Shear reinforcement yield stress 4

[Gus14]

If the same rebar being used for longitudinal and shear reinforcement is of 420 MPA yield stress. For shear design, do you reduce it or keep it 420 ?. I was taught to reduce it to 280.

 

[KootK]

If you did reduce the yield stress, what would be the reasoning for it?

1) High seismic application?

2) Bent up bars where the flexural reinforcing becomes the shear reinforcing?

3) Using the flexural bars to resist shear vial dowel action rather than as stirrups?

We may need a better feel for what your up to in order for us to provide competent guidance.

 

[Gus14]

Thank you Kootk for replying. None of the reasons you have mentioned. The engineers who taught it to me claimed that shaping the stirrups and bending the bars reduces the steel yield stress. I have been trying to find a reference for this but so far unsuccessful.

 

[bugbus]

The engineers who taught it to me claimed that shaping the stirrups and bending the bars reduces the steel yield stress.

This sounds like baloney

Even with low ductility bars there is no reduction in (allowable) yield stress for a bent stirrup.

 

[Gus14]

My theory is that there is a misunderstanding between engineers and contractors because most local engineers follow design books that refer to 280 MPA grade steel for stirrups to ease bar bending. While some contractors do order 280 MPA steel other order 420 MPA steel.

 

[HTURKAK]

When you say shear reinforcement and if you do not define the type of member ( beam ,column, slab etc..) then ,one would understand the stirrups with two legs.

ACI 318 suggests the beam’s nominal steel shear strength calculated by Vs = Avfytd/s. ( fyt = specified yield strength of transverse reinforcement )

If you use the same steel for longitudinal and shear reinforcement ( in this case 420 MPA fy) , You are expected to use fyt=420 MPa.
There is S 220 ( fyk=220 MPa ) at some countries ..You may prefer to use for transverse reinforcement if available.. But ,be careful that ,S 220 in general plain bar and detailing rules differ..

 

[KootK]

I'm with Gus14 on the shaping... baloney.

Sure, if there's a legitimate risk that your project will be supplied with lower strength bars than you specified, that would be valid reason for the reduction.

 

[dold]

Our notes give contractors the option to substitute Gr. 40 bars for field-bent dowels, with a proportional reduction in spacing. That is just to make it easier for them to bend it. Bending the bar has no effect on the yield stress as far as i'm aware, unless you bend it back and forth a bunch of times. Typically bars should only be bent once to form the bend.

For example, wall/slab dowels at the top of concrete shearwalls are sometimes cast in with the hook placed parallel to the wall. once the forms are stripped, they will bend (twist) the bar to turn the hook perpendicular to the wall. I've never been a huge fan of it, but that's just the way it is.

But...I don't know why you would use lower grade for shear reinforcing.

Or are you considering your strength reduction factor 420*0.65=~280? I am not familiar with non-US codes, so i don't know what your phi for shear is.

 

[Gus14]

Thank you everyone for replying.

If you use the same steel for longitudinal and shear reinforcement ( in this case 420 MPA FY), You are expected to use fyt=420 MPa.

yes this is what I mean, but I was told to use 280 MPA.

Sure, if there's a legitimate risk that your project will be supplied with lower strength bars than you specified, that would be a valid reason for the reduction.

. Not something that can't be specified but I don't want other offices to worry the client over it, and I do limit the increase in the stirrups area only to the location with high shear requirements.

Or are you considering your strength reduction factor 420*0.65=~280? I am not familiar with non-US codes, so I don't know what your phi for shear is.

No I am referring to the Fyt before reduction.

Sidenote, After further review, I think this is something engineers used to do in the past because they used 280 MPA grade stirrups but stuck to it without noticing that higher yield stress steel is being ordered for stirrups.