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  • 1.  Analyzing Building Frames

    Posted 12-30-2022 02:19 PM

    While analyzing building frames, thickness of the joints is ignored usually which an error on the safe side but is it has some benefit if we're given more thickness to space rebars. 

    Alex Thomas REng, CEng, M.ASCE

    Alex Thomas R.Eng, C.Eng, M.ASCE
    Engineer Vistar Associates
    CochinAlexThomasR.Eng, C.Eng, M.ASCEIndia

  • 2.  RE: Analyzing Building Frames

    Posted 01-03-2023 08:03 AM

    My experience with analysis is, that when designing we simplify frames and make simplified assumptions (such as pinned joints) to generate forces to which we design to, however if we have to assess existing no simplifying assumptions can be made.  I have found assessing an existing structure and going through the structures load path to failure to be much more challenging and educational.   You gain a greater understanding of how a frame works and whether the original analysis assumptions were valid.
    If you want to gain greater understanding of your observations,  you may want to look at your frame do a step by step analysis  see when and where your frame cracks, and when  and  where you have plastic joints that will lead to a collapse mechanism.  I realize that you may not be provided the time to do this but it is a really worthwhile exercise for future designs.

    David Thompson

    David Thompson P.E., M.ASCE
    KTA Structural Engineers Ltd.
    Calgary AB

  • 3.  RE: Analyzing Building Frames

    Posted 01-05-2023 12:56 PM
    Like David, when modeling I had always modelled up to the point of where the cross-sections of the members were was no longer constant. Of course, it is an iterative process to define that desired combination of member length and X-section. Rigid elements (program provided or user created) were used back to the joint and end releases selected based on the conditions at the joint.

    Back in the day and depending on the program, the user created rigid elements allowed one to vary stiffness or spring constants. Of course, this was all linear analyses. The forces of the member or reactions at the joint was then extracted and used to design connections. Of course, if budget allowed, one could switch to the finite elements and model joint details (plate models for steel). Thankfully, MathCAD was around (consider this portion of text a declaration of age).

    I have attempted to use some 3-D solid modeling analysis programs (contact surfaces) for both concrete and steel design. Even if the programs include joint analyses, read the disclaimers and visit the website's user support section. This is where you discover the real limitations identified by the SUPER USES. I have never come across software that did not provide a warning that the software was not perfect and place the burden of verifying answers back on me.

    James Williams P.E., M.ASCE
    POA&M Structural Engineering, PLC
    Yorktown, VA