www.sefindia.org

STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

 Forum SubscriptionsSubscriptions DigestDigest Preferences   FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups  RegisterRegister FAQSecurity Tips FAQDonate
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log in to websiteLog in to websiteLog in to websiteLog in to forum 
Warning: Make sure you scan the downloaded attachment with updated antivirus tools  before opening them. They may contain viruses.
Use online scanners
here and here to upload downloaded attachment to check for safety.

Torsion in beams and staad modelling

 
Post new topicReply to topic Thank Post    www.sefindia.org Forum Index -> SEFI General Discussion
View previous topic :: View next topic  
Author Message
Novice
SEFI Member
SEFI Member


Joined: 04 Jun 2010
Posts: 5

PostPosted: Fri Jun 04, 2010 4:31 am    Post subject: Torsion in beams and staad modelling Reply with quote

Dear Sefians
                     This is my first mail in forum. I don't have much experience in structure design. Recently, I have joined a small structural design firm. We are using staad pro for modelling multistorey buildings. We only model frame structure containing beams and columns . For slabs, we apply floor load. Some times, Staad shows beams failed because torsion moment Mx is quite high. To counter it we reduce the torsional stiffness in beam i.e. Ix to about zero(1 e-6) and increase Iy substantially(1 e6). When I asked my senior about it, he says, we are taking diaphram stifness of slab, so that all of the torsion is attracted in the direction of slab.

Is this concept OK ?


regards

Rinku
Back to top
View user's profile Send private message
pushpinder84
Progressive Member
Progressive Member


Joined: 08 Mar 2010
Posts: 42

PostPosted: Fri Jun 04, 2010 4:51 am    Post subject: Reply with quote

Dear SEFIan,
I am not sure how this concept works. From what i know, torsion in beams is normally due to unsymmetrical loading on the slabs it is supporting, and thus slab should not contribute towards stability.
However if there is torsion due to lateral (earthquake or wind) loads, then slabs contribute towards stability through diaphragm action (helping in distribution of loads to vertical elements and relieving the beams of some stresses) and this concept should be valid to some extent.
So i guess it is up to the judgment of the engineer to see how the design process should proceed. But one thing i can say for sure is, that do not do anything just because the software says so, or your seniors say so. Always look for explanations and technical references and rely on your own judgment.
Regards
Er. Pushpinder Chauhan
Back to top
View user's profile Send private message
Novice
SEFI Member
SEFI Member


Joined: 04 Jun 2010
Posts: 5

PostPosted: Fri Jun 04, 2010 5:20 am    Post subject: Reply with quote

Dear sir
               Most of the buildings we design are in seismic zone - 4 & 5. Torsion is due to the load combinations with  earthquake loads. IS-456 also provide emperical formula to convert torsional moment to moment about major axis of beam. Then, as per code this torsional moment should be added to MZ.  But by increasing IY, STAAD is extracting this torsion in MY direction.

Reducing Ix and increasing Iy, results in torsional moment 'MX' to about 0 , 'MY' increases significantly and 'MZ' remains more or less same. Since Slab provides enormous stiffness in lateral direction to beams, we just ignore this value of MY for design.  By judgement also, it looks that such a stiff slab(most of the time slab spans more than 4m c/c in each direction and no. of pannels are also more than 3) would not alow beam to be twisted.

Is this concept OK ?

regards

Rinku
Back to top
View user's profile Send private message
debayan
...
...


Joined: 25 Jul 2008
Posts: 88

PostPosted: Sat Jun 05, 2010 4:14 am    Post subject: Reply with quote

If proper grid arrangment of your main beams and supporting columns is formed. Then ideally there should be no torsion in the internal beams except for the edge beams. Please check whether you have torsion in internal beams or not. If you have, then probably you should check your grid arrangment once again.

If torsion is present only in the edge beams then it is an expected behaviour and it is better to design the peripheral beams for torsion if the values are too high.

Also I doubt whether EQ forces alone are responsible for your torsional moments. If I may say so without any knowledge whatsoever of your load combinations, I expect gravitational loads to cause this torsion in peripheral beams. It might be possible that you are checking a load combination with EQ forces persent in it.

Regards,
Debayan
Back to top
View user's profile Send private message
debayan
...
...


Joined: 25 Jul 2008
Posts: 88

PostPosted: Sat Jun 05, 2010 4:17 am    Post subject: Reply with quote

Also I feel that if proper grid arrangment is present then lateral forces will only cause greater My moment and not torsional moment Mx.
Back to top
View user's profile Send private message
Novice
SEFI Member
SEFI Member


Joined: 04 Jun 2010
Posts: 5

PostPosted: Sat Jun 05, 2010 6:22 am    Post subject: Reply with quote

Dear Sefians
                      Ofcourse, moment 'MY' will be due to lateral seismic inertia force.
My question is different. Look following input command given in my staad file for beams:

607 TO 640 PRIS IX 1e-007 IY 1e+007 YD 0.6 ZD 0.4.

In STAAD, direction 'X' is along length of the beam, 'Z' along width and 'Y' along depth.

There is some torsion 'MX' in these beams. When I add additional moment due to this torsion as per clause 41.4 of IS-456 to MZ, these beams do not pass even for dobly reinforced section.

Since there is slab diaphragm monolithicaly connected to beams with enough rigidity at joints more close to fully fixed than simply supported condition, actual 'IY' is significantly higher. STAAD will compute IY based on the dimensions of beams not considering the effect of slab which results in much lower 'IY' value than actual.

Beams without giving prismatic property of IX 1e-007 and IY 1e007, results in some MY due to lateral seismic inertia forces.

             When I reduce torsional stiffness IX to 1e-007 only(607 TO 640 PRIS IX 1e-007 YD 0.6 ZD 0.4), Staad do not give any torsion(MX) at all. Torsion is distributed in MZ and MY as per the stiffness in respective directions i.e I get more MY and MZ.

Now if I increse stiffness IY significantly to 1e+007 with reduced IX ,  I get MY much higher but MZ value is reduced. This is because moment distribution factor is more in the direction of slab and almost all the moment due to torsion is extracted in MY direction.

My question is can we consider the stiffness of slab like that in STAAD i.e. increasing IY to some significantly higher value ? Are the results close to actual behaviour by doing it? If this approach is realistic and structurally safe we can have more economical sections of beams . I have not seen any literature or expert comments of modelling beams like this in analysis software. So, I'm not confident about this approach.  

By judgement also it looks that slab diaphragm would not allow the beams to be twisted or bent in the lateral direction provided slab is rigid. By rigid, I mean deflection of slab is less than 2times the deflection of centre of mass of the storey.

regards

Rinku
Back to top
View user's profile Send private message
Novice
SEFI Member
SEFI Member


Joined: 04 Jun 2010
Posts: 5

PostPosted: Sat Jun 05, 2010 6:52 am    Post subject: Reply with quote

Dear Sefians

                      Please ignore my pevious mail. I have redrafted my previous mail for better understanding.

                 
                      Ofcourse, moment 'MY' will be due to lateral seismic inertia force.
My question is different. Look following input command given in my staad file for beams:

607 TO 640 PRIS YD 0.6 ZD 0.4.

In STAAD, direction 'X' is along length of the beam, 'Z' along width and 'Y' along depth.

There is some torsion 'MX' in these beams. When I add additional moment due to this torsion as per clause 41.4 of IS-456 to MZ, these beams do not pass even for dobly reinforced section.  There is some 'MY' also due to lateral seismic inertia force.

Since there is slab diaphragm monolithicaly connected to beams with enough rigidity at joints more close to fully fixed than simply supported condition, actual 'IY' is significantly higher. STAAD will compute IY based on the dimensions of beams not considering the effect of slab which results in much lower 'IY' value than actual.



Now, look at the following input commad is STAAD manually providing IX to about 0.



607 TO 640 PRIS IX 1e-007 YD 0.6 ZD 0.4.

             When I reduce torsional stiffness IX to 1e-007 only, Staad do not give any torsion(MX) at all. Torsion is distributed in MZ and MY as per the stiffness in respective directions i.e I get more MY and MZ.

Now, consider following input commad is STAAD manually providing IX and IY.



607 TO 640 PRIS IX 1e-007 IY 1e+007 YD 0.6 ZD 0.4.


      if I increse stiffness IY significantly to 1e+007 with reduced IX ,  I get MY much higher but MZ value is reduced. This is because moment distribution factor is more in the direction of slab and almost all the moment due to torsion is extracted in MY direction.

My question is can we consider the stiffness of slab like that in STAAD i.e. increasing IY to some significantly higher value ? Are the results close to actual behaviour by doing it? If this approach is realistic and structurally safe we can have more economical sections of beams . I have not seen any literature or expert comments of modelling beams like this in analysis software. So, I'm not confident about this approach.  

By judgement also it looks that slab diaphragm would not allow the beams to be twisted or bent in the lateral direction provided slab is rigid. By rigid, I mean deflection of slab is less than 2times the deflection of centre of mass of the storey.

regards

Rinku
Back to top
View user's profile Send private message
chetan.solan
SEFI Regulars
SEFI Regulars


Joined: 26 Jan 2003
Posts: 33

PostPosted: Mon Jun 07, 2010 4:10 am    Post subject: Reply with quote

When I was in Mehro Consultants few years ago, We used to design beams for DLF buildings like that only(by reducing IX to about 0 and increasing IY to some large value). Design used to be proof checked by reputed consultants, some times by professors of IIT. Till I was there, they had never commented on this approach.


regards

Chetan
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topicReply to topic Thank Post    www.sefindia.org Forum Index -> SEFI General Discussion All times are GMT
Page 1 of 1

 

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


© 2003, 2008 SEFINDIA, Indian Domain Registration
Publishing or acceptance of an advertisement is neither a guarantee nor endorsement of the advertiser's product or service. advertisement policy