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CROSS Safety Report

School roof timber truss collapse

Report ID: 249 Published: 1 January 2012 Region: CROSS-UK

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Overview

A gym/assembly hall roof to a school collapsed suddenly and without warning overnight when fortunately the building was unoccupied.

Key Learning Outcomes

For 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 the potential for snow drift to occur

Full Report

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A gym/assembly hall roof to a school collapsed suddenly and without warning overnight when fortunately the building was unoccupied (Figure 1). The building dates from the early 1960s and, in the view of the reporter, the collapse was due to a failure of the mid-span splice connections in the top and bottom chords of the bolted timber trusses which span the length of the hall (approximately 12m).

Image
Figure 1: School roof truss collapse

Snow drift loading

The roof is flat over two thirds of its length but slopes upwards at 45 degrees at one end. A subsequent analysis of the trusses confirmed that the design complies with the codes of practice around at the time of construction, namely CP 112 (1952) and CP 3 (1952). However, CP 3 did not require the designer to consider the effects of snow drift loading. When snow drift loading is considered in the analysis, the mid-span splice connections to the top and bottom chord members are overstressed and this appears to be the reason for the failure.

There was no snow on the roof at the time of the collapse. However, during the previous winter, the region where the building was located had experienced what is considered to be the heaviest and most prolonged period of snow fall for many decades. It is the reporter’s opinion, that this caused damage to the timber, causing it to split at the mid-span splice connections.

Are roofs of a similar period at risk?

High winds during the winter may have exacerbated the problem. There is significant damage to both top and bottom chord splice connections. The reporter believes that the bottom chord splice failed first. The damage to the top chord appears more dispersed and 'collateral' in nature. The reporter’s concern is that any roofs of a similar period supported by bolted timber trusses and designed using the imposed load criteria of CP 3 could be vulnerable if the shape of the roof is such that significant snow drift loading can occur.

Expert Panel Comments

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It would be interesting to know if there were any indications of imminent failure. What was it on the night that precipitated the collapse? If a structure is so badly overstressed as to be on the point of failure, there will usually be warning signs.

Such roof structures are normally very robust and there may have been an underlying problem - perhaps with the joint detail. Were these designs based upon bolted joints or were timber connectors (toothplates/shear plates) used in the joints? One of the CROSS Expert Panel members some years ago came across rare cases where shear plate connectors had been used with timber members and steel splice/jointing plates. Some of the CP 112 design loads for shear plates were, they believe, stated in that code slightly on the high side. If shear plates were used in joints with load along the grain direction, and the end distance from connector to timber was the minimum allowed, and the timbers had maximum permitted grade defects (knot size) over the jointing area, then failure could occur even if design was fully to the code of practice.

The drift loading issue may not be the main issue here as many similar structures were designed to CP 3 and there do not appear to have been many failures from the period. Another point is that in this period, diagonal bracing use was not commonly used to these types of roof structure but the timber widths were thicker than modern trussed rafters and the overall construction was probably more robust.

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