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Release of MX in STAAD
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ykalamkar
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Joined: 26 Jan 2003
Posts: 108
Location: Nagpur

PostPosted: Wed Sep 17, 2008 8:13 am    Post subject: Release of MX in STAAD Reply with quote

Dear all,
We are designing a building which is modeled in STAAD as space frame. Load of slab is applied as floor load. Normally we release torsion in the beam due to slab load. hence we release MX( Torsion) in all beams. This makes secondary beam connection as simple support. Structure do not show any instability warning and even deflected shape do not show any distortion. While proof checking at a reputed consultant, they are not ready to accept the release command. They are saying that you can release MZ for secondary beam but do not release torsion in all beams.
My question is "Will it affect the structural behavior?" or is it wrong to release MX in beams?
Regards
Yogesh
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jiwajidesai
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PostPosted: Wed Sep 17, 2008 8:43 am    Post subject: Release of MX in STAAD Reply with quote

Dear Yogesh
Torsion in space frame beams may arises due to unsymmetrical loading, irregularities in geometry (spans and loadings in beams meeting at columns at right angles for example), lateral loads etc. This torsion has to be considered as otherwise the transfer of external loads is not realistic - the physical members do not behave like hinged at ends in these cases. Space frames are resorted to to take care of these situations.
Regards
J Y Desai

________________________________

From: ykalamkar [mailto:forum@sefindia.org]
Sent: Wed 9/17/2008 1:44 PM
To: general@sefindia.org
Subject: [SEFI] Release of MX in STAAD


Dear all,
We are designing a building which is modeled in STAAD as space frame. Load of slab is applied as floor load. Normally we release torsion in the beam due to slab load. hence we release MX( Torsion) in all beams. This makes secondary beam connection as simple support. Structure do not show any instability warning and even deflected shape do not show any distortion. While proof checking at a reputed consultant, they are not ready to accept the release command. They are saying that you can release MZ for secondary beam but do not release torsion in all beams.
My question is "Will it affect the structural behavior?" or is it wrong to release MX in beams?
Regards
Yogesh

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Jayant Lakhlani
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Joined: 26 Jan 2003
Posts: 191
Location: Rajkot, Gujarat, India

PostPosted: Wed Sep 17, 2008 10:35 am    Post subject: Release of MX in STAAD Reply with quote

Dear Yogesh,

It is perfectly alright to ignore Mx in the analysis but with a constraint in certain situations.

Last paragraph of cl 21.1 of SP : 24 - 1983 says,

"In analysing buildings built with a skeleton consisting of a series of plane frames connected by transverse beams, it is ususal practice to ignore the torsional resistance or stiffness of the transverse beams. In such cases, that is, when the stability of the system does not depend on the torsional strength or certain members, the torsional phenomenon as a whole can be ignored in analysis as well as in design. However, in structures such as beams curved in plan, euilibrium itself is not possible without torsion in the members. In such cases the member should be analysed and provided for torsion"

I am sure that your proof consultant can't deny this clear cut guidelines of the code itself. (And you owe me a party when he accepts this. Because I know it very well sometimes it becomes very difficult to get the design approved with proof consultants)

Regards.

Jayant Lakhlani
For
Lakhlani Associates
www.lakhlani.com

--- On Wed, 17/9/08, ykalamkar <forum@sefindia.org> wrote:

Quote:
From: ykalamkar <forum@sefindia.org>
Subject: [SEFI] Release of MX in STAAD
To: general@sefindia.org
Date: Wednesday, 17 September, 2008, 8:14 AM

Dear all,
We are designing a building which is modeled in STAAD as space frame. Load of slab is applied as floor load. Normally we release torsion in the beam due to slab load. hence we release MX( Torsion) in all beams. This makes secondary beam connection as simple support. Structure do not show any instability warning and even deflected shape do not show any distortion. While proof checking at a reputed consultant, they are not ready to accept the release command. They are saying that you can release MZ for secondary beam but do not release torsion in all beams.
My question is "Will it affect the structural behavior?" or is it wrong to release MX in beams?
Regards
Yogesh






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ashutosh.jain
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PostPosted: Wed Sep 17, 2008 11:34 am    Post subject: Release of MX in STAAD Reply with quote

Dear Jayant Lakhani Sir,

The code says that we can ignore the torsional resistance of transverse beams, now if my building is square shaped with equal spaced columns and I am designing the building for wind and earthquake forces in both X and Z direction, which direction beams shall I take as transverse beams, and which beams as part of plane frame.

Regards,  


ASHUTOSH JAIN


From: jlakhlani [mailto:forum@sefindia.org]
Sent: Wednesday, September 17, 2008 4:35 PM
To: general@sefindia.org
Subject: [SEFI] Re: Release of MX in STAAD


Dear Yogesh,

It is perfectly alright to ignore Mx in the analysis but with a constraint in certain situations.

Last paragraph of cl 21.1 of SP : 24 - 1983 says,

"In analysing buildings built with a skeleton consisting of a series of plane frames connected by transverse beams, it is ususal practice to ignore the torsional resistance or stiffness of the transverse beams. In such cases, that is, when the stability of the system does not depend on the torsional strength or certain members, the torsional phenomenon as a whole can be ignored in analysis as well as in design. However, in structures such as beams curved in plan, euilibrium itself is not possible without torsion in the members. In such cases the member should be analysed and provided for torsion"

I am sure that your proof consultant can't deny this clear cut guidelines of the code itself. (And you owe me a party when he accepts this. Because I know it very well sometimes it becomes very difficult to get the design approved with proof consultants)

Regards.

Jayant Lakhlani
For
Lakhlani Associates
www.lakhlani.com

--- On Wed, 17/9/08, ykalamkar wrote:    
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Jayant Lakhlani
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PostPosted: Wed Sep 17, 2008 1:14 pm    Post subject: Release of MX in STAAD Reply with quote

Dear Ashutosh,

When you are considering a particular direction (X or Z), the other orthogonal direction beams (Z or X) are transverse beams for that particular direction.

If you read other half of the clause, it says that when stability of the structure is not dependent on the torsional stiffness of certain members, the whole phenomenon of torsion can be neglected in analysis and design. So, you just need to check for your structure that if you neglect torsional stiffness, is it going to make your structrue unstable. That's all.

Regards.

Jayant Lakhlani
For
Lakhlani Associates
www.lakhlani.com


--- On Wed, 17/9/08, ashutosh.jain <forum@sefindia.org> wrote:

Quote:
From: ashutosh.jain <forum@sefindia.org>
Subject: [SEFI] Re: Release of MX in STAAD
To: general@sefindia.org
Date: Wednesday, 17 September, 2008, 12:34 PM

Dear Jayant Lakhani Sir,

The code says that we can ignore the torsional resistance of transverse beams, now if my building is square shaped with equal spaced columns and I am designing the building for wind and earthquake forces in both X and Z direction, which direction beams shall I take as transverse beams, and which beams as part of plane frame.

Regards,


ASHUTOSH JAIN


From: jlakhlani [mailto:forum@sefindia.org]
Sent: Wednesday, September 17, 2008 4:35 PM
To: general@sefindia.org (general@sefindia.org)
Subject: [SEFI] Re: Release of MX in STAAD


Dear Yogesh,

It is perfectly alright to ignore Mx in the analysis but with a constraint in certain situations.

Last paragraph of cl 21.1 of SP : 24 - 1983 says,

"In analysing buildings built with a skeleton consisting of a series of plane frames connected by transverse beams, it is ususal practice to ignore the torsional resistance or stiffness of the transverse beams. In such cases, that is, when the stability of the system does not depend on the torsional strength or certain members, the torsional phenomenon as a whole can be ignored in analysis as well as in design. However, in structures such as beams curved in plan, euilibrium itself is not possible without torsion in the members. In such cases the member should be analysed and provided for torsion"

I am sure that your proof consultant can't deny this clear cut guidelines of the code itself. (And you owe me a party when he accepts this. Because I know it very well sometimes it becomes very difficult to get the design approved with proof consultants)

Regards.

Jayant Lakhlani
For
Lakhlani Associates
www.lakhlani.com

--- On Wed, 17/9/08, ykalamkar wrote:
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akbar.civil
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Joined: 26 Jan 2003
Posts: 36
Location: New York

PostPosted: Wed Sep 17, 2008 4:11 pm    Post subject: Release of MX in STAAD Reply with quote

Dear All,

Isn't it ( release of torsion in straight beams ) a common assumption that we make every time. I would be interested in knowing from Mr. Yogesh as to whether the proof consultant is from some other country.

Regards,
Akbar
Calicut, Kerala


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Dr. N. Subramanian
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PostPosted: Wed Sep 17, 2008 4:19 pm    Post subject: Release of MX in STAAD Reply with quote

Dear Mr. Yogesh,

I feel that the Proof checking consultant is technically right. If you release MX( Torsion) in all beams,the advantages of using a space frame analysis is lost- you are simulating a plane frame analysis using space frame analysis! Why do you want to release MX in all beams? In fact the spandrel beams (edge beams in buildings)will be subjected to torsion and have to be designed for them, though many designers in India do not take it into account.

Best wishes
Subramanian

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Wed, 9/17/08, ykalamkar <forum@sefindia.org> wrote:
Quote:
From: ykalamkar <forum@sefindia.org>
Subject: [SEFI] Release of MX in STAAD
To: general@sefindia.org
Date: Wednesday, September 17, 2008, 1:44 PM

Dear all,
We are designing a building which is modeled in STAAD as space frame. Load of slab is applied as floor load. Normally we release torsion in the beam due to slab load. hence we release MX( Torsion) in all beams. This makes secondary beam connection as simple support. Structure do not show any instability warning and even deflected shape do not show any distortion. While proof checking at a reputed consultant, they are not ready to accept the release command. They are saying that you can release MZ for secondary beam but do not release torsion in all beams.
My question is "Will it affect the structural behavior?" or is it wrong to release MX in beams?
Regards
Yogesh
     



     


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Jayant Lakhlani
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PostPosted: Thu Sep 18, 2008 4:54 am    Post subject: Release of MX in STAAD Reply with quote

Dear Dr. Subramanian,

What you have quoted is the case when your analysis assumptions are totally matching with the actual space frame behaviour.

Now, when Mx is released from beams, at a beam-column junction of space frame, the total unbalanced moment at the junction gets transferred to columns only instead of getting divided between column and transverse beams.

So, this assumption of releasing Mx indirectly helps to achieve strong column - weak beam situation. Secondly, if we analyse without releasing Mx in a space frame, the magnitude of torsional moments in beams is very less compared to bending moments. Considering this, taking support of the codal provision, I am for releasing Mx in space frames.

Regards.

Jayant Lakhlani
For
Lakhlani Associates
www.lakhlani.com


--- On Wed, 17/9/08, drnsmani <forum@sefindia.org> wrote:

[quote]From: drnsmani <forum@sefindia.org>
Subject: [SEFI] Re: Release of MX in STAAD
To: general@sefindia.org
Date: Wednesday, 17 September, 2008, 5:02 PM

Dear Mr. Yogesh,

I feel that the Proof checking consultant is technically right. If you release MX( Torsion) in all beams,the advantages of using a space frame analysis is lost- you are simulating a plane frame analysis using space frame analysis! Why do you want to release MX in all beams? In fact the spandrel beams (edge beams in buildings)will be subjected to torsion and have to be designed for them, though many designers in India do not take it into account.

Best wishes
Subramanian

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Wed, 9/17/08, ykalamkar wrote:
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vikram.jeet
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PostPosted: Thu Sep 18, 2008 5:04 am    Post subject: Release of MX in STAAD Reply with quote

Carrying the load transfer through torsion is costiliest option
unless the torsion is Equilibrium torsion which can not
be avoided

IS 456 clearly allows the release of compatibility torsion and  
release of Mx is an economical option without compromising
with the safety of structure

MX thus released will result in additional flexural moments in
other direction beams but still economical. This is a case
somewhat similar to redistibution condition (here it is torsion
in one direction to flexure in perpedicular direction)

Natural behaviour of structure , if we talk, will need consideration
of all torsions to be accounted for
i) even from end slab panels on edge beams due to partial fixity
ii) secondary beam resting at main beam  


regards

vikramjeet

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amritaban
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Posts: 10

PostPosted: Thu Sep 18, 2008 7:04 am    Post subject: Release of MX in STAAD Reply with quote

Can anybody please explain how a RCC beam can be constructed with ZERO torsional stiffness?

In STAAD we simply tick a button to release torsional stiffness. But physically it has the stiffness. Then there must be some amount of torsional moment. I agree with Mr. Lakhlani that the magnitude of torsional moment for a more or less uniformly loaded structure will be less. But why should we ignore it when we have highly competent tools which can help us design the beams even after taking them into consideration.

Being good designers, does it not become our responsibility to design a beam or any structure taking into consideration even the minutest load that it would experience in its lifetime?


With Regards

Amrita

DCPL

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