Footing at Moment Frame and grade beam 2

[cenee]

I am working on this moment frame that is taking a large axial load and so the moment going to the footing is large. When I try to size the footing (using vertical reactions and moment at footing), it turns to be a large footing, too large. What is the approach for a situation like this?
Is it :
- after running the model on risa, I get the worse case loading on the footing (only vertical, no moment) and use that load to design my footing (for my case is DL and RLL). Then, what’s going to take the moment??. I am using the grade beam. I take the EL load on the frame, times the height of the frame to get a global moment. Use the vertical load, grade beam weight and find the eccentricity and check that the soil pressure”Q” won’t exceed the allow.. Then use the pressure “Q” (as a distributed load on the grade beam) to calculate the reinforcement on the grade beam that would take the global moment.
Is this the right approach??
I will appreciate any impute
Thanks.

 

[DaveAtkins]

I'm confused. Do you have fixed bases on your moment frame that are causing moment at the grade beam? Or does the column come down away from the center of the footing, causing the moment in the grade beam?

DaveAtkins

 

[jike]

I am also confused by your description!

Run your moment frame with fixed bases and then take the most critical load combination (axial, moment and shear) to design your footing or grade beam. Then take the other load combinations to check your footing.

Please notethat the most critical load combo for the moment frame may not be the most critical for the footing.

I usually try an individual rectangular footing for a footing with moment not a square footing. If that doesn't work, try a combined footing or a strap footing or a grade beam.

 

[cenee]

The M-F is just one frame (2 col, 1 beam) and the columns are fix, with footings ( which are connected with a grade beam that runs from col. to col.).
When calculation the footing, the moment at the base of the col. is making the footing too big and so my approach is just to calc the footing with the worse load combo (with vertical loads only) and say it turns to be a 5x5 footing. Now, take the earthquake load on the moment frame times the height, giving me a global moment. Then I check the pressure in the soil (by using the DL: vertical load and weight of the grade beam, which with the global moment I can find the eccentricity on the grade beam (e=M/P) and then check the b/6 and use the” Q” formula that applies. Q<allow soil pressure).
Then calculate the max moment on the grade beam to get reinf.
So, I use the grade beam to take the moment on the whole frame system (which is the moment frame, footings and grade beam, all forming a kind of rectangular frame). So, I am not taking the moment from the col. to size the footing,
(ORMF using LRFD).
Thank you.

 

[oldrunner]

Just do a free body diagram of the footing only - ignor for the time being whatever the superstructure is doing. Then it becomes a simple hand calc of P/A +/- M/S, using the appropriate load factors.

 

[jike]

Column footings can be designed for axial load only if the grade beam in combination with the footings are designed to to take out the overturning moment.

 

[DaveAtkins]

cenee,
I think that, yes, if you do the statics properly, you can approach the problem the way you propose. But I think a large, rectangular footing under the moment frame might be more efficient than two small footings and a grade beam. There would be less reinforcing, no stirrups, etc.

DaveAtkins