04-08-14 Building Codes Appeals Board

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ASCE 7-10 Wind Provisions and Effects on Wood Design and Construction Philip I..,ine, P.E.3 William L. Coulbourne, P.E. M.ASCE2 ABSTRACT It is well known that the major change for wind design in ASCE 7-10 Minirnarrrr design I oads for Buildings and Other Structures is the introduction of new wind speed wraps that are referred to as ultimate wind speed maps in the 2012 International Building Coale (IBC). Several other coordinated changes include: • revised load factors for wind in allowable stress design (ASD) and load and resistance factor design (LRFD) load combinations, • removal of the Occupancy Factor for wind, • reinstating applicability of Exposure D in hurricane prone regions, d revised wind speed triggers for definition of hurricane prone region and wind-borne debris region, and, 6 revised pressure values for ininimunn design loads. This paper will explore the net effect of these changes oil calculated design velocity pressures and provide comparison tables for select geographic locations. It will also compare select provisions of ASCE 7-10 with similar provisions in ASCE 7-05, and discuss implementation of ASCE 7-10 in model codes and the Wood Frarne Construction t llanual for• One -and Tito-farndjj Drtellings (YT'FC'M). INTRODUCTION Wind design in ASCE 7-10 incorporates several major changes. Among the changes are new wind speed maps that vary by risk category (e.g. separate maps are provided for each of the following risk. categories: 1, 11, and 111, and IV) and incorporation of uniform recurrence interval wind speed contours throughout all geographic regions including hurricane prone regions of tine U.S. These changes directly affect calculation of unfactored wind loads. Revised load factors for wind in ASD and LRFD load combinations were coordinated to compensate for the new wind speeds, resulting in design velocity pressures that are very similar to those calculated usirng provisions of ASCE 7-0.5 for most US, regions, hi addition, recent studies of hurricane winds over open water resulted in changes to hurricane wind modeling that, in general,increased wind speeds near the hurricane "eye,' reduced wind speeds over the broader storm area, and revised the definition of Exposure D so that it is no longer precluded frorn being applicable in hurricane prone regions. The outcorne of these changes are that design velocity pressures are reduced in some hurricane prone regions while design velocity pressures remain largely unchanged in non -hurricane prone regions. To confirm this, design velocity pressure per ASCE7-10 was calculated and compared to design velocity pressure determined in accordance with ASCE 7-0.5. In addition to serving as limited confirmation of generally expected outcornes, the purpose of the comparison is two -fold: to illustrate where differences in calculations occur; and provide insight into the effect of changes oil calculated pressures for specific locations and buildings of varying risk categories.