EARTHQUAKE PROVISIONS IN BUILDING CODES - Geotechnical Earthquake Engineering - page 465

TABLE 14.6 Presumptive Load-Bearing Values

Vertical Lateral bearing Lateral sliding

foundation pressure resistance

pressure (psf/ft below Coefficent Cohesion

of friction a (psf) b Class of materials (psf) natural grade)

1. Crystalline bedrock 12,000 1200 0.70 —

2. Sedimentary and foliated rock 4000 400 0.35 —

3. Sandy gravel and/or gravel (GW 3000 200 0.35 —

and GP)
4. Sand, silty sand, clayey sand, silty

2000 150 0.25 —

gravel and clayey gravel (SW, SP, SM,
SC, GM, and GC)
5. Clay, sandy clay, silty clay, clayey 1500 100 — 130

silt, silt and sandy silt (CL, ML, MH,
and CH)

For SI: 1 pound per square foot (psf) 0.0479 kPa. 1 pound per square foot per foot of depth (psf/ft) 0.157 kPa/m.
a. Coefficient to be multiplied by the dead load.
b. Cohesion value to be multiplied by the contact area, as limited by Sec. 1806.3.2.
Source: Table 1806.2 of the International Building Code (2012).

Presumptive load-bearing values shall apply to materials with similar physical characteristics
and dispositions. Mud, organic silt, organic clays, peat or unprepared fill shall not be assumed to
have a presumptive load-bearing capacity unless data to substantiate the use of such a value are
submitted. Exception: A presumptive load-bearing capacity shall be permitted to be used where
the building official deems the load-bearing capacity of mud, organic silt or unprepared fill is
adequate for the support of lightweight or temporary structures.

Table 1806.2 from the International Building Code (2012) has been reproduced as
Table 14.6 in this topic. The presumptive load-bearing values listed in Table 14.6 are
based on the type of rock (crystalline bedrock or sedimentary and foliated rock) and clas-
sification of soil using the Unified Soil Classification System. According to Sec. 1806.1 of
the International Building Code , when using the alternative basic load combinations that
include wind and earthquake loads, the vertical foundation pressures and lateral bearing
pressures in Table 14.6 can be increased by one-third.
As indicated in Table 14.6, the vertical foundation pressure can vary from 12,000 psf
(570 kPa) for crystalline bedrock to 1500 psf (72 kPa) for cohesive soil. Some of the limita-
tions of the vertical foundation pressures listed in Table 14.6 are as follows:

1. Settlement considerations. Different structures may have different requirements in
terms of the maximum amount of acceptable settlement. But the vertical foundation
pressures in Table 14.6 do not consider the amount of settlement that would occur
under the applied foundation loads.
2. Material density. The vertical foundation pressures do not consider the weathered
condition of the rock or the density of the soil. For example, the values listed in
Table 14.6 may be too high if the rock is highly fractured or weathered. Likewise,
the values may be too high for soil that is in a loose or soft state.
3. Size and depth of footing. The vertical foundation pressures in Table 14.6 do not
consider the size or depth of the footing. Thus for large size footings or where the