Rc Design Beam Shear design module doubts

[Anonymous]

Greetings, my problem is the following: I am doing a very detailed revision on the calculations in robot's RC design module (provided reinforcement) and I have found the following questions I would like someone could answer me, I really want to keep learning this software because I think it's better than ETABS, hence I am comparing both plus I am doing manual calculations on my own. The things I have encountered so far are the following: 1) I am trying to design a Special Moment Frame Beam in ACI 318 code, that means I have to select the option High risk or category D,E,F. When I do this I am also considering the plastic moments at the sides of the beam that generate a shear named Ve. I read in the manual that Robot needs a load combination case named SEISMIC SHEAR to calculate this force and the type of the load must be ACC. When I choose the code ACI 318-08 with simples cases option when I am tranfering the beam to the module, the software considers this load because I can see it in the results when I click on ALS layout and see the different combinations. The problem is that the value of shear V or Vc that the program is calculating is way too high from the one that ETABS or my own calculations based on the code. It should be around 10.000 Kg and Robot it's showing 25.000 Kg. When I read the manual about the nature of this force, it says "The formula from ve has been extended". Does this mean Robot is not using the exact formula of the ACI code? Can I see which formula is the program using to bring such a high result? 2) Another problem that is bothering me is that when this SEISMIC SHEAR combination is present (consider plastic moment option) the software automatically puts an addiotional stirrup column on the arregement when I use more than 2 longitudinal bars on either bottom or top fibers. When I try to delete this extra transversal reinforcement the program sends a warning saying that the cross section dimensions are too small. I don't understand this because if I decide to not consider the plastic moment such reinforcement is not needed. I am guessing that the program is doing this because Ve is way too high. I realy would like someone could help me with this, it's the only thing that I am missing so I can finally close the chapter in the beam module and conclude to my collegues that this software is bringing results very similar to the ones we do doing manual shear design calculations (Normally I wouldn't bother writing all this but the program does not show how it does these calculations, I managed to calculate the longitudinal reinforcement ones and everything was fine, but in this case, the Shear is way too high). Thanks for reading this, I wait your response!

[Anonymous]

Ak6, in the results table, the variable is named "Vc (Stirrups)". Also, the concrete shear strength must be 0 if two conditions are met:

1- Vp (Plastic moments shear) > Vg (Gravity shear, named "Seismic Shear" by robot)

2- Members axial force is less than Ag*Fc/20

Both conditions are met in this beam's case, so Concrete shear strength is 0. This also indicates that Vc is the bars strength. Robot also shows another variable named "Vc (Total)". I am guessing that this one is the concrete shear capacity (0 in this case) plus the bars one (32.000 Kg according to robot).

Etabs does not uses the provided reinforcement, it is limited to only calculating the required shear reinforcement. So, the model in that software cant be used to make comparations at this point.

[Alireza.Kord]

There is no SP4 for RSA2016 in autodesk support site

the last one is SP3

Aug 28 2015

Service Pack 3 for Autodesk Robot Structural Analysis Professional 2016

are u testing pre-released version?

[Artur.Kosakowski]

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@Artur.Kosakowski wrote:

Some investigations take more time 

In Robot the shear force is incorrectly multiplied by 2 which should be done for precast beams only. 

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Corrected in SP4 for RSA 2016

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[Anonymous]

Arthur, did you checked the 3 points I mentioned on my previous post? About the third pin in the ACI code and the other things I mentioned

[Artur.Kosakowski]

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Alright Arthur, I will comment on the 3 points you made:

1- I will download Robots 2016 Educational version then, now that you have told me that this was fixed in SP4.

What was the result of your test?

2- The thing about Vc 

See the attached document. 

3- pins

I have asked the development team to remove the provisions of having all main bars tied for ACI as excessive. 

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[Anonymous]

Greetings Arthur,  I will comment on the 3 points you did in the previous reply: 

1- What was the result of your test?

I have been very busy lately with some work, I did download the SP4 on Robot 2016 but I haven´t tested the fix about the Ve shear I mentioned you at the start of this wonderfull conversation. When I have the time, I will, but I don´t know when I can yet, paid work is demanding my attetion hehe.

2- See the attached document. 

Alright, I really apreciate this information. Now I can clearly see what is happening (I made the same calculations on excel when I was testing RSA) but still I would like to discuss this procedure because just to be sure that RSA is considering every article.

A- First of all, there´s an important seismic provisions article about considering the Vc (Concrete Capacity) or not, the article is 21.5.4.2 that states the following:

"a" states that the concrete shear capacity can't be considered if Vp (Plactic Moments shear) is bigger than Vg (Factored Tributary Gravitational Load that comes from the RSA SEISMIC SHEAR combination). In this case, Vp is is lower that Vg, so that would mean that Vc is not 0 but I don't know if RSA will put Vc=0 if this condition is met because in your document it's not mentioned this previous check before calculating Vc.

"b" states that the axial force Pu must be lower than Agfc/20, for most beams, this is always true because there's no axial force due to the loads adn geometric configuration.

B- Now, focusing on our case I managed to notice this:

Everything about the results is correct up to the point where you obtain Vc and Vs, after that I i think there might be an error. In the document you published it's stated that Vc= 86 Kn and Vs= 627 Kn (In the metric system is 8790 Kgf and 63844 Kgf) This values are definetly correct, I had the same values on my excel calculation.

Now the thing I noticed that might be wrong is the following, in the word document you attached the f.Vn is obtained like this:

fVn = f*(Vc+Vs) = 0.75*(86[kN]+627[kN]) = 322 [kN] / 0.00980…= 32905 kgf

But 0.75*(86+627) is actually 534 Kn that is 54.452 Kgf, a value that is totally different from the RSA output. So this might be a error that you may need to fix if I am right.

C- Lastly, I would like to once again mention another important seismic provisions article that I think RSA might be missing again because in the example you made, you put "shear phi" value as 0.75 and the document doesn't mention a previous check. ACI states that this value might vary from 0.75 to 0.6 in 9.3.4. 

This article explains that when the seismic provisions is high risk (Special Moment Frame, our example) Shear Phi could change from 0.75 to 0.6. ETABS and SAP2000 are considering this article, i am not aware if RSA is doing so.

3- I have asked the development team to remove the provisions of having all main bars tied for ACI as excessive

I am very happy to hear this really, with all this fixes the beam module will be perfect in no time!

So to conclue Arthur, I will really like you to read all of this and discuss with me the things I mentiond on the second point. There might be a error around again. After we conclude all of this, I will comment now about some things on splices I also think that might be not right (Yield zone lenght and lap splices location).

I await your response about the Vn results. I am looking forward to it.

[Artur.Kosakowski]

I'm attaching the updated version of the document as we noticed some mistakes we made in it. 

For C: 

Currently RSA always assumes 0.75. 

I'm sorry for my limited knowledge of ACI but I assume tat there should be an additional selection which defines the support types:

and then the corresponding checks of (a) through (c) should be made which also means that the beam itself should be classified as resisting E or not. 

Or alternatively there should be an option to select fi manually from 0.6 to 0.75 range. Is this correct?

[Anonymous]

You could add the selection from the user manually to choose from 0.6 or 0.75 but the best competitive idea would be that RSA could do it by itself (Like I said before, Etabs and Sap2000 check this condition automatically).

Also, did you checked that RSA does the verification to decide if Vc=0 or not?

[Artur.Kosakowski]

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You could add the selection from the user manually to choose from 0.6 or 0.75 but the best competitive idea would be that RSA could do it by itself (Like I said before, Etabs and Sap2000 check this condition automatically).

Yes, I do agree that this would be the best option. Could you tell me how exactly ETABS or SAP determine that the beam "belongs" to one of these structures

Also, did you checked that RSA does the verification to decide if Vc=0 or not?

I'm sorry for not putting this plainly in my previous answer: Yes, it does. 

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[Anonymous]

Alright Arthur, I have been trying to find more info about this and I can´t belive no one really talks too much about it. Nonetheless, I think I can tell you the basic idea.

As we know the article states this:

That means that there are three types of structures that must be considered in the first place in order to change their shear reduction phi factor. These structures are:

1- Special moment frames (In our example, our structure belong here)
2- Special structural walls
3- Intermediate precast structural walls in structures
assigned to Seismic Design Category D, E, or F

The posible changes in the shear factor these structures could present are the following:

Now, it can be tricky to think at first that the three types of structures must change their shear reduction factors as a,b and c dictates but the ACI commentary says the following:

Taking the commentary in mind, we can conclude that the conditions a and b are for walls only and the condition c is for special moment frames.

So, RSA should verify first if the structure belongs to one of these three and then apply the conditions a.b or c as the code dictates.

In our case, a normal framed beam will always have 0.75 as reduction factor. And the change will only be to the joints, as these will have 0.85. If RSA does not design these diagonally couping beams, then there's no need to verify anything on the beam shear reduction factor because all of the beams RSA can design always have 0.75 because they are normal reinforced special moment frame beams.

[Artur.Kosakowski]

Thank you for helping me to understand this code provisions.

In our case, a normal framed beam will always have 0.75 as reduction factor. And the change will only be to the joints, as these will have 0.85. If RSA does not design these diagonally couping beams, then there's no need to verify anything on the beam shear reduction factor because all of the beams RSA can design always have 0.75 because they are normal reinforced special moment frame beams.

As the coupling beams are not designed and we can assume that beams designed in RC Beam Reinforcement module are always normal reinforced special moment frame beams  the the assumption of using 0.75 for shear (as it is now in RSA) is as far as I understand correct. 

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[Anonymous]

Lately I have been having some free time to check on RSA 2016 SP4. I noticed the following problem now that I have installed the eduactional version on my windows 8.1 Laptop.

When I select the "Typical Reinforcement Option" the following error happens and it closes the whole program.

Do you have any idea why this version is giving me this error? The previous one didn't have this problem.

[Anonymous]

Also, I am experimenting other problems I didn't have before. For some reason, RSA results are saying that there are some spans in the beam that have MC=0 when we can clearly see that the reinforcement is there.

[Anonymous]

I opened a RSA 2015 file and then saved it under the new version. I don't know if this might be the reason of this sudden errors but as you can see they seems to be critical because I can't use an option and the MC is equal to 0 on the support when we can see that there is some reinforcement bars in that location.

I will attach the file too, I await your answer as always. Thanks in advance.

[Artur.Kosakowski · Accepted solution]

I opened a RSA 2015 file and then saved it under the new version. I don't know if this might be the reason of this sudden errors but as you can see they seems to be critical because I can't use an option and the MC is equal to 0 on the support when we can see that there is some reinforcement bars in that location.

I will attach the file too, I await your answer as always. Thanks in advance.

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I'm sorry for not being able to investigate thsi earlier. The reson for lack of rebar capacity is the fact that Robot sees these rebars as having hooks with -1 length. My assumption is that this may be caused by some data corrruption in your reinforcement bar datanase which also shows no steel grade when I open your file on my computer.

Could you attach it please?

The workaround I have managed to find is to temporarily change reinforcement bars diameter whch I illustrated on teh movie below:

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Artur.Kosakowski]

When I select the "Typical Reinforcement Option" the following error happens and it closes the whole program.

Do you have any idea why this version is giving me this error? The previous one didn't have this problem.

We will investigate this. Thank you for indicating this situation. 

[Anonymous]

So it might be corruption on the rebar database? What I find weird is that in the previous version I was using that same databse and there were no problems at all. I will try doing what you said and I comment you on the results when I can.