Master of Engineering Science
Civil and Environmental Engineering
Kopp, Gregory A.
El Ansary, Ayman
Light-frame wood construction comprises nearly 90% of the housing industry in
Canada and the United States. The roofs of these houses can be constructed either
entirely on site or using prefabricated trusses. Assembling the roof structure on site,
otherwise known as stick-framing, is a framing technique with current code guidelines
that are based on past practice and limited consideration of wind loads. This makes
these roof structures susceptible to failure in high-speed wind events, such as tornadoes. Around 90% of all tornadoes in Canada fall in or below the EF-2 category,
making it a possible target for design of wood-frame houses to be resistant to these
events. This research proposes improved stick-framing guidelines that would work for
EF-2 tornadoes. Using non-linear finite element analysis, a stick-framed roof was designed following the guidelines in the National Building Code of Canada. Non-linear
links were used to model all of the connections between the members in the roof structure, with frame elements used to represent the members. Increasing wind loads were applied to the structure and the first elements of the roof that failed were improved using an iterative performance-based design approach until the performance target of resistance to EF-2 tornadoes was achieved. The failure of the roof-to-wall-connections and the lack of members used in the framing were the two main issues highlighted and addressed. Damage survey photos were used to compare failures observed in the model with failures after real tornado events, which demonstrate many similar failure modes. This research recommends the requirements to ensure stick-framed roofs can withstand EF-2 tornadoes. Most notable is an improved gable end frame, which gives the structure more roof-to-wall connections, as well as a more structurally sound frame where wind loads are the highest. Other additions include struts, hurricane ties at all roof-to-wall connection locations and increased number of nails in various connections throughout the repeating inner frames. Minimum member sizes and strengths for each type of member used in the roof structure are recommended.
Summary for Lay Audience
Wood-frame houses are very common in North America. They can be constructed
either using wood framing members and nails on site, or assembled off-site using truss plates. The on-site construction method is called stick-framing, and the current National Building Code of Canada guidelines for stick-frame construction are not strong enough to be resilient against high wind speeds. Making all types of wood-frame constructed houses resilient against EF-2 tornadoes, a category in which 90% or more of tornadoes in Canada fall under or below, would ensure that fewer people have their house destroyed in these high-wind events. The losses that come from roof failure are severe, as anything of value in the house is exposed to the elements. Many residents also become displaced after their house structures fail, and are uprooted from their lives. To mitigate all these issues, this research aims to suggest new stick-framing guidelines in order for stick-framing to be resilient against EF-2 tornado wind loads. This is done by modelling a gable roof structure in SAP2000, a structural analysis program, following the current stick-framing guidelines available in the National Building Code. Then EF-2 tornado level wind loads are applied to the model, and the locations of failure are analyzed and improved upon. Both connections and members may be susceptible to failure in the structure and their capacities will be checked against the load demand on them. The research suggests new stick-framing guidelines to be implemented, including additional members to be used and various connections to be improved.
Bain, Caspar, "Design of Stick-Framed Wood Roofs Under Extreme Wind Loads" (2022). Electronic Thesis and Dissertation Repository. 8929.
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