This factor is listed in Table 26.9-1 in ASCE 7-16.

Over 20 states have a mean elevation over 1000 feet, so a significant part of the country could take advantage of the reduced velocity pressure created by the new K e factor. This reduced pressure considers the lower air density at higher elevations thus this factor modifies the 0.00256 multiplier used in the velocity pressure equation. There is an elevation factor, K e, that reduces the velocity pressure q for a site elevation above 1000 feet.Now that the K z coefficient is the same for both the Main Wind-Force Resisting System (MWFRS) and C&C, that table has been moved to Chapter 26. The K z coefficient used in ASCE 7-10 for C&C was listed in Chapter 30 for use with C&C design. Table 26.10-1 in ASCE 7-16 provides the K z coefficients. Since most buildings are in Exposure B, this reduction in K z will affect many structures. The elevation/exposure coefficient, K z, has been reduced for C&C pressures for Exposure B buildings up to a height of 30 feet.Wind speeds in the Midwest and west coast are 5-15 mph lower in ASCE 7-16 than in ASCE 7-10. Wind speed maps west of the hurricane-prone region have changed across the country.There are several compensating changes in other wind design parameters that reduce these design pressures in many parts of the country. Comparative C&C negative pressures, 140 mph, 15-foot mean roof height, Exposure C. In some cases not shown in Table 1, such as for Zone 1, the revised coefficients produce an approximate doubling of roof pressures. Pressure increases vary by zone and roof slope. This Table compares results between ASCE 7-10 and ASCE 7-16 based on 140 mph wind speeds in Exposure C using the smallest EWA at 15-foot mean roof height in Zone 2.

An example of these wind pressure increases created by the increase in roof pressure coefficients is illustrated in Table 1. To resist these increased pressures, it is expected that roof designs will incorporate changes such as more fasteners, larger fasteners, closer spacing of fasteners, thicker sheathing, increased framing member size, more closely spaced roof framing, or a change in attachment method (e.g., change smooth shank nails to ring shank nails or screws). The significance of these changes is the increase in pressures that must be resisted by roof construction elements subject to component and cladding wind loads including but not limited to roof framing and connections, sheathing, and attachment of sheathing to framing. Examples of ASCE 7-16 roof wind pressure zones for flat, gable, and hip roofs. The coefficients for hip roofs are based on the h/B ratio (mean roof height to the building width ratio) and, for roofs with slopes from 27° to 45°, the coefficients are a function of the slope.įigure 1. Hip roofs have several additional configurations that were not available in previous editions of ASCE 7. And, the largest negative external pressure coefficients have increased on most roof zones. For gable and hip roofs, in addition to the changes in the number of the roof wind pressure zones, the smallest and largest effective wind areas (EWA) have changed. These changes are illustrated in Figure 1. Additional edge zones have also been added for gable and hip roofs. For flat roofs, the corner zones changed to an ‘L’ shape with zone widths based on the mean roof height and an additional edge zone was added. Determining Components and Cladding Roof Design PressuresĬomponent and cladding (C&C) roof pressures changed significantly in ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures.

Asce southeastern conference 2018