Braced Frame foundation

[hello12345hello]

I want to design the spread footing for steel diagonal braced frame. I just read this one thread on eng-tips about footing design.

http://www.eng-tips.com/viewthread.cfm?qid=228071&page=5

thread486-228071

I have braced frame footing with lateral load of 67 kip. I am tying both footings together. so there will be axial load in beam. As i have grade beam running between the footing so that there will be uplift on one footing and downward load on other footing. TO check against overturning, i will make sure my footing weight is good enough for uplift. To check against sliding i will use passive pressure and sliding coefficient times vertical downward load on the opposite footing.

Now do i have to check that as isolated footing and consider moment at base of the footing M = Horizontal load*depth of footing. To check footing soil pressure do i have to consider this moment from horizontal lateral load.

q = P/A +- M/S.

or just q = P/A.

Thank you

 

[ishvaaag]

See attachment. It is better count some moment.

I think many wouldn't count both friction and passive push to equilibrate the horizontal loads, but it may be warranted since friction will mobilize passive push from deeper layers than just the front of the lead footing.

 

[ishvaaag]

When accounting for the reduction of moment respect the base, the height of friction respect base is zero and wouldn't reduce the applied moment; so only the resultant of the passive push is of use to such purpose. It is also worth to note that the passive push represents the maximum reaction that the soil in the front of the footing can develop, but at the applied loading may not be developed in full extent. Hence accounting for it at maximum value may result in underestimating the maximum vertical pressure on the soil respect what a more accurate model would show.

 

[ishvaaag]

... however seeing how the allowable/limit bearing stresses are determined, what involves the displacement of a passive wedge, quite likely the use of the whole passive push -that also uses such kind of wedge- to diminish the applied moment may be in the end acceptable.

 

[ishvaaag]

The line of thought behing the previous entry is "when the soil is bound to fail, the whole passive earth will be being delivered, and acting on whatever the footing is doing", so the whole upper value of passive push will be available for reduction of the applied moment when one is going to check against the applied loads.

All the process to be done with the factors mandated by the codes that distinguish their favourable or unfavourable behaviour.