I have analyzed a mat foundation in STAAD.Pro. How do I check the stability requirements and find out the factors of safety ?
Although STAAD.Pro does not report serviceability checks but these checks can be carried out pretty easily using the results of the analysis. This is explained with a simple example model with loads as mentioned below
LOAD 1 GRAVITY LOADLOAD 2 LATERAL LOADLOAD 3 GRAVITY + LATERAL LOAD
Check for DisplacementsTo begin with, one can check the displacement corresponding to the service cases by going to the Node > Displacement page within the Postprocessing Mode. The Summary tab within the Node Displacements table would be the best way to do a quick check on the maximum displacements. The results can be filtered based on the service cases using the menu option Results > Select Load Case which would bring up the Results Setup dialog box. Similarly the results can be filtered for just the mat foundation nodes using the Range tab within the same Results Setup dialog box. The maximum displacement value thus obtained, can then be compared to any allowable displacement value.
Check for Bearing PressureThe maximum bearing pressure can be checked by going to the Node > Base Pressure tab within the Postprocessing mode and taking a look at the Summary tab within the Base Pressure table as shown below
Contour plots of bearing pressures can also be plotted for any service case as shown below
This value can be compared to the allowable gross bearing pressure.
Check for SlidingThe total force causing the sliding can be checked by going to the Node > Reactions page within the Postprocessing mode and viewing the Statics Check Results table as shown next. The summation of the applied loads for the lateral load case ( Load case 2 in the example ) can be obtained as marked in the picture below. In this example the value is 54 Kip. Similarly the resistance to sliding can be obtained by finding out the total vertical load due to gravity and multiplying that by the coefficient of friction. In this example total gravity load = 235.4 Kip. If coefficient of friction is 0.5 ( say ), the total resistance to sliding would be = 0.5 x 235.4 Kip = 117.7 Kip. The Factor of safety against sliding can then be obtained as 117.7/54 = 2.18 which can be compared to the allowable FOS for sliding.
Check for Overturning The overturning moment due to any lateral load case can be checked from the same Statics Check Results table. The moments reported are with respect to the origin which is 0,0,0 by default. In the example, the Mz =1080 Kip-ft represents the overturning moment due to the lateral load case 2 ( the negative sign indicates that the moment vector is along negative global Z ). The restoring moment can be found by checking the moment due to the reaction forces corresponding to a gravity loading which is the case 1 in the example. These are marked in the picture below. So resisting moment = 3531.082 Kip-ft. The FOS against overturning is 3531.082/1080 = 3.27 which can be compared to the allowable FOS for overturning.
The STAAD.Pro model used for this wiki is also attached for reference