The big box store is 290’ long x 169’ wide x 24’ tall, and the apartment building is 100’ long x 40’ wide x 55’ tall. Wind speeds were determined using ASCE7hazardtool.online, an online tool.

DWPs can be determined by using proprietary third party methods (e.g., RoofNav), simplified calculation tools (e.g., RoofWindDesigner), or hand calculations following the methods within the ASCE 7 standard.

For this case study, DWPs were determined using hand calculations via an Excel spreadsheet. The DWPs are slightly more refined since there was no rounding of wind speeds for ASCE 7-16. Both RoofNav and Roof Wind Designer round to the nearest 5 or 10 mph. The maps in ASCE 7-16 have been revised, not only from a wind speed perspective but from the number of wind isobars on the maps. The ASCE 7-16 maps are more refined which allows for more exact wind speed values.

Figure 5: ASCE 7-10 Map, Risk Category II, showing a limited number of wind-speed isobars; ASCE 7-16 Map, Risk Category II, showing many additional wind-speed isobars.

In total, 72 different sets of DWPs were calculated. Buildings over 60 feet tall are designed the same in 7-16 as in 7-10; therefore, this article focuses on buildings less than 60 feet tall.

For this article, the design wind pressure analysis for the big box store in Kansas City, Mo., and the 5-story apartment building in Mobile, Ala., will be presented. The full study can be found here. Figure 6 shows the DWPs for the Big Box store in Kansas City, Mo., and Figure 7 shows the DWPs for the 5-story apartment building in Mobile, Ala.

Figure 6: Design wind pressures based on ASCE 7-10 and 7-16 for a big box store in Kansas City, Mo.

Figure 7: Design wind pressures based on ASCE 7-10 and 7-16 for a 5-story apartment building in Mobile, Ala.

Comparing Percent-Increases in DWP. Using the design wind pressures (for Risk Category III, Exposure C, Enclosed), the percent increase in DWP was calculated for each roof zone for the two case study buildings. The percent increases in DWP were calculated by dividing the ASCE 7-16 value with the corresponding ASCE 7-10 value for each roof zone. However, because there are 4 roof zones in ASCE 7-16 (and 3 roof zones in ASCE 7-10), both Roof Zones 1’ and 1 were divided by the value for ASCE 7-10 Roof Zone 1 to determine the percent increase. Values in the following charts that are below 100 percent (shaded gray) indicate a reduction in DWP from ASCE 7-10 to -7-16. Figure 8 shows the Percent Increases in DWP per Zone for the big box store in Kansas City, Mo., and Figure 9 shows the same for the 5-story apartment building in Mobile, Ala.

Figure 8: Percent increases in DWP per roof zone for a big box store in Kansas City, Mo.

Figure 9: Percent increases in DWP per roof zone for a 5-story apartment building in Mobile, Ala.

For the big box store scenario, the percent increases in DWP per roof zone varies from 84 to 164 percent. The percent reductions (values less than 100 percent) are all in Roof Zone 1’ while Roof Zones 1, 2 and 3 have increased DWP. Roof Zone 1 percent increases are largest, and Roof Zone 3 increases are smallest.

For the 5-story apartment building scenario, the percent increases in DWP per roof zone varies from 91 to 177 percent. The percent increases in Roof Zone 1, again, are the largest while the smallest are in Roof Zone 3. Reductions, again, only occur in Roof Zone 1’.

It is worth noting that the increases in Pressure Coefficients from ASCE 7-10 to 7-16 (Figure 3) are, in fact, relatively indicative of the increases in DWP. For more on Pressure Coefficients and the wind design process, see this blog.

These two case studies show the 2016 version of ASCE 7 imposes higher DWPs in Roof Zones 1, 2 and 3. However, given the substantial reductions in Roof Zone 1’, it could be expected that installed roof system capacity will be lower. While that is hopeful, the roofing industry has a minimum-capacity backstop of 60 psf. This means that any calculated DWP at or below 60.0 psf (even for DWP values as low as 23 psf [Figure 6]) will have a 60-psf-capacity roof assembly installed to meet building code requirements. The reality is many buildings are required to use a 60-psf-capacity roof system when in fact ’60 psf’ is more capacity than needed based on calculated DWPs.

Comparative Analysis of Roof Zones and Uplift Ratings

Analyzing the roof zone layouts and the associated wind-uplift ratings based on the 2010 and 2016 versions of ASCE-7 provides a more direct comparison for each of the two case studies.

Big Box; Kansas City, Mo. The big box store in Kansas City, Mo. saw the DWP increase from 38 psf to 56 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 32 psf in Roof Zone 1’. Because the lowest available wind-uplift rated roof system is rated to 60psf, the increases or decreases in Roof Zone 1 and 1 Prime are rendered ineffectual. A 60-psf-capacity roof should be installed in both Roof Zone 1 and Roof Zone 1’ per ASCE 7-10 and 7-16. Figures 10 and 11 show the roof zone layouts, associated design wind pressures, and the minimum wind-uplift rating for each roof zone for a big box store in Kansas City, Mo.

Figure 10: Roof zone layouts, design wind pressures, and the minimum wind-uplift rating for each roof zone for a big box store in Kansas City, Mo. based on ASCE 7-10.

Figure 11: Roof zone layouts, design wind pressures, and the minimum wind-uplift rating for each roof zone for a big box store in Kansas City, Mo. based on ASCE 7-16.

For Roof Zones 2 and 3, the DWPs are not only increased, but the sizes of the roof zones that require higher capacity are also increased. Roof Zone 2 per ASCE 7-10 required a 75-psf-capacity roof, and a 75-psf-capacity roof will be needed per ASCE 7-16.

In this case, because of the increased size of the roof zones (specifically Roof Zone 2), approximately 9 percent of the roof area will need a higher capacity roof. Figure 12 shows roof area percentages based on the required roof system capacity for this scenario.

Big Box Roof Example (Kansas City, MO)

Figure 12: Required roof capacity and the roof area (%) where each is required.