tension on the soil 1

[h-badawy]

dear Experts,

i am designing isolated footing for steel car parking shade , and i checked on the bearing capacity and the sliding & overturning stability for the footing with 1.5 factor of safety and everything safe , but the thing is that the stress between the footing and soil still tension and reached to almost half of the footing , is there any limitation in any code to control this tension or if all stability and bearing checks are ok no need to worry about this tension stress?.



Thank you

 

[oldestguy]

For me I suspect your word "tension" really means shear??

 

[h-badawy]

Not shear , i mean when we are checking the allowable bearing pressure on the soil with the existing of moment force part of the footing subjected to compresion stress and other side subjected to tension stress , the part of tension is very big .

Thank you

 

[oldestguy]

Usually for footings in bending, one part in compression and the other part tension but all the tension taken in steel reinforcing. No concrete is considered as good for tension usually, unless very low in those cases. Add steel and that is a common technique.

 

[hokie66]

To resist overturning, you can only count on the mass of the footing, not any contribution from the soil below in tension. I suspect what you are seeing is a triangular pressure distribution, but the part beyond where there is compression below the footing cannot be used. Maybe if you can provide your calculations, we can advise further.

 

[oldestguy]

OK this is pressure under the footing. There obviously is no tension from soil below. Then your pressure distribution from the loads is a triangular distribution with the resultant single center of loading outside the middle third of the footing. A common assumption is that the highest computed pressure is at the edge of the footing, but for highest actual soil bearing pressure I would round off that peak and use an average for the whole zone portion so loaded (roughly half the peak pressure of that triangular zone only and none for the zero pressure zone). In effect the only effective bearing area from the loads is the zone with the triangular pressure part. Of course if that average pressure exceeds the ultimate soil bearing capacity, you will get excessive settlement. I'd compute how much settlement the pressure applied (as suggested here) and use that as controlling whether it is safe. I'd not rely on a safety factor against ultimate failure. At the allowable settlement, it is usually likely that the resulting soil pressure is far below ultimate.

Edit: To further discuss this take the example where you wish to walk across a very soft ground area. So you borrow a piece of plywood one foot square and walk on that and it sinks in an inch or so. Then you take a 4 foot by 12 inch wide plywood and walk 6 inches from one end. It works and in both cases of plywood there is no tension needed to keep it there and each one sinks in an inch.. So take that footing you are now concerned with and consider the end of the footing that does not apply soil pressure as needless. You don't need any tension since the footing is stable at one inch settlement. Forget about safety factors, only consider settlement.

 

[h-badawy]

overturning for the footing is ok but my concern is the tension under the footing
see the attachment photos-1

Thank you

 

[h-badawy]

attcment-2

Thank you

 

[Lomarandil]

OK --

It appears that you have a large region of your footing in "uplift". As long as you understand there is no "tension" reaction between the concrete and soil in that area, I think we are all in agreement (and just caught up in terminology).

I don't know what software you are using, but this is a case where I always feel better checking the result with a free body diagram (it will look something like this)

This can be OK, (and is usually unavoidable in structures with large base moments like your parking shade. I just had the same issue with one I designed last week).

As oldestguy mentions, the critical limit is often not the allowable bearing pressure, as much as how much settlement is allowable. If you have a geotechnical engineer involved, you may want to consult with him about this. For structures with large base moments like this, don't forget that settlement actually means rotation at the top of your structure. However, parking shades tend to have some room to move around, so I tend to not be as concerned with those as I would be with a transmission line, etc.

----
The name is a long story -- just call me Lo.

 

[Ingenuity]

What Lo said.

From you graphic - assuming your calculations are correct - your peak pressure is only 100 kPa - pretty small.