CROSS Safety Report
Contractor notches steel transfer beam
A transfer beam with 50mm thick flanges supporting a four-storey building was notched, without approval from the design engineer, to accommodate suspended ceiling fixings.
Key Learning Outcomes
For clients appointing contractors:
Competent contractors should be appointed to undertake and deliver the project
For designers working with engineers:
Check with the structural engineer for any special requirements associated with transfer structures and ensure careful consideration is given to tolerances for service zones
For civil and structural design engineers:
The design of transfer structures has stricter criteria than for normal beams
Be aware that deflection can be a major constraint for the design of transfer beams and guidance is available
Proper coordination and detailing can ensure that tolerances that may be required for secondary structures and fixings are provided
For the construction team:
Never make unauthorised changes - always seek approval from the design engineer
Quality control and competent supervision of subcontractors on site can help prevent unauthorised alterations from occurring
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Due to planning restrictions on height, says a reporter, an engineer designed a shallow depth steel transfer structure to support a four storey building above. The main beams had flanges of 50-60mm thick. When the building was structurally complete, there was obviously a certain amount of deflection in the main beams.
A follow on contractor erecting aluminium T-section ceiling grids, found that the legs of the T-sections fouled the beam’s bottom flange. To overcome this, the contractor simply cut notches in the flange. Fortunately, this was spotted and repaired.
Expert Panel Comments
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This is clearly a matter of competence of the contractor but raises several questions:
Was the contractor selected on the basis of their experience or was it simply a matter of cheapest cost?
Was the designer of the structure aware of the ceiling grids, and did they allow for a deflection zone?
What were the quality assurance processes on site?
What was the level of supervision of the contractor?
A simple report of a simple error, but one which could have had serious consequences.
Is education the key to mitigating basic errors?
The operatives undertaking this work may have been trying to make things easier for themselves to get the job done with a lack of appreciation for the consequences of their actions. Had there been any engineering presence on site, or competent supervision, this could have been prevented. The industry needs to step up by educating construction teams not to make such basic errors which could have catastrophic consequences.
Had there been any engineering presence on site, or competent supervision, this could have been prevented
CROSS has seen similar examples which would also have potentially very serious consequences. Less competent contractors seem to think that designers use excessive factors of safety and therefore a notch here and there is not going to be a problem.
The need for proper coordination and detailing
The basic safety violation is clearly that of making unauthorised changes. In this case that was especially dangerous since alterations were made to a member that was obviously a major structural item.
But another overall comment can be made; design is not just sizing members. Design includes the activity of co-ordinating a structural system so that all components fit together properly, negating the need for alterations. Necessarily, that process involves a proper appreciation of any movements and tolerances that might affect alignment.
Deflection of transfer beams
A wider issue is the deflection in transfer beams. The use of engineering judgement is signalled by BS 5950, EC3 and numerous other reference documents regarding deflection control.
Some would argue for transfer structures, a limit of between L/500 to L/1000 for total deflection, depending on the span or the nature of the loads and structure supported; or an absolute limit of 15mm rather than a proportion of the span. These figures aim to be comparable with the stiffness of foundations that may typically settle a maximum of 12-15mm in low rise, 3-4 storey buildings.
There is guidance available on the design of transfer structures including the paper Structural Design of Transfer Structures 1. The extract below, is worth reading in this context:
‘In general, building codes refer span/250 as an appropriate limit value for the vertical deflections of beams and slabs, for the quasi-permanent loads. Furthermore, span/500 is normally an adequate limit for deflection after construction, meaning the deflection which occurs after the addition of partition and finishes’.
Currently, there is not much guidance as to the deflection limits for transfer structures, although it is commonly accepted that their design should follow more severe criteria than normal beam or slab elements. This can be achieved either by imposing stricter limits or by designing for a more severe load combination, such as frequent loads or characteristic loads. Despite this, the serviceability criteria for global transfer structures must be specified for each project and agreed with the client.
The reporter does not mention how the remedial work was done. Repairs to a notch in a thick steel flange in tension requires care and consideration.
1. Structural Design of Transfer Structures, Ribeiro. G. Technical University of Lisbon (2018)
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