ASCE 7/37 Wind Loads

[Steve1415]

thread507-307748

See the reference thread.

I am working on developing some wind pressure profiles for temporary applications. I am running into the same question as the original poster for the referenced thread.

ASCE 7-05 (as well as previous versions) provides a table (C6-7) in the commentary allowing one to adjust the basic design wind speed for various Mean Recurrence Intervals. ASCE 37-02 provides a table based on the construction period, also allowing for basic design wind speed adjustment. The issue I see is that the values for similar time periods do not match up. For example, out of ASCE 7-05, for a mean recurrence interval of 5 years, the reduction factor would be 0.78. ASCE 37, however, recommends a factor of 0.9 for a construction period of between 2 and 5 years. Why the discrepancy?

I understand that there are two different committees responsible for the development of these documents. However, ASCE 37 references ASCE 7, and as such, one should be able to reasonably assume that the design standards jive. Can anyone provide any clarification?

 

[UcfSE]

What is the ASCE 7-05 factor for MRI = 2 years?

 

[WillisV]

I think it is important to define MRI and how it relates to probability. The mean recurrence interval is the inverse of the probability that a wind event of a certain magnitude will be exceeded in any one year. For example, a 50 year MRI means that during a single year, the probability of an event of this magnitude occurring is 2% (1/50 = 0.02). For a 5 year MRI, the probability of an event of this magnitude occurring at any time during a single year is 20% (1/5 = 0.2). Note that the design wind speed in ASCE 7-05 and before is based on a 50 year MRI.

In determining the wind speeds to use for construction periods in ASCE 37, the idea is NOT to determine the MRI for shorter and shorter recurrence intervals (which is what Table C6-7 in ASCE 7-05 does), as this would be essentially designing for smaller and smaller storms, the idea is to match the PROBABILITY that the storm damage that you typically design the building for during its 50 year life for will occur during a much shorter reference period (the duration of construction). I believe that is the distinction you may have been missing. We are not reducing the size of the wind event (MRI), we are just trying to match the reduced probability that this same size event will occur during a very brief reference period of time as compared to the 50 year presumed life of the structure

 

[WillisV]

The second important piece of the discrepancy relates to the fact that the equations used in the ASCE commentary associated with the reductions are based on a best fit average distribution for non-hurricane wind speeds across the country. The velocity reduction values presented in ASCE 37-02 represent the minimum reductions found at any site by analyzing real wind test data for a bunch of different sites.

 

[NOLAENG]

I agree with WillisV's first post. See the commentary in ASCE 37-02 (C6.2.1).