Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Bitsuamlak, Girma

Abstract

Elevated low-rise buildings are vulnerable to cladding damage underneath the structure due to extreme winds and the absence of proper wind loading codes and standards such as the National Building Code of Canada (NBCC). The space between the ground and the horizontal base surface of an elevated structure affects its aerodynamics differently compared to a ground-mounted structure. Despite the widespread use of elevated low-rise buildings, there is still limited understanding of the wind interaction across building surfaces for different stilt heights and wind types. This research aims to evaluate the impact of atmospheric boundary layer (ABL) and downburst wind loads on an elevated low-rise building with typical northern Canadian architecture, using experimental testing facilities at Western University. The Boundary Layer Wind Tunnel Laboratory was used to simulate ABL winds in open terrain and to measure the external pressure coefficients on the building model. Additionally, the WindEEE Dome was used to generate downburst-like winds and measure their resultant wind loads. The analysis of both datasets indicates that stilt height has a significant impact on surface pressures on the base surface of the building, resulting in increased peak suction near the corners and edges when the stilt height is increased. The wind loads from both test series were compared to the newly introduced ABL wind loading provisions for elevated structures in ASCE 7-22 to assess the adequacy of these design pressures on the study building. The enveloped negative external pressure coefficients due to both ABL and downburst winds were effectively covered by the ASCE 7-22 design loads for stilt heights below 2.5 m. However, 2.5 m and 3 m stilt heights produced external pressure coefficients which exceeded the design pressures of ASCE for tributary areas below 5 m2. Therefore, further refinement of external design pressures and components and cladding zones may be necessary to ensure a more conservative design of elevated structures. The results of this study can be used to improve the NBCC by incorporating aerodynamic information for elevated buildings.

Summary for Lay Audience

Elevated structures, such as those used in arctic and coastal communities, provide climate-resilient infrastructure that reduces permafrost degradation and withstands flooding. These structures have their lowest floor elevated above ground level using compression-resistant structural members, such as piles or columns, allowing air/water flow between the ground and the base of the building. However, under extreme winds like downbursts and hurricanes, high-velocity winds can damage the cladding on the underside of the structure. Therefore, it is crucial to understand the aerodynamics and wind loading of elevated structures to establish appropriate engineering standards.

This study experimentally simulated both straight-line winds and downbursts to evaluate resulting wind loads on an elevated low-rise building. Straight-line winds were used to determine wind loading and provide a benchmark for comparison with downburst loads. Downbursts are severe wind events associated with notable structural damage, creating multi-directional high-velocity winds over a short duration. Pressure measurements were obtained on a building model to determine the external pressure coefficients across the building surfaces for both wind types.

The downburst-induced wind loads were compared to those of straight-line winds, revealing higher loads on component and cladding elements under downburst events. The datasets were then compared to current design pressures for elevated structures in ASCE 7-22 from the United States and Canadian wind loading provisions for seated buildings in NBCC 2020. The results indicated that the standards may underestimate the negative pressures on buildings with stilt heights above 2 m and generally underestimated positive pressures on cladding exceeding an area of 5 m2. This study provides insight into wind loads on elevated structures and produces new aerodynamic data for codification in NBCC.

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