shear reinforcement - vertical stirrups in tension and shear - interaction? 1

[greznik91]

I want to discuss about shear reinforcement - vertical stirrups.


Based on Eurocode 2, shear capacity of RC elements are based on 2d truss model as shown in picture bellow.

Because of vertical shear forces we have stirrups in tension.

In case we are making a composite (existing concrete + new concrete) and we use the same stirrups as connectors/dowels we are also having a horizontal shear forces on stirrups (because of a horizontal shear flow between both concrete elements).

I was reading a similar topic not long ago about that and some of you suggested that there is no interaction between shear-tension forces in a stirrup/rebar but I kinda disagree.

Anyway... Horizontal shear forces on a single stirrup is based on geometry, vertical forces and spacing between them.
But Im wondering how can I get a tension forces in a single stirrup on a 2d truss model from knowing what is the max bending moment and max shear force in a beam?

I know how to consider bending moments (N+ = N- = Md/z) but I dont know how to consider vertical reaction (shear forces) on a truss model.

To make things clear - I want to know how to get a tension force in a single stirrup based on bending/shear diagram.
I need this so I can make interaction shear - tension.

 

[KootK]

I was reading a similar topic not long ago about that and some of you suggested that there is no interaction between shear-tension forces in a stirrup/rebar but I kinda disagree.

That was probably me but we'll leave that alone for now.

To get the tension on each stirrup tie set, divide the design shear force at a given location by the number of ties set that you are assuming cross each diagonal tension shear crack. Then divide that force by the number of stirrup legs in each tie set to get the force per bar.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I just noticed that you seem to be putting shear stress into your bars. Normally, in North America, we use shear friction for composite shear flow which does not stress the dowels in shear. Perhaps it's done differently in Europe.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

Kootk - I'm not quite sure if I understand you... Are you saying that you don't consider shear forces (as a result of horizontal shear flow between existing and new concrete element) on each stirrup? So when you are connecting new and existing concrete you do not consider horizontal forces on stirrups? (stirrups are only in tension - no interaction between shear - tension).

Is friction between concrete enough/sufficient? As I said I'm not sure what are you saying.

 

[KootK]

You've interpreted my comments exactly right greznik. Are you familiar with the shear friction concept that we use in north america? That's how it works: the concrete mating surfaces deal with the shear and the dowels provide only a clamping force. That doesn't mean that you couldn't treat the rebar as true dowels as you've suggested. It's just uncommon where I practice and, to my knowledge, we don't really have an accepted methodology for doing that.

With shear friction, by the book, you're supposed to develop your dowels for Fy on either side of the joint. That may prove difficult in your situation. If so, maybe a true dowel method is more appropriate.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[greznik91]

Thank you for explanation. Does your code contain an equation about this shear friction between concrete elements (im wondering what it looks like) or is it a common assumption?

I dont think Eurocode 2 mention anything like that. Will check it.
Does anyone who use EC2 has something to say about this?

 

[Shu Jiang PEng SE]

As shown in your drawing, the "U" shape stirrups should anchored to the concrete compression fiber some distance (refers to development length for the steel bars in tension). It is hard to achieve this in your case. You may consider use the new concrete beam take all the possible shear.

 

[Klitor]

Check section 6.2.5 of ec2

 

[Teguci]

I'd think the force on a stirrup would be something like -

INT (from x-s/2 to x+s/2) [ R - w x) dx
x is the distance from support to stirrup
s is the spacing of the stirrups
R is the reaction at support
w is the load per x

The horizontal shear is locked in by the compression struts

 

[KootK]

Does your code contain an equation about this shear friction between concrete elements (im wondering what it looks like) or is it a common assumption?

The codes in Canada and the US do have provisions for shear friction. It's probably the the simplest structural engineering formula out there. Coefficient of friction-ish looking like 8th grade physics.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

The shear friction concept does not really exist outside North America!

Eurocode clause 6.2.5 covers the design of the strength calculations for this interface!

 

[slickdeals]

How do you get around the requirements for anchorage of shear reinforcement in these situations? ACI requires that the stirrups be anchored by providing a hook around the longitudinal bar.

 

[Settingsun]

Rapt,
Isn't the longitudinal shear resistance from AS 3600 essentially shear friction? The commentary calls it 'frictional shear resistance' and it relies on the same clamping due to transverse reinforcement across a rough interface.