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Regarding P-D analysis.
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JVCSNL
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Joined: 26 Jan 2003
Posts: 159

PostPosted: Thu Jul 08, 2010 3:25 am    Post subject: Regarding P-D analysis. Reply with quote

Dear SEFI members,

I suggest the members to go through the discussion on SEFI during Sep. 2003.

The topics were P-Delta analysis, Slender Column etc. This point was discussed at length under these topics.

Regards,

Jignesh Chokshi

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lele_raj
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Joined: 26 Jan 2003
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PostPosted: Thu Jul 08, 2010 6:09 am    Post subject: Regarding P-D analysis. Reply with quote

Thanks for the heads-up. My concern is that p-delta effect is thought to be present / effective only in the cases of long slender column. Examples I provided show that this is not the case, it's present even in latticed structures (under 100% design loads, though). And in fact, it needs to be considered in conjunction with two types of non-linearities and the structure needs to be analysed appropriately.

I'm not sure whether these aspects were covered in the discussion of Sep03.


Best regards,

Rajendra (Raj) Lele

  
P Please consider the environment before printing this e-mail or any attachment/s.



From: JVCSNL <forum@sefindia.org>
To: general@sefindia.org
Sent: Thu, 8 July, 2010 2:58:07 PM
Subject: [SEFI] Re: Regarding P-D analysis.

Dear SEFI members,

I suggest the members to go through the discussion on SEFI during Sep. 2003.

The topics were P-Delta analysis, Slender Column etc. This point was discussed at length under these topics.

Regards,

Jignesh Chokshi

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hemal
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Joined: 01 Apr 2008
Posts: 129

PostPosted: Thu Jul 08, 2010 1:24 pm    Post subject: Regarding P-D analysis. Reply with quote

Dear SEFI members,

As i told earlier from P-D analysis we will get global displacement (delta) and moments due to this delta caused by gravity loads (1.5DL or 1.2DL+1.2LL) for columns (no matter slender or shorter columns).

From value of global displacement (delta) for particular storey, we may determine stability index (Q) from ANNEX E IS:456-2000 (page no-92). From value of Q we may know weather columns are sway or non sway and we may use effective length factors accordingly from WOOD'S chart (FIG-26,27) from IS:456. I don't think any software has utilize this clause regarding STABILITY INDEX. Generally we need to input eff. length factor manually. Some softwares calculate eff. length factor automatically for sway (unbraced) or non-sway(braced) columns automatically but decision of weather column is sway or non-sway is left to user (i.e. no use of Q).

For slender columns local displacements (due to buckling) will be more so secondary P-D moments calculated by cl. 39.7.1 of IS:456.

SEFI members may correct me if i am somewhere wrong.

Regards

hemal mistry
Surat

--- On Thu, 8/7/10, lele_raj <forum@sefindia.org> wrote:
Quote:

From: lele_raj <forum@sefindia.org>
Subject: [SEFI] Re: Regarding P-D analysis.
To: general@sefindia.org
Date: Thursday, 8 July, 2010, 4:53 AM

Thanks for the heads-up. My concern is that p-delta effect is thought to be present / effective only in the cases of long slender column. Examples I provided show that this is not the case, it's present even in latticed structures (under 100% design loads, though). And in fact, it needs to be considered in conjunction with two types of non-linearities and the structure needs to be analysed appropriately.

I'm not sure whether these aspects were covered in the discussion of Sep03.


Best regards,

Rajendra (Raj) Lele

P Please consider the environment before printing this e-mail or any attachment/s.



From: JVCSNL
To: general@sefindia.org
Sent: Thu, 8 July, 2010 2:58:07 PM
Subject: [SEFI] Re: Regarding P-D analysis.

Dear SEFI members,

I suggest the members to go through the discussion on SEFI during Sep. 2003.

The topics were P-Delta analysis, Slender Column etc. This point was discussed at length under these topics.

Regards,

Jignesh Chokshi
     



     



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Rajiv
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Joined: 04 Aug 2008
Posts: 52

PostPosted: Fri Jul 09, 2010 4:30 am    Post subject: Effective Lengths for columns, Sway/No Sway and P Delta Reply with quote

Dear All:

Please note that currently ETABS is doing all these calculations as desired by Hemal for column design. ETABS computes Story index Q and determines if a story is Sway/No Sway and calculates Effective Length factors accordingly. User has a choice of enhancing effective length factors. Many additional things for column and shear wall designs are now available.

If you are an ETABS user you can write us at support[at]csi-india.com. Just change the word [at] with @.

More on these important aspects in our support mails to you!!

Best regards

Rajiv
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PSRaju
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Joined: 02 Jun 2008
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Location: HYDERABAD

PostPosted: Fri Jul 09, 2010 5:50 am    Post subject: Re: Regarding P-D analysis. Reply with quote

Dear Er. Hemlal,

Thank you for information on stability index.

Regards,
Raju

hemal wrote:
Dear SEFI members,

As i told earlier from P-D analysis we will get global displacement (delta) and moments due to this delta caused by gravity loads (1.5DL or 1.2DL+1.2LL) for columns (no matter slender or shorter columns).

From value of global displacement (delta) for particular storey, we may determine stability index (Q) from ANNEX E IS:456-2000 (page no-92). From value of Q we may know weather columns are sway or non sway and we may use effective length factors accordingly from WOOD'S chart (FIG-26,27) from IS:456. I don't think any software has utilize this clause regarding STABILITY INDEX. Generally we need to input eff. length factor manually. Some softwares calculate eff. length factor automatically for sway (unbraced) or non-sway(braced) columns automatically but decision of weather column is sway or non-sway is left to user (i.e. no use of Q).

For slender columns local displacements (due to buckling) will be more so secondary P-D moments calculated by cl. 39.7.1 of IS:456.

SEFI members may correct me if i am somewhere wrong.

Regards

hemal mistry
Surat

--- On Thu, 8/7/10, lele_raj <forum> wrote:
Quote:

From: lele_raj <forum>
Subject: [SEFI] Re: Regarding P-D analysis.
To: general@sefindia.org
Date: Thursday, 8 July, 2010, 4:53 AM

Thanks for the heads-up. My concern is that p-delta effect is thought to be present / effective only in the cases of long slender column. Examples I provided show that this is not the case, it's present even in latticed structures (under 100% design loads, though). And in fact, it needs to be considered in conjunction with two types of non-linearities and the structure needs to be analysed appropriately.

I'm not sure whether these aspects were covered in the discussion of Sep03.


Best regards,

Rajendra (Raj) Lele

P Please consider the environment before printing this e-mail or any attachment/s.



From: JVCSNL
To: general@sefindia.org
Sent: Thu, 8 July, 2010 2:58:07 PM
Subject: [SEFI] Re: Regarding P-D analysis.

Dear SEFI members,

I suggest the members to go through the discussion on SEFI during Sep. 2003.

The topics were P-Delta analysis, Slender Column etc. This point was discussed at length under these topics.

Regards,

Jignesh Chokshi
     



     



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hemal
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Joined: 01 Apr 2008
Posts: 129

PostPosted: Sat Jul 10, 2010 1:20 am    Post subject: Regarding P-D analysis. Reply with quote

Dear Mr. Rajiv,

Thanx for the information on ETABS. It has many gifts for indian standards for frames and shear walls.

I have gone through concrete design manual of etabs for Indian codes (I think it is first time released). It is mentioned that "Etabs by default considers all columns as sway columns and calculates eff. length factors accordingly (varies from 1 to infinity, sometimes unusual and very high, needs overrides as mentioned there). But it is mentioned that if u perform P-D analysis, it will assume that further sway is prevented and will take eff. length factor 1 conservatively.

However, i have tried P-D analysis in a trial version but it takes eff. length factor for sway columns even after performing P-D analysis.

I think there is nothing mentioned about STABILITY INDEX (Q) which decides sway or non-sway column.

Regards
Hemal

--- On Thu, 8/7/10, Rajiv <forum@sefindia.org> wrote:
Quote:

From: Rajiv <forum@sefindia.org>
Subject: [SEFI] Re: Regarding P-D analysis.
To: general@sefindia.org
Date: Thursday, 8 July, 2010, 11:30 PM

Dear All:

Please note that currently ETABS is doing all these calculations as desired by Hemal for column design. ETABS computes Story index Q and determines if a story is Sway/No Sway and calculates Effective Length factors accordingly. User has a choice of enhancing effective length factors. Many additional things for column and shear wall designs are now available.

If you are an ETABS user you can write us at support[at]csi-india.com. Just change the word [at] with @.

More on these important aspects in our support mails to you!!

Best regards

Rajiv
     



     


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Rajiv
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Joined: 04 Aug 2008
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PostPosted: Sun Jul 11, 2010 3:39 am    Post subject: ETABS and Concrete Design Issues Reply with quote

Dear Hemal:

Originally we had adopted the method that a user would be doing a P-Delta analysis always, since it is what our code requires and moreover second order analysis is now common and recommended in most advanced codes. Since our code gives no specific guidelines and rather intimidates a new user (See Explanatory Notes published in 1978), we had noticed that most users were NOT doing it due a sense of fear.

ETABS' original method was to assume that the user would be doing a P-Delta analysis and then K factor would be taken as 1.0. This is similar to ACI code. But ETABS would be required to magnify these moments to meet the demand of P-Delta (local) deformations. So ETABS would always be adding additional slenderness moments based on an effective length factor of 1.0. This was considered conservative.
However after conducting many seminars all over the country we realised that very few engineers were actually doing a P-Delta analysis correctly, either due to lack of interest or lack of guidance or simply due a very short time allocated for design. So to safeguard the building against a possible misuse of ETABS, we decided to update the program to have a 100% codal compliance.

In ETABS 9.7 we made computation of effective length factors the default method and P-Delta was optional. When a P-Delta analysis is done then the user should use overwrites to change the effective length factor to 1.0 on his own.

Our new update 9.7.1 has a more refined and a very robust design for concrete elements for Indian code. In that we have added Q factors, effective length factors, Sway/No-Sway issue and we have even improved our section designer for a more optimized search for steel in column sections which can be of any shape. This has been completed and is to be released soon.

Our code wants to include P-Delta effects for story sway as well as P-Delta due deformations within the column length within a story. But somehow our code writers have not paid much attention to write the specification of such an analysis for the reasons best known to them.
So what is needed is a comprehensive review of P-Delta analysis and the underlying issues and we need to educate our design engineers and design managers at all levels on how to include it in design correctly.

Best regards

Rajiv
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hemal
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Joined: 01 Apr 2008
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PostPosted: Sun Jul 11, 2010 4:51 pm    Post subject: test Reply with quote

Dear Mr. Rajiv,

Thanx for the really fruitful explanation on P-D analysis, Q, k related to ETABS.

I have observed that due to consideration of column as sway column by default in v9.7, main reinforcement of column is veeery high (many times @ 2 to 3 times) as compared to v9.5. However overwriting k values to 1 in v9.7 gives 100% same results in v9.5 & v9.7. But, if we have 100 columns in a 100 story building, how much time it will take for overwriting k to 1 for all columns?

As mentioned in previous post, it is mentioned in manual somewhere that if P-D analysis is performed k will be taken as 1 by default. But in v9.7 even after performing P-D analysis, it takes k for sway columns (>1). Can u explain is it so or i am making any wrong statement.

I would like to suggest that in "preferences" u may put one variable for k as below

(1) calculate k based on Q values (sway or non-sway). (if P-D analysis not performed)
(2) Take k=1 (sway or non-sway). (if P-D analysis is performed)
(3) calculate k for sway columns
(4) calculate k for non-way columns

option-3 and 4 are not logical as user can not decide weather column is sway or non-sway. but, may software gives user this choice. Actually it is somewhat dangerous because many times people takes 0.65 or 0.85 or 1 factor without considering storey height, support condition, beam framing etc.

Option-1 will give k<1 if non sway column (Q<0.04), which will be advantageous. which we are not having in present version.

If all above suggestions are incorporated in v9.7.1, then i think it will be the first software to address this critical issue.

Regards

Hemal

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Rajiv
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PostPosted: Mon Jul 12, 2010 5:05 am    Post subject: P-Delta Analysis in ETABS Reply with quote

Dear Hemal:

Fixing overwrites in ETABS takes a very small time of about a few seconds only. Select all column and assign them an overwrite.

All other things have been taken care of in ETABS 9.7.1, and it offers an improved   shear wall design too!

Just wait for a few days only.


Best regards

Rajiv
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bijay sarkar
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Joined: 14 Dec 2009
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PostPosted: Mon Jul 12, 2010 12:31 pm    Post subject: Reply with quote

Dear all,

My observation on the subject matter are as follows for your comment :

Analysis which we normally do, is called an elastic linear analysis which does not take care of the deflections of the structures, be it from sway or from slenderness/buckling.
P-Delta Analysis is called a non-linear analysis which incorporates the effects of deflections of the structure.
Now there are two types of deflections : (i) Structural deflection or Global deflection or what we normally call structural “sway”.  (ii) Member deflection or local deflection or slenderness deflection. Additional moments comes from these deflections which are also to be considered for design.
Now, this is pertinent to be confirmed whether the frame is a “sway frame” or a “non-sway frame” for deciding which Fig. (26 or 27) is to be used for effective length factors for columns. For identifying a “sway” or “non-sway” frame, IS 456 has directed to calculate an index Q which depends on the elastic linear analysis of the structure.
If the structure is a “sway frame”, then only we may go for P-Delta analysis, otherwise, elastic linear analysis is enough.
But P-Delta analysis of Staad does not cover the deflection due to slenderness effect of the members which requires a buckling analysis of the frame. Therefore, we may say that after P-Delta analysis, additional moments due to slenderness/buckiling are yet to be added, if any.
For this slenderness/buckling moments, we require to calculate the effective lengths of the members. For “sway frame”, effective lengths shall be in accordance with Fig. 26 of IS 456 and for “non-sway frame”, Fig. 27 shall be used. But as we have already analysed the structure for P-Delta effects, this is my personal opinion that, now, we shall go for Fig.26 only i.e. for non-sway frames to calculate the effective lengths to find out the additional moments for slenderness/buckling. If P-Delta analysis is not carried out and the frame is a sway frame, then only we will go for Fig. 27 to cover up both the deflections as we normally do.
I am submitting my observations stepwise for comments of the sefians :
a)  Conduct elastic linear analysis what we normally do.
(b) Find out Q to decide the type of frame (sway or non-sway).
(c) If non-sway frame, P-Delta analysis is not to be carried out. Effective lengths of members shall be based on Fig. 26 of IS and additional moment due to slenderness/buckling shall be added to the normal elastic linear analysis to find out the design moments.
(d) If sway frame, we may carry out P-Delta analysis or may not.
           (i)If P-Delta analysis is carried out, calculate effective length factors based on Fig. 26 of IS 456 which is for non-sway frame for calculating additional slenderness/buckling moments. Add this moment with the P-Delta analysis. Is it right or Fig. 27 is yet to be used?
          (ii)If P-Delta analysis is not carried out and the frame is a sway frame, calculate the effective length factors based on Fig. 27 of IS 456 which is for sway frame and calculate the additional moment based on the buckling/slenderness equation provided in the IS. Consideration of Fig. 27 of IS will cover both the deflections i.e. from sway of structure as well as slenderness/buckling of the members to find out the additional moments.


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