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 (cor-ner)—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 require-ments.

A PROTOTYPE BUILDING As an example, consider an office build-

ing 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

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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. Users of the ASCE 7 standard should be aware that prior to the 2010 edition, the wind veloci-ties used and the pressures cal-culated are ASD pressures, and

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.

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Figure 2 – C&C roof zones – ASCE 7-16.

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 author-ity having jurisdiction) requires the existing steel roof deck or other areas to be able to resist these same higher pressures.

Figure 3 – ASCE 7-16 pressures over ASCE 7-10 pressures.

Table 2 – ASD C&C wind uplift pressures -

ASCE 7-16.

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

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