Design of Stick-Framed Wood Roofs Under Extreme Wind Loads
Abstract
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.