Beam Fail In Torsion And Shear In Etabs Then Why Torsion Modifier Reduce To 0.001

[kHURRAM ALI]

many engineers in pakistan use this practice when modeling a beam in etabs , that if beam is fail in shear and torsion than they just change the torsion modifier from 0.35 to 0.001 , and then redesign that beam , after designing that beam appear to be pass in shear and torsion , also showing zero r/f required for this particular beam.

 

i want to know that is this the right way to model beam, does code provide such kind of solution ,or it has no meaning.

[WR1]

They are doing it right. It depends on you. It is the beauty of the structures that they will behave the way you designed them. 

 

When reducing the torsion modifier for beams that are failing to a value approx equal to 0 then watch for the increased moments in slabs. If you put the reinforcement in slab for additional moment then it is ok!

 

It depends upon the relative stiffness of beam and slab that how much load beam will take (Torsion, moment etc).

[Muneeb Badar]

One more thing I want to add, although I am not sure about this.
 
 
In ACI code there are two methods of calculating the torsion for a beam in ACI 11.5. One is slab resting on beam and there is torsion in the beam ACI Fig R 11.5.2.2 and second method is slab cantilever from beam Fig R 11.5.2.1.
 
ETABS normally design the each beam by the second method which is the conservative one. So this the reason beams mostly fails in torsion effect. This is the reason we reduce the torsion modifier otherwise no code allows to reduce modifier to such extent.
 
This is my concept any body can comment......   specially Rana Waseem

[WR1]

One more thing I want to add, although I am not sure about this.

 

 

In ACI code there are two methods of calculating the torsion for a beam in ACI 11.5. One is slab resting on beam and there is torsion in the beam ACI Fig R 11.5.2.2 and second method is slab cantilever from beam Fig R 11.5.2.1.

 

ETABS normally design the each beam by the second method which is the conservative one. So this the reason beams mostly fails in torsion effect. This is the reason we reduce the torsion modifier otherwise no code allows to reduce modifier to such extent.

 

This is my concept any body can comment......   specially Rana Waseem

 

In my understanding these are NOT 2 different methods;

 

This is just a differentiation;

 

There are two torsions; 

 

1. Compatibility torsion (where redistribution of moments take place) like slab on beams

 

2. Equilibrium torsion (where there is no path available for redistribution of moments, like a cantilever slab resting on a beam)

 

These are not two different methods of analysis in ACI or ETABS. This is just to distinguish the cases.

 

That is why it does not matter in ETABS because in ETABS loads will follow the paths that is available.

 

So does not matter if it is case 1 or 2, apply J modifiers but watch for slab moments.

 

Also make sure your detailing handles all these issues.

 

For example if the beam is torsionally too stiff as compared to slab, it will take more moment as compared to slab, and if you are applying less J modifier to beam then make sure the detailing also follows the same approach. (try to increase bottom reinforcement of slab).

[Muneeb Badar]

Dear Rana, I partially agree and partially disagree

 

You are right about the explanation. This is much informaticve for me. but when ETABS will check the torsion, how it will check. In ETABS its is not coded to check the beam torsion by which way. Neither ETABS nor we can ask it to do what we want. So ETABS takes it conservative. I am also not 100% confirm but this is my opinion.

 

About the modifier you are saying, I think no code allows to reduce it to 0.001. This is just a practice to handle ETABS issue.

 

Actually, I am not saying it by myself, I asked this question from the person that used to work for CSI & ETABS as you know that was my last office.

 

Thanks

 

Muneeb

[Muneeb Badar]

Dear Rana,
 
Please check the reference from ETABS Concrete design manual. This is what I am saying.
 
Thanks
 
Muneeb

 

[Mehreen Iqbal]

I am really confuse it we dont design beam for torsion than how we are going to design our slab for the torsion that is coming from beam.

providing how much amount of moment near support in slab.

Plius this is the design practice in pakistan that we reduce torsional modifier to 0.01 in beams but for slab we are designing it manually...not taking into consideration that we ignored it beam...

is this a right practice.. 

[ILYAS]

justified and reasonable practice as explained above the concept of equilibrium torsion

[Eman]

On 6/6/2015 at 11:54 PM, Muneeb Badar said:

Dear Rana,
 
Please check the reference from ETABS Concrete design manual. This is what I am saying.
 
Thanks
 
Muneeb

 

Dear engineers, can somebody clarify the above. What is the difference between the analysis results of a member released for torsion and a member with torsional stiffness modified in a statically indeterminate structure? I have had the understanding that both of them have same effect. 

[Ayesha]

On 4/4/2018 at 2:42 PM, Eman said:

Dear engineers, can somebody clarify the above. What is the difference between the analysis results of a member released for torsion and a member with torsional stiffness modified in a statically indeterminate structure? I have had the understanding that both of them have same effect.

Likely.

[WR1]

On 4/5/2018 at 12:42 AM, Eman said:

 

The paper says that upon cracking the internal redistribution occurs. However like for moments, ETABS does not redistribute Torsion automatically. If you want it, then release (or apply modifiers) to redistribute forces. That is why I said, after this, you need to check for increased forces in slab. Its that simple.

[Eman]

Thaks engineer. But do you mean that for flexural monents unlike for torsional moments, the program has a capability to redistribute the values automatically; witout providing release/stifness modifier?

[WR1]

The analysis is Linear. Automatic redistribution would have been possible if analysis were Non-Linear. In order to approximate redistribution (during cracking), the USER has to put some kind of stiffness modification. This applies to shear, torsion, moments, axial...everything.

[Eman]

But, as my understanding stiffness modifiers has nothing to do with redistribution of moment for statically indeterminate structure. As stiffness modifiers are applied through out the length of the member (here in this case i am addressing a beam particularly) to account for cracks which might occur whether at mid-span or support, whereas redistribution of mement due to cracking too should be applied through the provision of beam-end releases only until the moment at support reaches the desired value; as per design codes the reduction is allowed up to 30% of the support moments. Thanks.

[Luk]

On 6/4/2015 at 12:57 PM, Rana said:

They are doing it right. It depends on you. It is the beauty of the structures that they will behave the way you designed them. 

 

When reducing the torsion modifier for beams that are failing to a value approx equal to 0 then watch for the increased moments in slabs. If you put the reinforcement in slab for additional moment then it is ok!

 

It depends upon the relative stiffness of beam and slab that how much load beam will take (Torsion, moment etc).

@Rana , i’ve try it to contact you in email and Skype , but without succes .

Can you help me with an Etabs&Safe Analisys for a verry small 2 leves building , 160qm , but with long opened corner cantilever 

please contact me an email : Luk2011 at yahoo dot com

I hope you will get this message and will want you to help me

[WR1]

On 10/20/2018 at 6:50 PM, Luk said:

@Rana , i’ve try it to contact you in email and Skype , but without succes .

Can you help me with an Etabs&Safe Analisys for a verry small 2 leves building , 160qm , but with long opened corner cantilever 

please contact me an email : Luk2011 at yahoo dot com

I hope you will get this message and will want you to help me

Please post the question here. I can't help preparing models for you but if you do have a specific question, please feel free to post in the relevant section of the forum. This post will be deleted after 3 days.

[Moutasem Abdulrahman]

On 6/4/2015 at 1:57 PM, WR1 said:

They are doing it right. It depends on you. It is the beauty of the structures that they will behave the way you designed them. 

 

When reducing the torsion modifier for beams that are failing to a value approx equal to 0 then watch for the increased moments in slabs. If you put the reinforcement in slab for additional moment then it is ok!

 

It depends upon the relative stiffness of beam and slab that how much load beam will take (Torsion, moment etc).

What if the beam fails in ETABS even with torsional stiffness modification factor in ETABS (0.01) but yet passes in SAFE, what to do? torsional  failure that is.