Wind Loads Guide To The Wind Load Provisions Of Asce 7-16 Pdf Download
Geralyn
The new ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures (Standard) is adopted into the 2018 International Building Code (IBC) and is now hitting your desks. The 2018 IBC and the referenced Standard are being adopted by a few jurisdictions and will become more widely used in 2019. Thus starts the time when practicing engineers learn the new provisions of the Standard and how they apply to their practices. To help in this process, changes to the wind load provisions of ASCE 7-16 that will affect much of the profession focusing on building design are highlighted.
New additions to the Standard are provisions for determining wind loads on solar panels on buildings. These provisions give guidance to the users of ASCE 7 that has been missing in the past. Previously, designers commonly attempted to use a combination of the component and cladding provisions and other provisions in the Standard to determine these loads, often resulting in unconservative designs.
wind loads guide to the wind load provisions of asce 7-16 pdf download
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There are two methods provided in the new Standard. One method applies specifically to a low-sloped roof (less than 7 degrees) (Figure 5) and the second method applies to any roof slope where solar panels are installed parallel to the roof. Each of these provisions was developed from wind tunnel testing for enclosed structures. Thus, these provisions are not applicable to open structures because the flow of the wind over the roof of enclosed structures and open structures varies significantly. Further testing is currently underway for open structures, and these results will hopefully be included in future editions of the Standard.
The wind loads for solar panels do not have to be applied simultaneously with the component and cladding wind loads for the roof. However, the roof still needs to be designed appropriately assuming the solar panels are removed or not present.
The component and cladding pressure coefficients, (GCp), for roofs on buildings with an h < 60 feet, have been revised significantly in ASCE 7-16. The new roof pressure coefficients are based on data from recent wind tunnel tests and then correlated with the results from full-scale tests performed at Texas Tech University. The full-scale tests indicated that the turbulence observed in the wind tunnel studies from the 1970s, that many of the current roof pressure coefficients were based on, was too low. Also, the technology available to measure the results of these wind tunnel tests has advanced significantly since the 1970s. Therefore, the new wind tunnel studies used flow simulations that better matched those found in the full-scale tests along with improved data collection devices; these tests yielded increased roof pressures occurring on the roofs. Thus, the roof pressure coefficients have been modified to more accurately depict roof wind pressures.
Previously, designers were required to use various provisions of overhangs, free roof structures, and more to determine the wind loads on canopies. Research became available for the wind pressures on low-slope canopies during this last code cycle of the Standard. This research was limited to low-slope canopies and only for those attached to buildings with a mean roof height of h < 60 feet. Research is continuing on sloped canopies, and the Committee hopes to be able to include that research in the next edition of the Standard.
In this article, an example wind load pressure calculation for an L-shaped building in Cordova, Tennessee will be shown. This calculation will be in accordance with ASCE 7-16 wind load calculations (directional procedure).
The first thing in determining the design wind pressures is to classify the risk category of the structure, which is based on the use or occupancy of the structure. Since this example is a plant structure, the structure is classified as Risk Category IV. See Table 1.5-1 of ASCE 7-16 for more information about risk categories classification.
In ASCE 7-16, the wind speed data can be obtained from Figures 26.5-1 to 26.5-2. From Figure 26.5-1A, Cordova, Memphis, Tennessee is near the red dot shown in Figure 3 below, and subsequently, the basic wind speed, \(V\), is 52 m/s. Take note that the values should be interpolated between known wind contours.
Depending on the wind direction selected, the exposure of the structure shall be determined from the upwind 45 sector. The exposure to be adopted should be the one that will yield the highest wind load from the said direction. The description of each exposure classification is detailed in Section 26.7.2 and 26.7.3 of ASCE 7-16.
The wind directionality factors, \(K_d\), for our structure are both equal to 0.85 since the building is the main wind force resisting system and also has components and cladding attached to the structure. This is shown in Table 26.6-1 of ASCE 7-16.
The velocity pressure coefficient, \(K_z\), can be calculated using Table 26.10-1 of ASCE 7-16. This parameter depends on the height above ground level of the point where the wind pressure is considered, and the exposure category. Moreover, the values shown in the table is based on the following formula:
Usually, velocity pressure coefficients at the mean roof height, \(K_h\), and at each floor level, \(K_zi\), are the values we would need in order to solve for the design wind pressures. For this example, since the wind pressure on the windward side is parabolic in nature, we can simplify this load by assuming that uniform pressure is applied on walls between floor levels. We can simplify the windward pressure and divide it into 2 levels, at the eave height (+5.0m), and at the mean roof height (+6.5m). Moreover, α = 9.5 and \(z_g\) is equal to 274.32 m since the location of the structure is classified as Exposure C.
This version reflects significant changes made to the wind load provisions from the previous version of the standard, ASCE 7-16. This guide has been reorganized to follow the chapter organization in ASCE 7-22 and the step-by-step procedures provided in the standard.
With 19 real-life design problems applying the appropriate use of analytical and simplified procedures for calculating wind loads for various common structure types, this guide is an essential reference for practicing structural engineers.
Wind load analysis, which involves the computation of forces exerted by the wind on structures, is a crucial aspect of structural engineering. Wind can exert significant lateral forces on a structure, which can cause it to sway or even collapse if it is not properly designed to withstand these loads.
The most critical aspect of wind load analysis is obtaining the basic wind speed, V V V, where the building is located. Wind speed is typically sourced from related code-based maps, such as that provided by ASCE 7-16 Figure 26.5-1A to D and Figure 26.5-2A to D (see Figure 1), or local meteorological records.
Wind directionality in wind load analysis refers to the consideration of how wind flows around and interacts with a building. It takes into account the shape, size, and height of the structure, as well as its surrounding terrain.
Another important factor to consider in wind load analysis is the building enclosure. The principle behind this is to consider the difference in pressure between the inside and outside of the building.
This article primarily focuses on applying wind load calculations for two commonly used systems in residential and light commercial projects: the main wind-force resisting system (MWFRS) and the components and cladding systems of structures.
The main wind-force resisting system, or MWFRS, is an ensemble of structural elements designed to take on lateral and vertical wind loads, including structural walls, columns, rigid frames, and bracing systems.
The MWFRS is vital as it forms the backbone of any structure, ensuring overall stability against wind forces by transferring these wind loads. The mechanism involves transmitting wind forces from their point of application, typically the external envelope of the building, through the structural system and ultimately to the ground, thereby ensuring the building remains upright and secure.
For the MWFRS wind loads calculation, ASCE 7-16 presents two methods: The Directional Procedure, which is widely used for all building types, and the Envelope Procedure, which specifically applies to low-rise buildings and is the primary focus of this article.
While the MWFRS ensures overall structural stability against wind loads, the significance of components and cladding in wind load analysis arises from their crucial role in withstanding localized wind pressures, safeguarding the building's integrity, and protecting its occupants.
There are distinct differences in the design considerations for the MWFRS and components and cladding systems. While the MWFRS is designed to counter wind loads from any direction, ensuring overall structural stability, the components and cladding system are primarily evaluated for wind loads that are normal or perpendicular to the surface.
However, additional factors come into play in the case of components and cladding systems. The effective wind area, which is the area that the wind load is distributed over, is a key factor in these calculations.
Similarly, the pressure coefficient values would differ for windows and doors, which often serve as pressure release points during extreme wind events. Their design must withstand not only the direct wind loads but also the rapid changes in pressure caused by gusty winds or the event of a sudden windstorm.
In the United States, the ASCE 7 guideline is the go-to resource for wind load calculations. This guideline takes into account several factors, including wind speed, wind directionality factor, exposure category, topographic factors, ground elevation, and building enclosure.
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