Joint reactions along local axes in ETABS

[Waqas Haider]

Assalam o alaikum,

I am having a building with a semi circle shape. None of the footing is either parallel to global X-axis nor to the global Y-axis. The plan of footings is attached. The problem I am facing is, ETABS reports joint reactions along global X-axis and global Y-axis instead of along the local axes of column. Since my columns are at a degree to global axis, the joint reactions also becomes at a degree from column and footing. Since to use simple combined stress formula of 

stress = P/A + M_x*Y/I_x + M_y*X/I_y 

I need forces along axis of footing. Or i will have to resolve either moment of inertia of footing along direction of forces or vice versa, which is quite tough and time taking. The other option I can go for is to design footing in SAFE. But again here, there i dont find any option to rotate footing at a certain degree to match orientation of columns. If i rotate local axis of footing, It only rotates its local axes and meshing but not the footing physically itself. The orientation of footing remains same. How can i solve this problem? Either having reactions in etabs along local axes of columns can solve my issue so that i can design it manually or rotating footing in safe to match local axes of columns can solve my issue. So can any one guide me how to do either option? Thanks.

[WR1]

Solutions;

1. Why dont you import the cad drawing (as shown in image) into SAFE and draw footings with proper orientation?

2. Like you got reactions under so many combinations from ETABS for each column, in the same way, you could select all columns and export to EXCEL the local forces at and then filter for zero location and proceed with manual design.

3. Get global reactions (like you did already) but one group at a time. By one group i mean all the columns on one radial grid line. All columns on that grid will have same orientation and angle right. Export to EXCEL, transform forces in XY to that angle. New rotated forces will be;

Fx' = Fx Cos theta + Fy Sin theta

Fy' = -Fx Sin theta + Fy Cos theta

there you go, you now have the new rotated forces. Repeat it for each radial line and then design footings manually.