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CROSS Safety Report

Cantilever canopy failure under snow loading

Report ID: 197 Published: 1 July 2010 Region: CROSS-UK

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Overview

A reporter shares their experience after they inspected a cantilever canopy structure that collapsed following a recent snowfall.

Key Learning Outcomes

For civil and structural design engineers:

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

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

  • Careful consideration should be given to canopies that abut adjacent structures, and there is a potential for snow drift to occur

For building owners/managers:

  • The specification of any inspection and maintenance requirements for fixings should be considered and recorded in the operation and maintenance manual

  • Fixings may require a combination of regular visual inspections with full inspections at appropriate intervals. They should be inspected by a person who is competent to do so.

Full Report

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A reporter inspected a cantilevered canopy structure that collapsed following recent snowfall (Figure 1). The canopy consisted of a cantilever arrangement that projected from the flank of a single storey cavity masonry school building.

Image
Figure 1: collapsed cantilevered canopy

Canopy support and fixings

The canopy projected over an area approximately 3.8m wide by approximately 15m long and was formed as a covering of GRP supported at intervals along its length by GRP spandrel beam elements bolted to steel brackets that were in turn fastened into the external brickwork skin using mechanical anchors.

The steel brackets were provided to allow stand-off of the GRP spandrel beams from the face of the building to accommodate the gutter. The canopy was seen to have failed as a consequence of a hinge or crease forming in these stand-off brackets along the line of the top most row of anchors. 

Inadequate steel support

The formation of this hinge or crease allowed the canopy to rotate toward the ground until it came to rest on its unsupported leading edge. The immediate impression, irrespective of failure, was that the steel section selected for use in these brackets would not be adequate when considered in relation to normal Codes of Practice for structural design. 

The 50mm x 50mm x 2mm thick square hollow sections adopted for use in these stand-off brackets is, according to the reporter, only capable of supporting a very modest load in addition to the self weight of the canopy itself.

Damage to masonry wall

Failure, or possibly just the presence of the canopy structure, has also resulted in damage to the masonry that supported it. This damage includes spalling of walling units within the external brickwork where mechanical anchors have been pulled out (anchor failure is the exception as for the most part the fastenings have remained intact).

The development of horizontal cracks within the internal blockwork where prying forces exerted by the canopy support brackets appear to have been transferred through the wall ties. This canopy is believed to have failed after receiving a snow load that represented only a small proportion of the load that it should have been designed to sustain.

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