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Section 1806 Presumptive Load-Bearing Values of Soils

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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 ASCE 7, Section 2.4 or the alternative allowable stress design load combinations of Section 1605.2. 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 allowable stress design load combinations of Section 1605.2 that include wind or earthquake loads.
1806.2 Presumptive Load-Bearing Values
The load-bearing values used in design for supporting soils near the surface shall not exceed the values specified in Table 1806.2 unless data to substantiate the use of higher values are submitted and approved. Where the building official has reason to doubt the classification, strength or compressibility of the soil, the requirements of Section 1803.5.2 shall be satisfied.
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
PRESUMPTIVE LOAD-BEARING VALUES
CLASS OF MATERIALS VERTICAL FOUNDATION PRESSURE (psf) LATERAL BEARING PRESSURE (psf/ft below natural grade) LATERAL SLIDING RESISTANCE
Coefficient of frictiona Cohesion (psf)b
1. Crystalline bedrock 12,000 1,200 0.70
2. Sedimentary and foliated rock 4,000 400 0.35
3. Sandy gravel and gravel (GW and GP) 3,000 200 0.35
4. Sand, silty sand, clayey sand, silty gravel and clayey gravel (SW, SP, SM, SC, GM and GC) 2,000 150 0.25
5. Clay, sandy clay, silty clay, clayey silt, silt and sandy silt (CL, ML, MH and CH) 1,500 100 130
For SI: 1 pound per square foot = 0.0479kPa, 1 pound per square foot per foot = 0.157 kPa/m.
  1. Coefficient to be multiplied by the dead load.
  2. Cohesion value to be multiplied by the contact area, as limited by Section 1806.3.2.
1806.3 Lateral Load Resistance
Where the presumptive values of Table 1806.2 are used to determine resistance to lateral loads, the calculations shall be in accordance with Sections 1806.3.1 through 1806.3.4.
1806.3.1 Combined Resistance
The total resistance to lateral loads shall be permitted to be determined by combining the values derived from the lateral bearing pressure and the lateral sliding resistance specified in Table 1806.2.
1806.3.2 Lateral Sliding Resistance Limit
For clay, sandy clay, silty clay, clayey silt, silt and sandy silt, the lateral sliding resistance shall not exceed one-half the dead load.
1806.3.3 Increase for Depth
The lateral bearing pressures specified in Table 1806.2 shall be permitted to be increased by the tabular value for each additional foot (305 mm) of depth to a value that is not greater than 15 times the tabular value.
1806.3.4 Increase for Poles
Isolated poles for uses such as flagpoles or signs and poles used to support buildings that are not adversely affected by a 1/2-inch (12.7 mm) motion at the ground surface due to short-term lateral loads shall be permitted to be designed using lateral bearing pressures equal to two times the tabular values.

Related Code Sections

Section 1806 Soils and Foundations, Presumptive Load-Bearing Values of Soils
, strength or compressibility of the soil, the requirements of Section 1803.5.2 shall be satisfied. Presumptive load-bearing values shall apply ...
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