Investigation of Damaged Structures Exposed to the Tornadoes in National Capital Region of Canada

Abstract

On September 21st, 2018, six tornados classified as EF-3, EF-2, and EF-1 touched down in the national capital region of Canada (eastern Ontario and western Quebec), injured about a dozen people, damaged or destroyed numerous buildings, overturned vehicles, and snapped many trees and hydro poles. The authors were involved in investigation and damage assessment of around 50 structures, mostly wood-frame residential buildings, that were subjected to EF-3 or EF-2 tornados in Ottawa and its rural neighbourhoods. The tornado-related damage to the investigated buildings varied from total destruction of garage buildings to cracking of interior finishes. It was observed that an EF-3 tornado directly causes structural damage to a residential building, but that was not the case for the buildings subjected to EF-2 tornado unless a structural deficiency pre-existed. The most common indirect structural and non-structural damage was found to be from tree and flying debris impacts, respectively. It was also observed that the garage buildings that were attached to the main buildings were the most vulnerable part of the structures when exposed to EF-3 tornado, and their collapse started with failure of overhead doors. The other forms of damage directly associated with tornados were displacement of gable end walls/trusses and hinge failure of their studs as well as loosening of the truss connector plates used to connects members of roof trusses. To avoid such failure, a continuous load path from the roof frame to the foundation walls should be achieved. This requires improved design and constructions method, especially at the connections, and new requirements in future versions of part 9 of the National Building Code of Canada (NBC) and Ontario Building Code.