Is Temp. & Shrinkage Reinf. Required in Both Directions? 1

[mdmiller]

I am designing a footing where Temperature and Shrinkage steel controls.

If say #5 @ 12" o.c. is required do I need to provide this steel in both directions or just one?

Thanks!!

 

[JAE]

Both

 

[JAE]

Well...if it is a spreaad footing - then both.

If it is a continuous footing, then many times the temp/shrinkage is required down the length of the footing, but not necessarily across the footing width as this might be fairly short and usually transverse bars are required only when the cross-bending moment requires it.

 

[oneintheeye]

would you not have minimal steel in transverse direction as you will get moments (poisson)? I suppose you go down road of mass concrete being adequate though? would try and use mesh, two layers if neccesary?

 

[JAE]

With transverse moments generally being quite small, you can get the Plain Concrete provisions to work without any reinforcing.

More like a mass/arch concrete condition.

 

[jike]

In a continuous wall footing of relatively narrow width, you can often get (bending and shear) it to work as plain concrete. If this is the case, then I add longitundinal steel for shrinkage and transverse bars at 48" oc just to hold them in position by wiring them together.

It drives me crazy when an engineer or architect just require longitudinal bars and expects the contractor to just lay them on concrete brick. The first thing that happens when you pour concrete is the bars will roll off the bricks into the sand!

 

[dik]

Can it be proportioned as a plain concrete footing? If so, then no rebar...

Dik

 

[concretemasonry]

Common sense says that transverse steel is not necessary for shrinkage and temperature and the longitudinal steel will depend on the code/application. If it is a narrow strip footing, longitudinal may not be required, but local codes could require it. - Code always applies even if nonsense.

Since you have a footing with stable temperatures and moisture, the real value of transverse steel is the constructability. Widely spaced transverse steel will keep the the longitudinal steel in a position where it will function. If you determine that longitudinal steel is not required for this project, the question of transverse steel is moot.

Often on

 

[mdmiller]

Thank you everyone for your responses!!

The footing cannot be proportion as plain, but if I increase the thickness it could be.

Which brings up the question: If a strip footing only has rebar in one direction then it does not meet the "reinforced concrete" requirments and is therefore considered "plain". Then what is the purpose of the longitudinal rebar? ...just for good measure?

Should there be a concern for some type of crack control in plain concrete? Both footings and piers.

For example: What if you have a very large pier, say 36"x36"x48"high (classified as a pedestal in ACI) on top of a footing. Plain concrete works for stresses, but using 1/2% of the gross Area for vertical steel seems excessive.

What should you do?

Thanks!!!

 

[civilperson]

Shrinkage steel is required is structural slabs, does not apply to wall footings.

 

[JAE]

civilperson,

I think many engineers view section 10.5.4 of ACI 318 as requiring temp/shrink reinforcing per 7.12.

In many cases, a wall footing, in the direction along the wall, does not have any significant bending moments, especially if the wall is very stiff. But for simple common sense in tying the bottom of the structure together with some reinforcing, section 7.12 is typically used.

 

[dik]

If the increase is small, then it may be less costly. You can also save some forming costs by using fabric forming using a geotech fabric...

Dik

 

[civilperson]

ACI Commentary for 10.5.4 says that slabs, mat foundations and other slabs.. should meet the requirements. Walls have their own minimum requirements for steel at right angles to the primary reinforcement and wall footings are not specifically addressed as to needing temperature and shrinkage reinforcement.
15.10.4 does express the 7.12.2 requirement for mat foundations, (says nothing about strip/wall footings).
15.8.2.2, Minimum vertical reinforcement is specified in wall/footing in 14.3.2.

 

[mdmiller]

Thank you JAE, Dik, CivilPerson and Everyone else!
-mdmiller

 

[JAE]

civilperson - granted.

But if you look at the practice of many engineers, and look at the way many software products are written, the 0.0018Ag is used. Just sayin'...

 

[civilperson]

If the software is wrong, why use it?

 

[civilperson]

Commentary R7.12.1 "The provisions of this section are intended for structural slabs only:.."

 

[JAE]

I've had discussions with RISA and RAM over this. RAM, in fact, seems to apply all sorts of large bending moments in wall footings such that T&S reinforcement wouldn't control anyway.

RISA Foot seems to use the 7.12 provision in all their footings and I seem to remember challenging them on this at one point.

I have rarely (actually never) seen wall footings without at least a couple of bars. In my book it is good practice to tie it together longitudinally. But you are technically correct that 318 doesn't directly mandate it.

 

[hokie66]

My 2 cents worth--I always use some longitudinal reinforcement in strip footings under masonry bearing walls and in slab thickenings under walls, but generally do not use any under concrete walls, except to assist in holding the starter bars in position. I figure the reinforcement does more good in the bottom of the wall. The exception is a cantilevered retaining wall.

 

[kipfoot]

"For example: What if you have a very large pier, say 36"x36"x48"high (classified as a pedestal in ACI) on top of a footing. Plain concrete works for stresses, but using 1/2% of the gross Area for vertical steel seems excessive."

mdmiller,
ACI 318-22.8: plain concrete pedestals are permitted with a couple conditions. One of which is: unsupported height / least lateral dimension < 3