Rebars at mid-height of rectangular RC beams

[Jason C]

I have not seen this before and I wonder how it serves. I understand the concept of using longitudinal rebar to take tension or to enhance the compression capacity of a beam. But a rebar right in the middle, ...isn't that some sort of an inefficient way to do things? Is there any benefit?

 

[EZBuilding]

ACI has requirements for skin friction reinforcement in deeper beams. Mid height reinforcement can also be utilized to resist out of plane moments, which could arise if the beams is also being utilize to resist wind pressures.

 

[JoshPlumSE]

I was going to say skin friction as the most likely option, as well.

Some other possibilities:
1) It could be there to control weak axis bending in a beam. It can certainly be used for strong axis moment as well, it's just not as effective for that.
2) If the beam has significant axial load, they may be detailing it more like a column. I'm thinking of beams that are used as chords or collectors.
3) Maybe they're concerned about crack control. I think this is one of the reasons for the skin friction requirements, but I'm not certain.
4) Goofy detailing reasons. Like you want some bars to be continuous through a joint, but the ones at the top and bottom are in a very congested area. Though I would think this would be localized to a specific joint.
5) If you want to attached things to the side of the beam with anchors or such, or you have some penetrations. In those cases, you might want some extra reinforcement around throw some bars in there.... Though I would think this would be very localized.

 

[KootK]

What is this "skin friction" of which you guys speak? Freudian slip? I assume that was a typo and we really just mean "skin reinforcing". If so, I agree that's the most likely explanation for the mid-height bars.

I doubt that it applies here but I also recall seeing in one of Paulay's books (I think) that bars placed closer to mid-height are more effective flexurally than one might think at the very high strain levels associated with plastic hinge development in seismic frame beams.

 

[EZBuilding]

Good catch. Skin Reinforcement is the correct term.

Now to spend some time thinking about how many awkward times I've called it the wrong name in the past.

 

[strucbells]

Could be driven by spacing requirements for longitudinal reinforcement for torsional loads as well. 12" max IIRC

 

[Tomfh]

As noted side face bars are common for crack control in deeper beams.

Sometime you see small beams (and thin slabs) with a single central layer of reinforcement. If it works it works.

 

[JLNJ]

You might want some mid height bars if you have torsion in the beam.

 

[jayrod12]

I'm fairly certain the CSA standard here in Canada requires skin reinforcement for beams deeper than 750mm. Which is almost 100% of our gradebeams.

 

[HTURKAK]

For deep beams, cracks may develop around the beam mid-depth, between the neutral axis and the bottom ( tension face ). Therefore, ACI 318 requires beams with a depth h > 36 in. to have ( web face or ) skin reinforcement.

This excerpt from ACI 318-14

(9.7.2.3 For nonprestressed and Class C prestressed beams with h exceeding 36 in., longitudinal skin reinforcement
shall be uniformly distributed on both side faces of the beam for a distance h/2 from the tension face. Spacing of skin reinforcement shall not exceed s given in 24.3.2...)

There is similar requirement at EC-2 and the following excerpt from EC-2 EN 1992-1-1:2004,
(...7.3.3 Control of cracking ...
(3) Beams with a total depth of 1000 mm or more, where the main reinforcement is
concentrated in only a small proportion of the depth, should be provided with additional skin
reinforcement to control cracking on the side faces of the beam. This reinforcement should be
evenly distributed between the level of the tension steel and the neutral axis and should be
located within the links. The area of the skin reinforcement should not be less than the amount
obtained from 7.3.2 )