Calculation of Gross Bearing Capacity for Various Load Cases
One of the checks that the program does for every load case –service as well as ultimate– is whether the maximum pressure exerted on the soil for any given load case exceeds the maximum permissible value that the soil can undergo for that load case. This maximum permissible value is termed as the gross bearing capacity of the soil (GBC).
If we call the GBC as term (a), and, the actual maximum pressure on the soil due to the vertical load and overturning moments as term (b):
The footing size is deemed to be adequate for that load case if (b) does not exceed (a).
Note that (a) as well as (b) are unique to each load case. The procedure used in calculating (a) is described here.
In the Cover & Soil page for various foundation types, one of the values that the user has to provide is termed "Base Value of Soil Bearing Capacity".
This base value forms the starting point for calculating the GBC. The GBC is obtained from this base value by applying the following transformations:
- What type does the Base
Value belong to? Gross or Net?
On the Global Settings dialog, on the Rigid Foundation Settings tab in the Bearing Capacity Settings, group you will find two options: Gross Bearing Capacity Input and Net Bearing Capacity Input.
If that setting is Gross, then, this base value is assumed to be of type "Gross".
If that setting is Net, then, this base value is assumed to be of type "Net".
If that setting is "Net", then, this base value is first converted to type "Gross", using the equation
whereGross = Net + λ ×Ds - λ
= - Unit weight of soil
- Ds
= - Height of soil above the base of the foooting
Thus, at the end of this operation, we have the Base Value measured in "Gross" terms.
- Application of the
multiplying factors
If the load case under consideration is of the service type, the GBC for that load case is obtained as the
Base Value of "Gross" as obtained from step 1 × factor a
If the load case is of the Ultimate type, the permissible maximum soil pressure for that load case is obtained as the
Base Value as obtained from step 1 × factor a × factor b
where- factor a
= - the multiplying factor for that load case as specified in the Pile/Soil Bearing Capacity Factors page
- factor b
= - the Multiplier on Soil Bearing Capacity for Ultimate Loads value specified in the Cover & Soil page
If you want all the ultimate load cases to have the same GBC, assign the multiplying factor accordingly to "factor b" and keep "factor a" as 1.0 for all ultimate load cases.
If each ultimate load case should have its own unique GBC, assign the multiplying factor accordingly to "factor a" and set "factor b" to 1.0.
If any of the service load cases too should have its own unique GBC, assign the multiplying factor accordingly for that load case to "factor a".