Soil Capacity and Stiffness
The soil capacity beneath a footing is calculated from the data that is defined by the user in the Foundation module. If a spread footing is assigned a soil capacity explicitly (without the use of a soil table), this value will be used for the design of the spread footing. In the case of a continuous footing, where the modulus of subgrade reaction is required to calculate the stiffness of the springs supporting the footing, an approximate method outlined below is used (J.E. Bowles Foundation Analysis and Design Fifth Edition):
| ks = 12(SF)s | Equation 4-1 |
Where SF is the safety factor taken as 2.
| Soil | ksf/ft | MPa/m |
|---|---|---|
| Loose sand | 30-100 | 4.8-16 |
| Medium density sand | 60-510 | 9.6-80 |
| Density sand | 410-810 | 64-128 |
| Clayey medium density sand | 203-510 | 32-80 |
| Silty medium dense sand | 150-300 | 24-48 |
| Clayey soil: | ||
| qa <= 30ksi , 200kPa | 75-150 | 12-24 |
| 30ksi , 200kPa < qa <= 120ksi , 800kPa | 150-300 | 24-48 |
| qa > 120ksi , 800kPa | >300 | >48 |
In the case in which a soil table is assigned to the footing, the soil capacity is found by interpolating between the actual footing minimum dimension/depth and the entries in the table to find the appropriate soil capacity. If the footing dimensions are less than the smallest dimensions in the table then the entry in the table with the smallest dimensions will be used. If the footing dimensions are larger than the largest dimensions in the table then the entry in the table with the largest dimensions will be used.
If the Modulus is defined for the footing that value will be interpolated using the same factors as the soil capacity. If there is no Modulus defined for the footing then the method above will be used to calculate the Modulus after the soil capacity has been calculated.