There are three different types of reductions applicable to overturning effects at the soil-foundation interface (also referred to as soil-structure interface or just foundation interface) in the 2012 IBC, as indicated below:

1. The first provision is found in 2012 IBC Section 1605.3.2, Alternative basic load combinations:

   “When using these alternative load combinations to evaluate sliding, overturning and soil bearing at the soil-structure interface, the reduction of foundation overturning from Section 12.13.4 in ASCE 7 shall not be used.”

   The “shall not be used” in the above sentence needs to be emphasized. 2012 Section 1801.2, Design basis, requires: “Allowable bearing pressures, allowable stresses and design formulas provided in this chapter shall be used with the allowable stress design load combinations specified in Section 1605.3.” This certainly is current practice. Thus the strength design load combinations of Section 1605.2 are not discussed here. Of the two sets of ASD load combinations, the reduction permitted by ASCE 7-10 Section 12.13.4 is applicable only with the basic ASD load combinations of Section 1605.3.1.

2. The second provision is found in 2012 IBC Section 1605.3.2, Alternative basic load combinations:

   “When using these alternative basic load combinations for proportioning foundations for loadings, which include seismic loads, the vertical seismic load effect, E_v, in Equation 12.4-4 of ASCE 7 is permitted to be taken equal to zero.”

   This relaxation is not applicable with the basic ASD load combinations of Section 1605.3.1.

3. The third provision, which results in an effective reduction of overturning effects at the soil-foundation interface, when the Table 1806.2 presumptive allowable soil bearing stresses are used, is found in 2012 IBC Section 1806.1, which reads as follows:

   “1806.1 Load combinations. The presumptive load-bearing values provided in Table 1806.2 shall be used with the allowable stress design load combinations specified in Section 1605.3. The values of vertical foundation pressure and lateral bearing pressure given in Table 1806.2 shall be permitted to be increased by one-third where used with the alternative basic load combinations of Section 1605.3.2 that include wind or earthquake loads.”

   Please note that the one-third increase allowed by this provision is permitted only for alternative basic load combinations of 2012 IBC Section 1605.3.2 including wind and earthquake effects when presumptive allowable soil bearing stresses are used.

Of course, allowable vertical bearing and lateral bearing stresses in soil are typically based on geotechnical reports, rather than on presumptive values given in 2012 IBC Table 1806.2. We are often asked if a one-third increase in allowable bearing stresses in design for wind or seismic effects is permitted when such an increase is provided for in the geotechnical report. The IBC is simply silent on this topic. According to 2012 IBC Section 1605.3.1.1, “Increases in allowable stresses specified in the appropriate material chapter or the referenced standards shall not be used with the load combinations of Section 1605.3.1, except that increases shall be permitted in accordance with Chapter 23.” However, that provision does not apply to allowable soil stresses, because there is no material chapter or referenced standard on soils. SKGA does not have a position on this specific issue at this time. The designer needs to make a decision concerning this allowable stress increase based on input provided in the geotechnical report, subject to approval of the authority having jurisdiction.

Reduction of Overturning Effects at Soil-Foundation Interface