Top Reinforcement in Footings

[bigbadwolf]

I'm having a debate at work, I was hoping for some help to resolve it?

My boss insists that including top reinforcement helps with the punching shear capacity for a footing (a regular spread footing with no uplift or negative moment). I'm not sure if this is true, because I know that longitudinal reinforcement does not count for shear in regular beams. Thanks in advance for any thoughts!

 

[hokie66]

I have no idea why he thinks the top bars would assist in punching shear. What they are prone to do is to result in plastic settlement cracking.

 

[msquared48]

Depending on the size of the footing and base plate, you could engage in shear anywhere from 2 to 8 bars in the top of the footing, but, considering the area of the pyramidal failure cone at the top of the footing, the overall contribution would be minimal compared to the concrete. You would not engage all the top bars of the footing.

Mike McCann, PE, SE (WA)

 

[bigbadwolf]

Hokie66, can you explain?

Msquared, thank you, that makes sense

 

[dik]

As others have noted, it is not a good idea because it wastes a resource. As Hokie noted, it can initiate plastic settlement and premature cracking. It does little or nothing to improve shear resistance, unless you have uplift. Generally it's a bad idea. It also increases the Dead Load <G>.

Dik

 

[civeng80]

There are so many unknowns in footing design that I would always have top and bottom reinforcement together with stirrups to tie them and make a cage and place in the trench.

 

[JAE]

We never have top reinforcement unless there is net uplift on the footing. (talking about individual column footings)

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[dik]

There are so many unknowns in footing design that I would always have top and bottom reinforcement together with stirrups to tie them and make a cage and place in the trench.

You're putting a lot of effort and cost into something that doesn't need it. Footings, including those with moments, are very straight forward and function as designed. There are relatively few unknowns... your design vertical load and moments are generally conservative (the exception being an error), your materials are generally as spec'd. There is no need for being 'ultra' conservative.

Dik

 

[hokie66]

civeng meant a strip footing rather than a pad footing.

 

[Tomfh]

They'd probably help a little bit, but it's not something I'd count on. If punching shear is at all an issue I'd just make the footing thicker.

 

[MotorCity]

For a spread footing, the top bars will not increase the punching shear resistance. If you have a pile cap, you could have punching of the column or punching of the piles. In that case, the top bars would be beneficial.

 

[DaveAtkins]

I disagree with msquared48. As far as I know, there is nothing in ACI 318 that indicates flexural reinforcing bars help with shear.

DaveAtkins

 

[dik]

Hokie... yup, but I've done lots of unreinforced strip footings with hooked dowels to support longitudinal rebar.

Dik