Transferring shear on strap footings

[ketanco]

Hello

on All in One latest edition page 384, the design of strap footings is asked.

In addition to axial loads, the also calculated a moment based on the axial load of one footing times distance to the other footing and borught it as a moment to second footing. and based on that moment, from moment arm, they calculated an additional axial load on first footing. then they deducted that transferred axial load from the first footing

sounds confusing but here is my question. how do we know from which footing to which footing to transfer the load? i mean why did we add axial load to footing 2 and deducted it from 1 and not the opposite

 

[civilized_naah]

The reason the moment is on the footing on the left is the space constraint on that side The prelim size of the footing A is 6 ft, which places the center of footing at 3 ft from the edge, while the column and therefore the load is 1 ft from the edge. The eccentricity of the load on footing A is therefore 2 ft. This causes there to be a counterclockwise moment of 180 k-ft on that footing in addition to the column load of 90 k. If the strap were not used, footing A would have to carry P=90 k & M = 180 k-ft by itself, this would cause uplift on the right side of footing A and max soil pressure (on the left) = 9 ksf (approx)

The use of the strap makes the 2 footing combination a 'system', which must resist this couple of 180 k-ft (the opposing couple would be an extra up force under (center of) footing A and an extra downforce under center of footing B). Since the moment arm for these forces is the center to center distance between footing centers (20 ft), the 'delta' force is 180/20=9 k. This force increment increases the soil reaction under footing A by 9k and reduces the soil reaction under footing B by 9k.

Note that, by using the strap beam, the soil pressures have been kept to approx 2.5 ksf (instead of the 9 ksf otherwise)

 

[ketanco]

ok i see. i now understand the logic there, so by using a strap beam, we are creating a moment arm of 20 feet, which is a moment couple, to resist the moment on footing A.i am clear on this mechanism now...

but then here is my next question (which relates to my original question): from your description above, we thought of the system from footing A point of view. We are treating footing B to be there to help footing A.

but how about we think of the situation from footing B point of view? such as, an axial load is being applied to footing B. and because of the strap beam, a certain portion of this vertical load will now be transferred to footing A, (just because of the axial load on B). so this is now a different calculation.

I mean i fully understand the moment resisting mechanism you describe above, but we treated the load on footing B as if it is being applied "later". but in this whole system, there is not only footing A, its load and moment and footing B, but also, there is that 180 kip load on footing B, which is the whole system. we thought everything from footing A point of view. I hope I am able to explain... i mean for instance, if you tried to draw a free body diagram of the whole thing, that axial load on footing B will also be in the picture and be a part of the whole calculation in the first place.