Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: ASCE 7-98 Wind Pressure Questions

[Subject Prev][Subject Next][Thread Prev][Thread Next]
>In C6.5.5 of ASCE 7-98, it states that design for applications
>of serviceability, design using maximum likely events, or other
>applications, wind speeds associated with the mean recurrence
>intervals other than 50 years may be used by multiplying the
>design pressures by the factors in Table C6-3.  Has anyone
>used this clause to justify using smaller (or larger) pressures? 
Yes. One example I can remember is in AISC Design Guide 10 Erection Bracing of Low-Rise Structural Steel Frames. There is a discussion of exposure period for an erected steel frame, which is comparatively much shorter than the exposure for that same frame in the completed building. Therefore, one can take advantage of the factors in the table you referenced to adjust the same basic wind exposures to use a lesser wind load in design for the erection phase. It does not, however, reduce the wind load on the final building.
>Another question, in ASCE 7-98, the directionality factor has
>been separated from the load factor and is now included in the
>equation for velocity pressure.  Consequently, the velocity pressure
>is reduced by 0.85 while the new load factor is increased from 1.3
>(in ASCE 7-95) to 1.6.  My question is why weren't the ASD load
>combinations affected?  Are the wind pressures now lower for
>ASD combos with 7-98 than with 7-95?
I asked the same question. No answer received to date. I think you are correct that the ASD combinations should have been affected as well. I will inquire again and report back if I get an answer.