Spread Footing Reinforcement 2

[dmitriy555]

I am looking at the design of a billboard foundation. The pad portion has two layers of reinforcement, top and bottom. The foundation is subjected to high moment. Can someone explain me why do we need top pad reinforcement? sketch is attached.

 

[bpstruct]

I've often wondered about this too. I have seen many footings that seem to be really overreinforced in the top mat. The only reason I can think of is if there is a net uplift at the column. I guess it might also be needed for extremely large eccentricities where the footing has a really big footprint and the resultant falls outside of the middle third of the footing.

 

[msquared48]

Even it it's inside the kern area, you still need to be able to develop the soil overburden to resist overturning, and sometines cannot do that without top reinforcing.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[bpstruct]

This thread below discusses this topic too, although not the focus of the original question.

thread507-323270 - sorry, don't know how to link to it.

Still don't really understand why you would need it if it fell within the kern. Doesn't seem like there would be a net overturning concern since there would be no net uplift on the heel of the footing.

 

[msquared48]

If you need the dead load of the soil to keep the resultant inside the kern, or just to keep the overturning FS greater than 1.5, you do need the top steel. If not, then you don't need the steel.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[delagina]

some engineers just put top reinforcement automatically whether required or not by analysis.

 

[hawkaz]

To put it simply- If you are using your footing to resist overturning, the top of the footing will be in tension- hence, top steel required.
If you are not using the footing to resist overturning, why is it so big?

This is an automatic fail question on the PE exams

 

[TLHS]

If your moment is actually high in proportion to your dead weight, then you'll have a zone on one side of the foundation where you've got tension on the top. Draw your free body diagram of the foundation and then a shear and moment diagram and see what's happening.

 

[ron9876]

In this situation where the top of the footing is in compression until a wind/seismic event I always debate with myself whether it is ok to check stresses in the top of the footing as plain comcrete. I often do this for footings sized to resist uplift due to the 0.6D condition.

 

[spats]

I agree with Ron. Just because there is tension in the top of the footing doesn't necessarily mean there needs to be reinforcing in the top. If that was the case, almost all single-story column footings would need top reinforcement. I think a lot of engineers overachieve on foundations. I even see some engineers who use stirrups in their footings, particularly continuous footings.

 

[ToadJones]

"This is an automatic fail question on the PE exams "

I didn't know they did that....

 

[TLHS]

Ron, given that this is a billboard base, I'm assuming the thing's a cantilever. As a result, you could get a lot of cycles of loading due to wind and if there's a reasonable height to it you could be in a significant moment case for a large portion of the lifespan of the structure. Personally, even if I could make the foundation for something like that work with plain concrete, I'd want to put reinforcing in along the top to get a ductile failure mechanism. I'd be worried that over multiple cycles I'd get some sort of progressive cracking along the upper face.

If you're talking about something where you don't uplift with unfactored loads but edge into it in a factored 1 in 50 wind event, then I can see not wanting to spend the extra money for that.

 

[ron9876]

TLHS I don't disagree. Like I said I always question myself. If there are any shrinkage cracking then plain concrete doesn't work at all. To add to your point a billboard likely doesn't weigh much so the compression in the top of the footing woud be fairly small compared to a building footing. I would probably also add some reinforcement in the top. Doesn't need to be hooked at the ends as shown in the detail.

 

[Ron]

Look back to msquared48's answer...he's right and in almost all cases you will have to resist the OT moment, so top steel is usually necessary. I do it as a routine for cantilevered signs, canopies, etc. as they are subject to reversals and usually a high OT moment. Keep in mind that the induced tension on reversal from wind gusts is actually a dynamic load that we analyze as a static load, so the tension gets "popped" into the concrete, not developed into it.

 

[tony1851]

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