Shallow footing design

[rlawrence67]

Typical spread footing with pedestal to grade. The building supplier provided the vertical and lateral loads. For design purposes can you reduce the lateral load applied at the pedestal height due to the constraint of structural fill and a slab on grade, or do you multiply the full lateral load by the height of pedestal and footing to determine the moment applied to the footer.

 

[jike]

I would not recommend reducing the lateral load due to the presence of structural fill nor a slab-on-grade.

 

[MikeE55]

Are you describing a foundation for a metal building system? If so, it is common to use a reinforcing bar wrapped around the anchor bolts to transfer lateral load into the slab-on-grade. I don't see any reason why you shouldn't use the slab to resist lateral loads, as long as you are providing a load path to get the force into the slab, and as long as the slab can resist the load in sliding.

 

[SlideRuleEra]

I would consider both the use and importance (per building code) of the structure. For an industrial structure where changes, over time, are likely or for essential services (say a hospital), jike's recommendations sound good.

For a competitively priced building, perhaps a small retail store, MikeE55's approach should be both satisfactory and cost effective.

http://www.SlideRuleEra.net

 

[SperlingPE]

Does anyone use full depth footings sized for the uplift, over-turning, and vertical downward force? My thought is that more concrete is used but the time to form the pier, the extra pour, and the extra cure time is deleted.

 

[archeng59]

I size footings for uplift, overturning and the downward force. Also, I use the hairpins in the slab whenever possible. If the horizontal forces are larger than what can be transferred using hairpins, I've used tie beams between the rigid frame columns.

 

[rlawrence67]

Do you have a reference for the use of hairpin in slabs. Won't that then present a problem with curling and shrinkage. Slab size is 90 x 75

 

[apetr26542]

Use of hairpin in slab is used to develop the dead weight of the slab and to restrain the top of the pier from the lateral forces by developing friction between slab and subgrade, however our office has started putting the vapor barrier under the slab and not the subbase, this reduces coeff. of friction, not sure to what, I thought I saw that on another thread. Anyone know?

 

[miecz]

I have a few basic questions about the use of hairpins. Do you eliminate slab joints so as to carry the tensile forces thru the slab? How do you deal with slab shrinkage. Doesn't a hairpin defeat the isolation joint between the slab and the foundation wall? How do you prevent the owner from cutting the slab in the vicinity of the column?

 

[apetr26542]

I tend to relocate the slab joints away form the columns. The hairpin does tie the slab to the pier/wall but I think it is still pretty local. You can not prevent the owner once it is in his hands, same could be said of tie rods. Its the plumbers you have to watch out for.

 

[jheidt2543]

I've used hairpins on many metal buildings without any problems. However, you must keep in mind the flow of construction. Many times the foundations are installed, backfilled and the building erected -- THEN the floor slab is poured under the cover of the roof. BUT, during that period between completion of the building shell and the curing of the floor slab the hairpins aren't effective. So, pray for no high winds and not heavy snows!

Also, as others have noted above, if the floor is cut at sometime in the future, the hairpins could also be cut too.

As to the use of a vapor barrier, yes it reduces the coeff. of friction between the slab and the subgrade, however you could compensate with longer hairpins tied to the slab reinforcing. Check the calcs once to verify the difference.

 

[apetr26542]

jheidt2543, thats a great point, Next time I get a prefab building, I will make sure the foundations can take the DL thrust and see what kind of FS I have.