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803 High winds cause parapet failure

Report ID: 803

Published: Newsletter 54 - April 2019

Report Overview

Exceptionally strong winds caused a fourth-floor level masonry parapet wall with a 1.7m high timber fencing on top to collapse.

Report Content

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 3-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.

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 affixed screening had been pushed over. The masonry had failed on the bed joint across almost the whole of its length. 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 and, 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.

Comments

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. Although this report is about a wind loading failure, it has parallels with an earlier CROSS report on balconies, where 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 in report 789 in this Newsletter.

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 and had there been casualties or fatalities, the responsibility would have come back to the owner of the premises.

 

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View other CROSS reports published in Newsletter 54

Images

Failure of masonry wall with fencing above



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