The wind loading provisions in the National Building Code of Canada (NBCC) have not been updated for decades despite developments in wind tunnel methods since the original source data. This study aims to review the Nbcc 2020 provisions compared to wind tunnel experiments, field measurements, and the ASCE 7-22 provisions for single and multi-span gable roof buildings. Field measurements and wind tunnel tests are used to collect data for an isolated building and later for a multi-span gable. The pressure coefficients from the field measurements were mostly higher than the wind tunnel, which is consistent with previous studies. The results provided evidence that NBCC provisions for single-gabled roofs (with slope 7(degrees) < alpha <= 27(degrees)) and multi-span gabled roofs (with slope 10(degrees )< alpha <= 30(degrees)) are currently underestimated. New provisions are suggested for the corner (c), edge (s), and field (r) zones of the single-span gabled roof and for gable-end-edge (s') zone of the multi-span. It is suggested to merge Zone s into Zone c to form a new perimeter Zone p for the single gabled roof and to merge Zone s' into Zone c for the multi-span gabled roof. The proposed changes correct the current provision underestimation and add simplicity for easier construction.

The wind load provisions for gabled roofs with a slope of 27° < α < 45° in the National Building Code of Canada (NBCC) were derived from wind tunnel studies conducted in the 1970s. The coefficients have remained basically unchanged until this time. The paper presents the results of full-scale measurements and wind tunnel experiments to verify the necessity to update the current NBCC provisions. A full-scale building with a gabled roof of 45° was instrumented with pressure taps, and the wind load was monitored for 15 months. Special attention is given to the roof overhang by considering the simultaneous pressure contribution from both the upper and lower surfaces. The building and surroundings were replicated at a wind tunnel scale and investigated by Concordia University and Western University. Based on these three sources of data, the study concluded that the corner and edge provisions for both cases—a roof with and without overhang—need to be increased. It is suggested to merge the corner and edge into a single “perimeter” load provision. This code simplification would help the roofing industry to reduce misleading interpretations of the code and minimize failure due to installation errors.

This study investigates wind loads on stepped roof buildings measured in the field and in an atmospheric boundary layer wind tunnel to verify the suitability of the North American codes and standards for the design of stepped roofs. Good agreement has been generally found between field and wind tunnel results. The field measurements and wind tunnel results were compared with the NBCC 2020 and ASCE/SEI 7–22 (2022) provisions. It was found that the Edge provisions of NBCC 2020 are currently underestimated and need to be increased. The study suggests merging the Corner and Edge zones into a single “Perimeter” zone in NBCC 2020 as this would resolve the current underestimation of the NBCC Edge wind loads and add simplicity to the Code provisions. Peaks of ASCE/SEI 7–22 (2022) were found conservative, except for the zone near the step wall on the lower roof; ASCE/SEI 7–22 (2022) positive peaks seem to be underestimated.

The current wind load provisions for low-slope roofs in the NBCC were established in the 70 s with wind tunnel experiments using analog data technology. These coefficients have remained almost unchanged. As the wind pressure measurement tools have significantly improved during the past 50 years, verification of the original pressure coefficients is required. The National Research Council of Canada (NRC), in collaboration with the Special Interest Group on Dynamic Evaluation of Roofing Systems (SIGDERS) consortium, conducted a wind load investigation to verify the suitability of the current wind load provisions. The investigation was carried out using both field measurements and wind tunnel simulations. An ideal full-scale representation of the low-slope roof specified in building code was instrumented and continuous data was collected for four years. A duplication of the building was evaluated by two wind tunnels to investigate the effect of critical wind directions. The paper presents the efforts made to make field measurements comparable to wind tunnel data and code provisions. It was observed that records with mean wind a speed above 22 mph (10 m/s) reduce the data variability and maximize reliability, which are important features for the code verification process. The goal of this research is to combine multiple sources of data to verify the suitability of wind load provisions on low-slope roofs. The research focuses on NBCC-2020 as an application. Evidence is presented that the NBCC-2020 provisions for the Roof Edge Zone is currently underestimated and needs to be increased. The study suggests taking the opportunity of Edge provision enhancement to merge Corner and Edge into a unique "Perimeter" load provision. The suggested code change addresses the current underestimation of the Edge Zone coefficients (wind load upgrade), and provides construction simplification to minimize roofing failure associated with labour-induced errors during cladding system installation (constructability upgrade).

Commercial rooftops provide extensive areas that represent the ideal platform to install a photovoltaic (PV) system. The combination of the roofing assembly and the PV system is termed a photovoltaic roofing assembly (PVRA). There was little guidance for determining the wind loads on rooftop PV systems for many years. With the efforts of the Structural Engineers Association of California and numerous wind tunnel studies conducted by reputed labs, the design methodology for determining wind loads on PV systems has been established, which has become ASCE 7-16, Minimum Design Loads for Buildings and Other Structures, and the National Building Code of Canada 2015. However, there is no guidance on the resistance aspect of the PVRA. During the life cycle of the rooftop solar array, the variable amplitude of the dynamic wind loading can lead to fatigue, which can accumulate damage in structure details. Such damages might lead to severe failures in the whole PVRA. Currently, there are no standardized test methods that determine the collective performance of a PVRA. In collaboration with the roofing and solar industry, the National Research Council Canada (NRC) is conducting a research study to address the missing link between the design and resistance of a PVRA. A unique, dynamic test method was developed to determine the wind pressure resistance of a PVRA. The test methodology applies uniform wind pressure on a 3 x 3 array. It provides a complete load path evaluating the capacity in members, connections, and PV attachments to the roof assembly. The current paper presents the PVRA test apparatus commissioning process. It details the experimental apparatus, the definition of the testing specimen, the establishment of a dynamic loading protocol, and quantification (measurements) of the system response. It also presents the results of pilot testing conducted as per the new test method. Once standardized, this new protocol will ensure that PVRA integrity is maintained and PV performance is optimized.

Flexible roofing systems (e.g., membrane roofs) are widely used in low-rise commercial and industrial buildings, accounting for over 60% of low-sloped building roofs in North America. Despite their wide usage, the effects of roof flexibility are neither thoroughly studied, nor accounted for in current building design codes and standards. To investigate such roof flexibility effects on wind-induced pressure, full-scale testing was conducted at the NHERI Wall of Wind (WOW) Experimental Facility (EF). The mechanically attached roof system (MARS) and partially adhered roof system (PARS), two of the most commonly used commercial roofing systems with different flexibility characteristics, were considered in this study. “U-shaped” pressure taps were used to measure the wind-induced pressure on the membrane roof, and the pressure coefficients were compared to those measured on a plywood roof. The effects of wind direction and wind speed on the wind-induced pressure coefficients of membrane roofs were studied. The results showed that the membrane deformation due to the membrane roof flexibility resulted in a reduction in the peak pressure coefficients as compared to those of rigid plywood roof. This reduction in wind loads is more significant for the MARS (27%) which has more flexibility than the PARS (19%). The membrane roof flexibility also modifies the non-Gaussian characteristic which results in a lower peak factor than the plywood roof. It was also observed that peak pressure coefficients on the flexible roof increase at higher wind speeds. This study provides an improved understanding of the effect of roof flexibility on wind-induced pressure coefficients. Further research is needed by testing more types and configurations of flexible roofs to formulate new code provisions for wind effects on flexible membrane roofs.

The North American Vegetated Roof Assembly (VRA-a.k.a. green roofs) market is dominated by two different types: modular vegetated roof assemblies (MVRAs) and built-in-place vegetated roof assemblies (BVRAs). In MVRA, the vegetation is pregrown in transportable trays and installed on the roof, whereas in the BVRA, the vegetation develops on the roof. In 2015, the National Research Council of Canada (NRC), in collaboration with the green roof industry and roofing industry, set the national standard for the wind resistance evaluation of MVRA. Practitioners and building authorities have widely used this standard. In 2017, NRC and the industry collaborators started a new research project on BVRA to expand the current standards. The wind performance of BVRA is highly dependent on the growth stage of the vegetation. This variable was investigated by testing four BVRA sources (S1, S2, S3, and S4) at three different growth stages, namely, as-built, 12 weeks, and 1 year of growth. The interaction between the approaching wind speed and the BVRA response was monitored by installing load cells underneath the test mock-up. The load cells were intended to capture the growth media loss as the wind flow develops over the BVRA mock-up. The wind speed at which the vegetated system's (VS) weight reaches 85% of the initial weight was used to compute the BVRA wind performance. The tests conducted at the three growth stages permitted developing an empirical growth modifier factor (GMF). The GMF is a practical engineering solution to estimate the wind performance of a 1-year BVRA using the as-built performance data. The paper presents the development of this new testing methodology and the standardization process. (C) 2021 American Society of Civil Engineers.

The National Research Council of Canada (NRC), as part of a consortium with members from vegetated roof industries and roofing associations, established a national wind resistance standard for modular vegetated roof assemblies (MVRAs), named CAN/CSA A123.24, "Standard Test Method for Wind Resistance of Modular Vegetated Roof Assembly." Currently, this standard is widely used on a voluntary basis by the Canadian roofing industry for wind performance compliance of MVRAs. CAN/CSA A123.24 was developed based on extensive data collected on various MVRAs by simulating wind in laboratory conditions. Toward codification of CAN/CSA A123.24, the consortia members decided to validate such collected data with in situ performance data. This paper presents and discusses the validation process. Different MVRAs were installed on the rooftop of a Humber College building, located in Toronto. The validation was conducted by comparing the three wind performances of MVRAs as follows: (1) pressure equalization (between the top and bottom of the vegetated system); (2) coefficient of lift, CL; and (3) estimated overturning wind speed, UOT, at different wind conditions. Data comparisons (laboratory versus field) showed consistent agreement in the global trend. Some discrepancies in the absolute values were observed; however, these discrepancies are due to the inherent variability of the field wind conditions and the limited amount of field data. The forecasting of the overturning wind speed provides a valuable tool to adequately evaluate MVRA design alternatives for acceptable wind resistance compliance in accordance with the National Building Code of Canada.

In North America over 60% of low slope buildings are roofed with membranes, and the market survey indicates that there is continuous growth in such roof assemblies. To the best knowledge of the authors, there is no in-situ measured data from such roofs. The National Research Council of Canada (NRCC), in collaboration with the Special Interest Group on Dynamic Evaluation of Roofing Systems (SIGDERS) consortium, has undertaken an in-situ monitoring program with the goal of providing valuable data to benchmark the current code provisions, as well as providing supporting data for existing wind uplift test methods to determine roof cladding resistance. This paper presents in-situ data of two most common commercial roof systems: mechanically attached roof system (MARS) and partially adhered roof system (PARS) and provides side-by-side comparison (partially refers to a system with and adhered membrane over some components that are mechanically attached). In order to provide more detail and controllable measurements, MARS and PARS mock-ups were evaluated at the open flow NHERI Wall of Wind (WOW) Experimental Facility (EF) at Florida International University (FIU). The investigations included visual observations (video) and pressure measurements at different roof locations in various wind directions and wind speeds. The visual observations confirmed that, due to construction characteristic, the membrane of the MARS deforms (balloons) under wind suction. The field and WOW pressure measurements revealed that the MARS generates systematic lower peaks pressure in comparison with the PARS. Data from field showed that the average reduction of the MARS is in the order of 20% in comparison to the PARS at locations close to the leading edge when wind is NW and WNW (close to normal). The WOW results showed that the pressure reduction of the MARS is in average 16% in comparison to the PARS when wind is W (normal), and the reduction would decrease to 12% when considering the worst case among all the wind directions tested in the lab.

The prediction of pollutant dispersion in urban environment is an extremely complex phenomenon, particularly in the vicinity of a cluster of buildings. Dispersion of effluents released from stacks located on building roofs are severely affected by adjacent surroundings. This paper investigates the impact of an upstream building on the near field of a pollutant source in terms of dilution distribution on the roof of an emitting building. The study was carried out using Computational Fluid Dynamics (CFD) approach with Realizable k–ε for turbulent flow modeling. A limited number of cases were also modeled in a wind tunnel for validation purposes. The study shows that when the source is located within the recirculation zone, dilution is highly sensitive to the height of the upstream building and much less sensitive to the width and length of the upstream building. It is also shown that dilution value has an asymptotic behavior which defines the particular point where dilution becomes independent of the upstream building configuration. Some discrepancies between CFD and wind tunnel data were found, specifically for extreme configurations e.g. significantly taller upstream building. These differences are mainly due to the inherent unsteady fluctuations in the wake of buildings which are not detectable by RANS.

The fermentation process in forage tower silos produces a significant amount of gases, which can easily reach dangerous concentrations and constitute a hazard for silo operators. To maintain a non-toxic environment, silo ventilation is applied. Literature reviews show that the fermentation gases reach high concentrations in the headspace of a silo and flow down the silo from the chute door to the feed room. In this article, a detailed parametric analysis of forced ventilation scenarios built via numerical simulation was performed. The methodology is based on the solution of the Navier-Stokes equations, coupled with transport equations for the gas concentrations. Validation was achieved by comparing the numerical results with experimental data obtained from a scale model silo using the tracer gas testing method for O2 and CO2 concentrations. Good agreement was found between the experimental and numerical results. The set of numerical simulations made it possible to establish a simple analytical model to predict the minimum time required to ventilate a silo to make it safe to enter. This ventilation time takes into account the headspace above the forage, the airflow rate, and the initial concentrations of O2 and CO2. The final analytical model was validated with available results from the literature.