Hi,
For footings subjected to an axial load, the footing dimension is determined using the service load combinations: Footing Area = (service axial load) / net allowable bearing capacity. However, with STAAD Foundation, I noticed that the software seems to be doing a different process. I tried looking for the computational process in the manual but it was not detailed. With that, I have some questions:
a) Why does it use the factored load combination for the final dimension (see image)?
b) Can somebody expound on how the software determine the “min. area required from bearing pressure” and “final footing size” (see image)?
c) What is the justification as to why SFA uses a different method in determining the footing area instead of the conventional formula (as described in the first sentence)?
3056.Isolated Footing - Axial.sfa
Answer to question (a)
Factored load combinations are used for final dimensions usually only if M/P is greater than 0.5 times the dimension calculated using the service loads for either of the two directions. Bending moments and shear forces for the factored load cases can be calculated only if M/P for those cases is smaller than 0.5 * footing dimension. That requires the program to increase the footing size if that condition is violated.
Based on user requests, the soil pressure check and a contact area check are done for factored loads too. However, for the majority of cases, the default multiplying factor of 1.7, and a minimum contact area of 0% will ensure that these conditions generally do not govern. But if either of them governs, the footing size will be increased to ensure a pass for these checks too.
Answer to question (b)
That is just a rough estimate that is used as a starting point for the footing size when the design type is "Calculate Dimension". A starting size is required because the final size is determined using an iterative method. I don't have the exact method at this point, but it is probably just the highest axial load for service cases divided by the allowable bearing pressure for service cases, with some rounding off to some slightly higher value.
Answer to question (c)
See answer to question (a).
Answer Verified By: Louis Flores
May I ask where did "0.5*footing dimension" come from? What is its significance?