Evaluation of K-factors for buckling analysis

[3JC]

Dear collages,

My name is Juan José Jiménez de Cisneros. I'm spanish and I'm mechanical engineer. I have participated in eng-tips formus for 2-3 years, but in ABAQUS section mainly. Congratulations to everybody.

I'm relatively new in SAP2000, and I use it for educational porpose. My problem is next: I want to compare SAP2000 results with other structure softwares (Power Frame and CYPE). I want to evaluate the buckling effective factors, but the procedure which is written in the documentation is very basic and is only valid in braced frames. I would like to evaluate these factors in an better procedure. I'm going to explain the procedure of the Power Frame (is described in its documentation): to evaluate the K factors, the software apply a distributed unitary load in a bar, and obtain displacements and rotations in the two nodes of the bar. With this information, is applied the differencial equation of buckling, and results of integration parameters are compared with the bar with two pins. This procedure, in my opinion, is very good: it is independent of the frame (braced or not). My question is next: shall anybody know how to do this (or something similar) in SAP2000?

Thank you very much.

Best regards and good week-end,

Juan José

 

[johnbridge231]

This is done implicitly in SAP200, when performing steel frame design. The calculated values can be overriden by the user under the Overwrites section of each element.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[3JC]

Dear Johnbridge231,

Thank you very much for your answer, but what I know is that it is a procedure valid only for braced frames, and where beams are horizontal (paralel to X or Y) and columns are vertical (parale to Z). But I what to know if exists a procedure (which would be automatic) similar to which I'm describing. Thank you very much.

Best regards,

Juan José

 

[stressed]

3JC, the Direct analysis method adopted by AISC (and other design standards?) and implemented with SAP2000 is another analytical approach which provides an alternative to using K factors. You may also experiment with SAP2000's Eigen buckling load case analysis.

As you probably already know, buckling analysis performed on an idealized perfectly symmetrical structure may calculate an artificially high critical buckling load. The Direct analysis method handles this through "notional" lateral loads with P-Delta analysis. To correct for the possibility of imperfections from construction tolerances, you would need to do something similar to notional loads, or modify the geometry of the structure to introduce imperfections or perturbations in the model for buckling analyses

 

[3JC]

Dear stressed,

Thank you very much. Do you have examples where it would be applied these methods? I would like to see the difference between direct and length effective method. Anyway I preffer this, I think that is more comprenssive to know the behavior of the structure, do not?. Thank you very much and merry Christmas for you and all members of Eng-Tips.

jJ

 

[stressed]

3JC, I don't have any examples that I can share with you, but if you google "Direct Analysis Method" you should find a wealth of information on it, as well as AISC 13th and 14th editions which cover this approach in detail. Also, if you use the SAP2000 Help menu, go to Documentation>Design>Steel frame design, and you should find an AISC Direct Analysis method Practical Design guide which is a 12 page summary on how to use Direct Analysis (DA) method with SAP2000. In AISC 13th edition (2005), the DA method was an alternative method detailed in an Appendix. With the current AISC 14th edition, Direct Analysis (DA) is now a standard approach, although effective length method is still an option.

With DA method, K=1, so application of K factors from alignment charts, which can be subjectively applied, particularly in structures which don't fit the charts, can instead be designed using the DA method. In moment frame, or mixed braced frame with moment frame structures where you have long columns which would otherwise have K > 1 with relatively heavy axial (gravity direction) loads relative to lateral stiffness, you will likely find significant differences between DA method and effective length method. Same with many irregular structures.

I did not fully understand the stability check method you described in your first post as implemented by "Power Frame". Has their approach been codified or otherwise widely accepted?

regards