OBC 4.1.7.4 & 4.1.7.5: Expert Guide to Wind Loads on Hills and Escarpments in Ontario
Sarah's Expert Insight
OBC Compliance Specialist
"When developing properties in hilly regions like Muskoka or near the escarpments in Oakville, ignoring topographic wind speed-up can lead to premature structural fatigue or building envelope failure. At empcontracting.ca Inc. your construction contractor in London, Ontario, we meticulously apply these Part 4 calculations to ensure that every hillside project is engineered to withstand the unique atmospheric pressures of the Ontario landscape."
Understanding Wind Velocity Speed-up on Topographic Features
Section 4.1.7.4 of the Ontario Building Code (OBC) specifies how wind velocity increases as it moves over hills and escarpments. For projects led by empcontracting.ca Inc. your construction contractor in London, Ontario, understanding these aerodynamic factors is crucial for ensuring the structural integrity of buildings situated on sloped terrains.
Parameters for Maximum Speed-up (Table 4.1.7.4)
The code provides specific multipliers for various topographic shapes to determine the maximum speed-up factor (ΔSmax). Key shapes include:
- 2-Dimensional Hills: Calculated using 2.2 Hh/Lh.
- 2-Dimensional Escarpments: Calculated using 1.3 Hh/Lh.
- 3-Dimensional Axi-symmetrical Hills: Calculated using 1.6 Hh/Lh.
For steep slopes where the ratio Hh/Lh exceeds 0.5, the code requires specific substitutions to maintain safety margins in structural calculations.
4.1.7.5 External Pressure Coefficients (Cp)
External pressure coefficients are essential for determining the wind loads on the building envelope. According to Article 4.1.7.5, these coefficients apply to various building shapes and are critical for selecting appropriate cladding, windows, and structural bracing. For low-rise buildings, additional guidance is found in Article 4.1.7.6, ensuring that localized pressures at corners and eaves are adequately managed.
Disclaimer: This content is generated from the Ontario Building Code for educational purposes. Always consult with a licensed professional engineer or your local building department before starting any construction project in London, Ontario.