Isolated footing design in staad foundation advanced

Question

while designing simple isolated footing, what does the type of depth- fixed at the top & fixed at bottom signifies in cover and soil properties? and also when I change from fixed top to fixed bottom, the results didn't change in both cases. can someone please help me understand this?

Kris Sathia · Answer

This term is used when the type of design is set to "Calculate Dimension". 
 For the "Fixed At Top" condition, if the footing thickness needs to be increased during the iterations, that increase will occur in the downward direction. Thus, the elevation of the top of footing remains unchanged, but that of the bottom of footing - founding level - will go lower. The height of soil above footing, which is the distance between top of footing and ground level, remains unchanged between each iterations. 
 For the "Fixed At Bottom" condition, the elevation of the bottom of footing - founding level - remains unchanged during the iterations. So, if the footing thickness needs to be increased during the iterations, that increase will occur in the upward direction. The height of soil above footing, and thus the weight of soil, keeps getting lower with each increase in footing thickness. 
 In both cases, it will lead to a re-calcuation of the moments to be used in calculating soil pressures if there are horizontal forces transmitted to the footing from the column. This is because, the lever arm, which is the distance from bottom of column to bottom of footing, increases with each thickness increment. 
 If you are seeing no change between these options, it may be because your starting thickness is sufficient and the program doesn't need to increase it. Try starting with a low value of thickness, say 100 mm, which will force the program to increase the thickness during the concrete design phase, and you should normally notice a difference in most results.

Modestas · Answer

You are using a very old version of STAAD Foundation Advanced. I think you should upgrade to the most recent STAAD Foundation Advanced version 9.4.0.20 where offset feature should work fine.