Applies To
	Product(s):	STAAD Foundation Advanced
	Version(s):	ALL
	Environment:	ALL
	Area:	General Foundation
	Subarea:	Isolated, combined footing
	Original Author:	Sye Chakraborty, Bentley Technical Support Group

I am not able to match the factored gross bearing capacity reported by SFA with my hand calculations. Can you please explain how SFA is calculating the factored gross bearing capacity.

My inputs :

Soil bearing capacity = 200kN/m2 ( I have chosen Net Bearing Capacity Input option from inside Global Settings and so this is considered as the net bearing capacity by SFA)

Depth of Soil above footing = 2m

Unit Weight of Soil=18kN/m3

Depth of footing = 0.5m

Multiplier on Soil Bearing capacity for Ultimate Loads = 1.5

So as per my calculations the factored gross bearing capacity should be = 1.5 [ 200 + 18 x (2+0.5) ] = 367.5 kN/m2. However SFA is reporting the capacity as 401.2464 kN/m2 as seen in the screenshot below

 

Response: 

In addition to the factors you have considered above, you also need to check if there are any Dead weight factors specified or not. In your model the ultimate load cases 20 and 21 have Dead Weight Factors specified as 1.5, as shown next

 

 

 Thus, the calculation done by SFA for conversion of net bearing capacity to gross bearing capacity is as stated next 

a * (200 + 2.5 * 18 * b) 

where a = Multiplier on soil bearing capacity for ultimate loads ( 1.5 in your case)

b = deadweight factor specified for the load case under consideration in the "selfweight and deadweight factor table" (again 1.5 in your case) 

Thus, the Gross Factored Bearing capacity for ultimate load cases = 1.5 * [200 + 2.5 * 18 * 1.5] = 401.25 kN/m2