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RE: Wind load cases

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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.

 

Rich

 

 

From: cairo briceno [mailto:cobad2008(--nospam--at)gmail.com]
Sent: Tuesday, March 09, 2010 11:14 AM
To: seaint(--nospam--at)seaint.org
Subject: Wind load cases

 

When using ASCE 7-05, load combination for wind case 2.4.7 as follow:

0.6D+W+H


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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