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CROSS Safety Report

Inadequate end bearings for transfer beams

Report ID: 644 Published: 1 October 2017 Region: CROSS-UK

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Overview

Whilst advising a steel fabricator on tendering for a transfer structure steel frame, a reporter's firm noticed a serious design error with the bearing pressures at the supports of a transfer beam.

Key Learning Outcomes

For civil and structural design engineers:

  • Connections can often be the weak link in structures and attention to detail is required to ensure what is designed can be fabricated

  • Careful consideration is required for connections, particularly at interfaces between different materials. The role of tolerances should not be overlooked.

  • An attribute of ‘safety’ is to assure that the design is not disproportionately vulnerable to minor error

Full Report

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Whilst advising a steel fabricator on tendering for a transfer structure steel frame, a reporter's firm noticed a serious design error. The structure comprises an irregular arrangement of large steel beams supported by steel columns and, at the building perimeter, bearing on to a reinforced concrete (RC) wall.

The beams support a slab and several stories of stud framed residential flats above. End reactions were shown on the drawings, with up to around 1,000kN design load specified for the bearing on the RC wall. The bearing detail was clearly shown and comprised beams resting on a recess cast in the RC wall. The specified end bearing was 100mm. A simple calculation showed the bearing pressure to exceed twice the permitted maximum value.

Can inadequate bearing design lead to failure?

Experience has shown bearing design to be a common cause of major structural failure, with inadequate provision for tolerance often cited as an issue. In this case were the works to be constructed as designed, tolerance issues could make an already bad situation much worse, with a reduction in bearing to 75mm being a distinct possibility even without unusually lax site control.

Experience has shown bearing design to be a common cause of major structural failure, with inadequate provision for tolerance often cited as an issue

Of particular concern is the fact that this defect was part of a structural design passed by a certification scheme; it thus illustrates the shortcomings in that process. As the 13th Biennial Review of SCOSS  (The Standing Committee on Structural Safety) pointed out, independent third party peer review is the gold standard in structural checking and the alternative of occasional sampling of in-house checking does not give the same degree of safety.

Expert Panel Comments

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As ever, safety is often dependent on attention to detailing and this report illustrates how the safety of a large structure can be jeopardised by inadequate attention to supports.

Five common themes emerge in such situations:

  1. It is frequently connections that are the weak link in any structure

  2. There are frequent problems at interfaces between different materials (here steel and concrete)

  3. There are frequent problems when the designer of a connection has been tasked with developing an ‘impossible connection’ because the structure has not been adequately conceived/designed ignoring practicalities of fabrication/connection

  4. The role of tolerances is often overlooked

  5. An attribute of ‘safety’ is to assure that the design is not disproportionately vulnerable to minor error

As ever it is essential that there is adequate quality control in place to ensure all design criteria are met.

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