Shear force at a distance of "d", why not shorter than that? 8

[Pretty Girl7]

Following equation is for the calculation of shear r/f for a beam according to Eurocode. The Eurocode expects us to calculate the shear force at a distance of "d" from the supports.

Let's suppose the shear force at a distance of "d" is 150kN, and red arrow shear force (from a point load etc) 400 kN, more close to the supports than a distance "d" shear force.

My question is, if there are any bigger shear force (from the red arrow point load) is present more close to the supports, aren't we missing the critical red shear force?

The critical shear force is not the shear force at the distance at "d" in this case, so why we take the shear force at distance "d"?

 

[centondollar]

The answer can be found in a textbook or your lecture notes from school. Shear design formulas in EC2 are based on strut-and-tie models, and when the distance of the critical section is "d" or less than that, a large part of the shear is transferred by arch action (a compression strut) directly to the support. Also, "d" is not the depth of the section (your figure makes it out to be the section depth), which you surely know.

You keep making threads containing basic questions about reinforced concrete design, with a fixation on formulas found in the code. It is better to study the mechanics first and to look at the code after that. The code will not teach you the principles of reinforced concrete design - it is just a tool to be used by engineers well versed in the subject matter of the code.

PS. If you are a student, redirect your questions to the student forum.

 

[Lomarandil]

In addition to the basic premise in Centondollar's first paragraph, while I dont know Eurocode, ACI would not allow taking the lower shear value at "d" from the support in this case with a point load present closer in. That provision is meant for cases with approximately uniform applied load.

Regarding CoDs last point, lets keep in mind that nowadays, it is possible to complete an undergrad degree without a concrete class (a problem for a separate discussion), or to have had a poorly taught class. These questions are similar to questions I commonly field from new graduate engineers.

And while I'd hope that young engineers have a supportive senior engineer in their working group to ask these questions, EngTips is the next best venue. I'd much rather help young engineers develop and connect their understanding of structural behavior on real-world problems here than have them go through the early years of their career without asking and discussing these questions, either never developing that intuition and stalling out in their career, or worse, pretending they understand.

 

[ThomasH]

The critical shear force is not the shear force at the distance at "d" in this case

What is that statement based on?

 

[Tomfh]

What is that statement based on?

A sufficiently large concentrated load near a support can overwhelm the enhancement in shear capacity you get near the support. Hence clauses like this in AS3600.

 

[IDS]

The short answer to the question is given in the introduction to the shear section in the code:

6.2.1(8) For members subject to predominantly uniformly distributed loading the design shear force
need not to be checked at a distance less than d from the face of the support.

... so if the loading is not "predominantly uniformly distributed" the design shear force does need to be checked at a distance less than d from the face of the support.

Specific provisions are given in clauses 6.2.2(6) and 6.2.3(8). Alternatively see 6.2.2(7):

6.2.2(7) Beams with loads near to supports and corbels may alternatively be designed with strut and
tie models. For this alternative, reference is made to 6.S.

It's also worth looking at the commentary to Eurocode 2, Clause 6.2.2.3 Loads near to supports.

There is no way that this is a simple problem that should be fully understood by all recent graduates. I graduated over 50years ago and I still struggle with it, as do design code writers around the world. Current international design codes include significantly different provisions, in fact I will be presenting a conference paper on that topic later this year.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Lomarandil]

IDS please do keep us appraised on that paper, it's an area of some interest to me.

 

[Tomfh]

I don’t like the way the codes default to “d” from support.

If I was writing the codes I’d say the critical section shall be taken at the support, *unless* etc etc.

As it stands the way the codes are written most everyone (and a lot of software) just takes it as “d” from the support regardless of concentrated loads.

 

[Pretty Girl7]

The answer can be found in a textbook or your lecture notes from school. Shear design formulas in EC2 are based on strut-and-tie models, and when the distance of the critical section is "d" or less than that, a large part of the shear is transferred by arch action (a compression strut) directly to the support. Also, "d" is not the depth of the section (your figure makes it out to be the section depth), which you surely know.

You keep making threads containing basic questions about reinforced concrete design, with a fixation on formulas found in the code. It is better to study the mechanics first and to look at the code after that. The code will not teach you the principles of reinforced concrete design - it is just a tool to be used by engineers well versed in the subject matter of the code.

PS. If you are a student, redirect your questions to the student forum.

Thank you for your reply.
Old textbooks might also be wrong/outdated anyways. Old research often being replaced with new research I felt it's odd to blindly follow "distance at d" rule.
I felt like if the concentrated load is large enough and near to the supports it can create issues. So I thought it's good to see what others say about this

 

[Pretty Girl7]

In addition to the basic premise in Centondollar's first paragraph, while I dont know Eurocode, ACI would not allow taking the lower shear value at "d" from the support in this case with a point load present closer in. That provision is meant for cases with approximately uniform applied load.

Regarding CoDs last point, lets keep in mind that nowadays, it is possible to complete an undergrad degree without a concrete class (a problem for a separate discussion), or to have had a poorly taught class. These questions are similar to questions I commonly field from new graduate engineers.

And while I'd hope that young engineers have a supportive senior engineer in their working group to ask these questions, EngTips is the next best venue. I'd much rather help young engineers develop and connect their understanding of structural behavior on real-world problems here than have them go through the early years of their career without asking and discussing these questions, either never developing that intuition and stalling out in their career, or worse, pretending they understand.

Thank you for your kind reply.

 

[Pretty Girl7]

What is that statement based on?

I felt the 'strut angle' can be like more than '45 degrees' (for the red concentrated load) and it can directly transfer to the bottom area before the supports.
Because, we are designing the beam for the shear load at a distance at "d" at that point the angle is ok, but we have a very large load before distance "d", that would not transfer to the supports as it definitely will not form a strut angle lower than 45 degrees. I might be wrong though.

 

[Pretty Girl7]

A sufficiently large concentrated load near a support can overwhelm the enhancement in shear capacity you get near the support. Hence clauses like this in AS3600.

Thank you for your reply.
Yes I was correct it seems, it can create issues if the concentrated load is large enough.

By the way, you had referenced the 'Australian Standards". I must mention that I feel "Australian codes" are one of the most organized and clearly explained engineering codes I've gone through. I had to follow some Australian Standards for some of my other works and I enjoyed reading them

 

[Pretty Girl7]

The short answer to the question is given in the introduction to the shear section in the code:

6.2.1(8) For members subject to predominantly uniformly distributed loading the design shear force
need not to be checked at a distance less than d from the face of the support.

... so if the loading is not "predominantly uniformly distributed" the design shear force does need to be checked at a distance less than d from the face of the support.

Specific provisions are given in clauses 6.2.2(6) and 6.2.3(8). Alternatively see 6.2.2(7):

6.2.2(7) Beams with loads near to supports and corbels may alternatively be designed with strut and
tie models. For this alternative, reference is made to 6.S.

It's also worth looking at the commentary to Eurocode 2, Clause 6.2.2.3 Loads near to supports.

There is no way that this is a simple problem that should be fully understood by all recent graduates. I graduated over 50years ago and I still struggle with it, as do design code writers around the world. Current international design codes include significantly different provisions, in fact I will be presenting a conference paper on that topic later this year.

Thank you for the kind reply and the references to the clauses. Hope you'll break a leg at the conference. Best of luck.

 

[ThomasH]

Like I said in a previous thread, the model for shear reinforcement is in the Eurocode based on a truss model. The effective height of the truss is "d", "d" is not the height of the beam. And as centondollar described it, for a load on top of the beam, a large portion of the load is transferred to the support trough an arc.

There are different approaches to shear in different concrete codes. I have worked with a code that added the capacity from the concrete to the capacity from the reinforcement and the capacity from prestressing. The approach was based on test results. The Eurocode uses a model based on equilibrium in a truss, as I mentioned in an earlier thread. But the truss is not locked in all respects. There are choices you can make based on your engineering judgement.

I notice that you have moved from load combinations to concrete snd shear. But I also see that you still just ask about the equations.

I asked you once if you are a student but I never got a straight answer. So I ask again, are you a student or what is the purpose of the questions?

 

[HTURKAK]

I did not read the previous posts and hope that my comment will not be repeat of the previous posts..

My points are ;

- The design shear force is checked at a distance ( d ) from the face of the support.

- The required shear reinforcement should continue to the support.

- The shear at the face of the support SHALL not exceed VRdmax . This criteria shall be verified



Say my understanding of EC-2..


EDIT:Compression strut capacity is VRd,max . Copy and paste of definition from the EC-2 ( : VRd,max is the design value of the maximum shear force which can be sustained by the member, limited by crushing of the compression struts.)






If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )

 

[Tomfh]

The design shear force is checked at a distance than d from the face of the support.

Not if there’s a concentrated load near the support. In that case you check it at the support.

 

[rapt]

Eurocode Clause 6.2.1(8) says only for predominately uniform loads, as Tomfh suggests!

 

[Just Some Nerd]

The answer can be found in a textbook or your lecture notes from school. Shear design formulas in EC2 are based on strut-and-tie models, and when the distance of the critical section is "d" or less than that, a large part of the shear is transferred by arch action (a compression strut) directly to the support.

I am not as far out of tertiary education as I think most of the members here are - I can definitively state that at no point was this taught to me in my studies. Perhaps more shockingly, there was no touching on strut & tie at all. Quite amusing for what is supposedly the best engineering uni in the country (Australia), though I do know of another uni doing some very basic S&T as one portion of a subject.





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Why yes, I do in fact have no idea what I'm talking about

 

[ThomasH]

The Eurocode expects us to calculate the shear force at a distance of "d" from the supports.

I had a few minutes, so I took a step back. That first statement that started the entire discussion, where does it come from? I can find a paragraph than can be interpreted like that, but it is not a general statement.

The equation you have for Asw/s includes "Ved", is that equation a combination of some other equations? What is the source for that equation?

Perhaps more shockingly, there was no touching on strut & tie at all.

If you studied shear models for reinforced concrete, you probably studied truss models. And a truss model can be considered a strut and tie model. The problem is sometimes that we don't look at the mechanics of a model and only see the "label". And if we don't recognize the "label" the method becomes unknown.
My first contact with strut and tie models was when I went to uni, and that was in the late 80's.

 

[Just Some Nerd]

If you studied shear models for reinforced concrete, you probably studied truss models. And a truss model can be considered a strut and tie model.

I can assure you that shear behavior of concrete might as well have been an magic black box, students just learnt a prescribed method of how to design to code requirements. It's quite amusing looking back at how we calculated the angle of inclination for compressions struts with nobody actually knowing what that angle meant at all. Couldn't tell you why a truss model for shear wasn't explained since it's not particularly difficult or long to explain, even the least bright student knew how a truss works.

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Why yes, I do in fact have no idea what I'm talking about