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Re: ASCE7/IBC 2000 minimum wind load

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Dwain,
Check out the archives around October 30 for "Minimum wind loads in the IBC". Jason Kilgore had a good answer to a similar question about IBC minimum wind loads. The bottom line was, determine the wind loads using the code provisions and then compare this total force to the total force calculated using 10 psf over the projected area and then use the larger case.

The previous thread was about the simplified method in the IBC, whereas your question is about the low rise method in ASCE 7-98, and there is a difference. The IBC method says to ignore negative horizontal roof loads which reduce the horizontal force on the structure. The ASCE 7 method doesn't say this, but it does have two provisions you should look at : see footnotes 4a and 4b under Figure 6-4.

Rick Burch
Columbia, SC


Dwain Hendershot wrote:

I am looking at wind loads on a 3-story building with a 4:12 gable roof
using the low-rise option in ASCE7 section 6.5.12.2.2.  The low slope of the
roof results in suction pressure on both sides of the ridge.  When the roof
loads are broken down into components on the vertical projected area the
resulting horizontal force is actually acting against the windward
direction.  IBC 2000 section 1609.1.2 states that the design of MWFRS shall
not be less than 10psf on the vertical projected area.

Does this mean that you use 10psf on the entire projected area vs. the total
of resultant forces from walls and roofs? Or do you use 10psf anywhere were
the horizontal load is less than 10psf, i.e. at the roof projection.  This
will increase the loads on the shearwalls significantly especially at the
3rd level.

Any comments are appreciated.

Regards,

Dwain Hendershot




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