shear reinforcment in concrete beams

[NS4U]

Attached a cross section of 5’ wide beam. My analysis indicates 2 legs of a stirrup are required for shear reinforcement.

My question: is there a problem with putting the reinforcement at one side of the beam as shown?

 

[DaveAtkins]

It doesn't seem correct to me to do what you are proposing. But I don't know of a Code provision that prohibits it.

DaveAtkins

 

[Lion06]

How deep is the beam? Why exactly do you want to do this? If closed stirrups around the whole beam is just too much steel, why not provide single 135 hooked bars on each side. That will give you both legs and keep the detailing more normal.

 

[apsix]

There may be no rule against it, but my common sense tells me it shouldn't be done.
The Aust. code AS3600 limits the transverse shear leg spacing to the lesser of 600mm and beam depth.

 

[NS4U]

apsix, if the code doesn't allow the leg spacing to be that far apart, but do they require legs be placed in the center of the beam?

The only problem I see with it, is that it throws your stiffness all off and can induce a torsion

Does anyone agree with that?

 

[dgkhan]

X-section without rebar can have diagonal shear crack first as load will follow path of least resistance. Once that rea is cracked, remaining x-section with rebar even might not be sufficient.

 

[JStructsteel]

can you look at that as more of a slab, and if I recall dont you get away from stirrups in slabs..just an idea

 

[haynewp]

The shear center is offset so I agree that torsion would be introduced.

 

[KCRatnayake]

If this spans between two columns, punching shear will be a problem. All depends on your loading and depth of the beam.

Most beams with small depths, probably you'll have doubly reinforced beams in which you need reinforcements to resist compression, every corner bar and each alternate bar in the outer layer of the beam, should be supported by a link. That's the BS 8110 code requirement for containment.

Clefcon

 

[miecz]

If torsion is introduced, then 7.11.2 requires the stirrups to enclose the flexural reinforcement. So, you could only count on the flexural reinforcement enclosed by the stirrups. Then your stirrups would need to resist torsion as well as shear.

 

[rday]

Well to start, if the top steel is compression steel I'm pretty sure it requires enclosure.

Section 11.5.4 provides spacing limits for shear reinforcing. Note that it does not define direction for that spaceing in section 11.5.4.1; only states a maximum of d/2 for non-prestressed members. One might argue that it applies in both directions from the axis of the reinforcing.

Of course the question I ask is; Why do your reinfocing that way?

 

[JAE]

I don't like it.

It just doesn't appear to engage the entire section. If you only were to count on the portion of the beam enclose in the stirrups for your [φ]Vc then you'd be OK perhaps for strength but the non-stirrup area would sort of "hang" off the side of the beam and be susceptible to cracking and even maybe separation from the main beam.

 

[rapt]

The ties MUST be spread across the beam width.

They must at least line up with the columns. JAE's suggestion to use a reduced width to calculate capacity used to be allowed by the Australian code as long as that width was used for all design calculations.

Think about how a concrete beam works in flexure and shear and how loads are being transferred (truss analogy etc) along the member and to the supports. This will tell you where you need the reinforcement and ties.

 

[NS4U]

what if this is a spandrel beam in a two-way slab. Does this change anything?

rday- your comment about top steel being in compression and needing enclosure. Can you elaborate? if this is just simple beam behavior, the top bars are of course in compression, but they are not relied on for calculating strength

 

[miecz]

If this is a spandrel beam in a two way slab, then you would have reinforcing perpendicular to the axis of the beam tying the slab to the beam. I think you could so this, provided you only count on the flexural reinforing enclosed by the stirrups.

 

[NS4U]

if the reinforcing perpendicular to the beam was fully developed over the "open" part of the stirrup, does this count as being fully enclose for torsion?

 

[Lion06]

If you have compression bars than you have to satisfy the column tie requirements. If you have bars in the compression zone that are bigger than a #4 I would consider them as compression bars even if you didn't count on them for strength.

 

[miecz]

NS4U

I think you would need fabricated closed stirrups for torsion.

EIT

ACI 7.11.1 reads "Such ties or stirrups shall be provided throughout the distance where compression reinforcement is [italic]required[/italic]." I don't think bars in the compression zone require stirrups, just because they are there.

 

[rday]

NS4U,

As stated by StructuralEIT compression steel in a beam requires ties as a column.

In the US the applicable code would be ACI318. Review chapter 7.

 

[kslee1000]

Answer your latest question: No, adding transverse top reinforcement does not help in torsion - torsion reinforcement shall be continuous around the beam section that is utilized to resist such force. But, for your beam size, and depending on the support conditions, it may not require torsional reinf.

For the early question: It looks like the beam requires minimum shear reinf. under the "Normal Beam (deepth larger than width)" theory, however, actually you have a "Wide Beam" which behaves quite differently. Check into requirement for the later category, if shear reinforcement is still required, I would provide more than one stirrups just to ensure there is no weakness at any location on the beam section. I would not off-set the shear reinforcement, because it creates zones with different rigidities (stirrups combined with the longitudinal bars and the concrete is essentially a composite truss, as opposed to regions without stirrops).