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Net Uplift on Ordinary Brace Frame Square Pad Footing

Net Uplift on Ordinary Brace Frame Square Pad Footing

(OP)
My questions are in regards to the design weight required to resist uplift on a brace frame column due to seismic loads. I've read a lot regarding wind loads but those are not based on the weight of the structure so I think that issue is different.

I am designing a structure in a high seismic hazard region. I do have net uplift on the footing.

My first question - I'm using the seismic load combinations from ASCE 7-05 section 12.4.2.3 Allowable Stress Design equation 8 - (06-.14Sds)DL + 0.7*Rho*Qe + H. I have no H force. My resultant from this equation is 0.44DL - 0.62DL (because the Qe force is based on the DL). Why do I have to use these combinations instead of the equations in section 2.4.1?

My second question - Do I have to reduce the weight of my footing by 0.44 (or 0.6 when using section 2.4.1)? That seems too conservative. My calculation based on the 0.44DL-0.62DL results in an uplift of 80 kips in the column. Does that mean my footing needs to weigh 80/0.44 = 181.8 kips? Please see my argument below.

Is the 0.44 to account for my over estimate of DL's? If so - then my seismic force is based on the over estimate and it would seem too conservative to over estimate the seismic force and turn around to under estimate the counter acting DL. Also - DL in the structure might not be so easy to confirm but I know the density of concrete and the size of the footing which isn't going to change so the unknown there is zero. Assuming it's built to my design.

Third question - I used to work for a company that did not include Rho in their OT and sliding design at the foundation. I can't think of any arguement to support that. Any comments on that?

RE: Net Uplift on Ordinary Brace Frame Square Pad Footing

Section 12.4.2.3 requires of the use of the load comb. in 12.4.  If .44 is what .6-.14Sds gives you then yes you would need to use it.  Some codes modifies ASCE load comb. you should check to see if the local code modifies the load comb.

The other company had no argument to not use it.

RE: Net Uplift on Ordinary Brace Frame Square Pad Footing

Have you considered reducing the overturning effect per section 12.13.4. Note that FEMA 450 commentary section 5.2.5 states:

"In the design of the foundation, the overturning moment calculated at the foundation-soil interface may be reduced to 75 percent of the calculated value using Eq. 5.2-14. This is appropriate because a slight uplifting of one edge of the foundation during vibration leads to reduction in the overturning moment and because such behavior does not normally cause structural distress."

RE: Net Uplift on Ordinary Brace Frame Square Pad Footing

tdstructural Q-#1 In chapter 12 E is defined as a function of both horizontal and vertical components of the ground motion.  Eqn 8 in 2.4.1 and Eqn 8 in 12.4.2.3 are the same.
Q-#2 See 12.4.2.2 Vertical seismic load effect, exceptions 2- The vertical seismic load effect Ev is permitted to be taken as zero where determining on the soil-structure interface of foundations.
Thus effective weight would be 0.6(DL_above + Self wt.of footing). The weight of the footing has to be multiplied by 0.6 to maintain a overall factor of safety of 1.67,  ( 1 / 0.6 = 1.67).
Q#-3 Section 12.3.4.1 Conditions where value of rho is 1.0 - This section lists eight conditions where rho is permitted to be 1.0. The foundation is not one of them. The foundation will be subjected to the same rho as the structure above.
As posted by wannabeSE, reduction of foundation overturning is permitted per section 12.13.4.
 

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