to ensure a design can be safely built. An outline construction methodology should be provided on the construction drawings. 

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This report concerns two steel beams used to support first floor masonry and the roof in a two storey house. The reporter says the fabricator provided splice and end plate details. The contractor installed temporary support and lifted two halves of one beam into position and inserted and tightened all bolts.

He then lifted the two halves of the other beam into position and was only able to insert half the total number of bolts required because he could not reach into the inner web area masked by the other beam. The contractor did not seek the reporter’s approval; the building inspector did not notice the lack of bolts; and the reporter only learnt of the situation from the building owner.

This was a near miss and in future the reporter's notes will include direction to contractors along the lines of....’the contractor shall plan and implement his temporary support and building methodology so as to ensure the permanent works are built as designed’. The incident highlights, says the reporter, how modern procurement without professional site supervision is potentially dangerous.

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This also raises the question as to what consideration the designer gave to construction difficulties arising from the design.  As has often been noted before with CROSS reports, and elsewhere, the designer should know that what he thought was being built was actually constructed.

This is however very difficult when the designer does not have a site role so contractors must be aware of the risks of making unauthorised changes. Here this was not so much a deliberate change as an omission made in order to construct the works. A designer should also ensure that his design can actually be built. Again, by making the changes the contractor may have attracted designer duties under CDM Regulations 2007.

This is however very difficult when the designer does not have a site role so contractors must be aware of the risks of making unauthorised changes

There have been many classic failures caused by unauthorised changes. For example, the Flixborough disaster in 1974 when there was a major explosion due primarily to a plant modification made without a full assessment of the potential consequences. Another was the Hyatt Regency Walkway collapse in 1981 when the method of suspending the walkway was changed without consideration of the design principles. In both cases there were multiple fatalities.

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