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thirumalaichettiar Silver Sponsor
Joined: 26 Jan 2003 Posts: 3478

Posted: Mon Oct 29, 2012 3:52 am Post subject: Difference between Buckling and PDelta analysis. 


What is the difference between Buckling and PDelta analysis?
When to use?
T.RangaRajan.


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Manoharbs_eq General Sponsor
Joined: 17 Jul 2012 Posts: 423

Posted: Mon Oct 29, 2012 7:26 am Post subject: Difference between Buckling and PDelta analysis. 


Dear sir,
Buckling causes column to deflect form its original position hence a deflected shape has some value delta.
to be clear when a column is loaded it either pails by crushing or by buckling. application of lateral load causes column to buckle.
PDelta
1) when column deflects from its original position i.e SEP. further when load is applied vertically. vertical load no longer passes through the centroid of column in SEP.
2) when load is applied on deflected column then the column is to face additional moment due to application of load. this effect is called pDelta. this is a second order effect.
On Mon, Oct 29, 2012 at 9:22 AM, thirumalaichettiar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:  What is the difference between Buckling and PDelta analysis?
When to use?
T.RangaRajan.[/color:c9eb4c3829]

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Rohinton Bhathena General Sponsor
Joined: 02 Mar 2009 Posts: 17

Posted: Mon Oct 29, 2012 8:08 am Post subject: Difference between Buckling and PDelta analysis. 


Dear Thiru,
For example you are analyzing a slender strut such as a Steel Pipe.This pipe is never 100% straight. Therefore you assume a 1/500 eccentricity. Hence a 3m. pipe will be eccentric by 6 mm. If you load this pipe with say 1000kg. An axial force of 1000kg is produced and a bending moment of 1000 x 0.006 = 6 kg.m. The force also produces a deflection d1. So now the eccentricity is 6mm. + d1. This produces another set of calculations. About 34 iterations will stabilize the forces. At that time the original 6mm. would have become 6+d1+d2+d3...dn.
For frame structures of critical nature; P Delta analysis gives a more realistic picture. It is time consuming hence it is replaced by extra factored load. Today's computers can perform calculations on hundreds of nodes by the time you read this sentence. So it is worth checking in this manner.
Regards
R Bhathena
On Mon, Oct 29, 2012 at 9:22 AM, thirumalaichettiar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:  What is the difference between Buckling and PDelta analysis?
When to use?
T.RangaRajan.[/color:c9eb4c3829]

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 2212

Posted: Mon Oct 29, 2012 9:54 am Post subject: Difference between Buckling and PDelta analysis. 


PDelta Effect
As a personal thinking,
both the phenomenons are as result of Axial force multiplied with lateral deflection
of member.In one case (PDelta)member drifts due to global effects under lateral
loading, whereas in other case (Buckling) , the member ends do NOT drift
but , slenderness under own compression load, causes member to buckle .
Global effect of horizontal deflection of members/frames (Drift) as result of
lateral forces (EQ/wind) may cause PDelta effects in structural members.
A vertical cantilever column of adequate dimensions under heavy load at its top
may not Buckle , but is subject to PDelta effect due to DRIFT at top under lateral
forces. Similarly a MS building under lateral forces drifts and horizontal deflection
causes Pdelta effect in members.
Column subject to vertical load ,due to slenderness , buckles. The P delta
effect is accounted by reducing the permissible stresses in the member
based on L/R in WSM . In LSM additional moments due to slenderness
are considered.
best regds
vikramjeet

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Deepan Shanmugasundaram SEFI Member
Joined: 20 Dec 2010 Posts: 1 Location: Bangalore (Weekdays) & Erode (Weekends)

Posted: Mon Oct 29, 2012 10:53 am Post subject: Difference between buckling and PDelta analysis.. 


Hi
Simply, to put..
Description  Buckling  PDelta  Whats it?  A failure mode  A method of analysis  Why it's done?  To evaluate strength  To evaluate demand 
Buckling: Buckling is an instability phenomena. It happens when there is a chance for a structural component to behave in an alternate manner for the same load level. (Technically, it said that, its happens at bifurcation point, a point where two equilibrium paths meet). Intuitively I understand the buckling phenomena by this, "A structure will behave in the easiest way possible, i.e., the behavior which needs less work". That means, say in columns, when the crushing becomes highly demanding, it buckles. Postbuckling strength can also be considered for design, in components like, plates. Buckling analysis is needed in almost all the cases of design, since it is a failure mode. The analysis can be omitted (based on engineering judgement) in certain cases (e.g. bulk concrete components) where it is obvious that buckling will not happen prior to other failure modes. PDelta Analysis: PDelta is an iterative analysis procedure. In typical linear structural analysis, the equilibrium equations are satisfied at undeformed configurations (exceptions are there: e.g. cables). In pdelta analysis, the equilibrium is considered in the deformed position. There are two types in it (explained here with ref. to columns): P (Small) delta analysis: Analysis done considering the deformation due to lateral bending of the columns. P (Capital) delta analysis: Analysis done considering the deformation due to lateral swaying of the columns.
I don't have enough experience to tell you about the situations where Pdelta analysis becomes necessary. The following are examples..
(1) Sway frames with high axial loads
(2) High plan unsymmetry in live loads
(3) Structures with flexible lateral load resisting systems.. etc
_________________ Deepan Shanmugasundaram
Last edited by Deepan Shanmugasundaram on Mon Oct 29, 2012 12:06 pm; edited 1 time in total 

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JVCSNL ...
Joined: 26 Jan 2003 Posts: 159

Posted: Mon Oct 29, 2012 11:03 am Post subject: Difference between Buckling and PDelta analysis. 


Buckling is the effect/resulting effect of members subjected to axial compression loads.
PDelta analysis is the methodology or consideration of buckling effect into analysis and design of the member subjected to axial compression loads to assess total effect. This analysis also include effect of drifts produced by lateral loads.
Regards,
Jignesh V Chokshi
From: vikram.jeet [mailto:forum@sefindia.org]
Sent: Monday, October 29, 2012 4:22 PM
To: general@sefindia.org
Subject: [SEFI] Re: Difference between Buckling and PDelta analysis.
PDelta Effect
As a personal thinking,
both the phenomenons are as result of Axial force multiplied with lateral deflection
of member.In one case (PDelta)member drifts due to global effects under lateral
loading, whereas in other case (Buckling) , the member ends do NOT drift
but , slenderness under own compression load, causes member to buckle .
Global effect of horizontal deflection of members/frames (Drift) as result of
lateral forces (EQ/wind) may cause PDelta effects in structural members.
A vertical cantilever column of adequate dimensions under heavy load at its top
may not Buckle , but is subject to PDelta effect due to DRIFT at top under lateral
forces. Similarly a MS building under lateral forces drifts and horizontal deflection
causes Pdelta effect in members.
Column subject to vertical load ,due to slenderness , buckles. The P delta
effect is accounted by reducing the permissible stresses in the member
based on L/R in WSM . In LSM additional moments due to slenderness
are considered.
best regds
vikramjeet

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thirumalaichettiar Silver Sponsor
Joined: 26 Jan 2003 Posts: 3478

Posted: Mon Oct 29, 2012 11:08 am Post subject: 


Dear Sefians,
Thanks for your immediate response.
I am not satisfied the with the replies but need more specific and better explanation since need to know when to do them or under what circumstances they should be employed? etc.
Also need to know in depth whether the Buckling and P Delta are related to column only or include for beams also.
Which analysis First order Elastic,First order Inelastic, Second order Elastic, Second order Inelastic or which analysis is good to get better results?
T.RangaRajan.
Last edited by thirumalaichettiar on Tue Oct 30, 2012 5:21 am; edited 1 time in total 

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nrupen yadahalli SEFI Member
Joined: 09 Sep 2009 Posts: 17

Posted: Tue Oct 30, 2012 5:21 am Post subject: Difference between Buckling and PDelta analysis. 


PDelta effect induces additional stresses in structure due to deflection/sway, whereas buckling is a geometric arrangement defining the max stress (may be compresion Strs may be flexural Strs) can be borne by member based on its compactness and slenderness. For Slendernessglobal behaviour or noncompact and belowlocal behaviour strength reduction will occur in any method WSM or Limit State....
Pdelta is only analysis method no reduction in stress/strenght is required if member is non slender n compact.
On Mon, Oct 29, 2012 at 4:22 PM, vikram.jeet <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:  PDelta Effect
As a personal thinking,
both the phenomenons are as result of Axial force multiplied with lateral deflection
of member.In one case (PDelta)member drifts due to global effects under lateral
loading, whereas in other case (Buckling) , the member ends do NOT drift
but , slenderness under own compression load, causes member to buckle .
Global effect of horizontal deflection of members/frames (Drift) as result of
lateral forces (EQ/wind) may cause PDelta effects in structural members.
A vertical cantilever column of adequate dimensions under heavy load at its top
may not Buckle , but is subject to PDelta effect due to DRIFT at top under lateral
forces. Similarly a MS building under lateral forces drifts and horizontal deflection
causes Pdelta effect in members.
Column subject to vertical load ,due to slenderness , buckles. The P delta
effect is accounted by reducing the permissible stresses in the member
based on L/R in WSM . In LSM additional moments due to slenderness
are considered.
best regds
vikramjeet


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Manoharbs_eq General Sponsor
Joined: 17 Jul 2012 Posts: 423

Posted: Tue Oct 30, 2012 6:27 am Post subject: 


Dear sir,
It is to be very clear, PDelta analysis is a analysis procedure for vertical supporting system column wall etc.
where as buckling is either local or global effect on the element or structure due to lesser stiffness to withstand own weight and lateral load.
Nonlinear analysis is the procedure for Pdelta
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zubairmeer1 General Sponsor
Joined: 27 Feb 2010 Posts: 88 Location: Hyderabad

Posted: Tue Oct 30, 2012 10:12 am Post subject: 


Dear rangarajan sir
i was also searching for this answer from quite a few days...and rightly said bu you a specific answer is required....lets wait and ses NS sirs reply..usually he has to the point answers....and the replies above were also excellent.. i shud accept it was a good piece of learning for me... and above all we require these types of discussions..in the forum....
what i see is P(small delta) effect also captures this buckling effect so why buckling analysis?
little bit off the topic but.. recently i was going thru past year discussions (20032007)...the discussion were of totally different level....and i learnt a lot from those posts...
i request administrator to just give those members a reminder so as to take active part in the discussions and help people like me...
regards
meer


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