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ASCE 7-05 vs ASCE 7-10 – Explaining Changes in Building Code & ASCE 7-10 Wind Uplift Calculations

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As you may already be aware, the International Code Council has published a new version of code, the 2012 ICC Codes.   Within this new publication, the 2012 International Building Code requires the use of a new version of the standard used to calculate wind uplift pressures on buildings and other structures, ASCE 7-10.  This version of ASCE 7 includes a different method of calculating wind uplift pressures when compared to ASCE 7-05.  The most noticeable difference is the design wind speed.

Currently, only Maryland and Florida have adopted the new ASCE 7-10.  However, as states begin to adopt the new 2012 IBC; this revised method of calculating wind uplift pressures will be required.

 

HOW DOES THIS AFFECT YOU?

There is not much of a difference in the results of the wind uplift calculations.  Roof system attachment design will remain largely the same when compared to roof system attachment requirements designed in accordance with ASCE 7-05.

 

MAJOR CHANGES EXPLAINED:

Currently, ASCE 7-05 references one wind speed map for the entire country.  This wind speed, along with a number of other factors, is used to determine the wind uplift pressures.  The new ASCE 7-10 involves three different wind speed maps: one for Importance Class I Buildings, one for Importance Class II buildings and one for Importance Class III & IV buildings.

– Importance Class I buildings are classified as structures that represent a low risk in the event of failure.  These include agricultural buildings and storage sheds. 

– Importance Class II buildings include the majority of structures.  These include residential buildings and small offices.

– Importance Class III Buildings include structures that pose a substantial risk to human life in the even of a failure.  These include schools and other buildings where large amounts of people can congregate in one area.

– Importance Class IV Buildings include structures that are considered essential facilities and can pose a substantial hazard to the community.  These include emergency health care facilities and facilities that are essential to national security.

For example: An Importance Class III school building in the Midwest United States is assigned to a design wind speed of 90 mph in ASCE 7-05.  ASCE 7-10 will assign this region a wind speed of 120 mph.  However, the new 2012 IBC permits an allowable stress design factor to be applied to the wind speed that actually reduces calculated wind uplift pressures to levels similar, if not less than the ASCE 7-05 version. 

Also, Category C is used for hurricane prone regions in ASCE 7-05.  ASCE 7-10 uses an exposure D along all coast lines.

Section 1609.1.1 of the 2012 International Building Code requires that "Wind loads on every building or structure shall be determined in accordance with Chapters 26 to 30 of ASCE 7 or provisions of the alternate all-heights method in Section 1609.6".  Whether you are dealing with projects that involve new construction, retrofitting an existing roof system or even complete tear-offs, wind uplift calculations need to be performed.  Codes are revised approximately every three years, and one must be confident that the roof systems that are being designed are compliant with the most recent code requirements.

 

By performing wind uplift calculations, you are assuring that your roof systems are being designed properly, and just as importantly, being installed in accordance with building code.

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