SlabWorks - Building Code Requirements

International Building Code (2006)

Foundation Design Requirements

The International Building Code (IBC) contains requirements for the design of foundations on expansive clay. The requirements for foundations on expansive clay are found in section 1805.8. Section 1805.8 allows for four design approached for dealing with expansive clay:

Removal of expansive soil to depth to a depth “sufficient to ensure a constant moisture content in the remaining soil.”
Stabilization of soil “by chemical, dewatering, presaturation or equivalent techniques. “
Use of slab-on-grade of mat foundation designed and constructed in accordance with the WRI/CRSI, PTI design methods or rational method accounting for soil-structure interaction, deformed shape of the soil support, and plate action of the slab in center lift and edge lift conditions.
Compliance with 1805.8.1.

Section 1805.8.1 Requirements

Section 1805.8.1 deals with the design of deep foundation and generally requires that the foundation be designed to resist the effects of volume change such that any structural movement does not interfere with the “usability and serviceability of the structure.” This section includes the following commentary:

1805.8.1 Foundations. Footings or foundations placed on or within the active zone of expansive soils shall be designed to resist differential volume changes and to prevent structural damage to the supported structure. Deflection and racking of the supported structure shall be limited to that which will not interfere with the usability and serviceability of the structure

Foundations placed below where volume change occurs or below expansive soil shall comply with the following provisions:

Foundations extending into or penetrating expansive soils shall be designed to prevent uplift of the supported structure.
Foundations penetrating expansive soils shall be designed to resist forces exerted on the foundation due to soil volume changes or shall be isolated from the expansive soil.

Drainage Requirements

The IBC code also contains requirements for drainage and grading in Section 1803.3 and 1805.3.4. Adherence to these requirements will minimize the possible of foundation failure due to poor drainage. Section 1803.3 generally requires a 5% slope ways fro the building perimeter for a distance of 10 feet. Section 1805.3.4 presents minimum foundation elevations, stating:

1805.3.4 Foundation Elevations. On graded sites, the top of any exterior foundation shall extend above the elevation of the street gutter at point of discharge of the inlet of an approved drainage device a minimum of 12 inches plus 2 percent.

International Residential Code (2003)

Section R403.1.8 of the International Residential Codes IRC requires that foundation on expansive clay “be designed in accordance with Section 1805.8 of the International Building Code.” The code also allows the local building official to approve alternative systems “which have performed adequately in soil conditions similar to those encountered at the building site.”

ACI 318-05 – Building Code Requirements for Reinforced Concrete

As discussed in the commentary, ACI 318-05 minimum steel requirements (0.18%) for elevated slabs are not applicable to slabs-on-grade. Instead, refer to ACI 224 “Control of Cracking” for directions on controlling cracking of slabs-on-grade. Generally, cracking is controlled with a combination of steel and control joints. According to ACI 224, 0.50%-0.60% steel is required to control cracking with steel alone.