I think that by reducing the footing dead load by 40% is an overly conservative design, since the forces that are being designed for are a statistical best guess. Also since the uplift forces for a shearwall or moment frame are cyclic and only last for a very short duration I really can’t see the footing, flying up into the air, especially if it is attached to the rest of the foundation.
This also brings up another issue that I have been thinking about a lot recently.
As engineers I think that very often we loose sight of the intent as to why we are consulted. Life safety first and foremost, but too many times I think we design with the intent of a lawsuit that may occur if anything goes wrong. Unfortunately this is the reality that there are too many greedy as**^le lawyers as well as people that are all too willing to sue at the drop of a hat.
From: Drew Morris [mailto:firstname.lastname@example.org]
Sent: Tuesday, November 22, 2011 1:03 PM
Subject: Re: FTG UPLIFT
We use the 0.6 for the footing and soil overburden weight. I always assumed the 0.6 was acting a "safety factor". I sat through a ASCE webinar a few years back where the presenter used a 0.6 factor for the building structure and associated loads and 1.0 for the footing and soil overburden. I'll try track down my notes for that presentation.
On 11/21/2011 5:26 PM, William Haynes wrote:
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?
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