huangyhg

eccentrically loaded spread footing design question
i am designing a spread footing for a column that is part of a moment frame and am assuming that the column/footing joint is rigidly connected to reduce the deflections of my frame.
say my service-level gravity force is 300kips and the associated moment is 150 ft-kips.
when checking bearing capacity, i can calculate my eccentricity to be e = m/p = 150/300 = 0.5ft and determine my bearing stress distribution.
however, i can't find a reference that comments on whether my factored bearing pressure distribution used to design for shear and flexure should be calculated using e = 0.5ft (from service-level forces) or if i calculate a different eccentricity associated with my factored loads such that e = mu/pu.
can anyone point me to a published reference that i can include with a submitted calculation book?
thanks.
check out our whitepaper library.
i don't know that there is a reference other than to think about it. the service level e is based on un-factored loads and will obviously be different than your strength level e based on factored loads (since the load cases get different factors).
i would always use the actual e for the condition you are checking. using the e=0.5' is valid for bearing stresses because that is a service load check, but when doing your strength design you need to use the appropriate e.
but you can make the argument that the service-level 'e' is the appropriate eccentricity because that is the eccentricity that the footing is supposedly seeing, we've just increased the magnitude of the load on it to provide a factor of safety.
if i was designing this before the lrfd method became the professional standard i would be using the same eccentricity for the concrete design and the geotechnical design and would account for my factor of safety by reducing the allowable stress in the concrete. therefore i would always be using the 'real' eccentricity rather than some artificial creation that has increased various loads based on statistical factors.
but if you use the factored axial load with the non-factored e, you are getting something smaller than the factored moment that you are supposed to be designing for. the loads are factored for a reason (based on statistical factors as you mentioned), and you can't just ignore it.
actually, now that i'm thinking about it, for a single spread footing the e won't change, just the magnitude of the loads. both m and p are being factored by the same amounts so those factors will drop out in the m/p=e calc.
i guess you could have a case where they weren't equal. if there is pdead, but no mdead, and some plive and mlive. then the factored and non-factored e wouldn't be the same. either way, when figuring out the factored soil bearing pressures to be used with the footing design (which is reinforced concrete), you need to factor the loads and change all parameter as required.
i would use the service e for both.
ucfse-
what is the rationale for that?
i guess i am just looking at this and saying that e is nothing more than a function of m and p. if you are factoring your loads to design your footing, then e is what it is. how do you arbitrarily say, "ah, i don't feel like using that e". that means you are not using the right moment (or axial load)
i would use different e values also.
i think analysis should be performed seperately for service loads to calculate bearing pressures vs. allowable and for ultimate loads to calculate ultimate bearing pressures for concrete design (so i would use different e's). soil distribution under moment is a nonlinear problem.
using the same "e" under service and ultimate load is like saying you are going to perform a frame p-delta analysis with service level forces and then factor the results to get your ultimate level forces including p-delta.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]