PEMB continuous footing to resist sliding

[cec17]

See below. In a traditional PEMB with perimeter column footings, do you ever consider the continuous footings to aid in sliding resistance? It's not something I have done, usually I design the column footing isolated to resist overturning and sliding, or hairpin to the slab for sliding resistance. I see no mention in Newman of using some portion of an adjacent continuous wall footing to resist sliding, not sure how that would be quantified (using the whole trib width?). Just wondering if this is something that others have considered as I can't see any reference to it.

 

[DaveAtkins]

I think it could be an acceptable load path, so long as you check lateral bending, shear, development, etc.

DaveAtkins

 

[canwesteng]

Does shear even need to transfer for sliding resistance? If you use friction with the soil, you just need to transfer vertical loads.

 

[dflva]

I have designed foundations for a handful of PEMB projects. I have not considered the continuous footings adjacent to the spread footing in resisting sliding, but I agree there is some legitimate amount of contribution. Determining a reasonable limit on the effective length of footing is a tough problem. I can say halfway to the next column is too much. I do like the stem wall tying everything together. Also remember that some amount of the wind load on wall (bottom half?, consider girt spacing) is resisted directly by the continuous footing and could be in the same direction as the D+L+W thrust.

 

[johnwow]

I do PEMB foundations regularly. I have a design to handle most building sizes where we use continuous footings to transfer all loads rather than isolated spread footings. The size of these isolated footings are wild considering the first thing that's going fly off on these tin cans is the metal roof. We do still add in hairpins for thrust.

 

[SE2321]

Yes, there will be some contribution from continuous footing to resist sliding. But, determining the exact amount is difficult. I have designed many isolated spread footings for PEMB structures, but, never considered any contribution from continuous footing.

 

[bones206]

I see no mention in Newman of using some portion of an adjacent continuous wall footing to resist sliding, not sure how that would be quantified (using the whole trib width?)

He actually makes passing reference to this in Design Example 7.1. In his example, he engages the full trib length of the stem wall to gain 23 kips of additional dead load for overturning resistance. He mentions in passing that engaging the passive pressure of the wall for sliding is an option that could also be pursued, but doesn't get into details on the effective length of wall for passive resistance.

My view is that it's ok to use the wall for this purpose, but to be conservative on both the effective wall length and the available passive pressure. I cut the passive pressure coefficient in half due to serviceability (lateral spread) concerns.

 

[Lomarandil]

I'll also vote for using it, but being sure to consider lateral spread/deflections under service as bones mentioned and considering what types of joints may be present between the footings. In some cases those end up being isolated to manage relative vertical deflections, which can limit how useful they would be for combined lateral resistance

 

[TRAK.Structural]

I think it's fair to account for some extra resistance from the continuous footing; but I can't imagine you will get much resistance due to the dead load away from frame columns will be very light. If you are just looking for a little extra to get you over the edge than go for it, if you are needing significantly more capacity than upsize the spread footing or consider hairpins.

 

[JAE]

I would consider the extended footings to contribute to uplift resistance and perhaps also some reduced passive resistance but NOT for sliding resistance...there's such little dead load pressure on the soil under those things that you wouldn't really have much to depend on.