CROSS Safety Report
High winds cause masonry parapet failure
This report is over 2 years old
Overview
A reporter highlights how exceptionally strong winds caused a fourth-floor level masonry parapet wall with a 1.7m high timber fencing on top of it to collapse.
The parapet overlooked a three-storey light-well that was situated within the confines of the building
At the base of the lightwell was a rooflight into an occupied space. Had the parapet slipped and fallen into the lightwell there could have substantial damage and possibly fatalities.
Key Learning Outcomes
For civil and structural design engineers:
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Adding to the height of cantilever walls should be carried out with care. If the additional load added is a UDL (like wind), then the bending moment at the base is proportional to the square of the height
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Be aware that attaching hoardings to walls can increase the windage on them and the walls may not be designed to take the load increase
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A design check should be carried out on an existing wall or parapet that have attachments added to them
Full Report
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A building has a roof top plant area above the fourth-floor level. The area is screened by timber fencing of approximately 1.7m above the top of the masonry parapet wall, which is in turn fixed to vertical posts that are secured to the wall. There was no evidence of raking bracing to the screen or support posts.
The parapet is approximately 0.9m high (excluding copings) and overlooks a three-storey light-well that is situated within the confines of the building and is south easterly facing. The parapet is approximately 225mm thick masonry in English bond. It is possibly lime mortar bedded and topped with rolled-top, terracotta, saddle copings.
Strong winds cause parapet to collapse
In 2018, there were exceptionally strong south-easterly winds with gusts of approximately 50+mph (80 km/hr). Under these conditions the wall collapsed, and inspection revealed that the parapet and attached screening had been pushed over. The masonry had failed on the bed joint across almost the whole of its length as shown in Figure 1. The left side of the parapet was a free edge with the right side bonded to a chimney.
It is assumed that the action of the strong winds on the screening allowed sufficient pressure to cause the screen and fence to be overloaded. As the wall has little tensile strength, the failure resulted along the bed joint closest to the roof level. At the base of the lightwell was a rooflight into an occupied space. Had the parapet slipped and fallen into the lightwell there could have substantial damage and possibly fatalities.
At the base of the lightwell was a rooflight into an occupied space. Had the parapet slipped and fallen into the lightwell there could have substantial damage and possibly fatalities.
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Feedback
Further to report 803 and the reports on freestanding walls there was a site I visited where a wire mesh fence had been affixed to the top of a masonry boundary (freestanding) wall to provide additional security. Subsequently ivy had grown through the mesh and then the wall suffered a partial collapse in high winds. The wall itself was relatively old and probably not designed for its own wind load. It is very unlikely the additional wind load and cantilever height loading the masonry was considered when the mesh was installed, let alone any potential wind load from climbing plants. In my view it is likely that no structural thought was given to the installation of the fence. Unlikely also that anyone suggested a maintenance regime to minimise loading. The general public, and unfortunately a large proportion of the wider industry have an unwarranted faith in the capacity of 'brick walls'.
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There is a similarity here to the collapse of a wall in Melbourne Australia in 2013. A 3.2m high advertising hoarding was up to 70cm taller than the ground level free-standing brick wall to which it was added. The wall blew down and three people were killed. There have been other cases where attaching hoardings have increased the windage on walls, which are vulnerable anyway, and have precipitated collapse.
Cantilever design issues
Although this report is about a wind loading failure, it has parallels with CROSS report 620 on balconies. In report 620 the reporter was concerned that the balcony designer had anchored their cantilever to some inner structure without verifying that it was adequate for the task. Every structure must be looked at as whole. Danger lurks at interfaces between units or at interfaces between divided responsibilities as highlighted in report 789 .
Adding to the height of cantilever walls must always be taken seriously as, if the load is a UDL (like wind), the bending moment at the base is proportional to the square of the height. It is not known if there was a designer in this instance but there should have been. If there had been casualties or fatalities, the responsibility would have come back to the owner of the premises.