Bearing pressure uplift 2

[DaveHolder]

I did design to foundation 5.5 by 5.5 meter and 2 meter deep, and I still get the uplift bearing pressure around -50 kN/m2!

Do I still need to increase the size of the foundation in order to remove the uplift? I can’t go down deeper than 2 meter as it is the mud stone level!

Any help is appreciated

 

[Ron247]

Brave heart:
You need to give the column forces acting on the foundation you want to design. If there are 4 columns on one foundation, yes it gets harder, but not impossible. You are still summing forces about each side for overturning. Stability checks for overturning and uplift do not used soil pressure at all in the calculations. Soil bearing is a separate design parameter you check.

 

[jayrod12]

What load combination in the program is giving you the uplift pressures.

 

[DaveHolder]

Factored Structure combination

 

[DaveHolder]

Un-factored Structure Combination

 

[DaveHolder]

Factored Geo Combination

 

[DaveHolder]

Unfactored Geo Combination

 

[Ron247]

Brave heart:
I do not know about the others, but I cannot tell anything about that computer output. A free-body diagram with loads and column locations would tell me more. I am also not sure what your problem is. I see the positive and negative numbers in the output but I need some reason that is a problem to your design.

 

[jayrod12]

It appears you have a combined footing already, I.E. multiple columns on a single large raft. As I indicated in my first post, technically you can have uplift on your foundation pad when it's due to overturning, but you must review the pressure distribution. You should change your support properties to compression only springs and check if it's stable, then if stable compare the maximum bearing pressure to the allowable from the geotech. If that all checks out then you are good to go.

However, it certainly seems as though you are relying quite heavily on software to perform your design/analysis. I suggest (read strongly recommend) you attempt to do some hand calculations to make sure that the values you are getting from the computer make sense. You likely won't be able to perfect replicate the software output, but you should be able to get close enough with hand calcs to make yourself comfortable the software is performing correctly.

 

[DaveHolder]

Jayrod,
I agree about changing the support properties to compression as I already did! Non linear analysis requires you to input only compression stiffness and zero tension but the program has failed to resolve the load cases!

I don’t rely that much on the software as you mentioned! I would always like to do hand calcs but in this case if I do hand calcs is going to take long time to finish it! So I resort to the software to save time!

I didn’t get what you mean by saying ( read strongly recommend)?

Ron,

I thought the foundation should not have any tension bearing pressure!

 

[CANPRO]

This is not difficult to get a rough check by hand if you assume your foundation is completely rigid - and given the dimensions of the footing you noted, I don't think that assumption is too far from reality.

Convert all of your point loads at various locations on the footing to a single point load and overturning moment (in both directions) located in the middle of your footing. Now its a simple case of axial load (P/A) combined with bending stress in both directions (Mx/Sx and My/Sy) - combine to get your results at the four corners and compare to your output.

Also, maybe this is just me being picky/a jerk - but you should try to limit your use of exclamation marks to maybe one per post, preferably none. A simple period will due for most sentences.

 

[Ron247]

Yes, Brave, it should not have any tension UNLESS you are using anchors. I agree with CANPRO, a hand check would probably be more useful. Unless I missed it, have you given us a Free Body Diagram? I think that will answer a lot of questions and permit better solutions.

 

[jayrod12]

Sorry I meant suggest=strongly recommend in that instance.

Regarding the program being unable to solve with compression only springs, then that means it's unstable and likely needs to be larger. If I'm seeing the load locations correctly perhaps the footing should be shifted until the centroid of footing lines up under the centroid of the loads.