Stuart, I would be interested in hearing the method that you are talking about.
From: Stuart, Matthew [mailto:MStuart@Pennoni.com]
Sent: Wednesday, November 23, 2011 6:25 AM
Subject: RE: FTG UPLIFT
I have a method of calculating a way to engage a large area of the slab on grade above as a part of the uplift resistance.
D. Matthew Stuart, P.E., S.E., F.ASCE, SECB
Structural Division Manager
Pennoni Associates Inc.
Office 215-222-3000 x7895 | Direct 215-254-7895
Fax 215-222-0789 |
http://www.pennoni.com | email@example.com
My office just served as the engineer for PEMB foundations on a large project. We did use the 0.6 factor on the self-weight of the footings, but there was a lot of discussion in our office about it. We could not find an authoritative source that would allow using 100% of the footing weight, although it seems to make sense for the reasons you describe. I'm interested to hear what others have done.
I wanted to see how many are using 0.6xFTG Self Weight of the footing when checking uplift due to wind or using the entire footing weight, 1.0xFTG Self Weight to help resist uplift. There appears to be several engineers that use 0.6x(everything else dead load except the footing) + 1.0x(FTG Self Weight) to check versus 1.0xGross wind uplift. The concrete has no allowance to be lighter than what you assume, and the footings have to be consistently dug to at least the dimensions you specify on your drawings. Is there anyone involved in forensics of metal buildings that have seen the footings uplift out of the ground when the code prescribed 0.6 wasn't factored on the footing self weight?