Tornadoes & building failures contributed: Vaughan damage report

The damage investigation following the August 21, 2009 severe storm that damaged areas of Vaughan, Maple Ridge and Woodbridge, Ont., shows that two separate tornadoes were responsible for damages. And that failure to adhere to building codes contributed to the damage.

The two areas of touch down—Martin Grove Rd. and Highway 7 and Jane Street between Teston Road and Major Mackenzie Drive—shows damage consistent with F2 tornadoes, write the authors of a report, Vaughan Ontario Damage Investigation, released by the Institute of Catastrophic Loss Reduction (ICLR) on September 10, 2009.

The structural damage to houses in Woodbridge was extensive, state the damage report investigators (and report authors) Gregory Kopp, Eri Lizumi, Craig Miller and Murray Morrison. All four report authors are from the Faculty of Engineering at the the University of Western Ontario, in London, Ont.

The investigators found that nearly all houses in this area were built with unreinforced masonry walls and pitched roof construction, prompting significant damage to roof shingles and sheathing. In some cases houses lost more than 50% of their roof shingles.

Downwind houses suffered damage from flying debris. For example, one residence had 2×6’s embedded in their roof.

Jane StreetDamage Site
Similar to the Woodbridge site, there was significant damage to roofs and failure of sheathing and shingles. Considering the fabrication of houses in this area was newer, roofs were built with trusses, rather than pitched roof construction, yet the failures (of structure and material) were similar to the Woodbridge site.

Building Failures Contributed
The report authors noted that “although the houses at each site were built differently failures observed at each site, were similar, involving the connections of the roof sheathing/shingles and connections of the roof structure to the walls.”

For example, the investigators found that a significant number of panels, from badly damaged houses in Woodbridge, had a large number of nails that had missed the roof trusses, reducing its hold down capacity. The findings were similar say the investigators to the findings in Florida in 1992, when examination after Hurricane Andrew found that many panels were not secured with nails completely missing roof trusses.

Another problem the investigators found was the lack of hold-downs used in the homes in both damage areas.

“While there were differences in construction at the two damage sties, the structural failures at both locations were mainly due to connections between the roof and the walls.”

For example, in Woodbridge, the houses were constructed with roof rafters spaced approximately one foot apart—which is closer than the typical two foot spacing commonly used for roof trusses. The investigators noted that this closer spacing would provide additional connection points to connect the roof to the wall and would make the roof much stronger. “However, in the case of the [house], few, if any hold downs, were observed attaching the roof to the masonry walls, as a result it is likely that the majority of the uplift resistance of this roof came from the roofs weight.”

In same cases portions of roof trusses were held down with only a single 8-d toenail—The National Building Code of Canada specifies that roof rafters/trusses be held down with three 8-d toenails.

Originally published in Canadian Insurance Business Magazine September 2009

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