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The evolution of wind provisions in standards and codes in the United States—Part 2

This two-part article provides an historic overview of the evolution of wind provisions in standards and codes in the United States. From the 1972 edition of the American National Standards Institute’s (ANSI A58.1) Minimum Design Loads for Buildings and Other Structures—which later became ASCE 7—to the current ASCE 7-05 and the International Code Council’s 2006 International Building Code (IBC), one trend is consistent. Through the evolution, the complexity of wind design has been steadily increasing. In this part of the article, I would like to make a plea for action leading to a way out of this complexity.

Part one of this series (which was printed in the December 2006 issue of Structural Engineer) discussed the history of the wind provisions in U.S. standards, specifically ANSI A58.1 and ASCE 7. Part two will focus on the evolution of wind provisions in the model building codes in the United States, such as the IBC and its three legacy model building codes.

Wind provisions in the model codes

The building codes of most jurisdictions within the United States used to be, and in some cases still are, based on one of three legacy model building codes: The BOCA National Building Code (BOCA/NBC) published by the Building Officials and Code Administrators International (BOCA) in Country Club Hills, Ill.; the Standard Building Code (SBC) published by the Southern Building Code Congress International (SBCCI) in Birmingham, Ala.; and the Uniform Building Code (UBC) published by the International Conference of Building Officials (ICBO) in Whittier, Calif. These three model codes, where still in effect, are in the process of being replaced by the International Building Code (IBC) published by the International Code Council (ICC), which has absorbed the former model code groups (BOCA, SBCCI, and ICBO). The following is an historical summary of wind design provisions in these model codes.

BOCA/National Building Code — ANSI A58.1-1972 was adopted by the BOCA/NBC in its 1978 edition, and retained in the 1981 and 1984 editions. Then ANSI A58.1-1982 was adopted in the 1987 edition, and retained in the 1990 edition.
In the 1993 edition, ASCE 7-88 was adopted and it was retained in the 1996 and 1999 editions. The 1999 edition was the last edition published before the integration of BOCA into the ICC.

Standard Building Code — The SBC adopted ANSI A58.1-1972 in the 1977 revisions to the 1976 SBC. The adopting language then appeared in the 1982 edition. Wind design using ANSI A58.1-1972 was permitted only for one- and two-story structures, provided the basic wind pressures from SBC Table 1205.1 were used. The 1982 SBC also adopted alternate wind load provisions (the Metal Building Manufacturers’ Association, MBMA, procedure) in Section 1206. This section was permitted to be used for the design of buildings with flat, single-slope, and gable-shaped roofs with a mean roof height of 60 feet or less, provided the eave height did not exceed the least horizontal dimension of the building.

The 1985 edition had three procedures that could be used. Two of the procedures were contained in Section 1205, Wind Loads, and the third was in Section 1206, Alternate Wind Loads for Low-Rise Buildings. The first option allowed under Section 1205 was use of the provisions within the section. The second option permitted by Section 1205 was to use the wind design provisions of ANSI A58.1-1982, provided the basic wind pressures of Table 1205.1 were used. Table 1205.1 was based on the basic wind speed map of Figure 1205.1 (same as the 100-year mean recurrence interval basic wind speed map contained in ANSI A58.1-1972), which differed from the 50-year mean recurrence interval map in ANSI A58.1-1982.

The alternate wind load provisions of Section 1206 (MBMA procedures) were permitted to be used for the design of buildings with flat, single-slope, and gable-shaped roofs with a mean roof height of 60 feet or less, provided the eave height did not exceed the least horizontal dimension of the building. Section 1206 contained its own basic wind speed map, which was taken from ANSI A58.1-1982.

The 1988 SBC permitted any building or structure to be designed using the provisions of ANSI A58.1-1982. In addition, Section 1205.2 had provisions based on the MBMA procedures for buildings with flat, single-slope, and gable-shaped roofs whose mean roof height was less than or equal to 60 feet. This edition did not require that the roof eave height be less than or equal to the least horizontal dimension of the building.

Section 1205.3 applied to buildings exceeding 60 feet in height, but not more than 500 feet in height, provided the roof slope did not exceed 10 degrees or was not an arched roof. Buildings between 60 and 500 feet in height and not meeting these limitations, and all buildings over 500 feet in height, had to be designed according to ANSI A58.1-1982. The basic wind speed map within Section 1205 was the ANSI A58.1-1982 map.

The 1991 edition was essentially the same as the 1988 edition, except ANSI A58.1-1982 was updated to ASCE 7-88. The basic wind speed map within Section 1205 remained unchanged from the 1988 edition, because the basic wind speed map did not change within ASCE 7-88 from what was in ANSI A58.1-1982.

In the 1994 SBC, ASCE 7-88 was adopted by reference to apply to all buildings and structures. An exception continued to permit the MBMA procedures in Section 1606.2 to be used for buildings with flat, single-slope, hipped, and gable-shaped roofs with mean roof heights not exceeding 60 feet or the least horizontal dimension of the building.

The 1997 edition was essentially the same as the 1994 edition, except that ASCE 7-88 was updated to ASCE 7-95. The basic wind speed map within Section 1606.2, Alternate Wind Loads for Low-Rise Buildings, remained unchanged from the 1994 edition. It is necessary to point this out because the basic wind speed map of ASCE 7-95 was based on the three-second gust wind speed.

The 1999 edition remained unchanged from the 1997 edition and was the last edition of the SBC.

Uniform Building Code — The wind design provisions of the UBC, through its 1979 edition, were based on ANSI A58.1-1955, the predecessor document to ANSI A58.1-1972.

The wind design provisions became based on ANSI A58.1-1972 in the 1982 edition of the UBC. The calculation procedure was simplified. Also, important changes proposed for ANSI A58.1-1982 were incorporated. Few changes were made in the 1985 and 1988 editions of the UBC.

The UBC wind design provisions became based on ASCE 7-88 in the 1991 edition. The calculation procedure was once again simplified. Minor changes were made in the 1994 edition, and no changes in the 1997 edition, the last edition of the UBC.

International Building Code — The first edition of the IBC, the 2000 edition, adopted ASCE 7-98 for wind design. However, Method 1, Simplified Design from ASCE 7-98, was not adopted. Included in Section 1609.6 of the IBC code was a different simplified design procedure, based on the low-rise analytical procedure (part of Method 2) of ASCE 7-98 and applicable only to simple diaphragm buildings, as defined in the code. For qualifying residential buildings, free of topographic effects, the SBCCI deemed-to-comply standard SSTD 10, Standard for Hurricane-Resistant Residential Construction, and the American Forest & Paper Association’s (AF&PA) Wood Frame Construction Manual (WFCM) also were allowed to be used. The 2000 IBC also added an alternative way of providing opening protection in one- and two-story buildings, included a conversion table between fastest-mile wind speed and three-second gust wind speed, and provided an optional design procedure for rigid tile roof coverings.

The second edition of the IBC, published in 2003, adopted ASCE 7-02 for wind design. There was still a simplified design procedure, applicable to simple diaphragm buildings, in Section 1609.6 of the IBC. But it was now very close to Method 1, Simplified [Design] Procedure of ASCE 7-02, because (as mentioned in Part one of this article) ASCE 7-02 discarded Method 1 of ASCE 7-98, and adopted instead the simplified design procedure in Section 1609.6 of the 2000 IBC with some modifications. Qualifying residential buildings free of topographic effects could still be designed by SBCCI’s SSTD 10 or AF&PA’s WFCM. The alternative way of providing opening protection in one- and two-story buildings, the conversion table between fastest-mile wind speed and three-second gust wind speed, and the optional design procedure for rigid tile roof coverings remained essentially unchanged.

ASCE 7-05 is adopted for wind design in the third edition of the IBC, which was published in 2006. Simplified wind design is no longer in the code; it is by reference to ASCE 7-05. Qualifying residential buildings free of topographic effects can still be designed by SBCCI’s SSTD 10 or AF&PA’s WFCM. The alternative way of providing opening protection in one- and two-story buildings is retained in a modified form in the 2006 IBC. The conversion table between fastest-mile wind speed and three-second gust wind speed is revised. The optional design procedure for rigid tile roof coverings remains unchanged.

1997 UBC versus 2006 IBC—A comparison

Design wind forces at the various floor levels of an example concrete building, the plan and elevation of which are shown in Figures 1 and 2, respectively, were calculated using the general analytical procedure (Method 2) of ASCE 7-05 (which has been adopted into the 2006 IBC) and the wind design procedure of the 1997 UBC, which is a simplified version of that in ASCE 7-88. The building is assumed to be located in suburban Los Angeles (three-second gust wind speed of 85 mph) and the exposure category is assumed to be B. The simplification of the analytical procedure of the 1997 UBC was the result of a joint effort by the Structural Engineers Association of California (SEAOC) and the Structural Engineers Association of Washington (SEAW). It can be seen in Table 1 that the UBC procedure produces slightly, but not overly, conservative results, as it should. The efforts involved in the two cases were not comparable, with the ASCE 7-05 design taking considerably more time and being more complex (even though the different load cases in Figure 6-9 of ASCE 7-05, other than Load Case 1 were not even considered). The primary reason that accounts for the additional time is that the simplifications made by SEAOC/SEAW to the provisions of ASCE 7-88 are not available to the user of Method 2 of ASCE 7-05. Also, as outlined in preceding sections, many complexities have been added to the wind design provisions of ASCE 7 between the 1988 and the 2005 editions. One example of the added complexity is the prescribed procedure for the computation of gust-effect factors for flexible buildings. The example building being flexible, the gust-effect factor had to be calculated. The calculation involves a large number of complex equations, and took an experienced engineer over an hour and a half to complete. Ironically, the factor turned out to be 0.87, which should be compared with the 0.85 prescribed for rigid buildings. While no generalization is possible on the basis of one example, the UBC procedure, which has been in the UBC since 1991, has been used in the design of a large population of structures located west of the Mississippi, in Indiana, and elsewhere. There is no record of distress that has been attributed to any deficiency in that design procedure.

When the state of Oregon adopted the 2003 IBC as the basis of the 2004 Oregon Structural Specialty Code, it made an amendment to the 2003 IBC allowing continued usage of the 1997 UBC wind design procedure (as adopted into the 1998 Oregon Structural Specialty Code). The state of Washington did not make a similar amendment when it adopted the 2003 IBC as the basis of the state code a few months ahead of Oregon. A simplification of the analytical procedures of ASCE 7-98 and -02 was under development by the SEAW for quite some time. The simplified procedure—SEAW’s Handbook of a Rapid Solutions Methodology for Wind Design—has recently been published. This procedure, however, does not appear ready for codification.

Conclusion

There is an urgent need for a design procedure in the IBC, which is similar to the design procedure included in the 1997 UBC. Its applicability, of course, would be somewhat restricted. The UBC design procedure itself cannot be used as it is. It will need to be updated because, for one thing, it is based on fastest-mile wind speed, which is no longer recorded by the National Weather Service. It is outdated in some other ways as well. The most effective way of accomplishing an update would be through collaboration among groups such as the Structural Engineers Associations of California, Oregon, and Washington. Early action to bring about such collaboration is strongly urged.

Acknowledgements
Much gratitude is expressed to Jim Messersmith and Steve Skalko of the Portland Cement Association and Susan Dowty of S. K. Ghosh Associates, Inc., for their contributions to this paper.

S.K. Ghosh, Ph.D., is the president of S.K. Ghosh Associates Inc., a seismic and building code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. He is active in the development and interpretation of national structural code provisions and can be contacted at skghosh@aol.com or via www.skghoshassociates.com.

FIGURES & TABLES

Figure 1: Plan of example concrete building

Figure 2: Elevation of example concrete building

 

Table 1: Comparison of computed wind forces for example building

Floor Level

Wind Forces (plf)

Wind Forces (plf)

2006 IBC/1997 UBC

 

1997 UBC

2006 IBC

 

R

235

214

0.91

20

314

286

0.91

19

310

284

0.92

18

306

281

0.92

17

302

278

0.92

16

298

275

0.92

15

293

272

0.93

14

290

269

0.93

13

285

265

0.93

12

282

262

0.93

11

277

258

0.93

10

272

254

0.93

9

267

249

0.93

8

261

244

0.94

7

253

239

0.94

6

247

233

0.94

5

239

226

0.94

4

229

218

0.95

3

218

208

0.95

2

243

233

0.96

X