Cantilever beam fails in ETABS no matter how big section I assign.

[Beenay Beenay]

cantilever beam fails in etabs no matter how big section I assign. Please help! my etabs file in attached herewith.

cantilever beam failure elevation view grid 2.bmp cantilever beam failure.Docx shear failure in cantilever beam.bmp

house for thulo kancha uncle 2.EDB

Edited August 22, 2019 by Beenay Beenay

[WR1]

Please elaborate. No one is going to download and check your model unless you post some snap shots and details of your problem.

[Beenay Beenay]

Actually there is nothing much to elaborate. I have designed many buildings in etabs. I came across this problem in two buildings and I cant find the cause. Cantilever beams just fail in shear no matter how big of a section I assign or how little load I apply. Its just a regular building. Nothing exceptional about it. I have remodeled it few times in Etabs 2015 and 2016 to see if there is some issue with the version of etabs that I am using. But I came across the same problem in both versions. But cantilever beams don't fail when I remove earthquake load pattern. Design summary show very very high unrealistic shear force. I don't know the reason why. thanks for showing interest.

[abbaskhan2294]

4 hours ago, Beenay Beenay said:

Actually there is nothing much to elaborate. I have designed many buildings in etabs. I came across this problem in two buildings and I cant find the cause. Cantilever beams just fail in shear no matter how big of a section I assign or how little load I apply. Its just a regular building. Nothing exceptional about it. I have remodeled it few times in Etabs 2015 and 2016 to see if there is some issue with the version of etabs that I am using. But I came across the same problem in both versions. But cantilever beams don't fail when I remove earthquake load pattern. Design summary show very very high unrealistic shear force. I don't know the reason why. thanks for showing interest.

Try section modifier for torsion as 0.01 and then check. It may help

[Beenay Beenay]

I did that. I provided torsional constant of 0.001.

[WR1]

22 hours ago, Beenay Beenay said:

Actually there is nothing much to elaborate. I have designed many buildings in etabs. I came across this problem in two buildings and I cant find the cause. Cantilever beams just fail in shear no matter how big of a section I assign or how little load I apply. Its just a regular building. Nothing exceptional about it. I have remodeled it few times in Etabs 2015 and 2016 to see if there is some issue with the version of etabs that I am using. But I came across the same problem in both versions. But cantilever beams don't fail when I remove earthquake load pattern. Design summary show very very high unrealistic shear force. I don't know the reason why. thanks for showing interest.

Never came across such an issue. Share the results may be?

[abbaskhan2294]

On 7/30/2019 at 12:42 PM, Beenay Beenay said:

cantilever beam fails in etabs no matter how big section I assign. Please help! here is my etabs file - https://drive.google.com/open?id=10Pr_WSKlv9L77CIb_aP9HbpYuux38fvA

Please have a look at the file now. As per my understanding it is very much okay now. You were inputting the wrong modifiers. For your reference I have also uploaded an image for appropriate section modifiers. Please find the attached files. Thanks

house for thulo kancha uncle 2-check.EDB

[Beenay Beenay]

On 8/17/2019 at 3:11 AM, abbaskhan2294 said:

Please have a look at the file now. As per my understanding it is very much okay now. You were inputting the wrong modifiers. For your reference I have also uploaded an image for appropriate section modifiers. Please find the attached files. Thanks

house for thulo kancha uncle 2-check.EDB 240.49 kB · 2 downloads

Thank you for your support. You guys in Sepakistan have been most supportive so far. I couldn't open the etabs file you uploaded because it might have been saved in the later version of etabs than mine. I did not modified any modifier from default value of 1 except for torsional constant in which I plugged in 0.001. I think its ok to leave all the modifiers at the default value of 1. Plugging in the value of 0.35 and 0.7 for beam and column respectively give less reinforcement. These modifiers are only supposed to be used in deflection check from what I have read in discussion forums. Only changing one modifier i.e. torsional constant to 0.001 to avoid torsion in beams, hence avoiding the beam failure in torsion(as beams can transfer torsion to columns and other beams as moment)  should suffice for a good design) from what I have read in forums. I was just wondering how can "not changing" the modifiers to the above values can show the cantilever beams as failed. I know modifying the torsional constant to negligible value  to avoid beam failure in recommended. But I think leaving other modifiers to value of 1 should not be a problem. I would appreciate if you can provide any explanation or clarification on this issue. Thanks a lot!

Edited August 18, 2019 by Beenay Beenay

[abbaskhan2294]

That is a very nice question asked. Well section modifiers just not only has to do anything with deflection but stiffness of the member too and stiffness in RCC is a major factor controlling the reinforcement. Just as u explained the torsional constant if it is input as 1 it means the beam is so stiff that all the torsional moment will be taken by this beam alone and it will not be distributed in the adjacent members thats why beams always fail in torsion when torsion modifier is used as 1. But if u start playing with the torsion modifier and decrease it u will see that the torsional demand acting on it previously has now been distributed amongst other members and as a result reinforcement in all the other adjacent columns and beams increases. This condition is known as compatibility torsions. 

Coming back to moment of inertia modifiers case, Building systems are broadly classified into moment resisting frames, Bracing system and dual systems. 

When designing columns in buildings with braced systems (shear walls), they recommend to use a section modifier for both Ixx and Iyy as 0.1 and when u do such , columns in buildings with greater heights will have 1% reinforcement for the sake that u have assumed that ur column stiffness is too less (from the section modifiers) and all the load is taken by the bracing systems alone. 

This is what i have understood from the case studies I have performed. sharing other experiences will be great addition. 

[WR1]

I think I misunderstood your question initially. I thought of it as a simple cantilever beam. However, from the trailing discussion I can discern the said beam was part of a building or frame. And yes, that's where the torsion stiffness play a role.

[UmarMakhzumi]

There has been very good discussion on this, I will just add my 2 cents. I don't have ETABS, so I have no idea about what the framing so the following comment *might* not be applicable if framing is not what I am thinking, however, please note that for beams supporting cantilevers, you shouldn't release the torsional moment or change its modifiers as the torsion would be equilibrium torsion not compatibility torsion. If your beam is failing, just add another beam inline with cantilever. See image below.

 

Also, I would suggest to *please* add images to question rather than ETABS model as not everyone has the software to check. Also checking someone else's file is cumbersome and time consuming, which a lot of people who volunteer here might not find time for as a result of that the thread may stay unresponsive.

Thanks.

 

 

[Beenay Beenay]

On 7/30/2019 at 1:27 PM, Beenay Beenay said:

cantilever beam fails in etabs no matter how big section I assign. Please help! here is my etabs file - https://drive.google.com/open?id=10Pr_WSKlv9L77CIb_aP9HbpYuux38fvA

cantilever beam failure elevation view grid 2.bmp 33.15 kB · 0 downloads cantilever beam failure.Docx 13.13 kB · 0 downloads shear failure in cantilever beam.bmp 38.85 kB · 0 downloads

I have attached the plan and elevation of the model showing cantilever failure. I have also attached the shear design summary of one of the cantilever beams. Shear force in it is 830 KN which is greater than the reaction at the base of the column for normal residential building of 4 story and 12ftx12ft grid.

[UmarMakhzumi]

10 hours ago, Beenay Beenay said:

Shear force in it is 830 KN

You need to check your applied load. Is this correct to say that cantilever beam is a balcony beam? If yes, the only load on the beam is the tributary balcony area that it is supporting and that can’t be 830kN. Check your area load units, material assignments, section properties. Clearly there is something wrong in the model. Another method would be to check which load combination is governing shear, then run individual basic load case in that load combination to trace error. 

Thanks.

[Beenay Beenay]

I have checked the loads and even remodeled it few times in two version of etabs ie 2015 and 2016. I hope someone can find my blind spot in modelling or loading.

[UmarMakhzumi]

On 8/20/2019 at 9:50 PM, Beenay Beenay said:

I have checked the loads and even remodeled it few times in two version of etabs ie 2015 and 2016. I hope someone can find my blind spot in modelling or loading.

Your applied loads need to match actual situation. I am not sure what is happening there, but if you have a simple residentail balcony, you can't have a 840kN shear due to area load (assuming regular areas). Did you just model the balcony by itself and compared it against hand calcs for shear? If not, please do so.

 

[mortezamobariz]

hello everyone 

regarding this issue I can not say for sure, unless I check the model, so please send me the Etabs Model

this problems seems un-logical. I checked the word file I guess the problem is the width of the beam; increase the width of the beam (at least 250mm) because 

beside that the problem may be in modeling and loading too 

[Beenay Beenay]

20 hours ago, mortezamobariz said:

hello everyone 

regarding this issue I can not say for sure, unless I check the model, so please send me the Etabs Model

this problems seems un-logical. I checked the word file I guess the problem is the width of the beam; increase the width of the beam (at least 250mm) because 

beside that the problem may be in modeling and loading too 

I have used 12 inchx14inch beam too. The the problem is the same. I have uploaded the model at the begining of this topic, 

[mortezamobariz]

I downloaded the model and ran the analysis, then designed no member failed; all member passed design

you can check pics

[Beenay Beenay]

@mortezamobariz Thanks a lot for taking time to check my model. But I think here is one problem. I mistakenly uploaded the etabs file in which I have deleted all the cantilever beams and just slab has remained. I am uploading the file that has cantilever beams. 

house for thulo kancha uncle 2.EDB

Edited August 22, 2019 by Beenay Beenay

[abbaskhan2294]

14 minutes ago, Beenay Beenay said:

@mortezamobariz Thanks a lot for taking time to check my model. But I think here is one problem. I mistakenly uploaded the etabs file in which I have deleted all the cantilever beams and just slab has remained. I am uploading the file that has cantilever beams. 

house for thulo kancha uncle 2.EDB 270.12 kB · 0 downloads

Please open the file which i uploaded. I have modified it with Etabs version 16.0. Download Etabstran2013 and convert the file to ur workable version.

Again I will suggest u that section modifiers have very important role to play. You need to take care of the case i-e whether it is equilibrium torsion or compatibility torsion in order to decide torsion modifier for section.

As far as Ixx and Iyy modifiers are considered, if u go with 1 as section modifier ur section has greater stiffness. Greater stiffness means greater attraction of loads towards itself. As it takes more load in case with section modifier 1 u need more shear reinforcement because more load means more shear demand on the member. Now in such cases the required shear reinforcement exceeds the section capacity and as a result section is dislayed as failed. Also these are short span members and such members are very critical to shear than flexure failure. This failure is referred to as deep beam failure. Please check definition for deep beam in Aci code.

If u go with the other option with section modifiers for moment of inertia as 0.35 (as suggested by ACI) which is pretty much valid for under reinforced cracked sections, it is actually the real representation of the beam element. In such cases less loads will be taken by the beams and consequently less shear demand. please use the section modifiers as suggested and then see ur structural elements. Thanks

[Beenay Beenay]

@abbaskhan2294

I downloaded the file you have uploaded in your previous reply, but it still says, it has been created in later version of etabs than mine.

Edited August 22, 2019 by Beenay Beenay

[abbaskhan2294]

5 minutes ago, Beenay Beenay said:

@abbaskhan2294

I downloaded the file you have uploaded in your previous reply, but it still says, it has been created in later version of etabs than mine.

change section modifiers in ur original model and then check. atleast u can do that

[Beenay Beenay]

Just now, abbaskhan2294 said:

change section modifiers in ur original model and then check. atleast u can do that

ok! Thanks a lot for your helping hand.

[Ayesha]

Apparently, you have a "load transfer problem".

The first and foremost thing that is expected is that reactions should equal applied loads. This should always be checked by design engineers unfortunately  this doesn't happen so often. If your applied loads don't match reactions, there is no point in discussion "Design Results" because your are designing for a model that doesn't represent "actual conditions".

For your situation, have you tried only modelling the balcony beams in a new ETABS model and check the reactions as suggested above? 

Or did you try isolating the load combinations and checking reaction for each load case against manual calculation as suggested above to pin down the load case that is causing problem?

Other items that you can do is apply your loads on cantilever as line loads.

I also want to comment that this thread is a perfect example of why SEFP should ban people from sharing their models. Everyone, expects someone else to check and figure out the problem and the discussion about potential problem is lost. Other items that are observed are that original poster (generally speaking) may only entertain the replies that require less to zero effort and totally avoid or not answer things that require him to crunch some manual numbers or to do math. Something that Admins/Moderators can consider. I believe this will improve the quality of discussion on this forum.

 

Edited August 23, 2019 by Ayesha

[mortezamobariz]

On 8/22/2019 at 10:07 PM, Beenay Beenay said:

@mortezamobariz Thanks a lot for taking time to check my model. But I think here is one problem. I mistakenly uploaded the etabs file in which I have deleted all the cantilever beams and just slab has remained. I am uploading the file that has cantilever beams. 

house for thulo kancha uncle 2.EDB 270.12 kB · 1 download

I suggest that redefine your section and remodel the members that fails

also don't forget to check your load pattern, load case and mass source carefully.