A New Way to Look at Wind – Changes in ASCE 7-10
Among changes to the 2012 IBC, recently incorporated into the Virginia Unified Statewide Building Code, is the method of wind load design. The reference to the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7) has been changed from ASCE 7-05 to ASCE 7-10. This article will look at the differences in how ASCE 7-10 assigns design wind loads, the rationale for those changes and what these changes will amount to for designers.
Under the previous version (ASCE 7-05) a facility was assigned an importance factor based on its occupancy. Facilities that would be more vital in the aftermath of an extreme wind event, such as a hospital or police station, or that would affect more people, such as a school, would be designated as of higher importance. A corresponding Importance Factor would be used in wind load calculations.
A single map indicated wind speeds assigned to various locations, using contour lines to indicate the various zones. These were nominal wind speeds – to be used for Allowable Stress Design (ASD) – the minimum standard. Where the Load Resistance Factored Design (Strength Design or SD) method was to be used – the designated wind speed (nominal wind speed) would be increased by a factor of 1.6. Either the result of this, or the nominal speed in the case of ASD, would then need to be multiplied by the structure’s Importance Factor.
The revised version assigns buildings to a Risk Category. All importance factors are 1.0 (basically, they are gone away.) Instead of one map for all categories – each Risk Category has its own wind speed map, based on wind events of different time periods of recurrence – or Mean Recurrence Intervals (MRI.) Higher risk categories are assigned wind maps based on longer recurrence intervals, in other words rarer, more severe wind events.
The wind speeds given by the new maps, “basic wind speeds” in ASCE 7 are “ultimate wind speeds” in the building code. These are to be used for SD without the need to multiply by a factor. For ASD, the wind speeds are reduced by a factor of .06. The wind speeds are higher than before, but the associated pressures are lower, reflecting different methods of understanding hurricanes.
The changes were largely implemented to clarify the process, and to remove chances for error due to the need to multiply by factors – particularly for the more critical SD method. The designer will need to be careful not to proceed by habit. Those used to applying numbers from the maps without needing to multiply will now need to reduce the wind speeds for ASD, and vice versa. The higher wind speeds combined with lower pressure will typically yield the same requirements as the previous version, but may reflect a clearer understanding of the workings of hurricanes and other wind events.