To me you could more accurately define the weight of the steel columns sitting on the footing (or the entire steel frame if you have this information available in a design-build situation from the metal building manufacturer) than you can the concrete for the footings.
You have safety factors for everything in the entire load path until you get to the footing. Then you can use what is left of the wind (1.0xWIND) versus (1.0xFTG SW.)?
But in a light building with high uplift, there is not much left for error in the wind calcs (or if the pressures are actually different for a design wind event than what ASCE calculates) and the footings are doing the majority of the resistance.
On Tue, Nov 22, 2011 at 10:14 AM, Bill Polhemus <firstname.lastname@example.org> wrote:
I think you have to be practical about this. Sometimes we - and I include myself - get caught up in the letter of the Code and disregard the spirit. We are engineers not clerks.For instance: why the load factor of < 1.0 for this combination? I believe it's to reflect the uncertainty of just exactly how much dead weight you're accounting for in the design.Doesn't it seem reasonable that you might have more confidence in the weight of the foundation than the weight of whatever building you're contemplating?Personally I would assume the weight of the footing times 1.0 for at least one of my combinations if that is going to yield a greater force through the column connection than otherwise.
William L. Polhemus, Jr. P.E.Sent from my iPhone 4
On Nov 22, 2011, at 8:05 AM, Daniel Popp <email@example.com> wrote:Will,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.Regards,Dan Popp
From: William Haynes <firstname.lastname@example.org>
Sent: Monday, November 21, 2011 8:26 PM
Subject: FTG UPLIFT
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?