huangyhg

wall footing for eccentric loading
hello all,
i am hoping to get some guidance here for a project i am working on. i am designing a masonry building to be located adjacent to an existing building. the wall of the new building is about 12" away from the existing one. i am currently designing the footing for the new wall, but i am having a little difficulty. the wall is sitting on the edge of the footing. its an 8" wall, i have assumed a 3' wide, 12" deep wall footing. i know in combined footings, we try to ensure that the centroid of column loads coincides with the centroid of the footing....does that apply here? i assume i would have to check for punching shear as a regular footing design (a distance 'd' from the wall face). but how (or can i) to check for two-way shear?
also, in designing for longitudinal reinforcement (parallel to the wall), is that steel just for temperature and shrinkage? or should i design steel in that direction as though in a beam directly supporting the wall? i hope that my questions and thoughts are clear.....and that i can get some help.
thanks so much!
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your questions are clear, but i think you are asking the wrong questions. with the wall on the edge of the footing, you need to find a way of rectifying this eccentricity if the footing is to be fully effective. possibilities are straps at right angles tied to other building footings, tying the footing to the floor slab to resist the bending, etc.
how did you select the proportions of the footing? is the 3 ft width required because of allowable soil pressure? would a narrower footing work?
shear in wall footings rarely controls, although it should be considered. the transverse shear check you describe is really beam shear. punching shear applies to concentrated loads. longitudinal shear would not be an issue with a strip footing because of continuous support.
longitudinal reinforcing for the wall footing would depend on a number of things. if you want to design for possible soft spots in the founding material, design for bending over the assumed area. if the block wall has enough reinforcing, you may need no reinforcing in the footing.
just design the footing with a triangular bearing pressure (make sure it doesn't exceed the max alloable).
you will get your shear and moment from that distribution. steel in the direction of the wall (long direction) is for t&s only.
there is no punching shear in a wall footing, check beam shear only.
if you do not rectify the moment due to eccentricity, this moment should transfer to the wall, which then you have to consider in the wall design!
shin,
there is nothing wrong with having a moment on the footing and no moment on the wall. the resultant of the triangular pressure distribution from the soil will simply coincide with the centroid of the wall. from that known point, verify that the max soil pressure does not exceed the allowable and you're good to go, right?
i would make sure you're within the kern distance. some authors and engineers will tell you that having the load resultant outside the kern forces the footing to rotate too much to do its job. while i haven't seen anything that makes a hard and fast rule, i believe it's good practice to design in the kern.
just check your footing with eccentric load due to an offset force. you'll have to provide a heavy enough footing to counter the offset wall location. it's easy to solve for the footing you need by setting up a spreadsheet.
or you can cast the footing monolithically with the slab and add top steel to take the tension force due to the eccentricity back into the slab. works for me. designed a tilt-up building property line footing that way 20 years ago and it is still there with no distress.
mike mccann
mccann engineering
structuraleit,
in the case posted here, if there is not rectifying moment, the total width of triangular bearing would be 1.5 x wall width, or 12". so the 36" wide footing doesn't compute.
i certainly agree that the 36" is overkill as the extra 24" is doing nothing for you. that being said, just for equilibrium the resultant of the soil reaction needs to be at the centroid of the wall unless you design the connection for the moment. if you don't and the max soil pressure isn't exceeded, you should be fine, correct?
thanks all for your help. my internet was down so i could not respond until now. i chose the 3 foot width because thats the width of the footing at the other walls for the building (due to bearing pressure on the soil). are you saying then that it would be better to make the footing narrower and design as a beam rather than a footing? i guess that would help reduce the eccentricity? the only issue there though is that the existing footing under the adjacent existing wall might be in the way if i increase the depth of the footing/beam.
i think you need to find a good foundation engineering text. from your posts i think you are missing some basic knowledge. braja das and coduto both have good foundation texts imo.
you should be solving for a footing width required so that your soil pressure works. you can do this with a numerical solver such as a spreadsheet or mathcad, a closed-form solution perhaps, or whatever means suits you best. you need to draw a fbd, show all your loads and locations, figure the eccentricity and then check the soil pressure as appropriate. as soon as you have your eccentricity and total load on the soil, the rest is fairly simple.

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