Interconnected Footings & Soil Expansion

[smokiibear]

Does IBC 2006 or CBC 2007 specify anything about interconnecting footings based on exapnsivity of soil?

As a rule of thumb, I think from a previous version of the UBC from the 80's (not sure), I deepen footings to 21" for medium, 27" for high, 33" for very high. If soil is low expansion of greater, I interconnect all exterior, interior and porch footings.

Do you have any comments? Do the interior footings need to be interconnected.

Any comments about IBC 2006 1805.8?

Thanks.

 

[ishvaaag]

Making a grid of the foundation is generally a good thing. One of the more common causes of ruin in buildings is the disorganization of the foundation, mainly through loss of the as projected (and hopefully built) geometry. This problem of the past (that also happened at floors through scarce embedment of wood logs in the masonry walls) is still a problem of today. Then, even the mere tie action, keeping the relative distances between foundation points fixed is a quite effective way of preventing foundation failure; if apart from axial stiffness the members tying footings or pilecaps have axial rigidity, they can also be a helper against general distortion of the floors at any level, since deformations at foundations are passed up through the columns. Hence having a stiff grill foundation pattern uses to be effective against the likely most common settlement pattern, dishing action; and is also effective in the inverted patterns when the corners or sides are sustained cantilevered from the center of the foundation, of which I have seen a case with four floors or more held by an unsupported corner of a caved-in (under that corner) structural mat foundation.

Whilst being in general a sound measure, depending upon your intent and some particular structure and pattern of expected heave or settlement it may be practical or simply, just economical, not tie the strong points of foundation in a grid; only a particular examination of the case may help to decide what one would do.

 

[smokiibear]

I apologize for not including this, but was especially concerned with this topic as it relates to residential design., but am open to hear about it as it relates to commercial as well.

 

[msquared48]

Colorado, for one, has a lot of expansive clays that have to be dealt with using periodic styrofoam void forms under the footings. The design methodology I am not familiar with, but have seen the design drawings.

Perhaps another could elaborate...

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

 

[tadeng]

For residential designs (and small commercial) here in Texas, we use stiffened (waffle) slabs alot. It allows the entire slab to move together. Depending on the soil, there may need to be some soil modification done beneath the structure. Typically either remove and replace with moisture conditioned soil to elevate the water and thus pre-expand the soil, or replace with select fill. The amount and type of modification comes from a geotechnical engineer.

We also use voids under slabs and/or grade beams like Mike stated in conjuction with drilled shafts. If the voids are only under the grade beams, some soil modification may be required under the slab to keep the slab from heaving.

The Texas Section of ASCE put out a publication entitled "Recommended Practice for the Design of Residential Foundations" for what I described above.