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shear reinforcment in concrete beams

shear reinforcment in concrete beams

shear reinforcment in concrete beams

(OP)
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?
 

RE: shear reinforcment in concrete beams

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

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

(OP)
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?

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

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

RE: shear reinforcment in concrete beams

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

RE: shear reinforcment in concrete beams

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

 

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

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?

RE: shear reinforcment in concrete beams

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 φVc 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.

 

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

(OP)
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

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

(OP)
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?

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

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.

RE: shear reinforcment in concrete beams

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).   

RE: shear reinforcment in concrete beams

The beam you have sketched is only two foot wide for flexure since the portion outside of the stirrups can develop a horizontal/45 degree shear crack when the shear is greater than the concrete shear strength.

RE: shear reinforcment in concrete beams

(OP)
civilperson, but was is your Vc? entire width of the x-section? or just the width of the stirrup?

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