This chapter presents the design wind loads from the National Building Code (NBCC, 2010) and compares these to house performance. Potential changes which could be applied in order to prevent these failures from occurring are also discussed.
10.1 NBCC 2010 Design Wind Pressure
Design of houses is based on Part 9 of NBCC, which is a prescriptive standard. The NBCC design wind pressure was calculated for this type of structure using Equation 10.1, which corresponds to the wind load calculation procedure presented.
To perform this calculation, the following assumptions are made: where Iw is the importance
factor, q is the reference pressure, Ce is the exposure coefficient and Cp and Cg are the pressure
and gust coefficients respectively, found in the building code commentaries
- Normal importance (Iw=1.0)
- Roof in place, because this is consistent with the design of the structure. - Suburban terrain (Ce)
- Structure located in London, where the reference pressure is 0.79kPa
- The structure is category 1 for internal pressures, with few openings (windows sealed). - The pressure (p) is multiplied by the ultimate limit states factor of 1.4.
The design pressures for the different walls are shown in Table 10.1. It is important to note that each wall requires designing for the worst positive and negative pressures. Because the structure is small, the edge zones of the walls encompass the entire wall, so the edge zone values apply across the full wall width.
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Table 10.1 - Factored design pressures
Factored Design Pressure (kPa)
Windward Wall 0.63
Side Wall -0.59
Leeward Wall -0.59
Given these design pressures, the tests indicate clearly that the walls without roof met or exceeded the design requirements when the catastrophic failure occurred, the first at a pressure of -1.5 kPa. Thus, the walls of a brick veneer, double-top-plate wood-frame house will most likely not fail at the ultimate code wind pressures specified by the NBCC. This implies houses constructed to code in Ontario are significantly stronger than what is specified by the minimum wind load requirements.
10.2 Tornado Resilience Design Recommendations
The recommendations presented herein are intended to be easily implemented and inexpensive to apply in order to improve the resilience of wood-frame houses to tornadoes:
1) Ensure adequate double top plate member overlap to increase the strength of top plate connections: this can be accomplished by either of the following methods.
a. Regulation of the overlap of the two members in a double top plate.
Specifically requiring a minimum distance between an interior to exterior wall connection and the end of the lower top plate member. This will prevent large wall rotations and increase shear force transfer between the two top plate members. As a result the walls will be stronger. This change requires no
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b. Required placement of metal connection plate (such as a Simpson Strong Tie mending plate) on top of top plate connections. This would increase the transfer of forces between the joints in the top plate.
2) Stiffening of long unsupported walls: Large local deflections were observed in the longest unsupported wall spans. The length of wall at which this becomes critical is unknown, however, and requires further research. The following two alternatives can achieve this goal:
a. Placement of additional studs and a third top plate member. This will increase the overall stiffness of the wall and create a stronger load path.
b. Mid-span lateral bracing of the top plate. Installation of a tie running through the ceiling which provides lateral bracing and stability to the top plate
member, creating a load path midway through the wall reducing maximum moments and shear forces.
3) Increase the strength of interior to exterior wall connections: This inexpensive change would increase the overall strength of an exterior wall. Interior walls provide a load path for wind-induced shear forces travelling through the exterior wall. Installation of Simpson Strong Ties at each connection; this would require very little effort or expense complete.
4) Extension of brick veneer around front corners: Change the relatively common practice of installing a brick veneer on the front of a house only. As has been clearly shown, brick extended around the corners would significantly increase the strength and stiffness of the corners and the adjacent walls. If it is impractical to extend brick
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around the corners, specifically for the back corners where both sides of the corner are vinyl clad, stiffening of the corner framing would also provide a strength increase. 5) Increase the Stiffness of the Back Corners: Because many houses are constructed with
vinyl siding on the back and sides, it is fairly impractical to suggest brick veneer be installed around the back two corners and not the remainder of the house (which would be expensive and time consuming). Instead, the addition of metal angles, wood beams or other ways to increase the stiffness and opening resistance of the back corners would positively effect the overall house strength.
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