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Re: Wind load cases[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Wind load cases
- From: cairo briceno <cobad2008(--nospam--at)gmail.com>
- Date: Sat, 13 Mar 2010 10:49:29 -0800
I thank you guys for provide me with reference. As Rich mention this combination yields problems with tall slender structures lightweight structures for wind design. It is an stability problem as you guys mention and it yield massive foundations and large soil pressures.
This was formally adopted by the 2009 IBC – see Section 1605.1.1.
The 1.5 FOS is maintained for retaining walls, but there are specific provisions in 1807.2.3 that adjust the load combinations.
GARY J. EHRLICH, P.E.
National Association of Home Builders
D 202 266 8545
Yes, the 0.6D + W load combination takes into account the old factors of safety of 1.50. If you read the ASCE 7 commentary you will see they have a brief discussion that the change was made around 2000 and they wanted to use a load combination equation instead of an arbitrary FS value. I don’t have my code with me, I’m going from memory right now. When you use this equation your FS=1.0
The challenge occurs when you have large uplift or overturning wind forces and it blows away your allowable bearing pressures. I have run into this a lot. I’ve decided that this is a safety stability issue, not a valid load case the footing will ever feel, so I tend to ignore the overload of the soil bearing pressure on this load combination.
When using ASCE 7-05, load combination for wind case 2.4.7 as follow:
from this load case analysis, reactions forces at foundation are generated. What are the proper safety factors:
Sliding: Safety Factor=1.0 or 1.5
Overturning: Safety Factor=1.0 or 1.5
Upward reaction: Safety factor 1.0 or 1.5
I think it could be 1.5, but my coworkers state the 1.5 has been built it into the load combination in the following form: 0.9D/1.5= 0.6D.
Does anybody has come across any ASCE reference or commentary about this subject?
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