Eccentrically Loaded Spread Footing Design Question
Eccentrically Loaded Spread Footing Design Question
(OP)
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.
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.






RE: Eccentrically Loaded Spread Footing Design Question
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.
RE: Eccentrically Loaded Spread Footing Design Question
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.
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
what is the rationale for that?
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
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.
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
I don't think this is a matter of being conservative. No one said to use factored loads to check the soil bearing capacity. It was only questioned that when factoring the loads to design the rebar and thickness of the footing which e to use. If you don't use the e from the factored loads then you are simply not factoring them correctly (because e is nothing more than M/P - if you use something other than the factored M/P then the loads aren't factored as required by the ACI/IBC strength load combinations).
I would never use factored loads and attempt to keep that soil bearing pressure below the allowable - THAT is extremely conservative.
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
Great question! I have never really thought of this before as I dont generally design fixed bases for portal frames. It would make some difference though.
RE: Eccentrically Loaded Spread Footing Design Question
Say in the service LC: D + 0.75 x (L + W)
And compare that to the similar strength LC: 1.2 D + 1.6 W + 1.0 L.
Your dead load contributions went up 60%, while your live went up 33% and your wind went up 213 %. Since D & L contributed to your axial, and the moment is likely due mostly to W, the ratio will change.
RE: Eccentrically Loaded Spread Footing Design Question
However, in ACI-318, it defines W as "wind load, or related internal moments and forces", and similarly for the other terms. This seems to allow either approach. I think this came up in a previous post about calculating moments in a footing with partial uplift.
RE: Eccentrically Loaded Spread Footing Design Question
Do I need to increase the size of my footing? I shouldn't have to because it is sized based on service loads. But I can't calculated a factored bearing pressure because as far as the calculations are concerned, a load can't be applied from outside the area of the footing.
So I really don't think it is as straight forward as some of these responses make it out to be.
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
My point is, there are many things we need to worry about. But a little extra reinforcing in a footing is not one of them. Many engineers analyze things to death which are relatively inconsequential to the overall performance of a structure.
RE: Eccentrically Loaded Spread Footing Design Question
1. Obtain service level soil stresses.
2. Multiply the service load stresses by a factor of 1.6, or 1.65 if LL>>DL, (since at the old time the DL factor was 1.4 & LL factor was 1.7)
3. Design the concrete footing use the factored stresses.
This method is acceptable for eccentricity/moment caused mainly by the transient loads (wind/earthquake). For permenent eccentrical loadings, you will need to find other solutions, best is to eliminate the eccentricity at the first place (using piles, ties, offset footing pad...).
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
By doing so, you are not necessarily design the concrete footing to withstand the whole factored loads (which is at the limit state), but you are sure that your footing will not fail before noticeable cracks suddenly developed on the walls/floors due to excessive progressive/permenent settlement, since the footing was sized against the allowable (elasticsteady state) soil pressure.
The unit load factor also ensure the footing has adequate strength to resist pressure above the presumptive static soil pressure at the split second of peak reversible loadings.
RE: Eccentrically Loaded Spread Footing Design Question
I would make a very large bet that if your e for service loads is 2.5' from centerline of a 6' square footing (the resultant is acting 6" from the edge of the footing) that your allowable soil stresses will rarely work.
This is not a hard discussion and I'm not seeing a case for using anything other than the factored loads. the factored loads are what they are. The e just comes along for the ride.
RE: Eccentrically Loaded Spread Footing Design Question
Trust me, it happens. Design a spread footing for a shear wall.
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
The two methods:
1) Determine soil bearing pressure with Service loads and pseudo-factor those pressures up to determine Factored level internal forces in the footing.
2) Determine the internal footing forces by using Factored level column forces, and the associated Factored level soil pressures.
None of the cases I ran showed more than a 10% difference in the critical internal footing forces. What was interesting was that the Overturning Safety Factor was up to 20% less when using method 2 (for higher Moment/Axial ratios). Of course there is some small error because my spreadsheet does not use any factoring for calculating footing self-weight.
I most agree with something kslee1000 touched on. The 10% discrepancy tends to indicate that the method selection does not substantially impact footing internal forces. I would focus my efforts on forcing the failure mode to behavior that warns the inhabitants of impending failure. Its scary to think that an overload situation would either break the footing or overstress the soil - because if that happens and the 'toe' moves... overturning stability disappears quicker than an Architect after receiving an invoice.
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
I don't think that designing this condition with applied factored loads is acceptable. It frustrates me that the building codes don't address things like this.
RE: Eccentrically Loaded Spread Footing Design Question
When designing the base for flexural steel, I then assume that the full allowable bearing pressure is developed over then entire base, convert to ultimate loads (i.e. multiply by 1.35) then design the flexural steel based on this loading.
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question
RE: Eccentrically Loaded Spread Footing Design Question