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Hasnain Khan

Design For Shear And Torsion Using Etabs

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Hello,

 

I have recently joined a structural engineer, and trying to get myself familiar with the norms of practice, so I have some basic questions which I haven't been able to get answers of sufficiently.

 

After modelling the structure in ETABS I'm trying to find out how to interpret the values of shear and torsion reinforcement given in design outputs. I have tried to find the answers by codes, manuals but haven't succeeded. Even asking my colleagues haven't given a satisfying answer, as everyone has his/her own way of interpreting these values.

 

Thanks,

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I have'nt understood your question. What do you mean by interpret; ETABS have clearly defined the symbols/acronyms that they use in design report. It is a common practice to add torsion reinforcement in flexural reinforcement, and divide it over the depth of beam: top bottom and middle. Stirrups  are also able to arrest the cracks due to torsion.

But the important point is to ascertain the importance of torsion reinforcement in a beam. The beam that transfers  load from a slab which is supported on four /three sides don't need all the torsion reinforcement reported by ETABS, as it can redistribute the stressed in the form of flexure and shear in beam and slab. Only minium torsion reinforcement is required in this case. In these cases you can run the analysis by reducing tosion stiffness of beam so that beam does not fail in shear+torsion in ETABS.

The Beam part of a cantilever slab has to transfer the moment through torsion as no other load path is available in this case, and redistribution is not possible.

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Hello,

 

I have recently joined a structural engineer, and trying to get myself familiar with the norms of practice, so I have some basic questions which I haven't been able to get answers of sufficiently.

 

After modelling the structure in ETABS I'm trying to find out how to interpret the values of shear and torsion reinforcement given in design outputs. I have tried to find the answers by codes, manuals but haven't succeeded. Even asking my colleagues haven't given a satisfying answer, as everyone has his/her own way of interpreting these values.

 

Thanks,

 

See this:

 

SEFP Consistent Design: Design for Torsion

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yeah every frame Element has its torsion constant which you can find in the Property Modifier of that Element.

 

At the time of defining any col or beam than giving the size and depths of that element you can see ''property modifier'' where you can find torsional constant.

 

My boss has told me to use 0.7 for col instead of 1 and 0.01 for beams instead of 1.

now i am trying to figure it out why we uses this..I will get you when i found this answer...:)

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For interpreting the ETABs Results see the 1st picture in Attaachment

As far as choosing a torsional constant for a member see the second doc. in attachmetns

 

Zain,

 

Could you post the source/ reference for the method described in Torsional Constant For Beams in ETABs.docx document?

 

Cheers.

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ASS-ALAMO-ALIKUM IF MY UNITS IN METER Suppose I use beam section 0.3m x 0.7m so for checking torsion constant i use the same doc sheet as mention above or may i use diffrent.

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Dear Zain,

 

I don't know the origin of document,you have uploaded for calculating torsional constant,but the methodology given therein is incorrect.As "Tcr" and "Tu" given therein are indeed threshold torsional strength and ultimate torsional stresses respectively, and are both design properties  not analysis properties. (See ACI 318-11 section 11.5.1).

 

Whereas the torsional constant, ETABS asks in "analysis property modification factors" is simply the torsional moment of inertia (J) used to determine torsional stiffness of a member (JG/L) i.e something else.

 

As long as its value is concerned,then in building structures it is a general practice to use a negligible value like .001 to nullify beam's torsional stiffness.In this way, the torsional stresses (if arising due to compatibility of deformation i.e compatibility torsion ) are transferred via alternate load path (i.e redistribution of torsional moments occurred), considering that beam is unable to provide torsional restraint and in other condition if torsional stresses in beam is required to satisfy equilibrium of structure (where redistribution is not possible) then torsional stresses in beams remains independent of whatever value of "J" you have selected as equilibrium equations are necessarily satisfied independent of stiffness as "Compatibility is optional and equilibrium is essential".

 

This approach of minimization of "J" economize beam sizes that arise from stringent combined shear and torsion requirement of building codes,but consequently beam sections designed in this way will start developing internal horizontal cracks (hairline cracks not affecting functionality of structure) due to torsional stresses and their torsional strength will continuously degrade till the design condition is achieved i.e negligible torsional strength of beam.But as the structure is designed to be stable without torsional stiffness of beam so it remain stable after this condition is achieved.However, the beam member itself cracks that doesn't affect the functionality of structure in any way.

 

A very descriptive and clarifying description is available in "Reinforced concrete design by Arthur Nilson".

 

As long as authentication of this approach is concerned then it is allowed by building codes as,

 

1, ACI-318-11 section 11.5.2.1 & 11.5.2.2.

 

2, UBC97 section 1911.6.2.1 & 1911.6.2.2

 

3, BS 8110-1 1997 section 3.4.5.13

 

Keeping in view above mentioned, it is a general practice to nullify torsional constant of beams in building structures and it is not required to use any iterative process to derive torsional constant of each beam section that is indeed not practical as there will be thousands of beam span in large structures.

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I am confused regarding this approach of reducing torsional modifier to such great extent. According to ACI 11.5.2.2

In a statically indeterminate structure  where reduction of the torsional moment in a member
can occur due to redistribution of internal forces upon cracking, the maximum Tu shall be permitted to be
reduced to the values given in (a), (b), or (c), as applicable:
(a) For nonprestressed members, at the sections
described in 11.5.2.4

φ4λ Sqrt(fc ′) A2cp / Pcp

 

It says that we can reduce torsional moment upto a specified limit. Not to zero or 0.001.  In its commentary it says, 

 

For this condition, illustrated in Fig. R11.5.2.2, the
torsional stiffness before cracking corresponds to that of
the uncracked section according to St. Venant’s theory. At
torsional cracking, however, a large twist occurs under an
essentially constant torque
, resulting in a large redistribution
of forces in the structure.11.34,11.35 The cracking torque
under combined shear, flexure, and torsion corresponds to
a principal tensile stress somewhat less than the
quoted in R11.5.1.
When the torsional moment exceeds the cracking torque, a
maximum factored torsional moment equal to the cracking
torque
may be assumed to occur at the critical sections near
the faces of the supports. This limit has been established to
control the width
of torsional cracks.

Also according to Nislon, this distribution is only possible after extensive cracking as highlighted in below pic. So I doubt the approach used to neglect torsion upto 0.001 level

What i got from the 2nd attachement of Zain Saeed the author is dividing Tcr with Tu to find how much reduction in Tu is needed to reduce torsion upto Tcr which is, as mentioned above, is necessary to keep torsional crack widths in control. and hence using the modifier for each section defined for beam. It might be a bit lengthy task to evaluate for each type of section ( most loaded members of a type of beam may be checked only), but the approach seems more realistic. 

Kindly comment as I think if even we reduce to 0.001, still this redistribution in torsion is not possible without large twisting which is not possible without excessive cracking. So reucing upto such a low value does not seem good. 

Untitled.png

Edited by Waqas Haider

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This is what i found in "Structural Concrete theory and Design by Nadeem Hussen"

 

Here it also reports the same thing that we should design for torsion for atleast 

φ4λ Sqrt(fc ′) A2cp / Pcp

Which is also specified by ACI that we can reduce our torsion upto phi.Tcr i.e. cracking torsion and not below this. Even if we neglect the remaining torsion for compatibility, we should atleast perform design for phi.Tcr or should at least provide minimum longitudinal and transverse reinforcemet for torsion. This is to control crack width to satisfy servicability. ACI, Nilson and Nadeem hussen all quote the same thing.

Hence reducing the modifier upto such a little value is not good at start. At start we should go for a value of 1, 0.3 or any other suitable value which user think is small enough to release majority of compatibility torsion and will sustain only smaler torsional moments. After designing, if still some indeterminate beams are being failed, then for the specific beams we can reduce value unless we get Tu equall to phi.Tcr or a bit larger than that. Because putting Torsional modifier to 0.001 would not report any torsional reinforcement and hence adding no torsional reinforcement at all would cause excessive crack widths affecting serviceability of the structure.

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When you are analyzing RCC frame members, you are concerned with flexural and shear demands in most cases, if serviceability and stability concerns have been addressed. The choice of modifier for torsional stiffness does not effect those results.

Choosing the modifier equal to 0.001 will lead to conservative results if your modelling the slab for serviceability and strength-checks.

As far as designing the beam for Tcr, in case of compatibility torsion, the code clearly directs the engineer to do that, as you have mentioned in your post..

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