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FOS forOvtg clarification[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: FOS forOvtg clarification
- From: <MSSROLLO(--nospam--at)aol.com>
- Date: Thu, 25 Jun 1998 22:03:44 EDT
I have got several responses concerning the background of this problem (wind loading, type of fdtn etc) and would like to clarify this. I really appreciate all the responses. I am trying to find a consensus on whether the vertical load should be considered an overturning force or not. For sake of this, I will say that the vertical load and horizontal load are actually a Dead Loads created by a cantilevered beam on a rigid frame. So all loads are Dead loads. The foundation is poured on top of the ground. Therefor there is no passive soil pressure present. Once there is somewhat of a consensus on this, I would like to change the problem a little, such as the force is a DEad and wind force and that the column is a little off-center. For now though, I am concerned with a centered column with a 10k upward load induced by dead load. #1. Is the vertical force an overturning force?..and why/why not #2. What would be the correct calculation for FOS about the right toe as shown by the loading in my AutoBadd sketch. I have included the original posting below.....Thanks again for the help...I am trying to fine tune a footing program that at least models what a consensus of experienced engineers beleive. I was hoping this group would at least clear up whether the force is an overturning one or not and the correct calculation for the FOS. have a question regarding the correct factor of safety for overturning for a foundation subjected to an uplift force. For purposes of this discussion, assume I have a single footing with a horizontal force or 3 kips (to the right) and a upward vertical force of 10 kips, both applied at the center of the top of the footing. Assume the footing is 15 kips in weight and is 8'x 6' x 2' thick. Pardon the phony graphics, I am using AutoBadd. ^ 10k | | ------> 3k --------------- | | | | 2' | | --------------- 8' 1. In calculating the FS for overturning, is it correct to call the 10k load an overturning load? The load is at the center of the foundation and does not actually induce any overturning except if you do statics about a toe. The load itself actually does not cause overturning. Even statics claims that is overturns about both sides simultaneously. I realize that the footing weight is always a stabilzing force in both directions of rotation. 2. Assuming you do think it should be included as an overturning force, would the correct calcualtion be Movtg = 3x2 + 10x4 = 46 and Mstab= 15x4 = 60, therefore FS = 1.304 OR is the correct one: Movt = 3x2=6 and Mstab= (15-10)x4=20 FS = 20/6 = 3.333. By definition, it would appear that the 1.3 answer is correct and that algebraically adding common forces is not permitted. 3. Assuming you do not think the load is an overturning load, what if I move it off center to the right 1". What is the magnitude of moment to attribute to overturning of the right side? Thanks for any help on this..
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