Slanted Stirrups - Beam Shear Reinforcment 2

[DavAD]

Hi,

When designing for shear in a beam I consider vertical stirrups at the required spacing.

Do you find that slanted stirrups (say 45degrees) are more effective to resist steel?

Thankyou, and Regards

 

[Tomfh]

If they’re slanted correctly they’re more effective, as they cross crack at a more perpendicular angle. Back in the olden days they used to routinely use diagonal ties.

If you specify diagonal ties you will soon encounter them installed the wrong way around, which renders them largely ineffective. That’s always a fun discussion convincing the builder that it makes a difference which way they’re tilted.

 

[jaskamakkara]

Not really sure what you're asking, but if it's to do with the angle of stirrups in RC members then the equations for shear resistance (crushing of concrete struts, and yielding of shear reinforcement) take into account the angle of the links:

 

[DavAD]

Thankyou for your replies.

@Tomfh
It all boils down to good workmanship agreed.
Having shear equal on both ends of the beam, would not make a difference in which direction the stirrup is inclined though?
or you would use vertical stirrups in such case?

@jaskamakkara
Thanks for this. Yes, that is what I was asking for.

Regards

 

[hokie66]

The shear on the two ends of the beam may be equal in magnitude, but not in direction. So the inclined stirrups would have to be leaned in opposite directions. If this is confusing for engineers, imagine how confusing and mistake prone it would be for builders.

 

[MIStructE_IRE]

Had to do this after all links were scheduled 200mm too tall. Its not a problem though, and can in fact be better, provided you incline them correctly. If they’re inclined along the same plane as the shear crack then you’re in trouble!

 

[Agent666]

In building frames subject to seismic, it's possible to get the shear reversing sign in beams if your beams are hinging each end. So consider that as well, you'd have better shear capacity for one direction of seismic loading, but worse in the other direction. Sounds a little undesirable to me.

 

[WARose]

Yes the stirrups at a angle (45^o IIRC) is the most effective. In fact if you look at older RC structures.....you see a lot of that in the plans. But at some point the lines crossed in terms of the cost to do it outweighed the benefit.

 

[STrctPono]

A lot of old continuous concrete bridges used to bend up their bottom longitudinal steel a set distance from the support and then run it over the pier on the top. This had the benefit of contributing additional shear reinforcing with a theta angle of 45 degrees and saved money on materials because they then transitioned their positive moment reinforcing into additional negative moment reinforcing. I imagine that buildings were the same.

As WARose said, at some point material got cheap and labor got expensive so it just didn't make sense any more.

Agent666 is also correct in that you really need to watch your detailing of 45 degree stirrups in high seismic regions where reverse curvature could create huge problems for your shear capacity.

 

[BridgeSmith]

A lot of old continuous concrete bridges used to bend up their bottom longitudinal steel a set distance from the support and then run it over the pier on the top.

We still do a similar thing in concrete bridge decks. Half of the transverse bars are what we call crank bars, which are bent so they're in the bottom between the girders and on top over the girders. I doubt it does much for the shear capacity, since the transition from bottom to top for moment capacity would be farther out in the span than where the max shear resistance is needed.

I've never seen any of the concrete girders in our state's bridges where they did what you describe; maybe it's a regional thing. For ours, it's fairly common to have extra straight bars on the top that only extend over the pier, sometimes 2 or 3 groups extending different lengths into the span, and the same on the bottom towards the middle of the spans.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[HTURKAK]

The OP is asking whether the inclined stirrups better than vertical ones..
I am familiar with both ACI 318 and EC-2 and both codes allow the use of inclined stirrups.

Regarding ACI 318 ,

Vs= Av fyt (sin α +cos α ) *d/ s
This formula gives the best angle for stirrups is 45 degr. and inclined stirrup with this angle 1.4 times effective than vertical..

Regarding EC-2 , Mr jaskamakkara also stated, 7.3.3 at this post.

If one compares the inclined stirrup with vertical ones ,will find easily best angle for stirrups is 45 degr,again 1.4 times effective..

But we have a problem that, the inclined stirrups say with 45 degr. are almost 1.4 times longer than vertical ones....

and the risks of inclined stirrups,

- In seismic regions , the shear force direction will change repetitively as Mr Agent666 pointed out also,
- With mistake at installation, the inclined stirrups could be oriented parallel to the potential shear cracks, so
stirrups provide no shear strength as Mr MIStructE_IRE pointed out.

IMO , the use of inclined stirrups will not make any positive impact and and no need to take risks ..


It was a tradition to use around 50% of span reinf . as bent ups at supports as tension reinforcement for hogging moments rather than shear.. and still used at some regions.