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actual surface areas (as opposed to projected areas) in each of the cases below. Also new in ASCE 7-16, a ground elevation factor (Ke) can be used to reduce pressures at higher elevations, or it can more conservatively be set to 1.0. Recognizing how a safety factor is included in the approval listing is critical to ensuring an appropriate roof system is selected and installed. This process results in Design Wind Pressures for each roof zone and determines the dimensions for each of the zones (although not discussed in here). they can be easily added vectorially. In the 2009, 2012, 2015, and 2018 versions of the IBC, for wind resistance of nonballasted roofs, the codes state that built-up, modified bitumen, adhered or mechanically attached single-ply roof systems, metal panel roof systems applied to a solid or closely fitted deck and other types of membrane roof coverings shall be tested in accordance with FM 44746, UL 5807, or UL 18978. ASCE 7-16 added another zone and it presents the potential to have four roof zones: interior, field, perimeter and corner, see Figure 3. For additional information regarding the changes to the 2005, 2010 and 2016 editions of ASCE 7, refer to the following articles by Thomas L. Smith published in Professional Roofing Magazine: “ASCE 7 update” (June 2008); “Mapping the 2010 wind changes” (August 2010); and “How do I load thee?” (October 2017), respectively. Figure 6: External Pressure Coefficients, GCp, for ASCE 7-16. The values for External Pressure Coefficients have been significantly increased in ASCE 7-16. Approval listings are maintained by various entities, such as government agencies, testing laboratories, and even a trade association. The dimensions of the zones are mostly determined by a building’s length and width. When the same pressure is applied to a different surface, we have chosen to internal pressure. Ensure that an appropriate safety factor is included on either the load side or the resistance side. Given:  The  enclosed office building shown in Figure Understanding the similarities and differences between the three versions of ASCE 7 provides for better recognition of the current version’s complexity and allows for more appropriate wind load determination. Figure defines the pressures (with the exception of the lateral/side wall pressures) that need to be computed for wind loading from the E/W direction. In situations where a specific version of ASCE 7 is not mandatory, using the most recent version of ASCE 7 is recommended. The net forces on each surface, in terms of direction relative the surface, James R. Kirby, AIA, is a GAF building and roofing science architect. Jim presents building and roofing science information to architects, consultants and building owners, and writes articles and blogs for building owners and facility managers, and the roofing industry. For the Windward wall (P1 & P2), Cp is 0.8 for all elevations. Use approval listings to select the appropriate roof system. You will also notice that the internal pressure has no effect on the net And with the latest version of ASCE 7, “Minimum Design Loads For Buildings and Other Structures” (ASCE 7), it has become that much more challenging for roof system designers and roofing contractors. rectangles, making the area calculation easier. We also need to know that h/L = 25.1'/50' = 0.50. coefficients are then combined with the gust factor and velocity pressures to In this case we combined all the leeward wall segments into one because they all have the same pressures. flat terrain. ASCE 7-16 has four wind speed maps, one for each Risk Category and they are also based on Strength Design. The Importance Factor was absorbed into the wind maps, which means for ASCE 7-10 and ASCE 7-16, the Velocity, V, is adjusted within the wind speed maps. internal pressure, Case III includes the maximum windward pressure (-17.6 psf) and negative ft. is typically used for roof systems. Height, Length, Width:  Determining the height, length, and width of a building should be straightforward and a vast majority of buildings are predominately square or rectangular in shape, or in general, have square or rectangular roof areas. because they all have the same pressures. All forces are Because of the different configurations of the roof zones and other factors that are intended to allow for a correction (i.e., a reduction) in velocity pressure, it is hard to state—broadly—a percentage that loads may increase. In this building all but the gable ends are This is the second step in determining design wind pressures. The loads acting on a roof must be calculated in order to select a roof system that has the necessary capacity (i.e., wind uplift resistance). Some of these selections may seem straight forward, but some impart a higher resultant design wind load, especially when compounded by similar risk-adverse choices. Figure Eventually, we will all use ASCE 7-16 as the basis for determining design wind loads for our roofs. The pressure coefficients for the walls are found in ASCE 7-05 Figure 6-6 (pg applications. ASCE/SEI 7-16 contains a number of revisions in the wind load chapters of the stan-dard. Kirby is a member of AIA, ASTM, ICC, MRCA, NRCA, RCI, and the USGBC. The tested roof systems are found in approval listings. These are shown in Figure 5. 49). In the end, the design architect’s responsibility is to provide the necessary design wind loads; the manufacturer is responsible for testing roof systems in order to determine wind uplift capacity (See Determining Resistance, below); and the roofing contractor is responsible for proper installation that follows the construction documents and installation instructions. are as follows: Restating the forces in terms of the global coordinate system we get: Mean Roof Height:  h = 2*11' + (3/12)*25'/2 = 25.1 ft, Mean 2nd Floor Height:  h = 11' + 11'/2 = 16.5 ft, Mean 1st Floor Height:  h = 11'/2 = 5.5 ft, I = 1.0 (ASCE 7-05 Table 6-1, Category II building), Case I includes the maximum windward pressure (-17.6 psf) and positive An architect/designer needs to know a building’s location; the building code that is in effect at the building’s location; its height, length, and width; the exposure category; the use and occupancy category; the enclosure classification; any topographic effects; and ground elevation in order to determine the wind loads acting on a roof. There are some noteworthy differences between the three ASCE 7 editions and they include: the wind speed maps, roof zones, enclosure classifications, and external pressure coefficients. It is often useful to resolve each force into it's global components so that Providing this information on the construction documents ensures the contractor and manufacturer (together or separately) can provide an appropriate roof system with tested capacity. Figure E/W Building Section Compute the Velocity Pressures, qz = .00256 Kz Kzt horizontal force. Wanted:  The wind pressures applied to the surfaces and For the roof, the slope angle is 14.0 degrees. areas over which they act. L/B. Therefore, step one is to determine the loads acting on the roof of a specific building. (See. In ASCE 7-05, Importance Factor is a stand-alone factor in the velocity pressure calculations, and why there is one map in ASCE 7-05. outward from the surface and a positive sign is inward. This presentation examines these revisions and how they impact low-slope roof assem-bly design in resisting wind uplift. If it were only that simple! Keep in mind that the wind speed maps in ASCE 7-16 are based on Ultimate Design and accordingly, design wind uplift pressures are often calculated and presented as Ultimate Design values. It’s important to recognize there are two basic steps used to determine design wind pressures acting on a roof. Note this is only done if the conditions and locations of the structures meet all of the conditions specified above and within Section 26.8.1 as ASCE 7-16. The building is located in a region with a wind speed (3-sec It is important that the testing method used to determine the capacity of a roof system is listed in the applicable building code. An abrupt change in the topography, such as escarpments, hills or valleys can significantly affect wind speed. gust) of 120 mph. As seen in Figure 6, the GCp values for Field of the roof increased by 70%, for the Perimeter by 28%, and for the Corners by 14%. It also describes wind uplift design of roof assemblies in accordance with ASCE 7-16, with several illustrative examples. walls will cancel each other. Sorry, your blog cannot share posts by email. Wind in the E/W Direction. Selecting an “Enclosed” or “Partially Open” building when it could become a “Partially Enclosed” building if doors and windows are blown out during a high wind event could result in a roof system without the appropriate capacity to handle the anticipated higher loads. The test methods to determine wind resistance are listed in the IBC Section 1504, Performance Requirements. The following table shows the computation results: Combining with the internal pressures you get the following four load cases Exposure D is applicable where Surface Roughness D prevails in the upwind direction for a distance greater than 5,000 ft. or 20 times the building height, whichever is greater. He has over 25 years of experience in the roofing industry covering low-slope roof systems, steep-slope roof systems, metal panel roof systems, spray polyurethane foam roof systems, vegetative roof coverings, and rooftop photovoltaics. The first step is to determine velocity pressure; the second step is to use velocity pressure to determine design wind loads for roof zones (e.g., field, perimeters, and corners). Post was not sent - check your email addresses! We can now compute the external pressures, qGCp, for each surface. The net force in the lateral direction is zero since the forces on the side And lastly, SPRI sponsors the Directory of Roofing Assemblies (DORA) which is an online database of tested assemblies. A terrain’s surface roughness is determined from natural topography, vegetation and the surrounding construction. However, the factors selected by a conservative owner (e.g., choosing Partially Enclosed) also have an effect on the design wind loads. https://www.gaf.com/en-us/document-library/documents/productdocuments/residentialroofingdocuments/roofdeckprotectiondocuments/deckarmordocuments/TechFlash___Ensuring_Maximum_Protection_when_Installing_Synthetic_Roof_Deck_Protection.pdf, Jeff, I'm having difficulty convincing a roofing contractor of the importance of using capped nails or capped staples when installing GAF's FeltBuster synthetic roofing felt […]. Note that some of the pressures are applied to differently oriented surfaces. Hello DW, we actually this technical document regarding this topic, here's a link from gaf.com below: ASCE 7 uses three Surface Roughness Category types—called B, C and D—which in turn, defines three Exposure Category types, also called B, C and D. Exposure Categories B, C and D are generally defined as follows: Use and Occupancy:  The use and occupancy of a building is used to determine the “Occupancy Category” in ASCE 7-05 or “Risk Category” in ASCE 7-10 and ASCE 7-16. Select roof systems that have capacity greater than the loads acting on the building. direction. Different editions of building codes exist, and therefore, different versions of ASCE 7 are being used in different parts of the country. This is close to 15 This is where much of the concern with ASCE 7-16 lies—the increase in the External Pressure Coefficients—and how the increases will affect design wind pressures. Copyright © 2018 GAF | All rights reserved. For more information about Resilient Roof Systems, read this blog. the net forces applied to the building. Using “Partially Enclosed” as the building type results in an increase of about one third in the design wind pressures in the field of the roof versus an “Enclosed” or “Partially Open” building—all other factors held equal.

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