INTRODUCTION Since the initial edition of ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, in 1988, the design component and cladding (C&C) wind pressures on a roof have varied little for a given location. While the method of determining wind speeds has changed from the “fastest mile” to the “three-second gust,” and the mean return interval has increased, the allowable strength design (ASD) pressures are remarkably consistent. The diagram for the pressure zones—Zone 1 (interior), Zone 2 (edge), and Zone 3 (corner)— has not changed since that initial ASCE 7-88. However, the new edition, ASCE 7-16, has implemented significant changes to the diagram for the pressure zones, which makes the design of reroofing a challenge, because the existing roof structure may not be able to resist the changes in wind uplift pressures within building code requirements. A PROTOTYPE BUILDING As an example, consider an office building located in Kansas City, MO. The building has plan dimensions of 100 by 100 ft., with a mean roof height of 30 ft., without parapets. The roof is “flat,” with minimal pitch for drainage provided by tapered insulation. For wind pressure design purposes, the Risk Category is II and the Exposure is C. For ASCE 7-10 and older editions, the roof zoning is shown in Figure 1, and the ASD uplift pressures are shown in Table 1. Users of the ASCE 7 standard should be aware that prior to the 2010 edition, the wind velocities used and the pressures calculated are ASD pressures, and would be multiplied by 1.6 to be converted to ultimate pressures. Starting with ASCE 7-10, the wind velocities used are higher, ultimate wind velocities. The calculated pressure is therefore an ultimate pressure, and would be multiplied by 0.6 to convert the pressure to an ASD pressure. Structural engineers would use an ASD pressure when using allowable strength design, and an ultimate pressure when using the load and resistance factor design (LRFD) method of design. As can be seen in Table 1, while the design wind velocity went from “fastest mile” in ASCE 7-88, to the “3-second gust” in ASCE 7-02, to a longer return interval in ASCE 7-10, the final design pressure did not change by more than 7% in any zone. Now consider the zone diagram (Figure 2) and ASD uplift pressures (Table 2) for this same building using the ASCE 7-16 standard. The implications of this change become apparent when the ASCE 7-16 pressures are overlaid upon the ASCE 7-10 pressures (Figure 3). You can see that large sections of the roof must now resist substantially higher uplift pressures. The result clearly shows that the roof 1 0 • RC I I n t e r f a c e J u l y 2 0 1 8 ASD Pressure (psf) ASCE 7-10 ASCE 7-02 ASCE 7-88 Zone 1 -20.0 -20.4 -21.1 Zone 2 -33.5 -34.2 -35.4 Zone 3 -50.4 -51.5 -54.2 V (mph) 115 90 78 Figure 1 – C&C roof zones – ASCE 7-10 and previous. Table 1 – ASD C&C wind uplift pressures, from 1988 to 2010. Photo credit: iStock.com/cash14 J u l y 2 0 1 8 RC I I n t e r f a c e • 1 1 structure that was designed for ASCE 7-10 or older wind pressures can be substantially overloaded when analyzed using ASCE 7-16 wind pressures. While it is quite possible to design the roofing membrane for the ASCE 7-16 wind pressures, there may be a problem if the permitting agency (the authority having jurisdiction) requires the existing steel roof deck or other areas to be able to resist these same higher pressures. Figure 2 – C&C roof zones – ASCE 7-16. Figure 3 – ASCE 7-16 pressures over ASCE 7-10 pressures. ASD Pressure (psf) ASCE 7-16 Zone 1 P -20.6 Zone 1 -35.8 Zone 2 -47.2 Zone 3 -64.4 V (mph) 122 Table 2 – ASD C&C wind uplift pressures – ASCE 7-16. 1 2 • RC I I n t e r f a c e J u l y 2 0 1 8 One instance where this structural retrofit may come into play is when Section 503.12 of the 2018 International Existing Building Code (IEBC) is enforced. When removal of roofing materials from over 50% of the roof area of a building occurs in an area where the ultimate wind speed is over 115 mph, the roof deck and connections must be able to resist at least 75% of the current wind loads. The 115-mph ultimate wind velocity encompasses the entire states of Hawaii and Florida, as well as the Atlantic and Gulf coasts for a Risk Category II (ordinary risk) structure. For a Risk Category III or IV structure (which includes essential facilities such as hospitals, fire and police stations, and structures designated as storm shelters), the entire U.S. falls into this condition. EFFECT ON THE STEEL ROOF DECK ANCHORAGE Let us focus specifically on steel roof decks. When IEBC Section 503.12 is applicable, the first thing to consider is if the attachment of the steel roof deck is adequate for the increased uplift pressures. For instance, there are regions of the roof that were in the previous Zone 1 that are now in the new Zone 2. This would increase the uplift pressure from 20.0 psf (ASCE 7-10) to 47.2 psf (ASCE 7-16). A 22-gauge, 33- or 40-ksi roof deck, welded to the supports using ½-in. visible diameter arc spot welds (“puddle welds”) in a 36/3 pattern, would have an allowable uplift capacity of 32 psf (RDDM, 2012). While a 36/3 pattern is not commonly used today, it can be found in some older structures. Seventy-five percent of 47.2 psf is 35.4 psf, which exceeds the 32-psf available capacity of this connection pattern. This would be adequate for the original design by ASCE 7-10 or older, but not adequate for ASCE 7-16, and would require additional deck anchoring. This may require removal of all existing roofing materials in order to add additional fasteners. Furthermore, concentrated line loading for uplift of mechanically attached membranes can severely overload the steel roof deck and its attachment, if the system was designed considering a uniformly distributed uplift load. This is addressed in an article originally published in Structure (Fisher and Sputo, 2017), which is reprinted in this issue of RCI Interface (see page 23). RECOMMENDATIONS When re-covering or replacing an existing roof, consideration must be paid to increased uplift pressures that can be required by code or the permitting agency. Additionally, the effect of concentrated line loads resulting from mechanically attached membranes also needs to be considered. Solutions do exist—some more costly than others. The services of both a roof consultant and a licensed structural engineer who are familiar with the design of both the proposed roofing system and the structural system involved should be engaged to determine options that will comply with the codes in effect. REFERENCES ASCE 7-10: Minimum Design Loads for Buildings and Other Structures. American Society of Civil Engineers, Reston VA, 2010. ASCE 7-16: Minimum Design Loads and Associated Criteria for Buildings and Other Structures. American Society of Civil Engineers, Reston VA, 2016. J.M. Fisher and T. Sputo. “Are Your Roof Members Overstressed?” Structure. March 2017. Roof Deck Design Manual, 1st Edition. Steel Deck Institute, Glenshaw PA, 2012. Thomas Sputo, PhD, PE, SE, FASCE, is the technical director of the Steel Deck Institute, a trade organization of steel roof and floor deck manufacturers. Additionally, he is a consulting structural engineer with the Gainesville, FL, firm of Sputo and Lammert Engineering, LLC, and an emeritus senior lecturer in the Department of Civil and Coastal Engineering at the University of Florida. He holds a PE license in twelve states plus an SE license in Illinois. Thomas Sputo, PhD, PE, SE, FASCE In April, firms in the United Kingdom revealed through their first government- mandated compensation reports that the construction industry has the largest gender gap out of 27 business sectors. More than 100,000 firms with 250 employees or more reported, showing a 24% median gap in hourly pay between men and women in the construction industry. Among firms in all sectors, the pay gap averaged about 10%. Balfour Beatty plc, the UK’s largest contractor, reported a 33% median gender pay gap and a 65% disparity in median bonus payments. Some 31% of its lowest-paid workers (lowest quartile) were female, while only 7% of the women were in the highest-paid quartile. To review gender pay gap data, visit https://gender-pay-gap.service. gov.uk/. Mandated Reports Reveal UK Construction Firms’ Gender Pay Gaps ENVIROSPEC INCORPORATED The PAVE-EL® Pedestal System Transforms flat roofs into attractive, maintenance-free, paver stone terraces. Elevates paver stones for perfect drainage. Levels paver stones and ensures their uniform spacing for an ideal roof terrace surface. A perfect solution for laying mechanical walkways for use by maintenance personnel. Ideal for laying paver walkways in roof gardens. Turn roof tops into beautiful deck areas Easy to Install 1-905-271-3441 • www.envirospecinc.com
