IBC - Load combinations when uplift is present ?

[SilverBeam75]

Hi,

I have a concrete beam on soil (expansive, uplift pressure)
Span = 30 feet (ends fixed)

if My loadings are (plf - pouds per linear feet):
WLu = 1500 plf (uplift load)
WL = 500 plf (live load - gravity)
WD = 600 plf

What if the load combination and what will be the factored linear load (LRFD) ?

Because I understand that the live load will counter some force of the uplift, meaning that WLu = (1500-500) = 1000 uplift

 

[larsacious]

Is the uplift pressure from soil or wind? I think you mean soil upward force due to expansion. Typically, "uplift" refers to a wind force. IBC 2006, 1605.2.1 has a list of load combinations (assuming you are working in the US). I would factor the loads treating the expansion force as "H."

 

[JAE]

There is not ONE load combination - but several that you have to check - see the applicable building code and/or ASCE 7 load combinations.

I've not ever designed for actual soil uplift (swell from expansive clays) before. I don't know how you can quantify it that exactly. Usually for expansive clays you try to avoid soil contact (crawlspace, etc.).

 

[SilverBeam75]

Yes the uplift is from the soil (expansive clay) it only affect the perimeter of the slab (2 feet wide) So this why I use a beam, I'm adding some stiffness

 

[hokie66]

You can use dead load to resist the swelling force, but not live load. It may not be there.

 

[msquared48]

What about mitigating the uplift by intermittently using compressible styrofoam under the footing. This is done all the time with the expansive clays in some regions of Colorado.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SilverBeam75]

Thank you for the advice. I will investigate...

 

[larsacious]

I suggest doing what JAE or msquared48 stated and providing a space between the soil and structure to avoid the expansive loads. You will need to provide piles/caissons to span the grade beam to for this approach.

I designed a post-tensioned slab on grade in Texas and recall that you are not designing for a force as much as you are designing for a movement. Consider the soil expansion to be similar to thermal movement and detail to allow the movement rather than trying to resist it.

 

[Ron]

Estimates of expansion pressure from the soil are not necessarily reliable nor consistent. They change with moisture content changes. Further, depending on the geometry of the section and the soil consistency, the expansive soils can exude rather than cause a lot of pressure on the member.

Design considers that expansion pressures exist, but not necessarily in quantifiable form, so separation as Mike and JAE have noted is appropriate for design.