DAM Appendix 7 Application to Step Columns
DAM Appendix 7 Application to Step Columns
(OP)
Does appendix 7 allow Kfactors for stepcolumns to be set = 1.0 for the upper and lower segments?
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RE: DAM Appendix 7 Application to Step Columns
I don't think step columns are ever officially mentioned. But, this is a good example where the DA method (with K=1.0) will probably be better / more reliable for ensuring stability than a K factor.
I'd use K = 1.0, but set the unbraced length as the distance between bracing... even if that is greater than the segment length.
There is another method to use for calculating the buckling capacity of this type of member. The method of successive approximations. This is described in the AISC design guide (number 25?) for tapered members. The method comes from one of Timoshenko's books (Elastic Stability?). And, the example that Timoshenko gives in his book is a stepped column.
RE: DAM Appendix 7 Application to Step Columns
I was just curious about DAM because Kfactors for step columns can become a real pain as the factors are load dependent and therefore sometimes you have different Kfactors for different load combinations.
RE: DAM Appendix 7 Application to Step Columns
Does DG7, the AIST Tech Report 13, or DG25 discuss this?
EIT
RE: DAM Appendix 7 Application to Step Columns
RE: DAM Appendix 7 Application to Step Columns
The concepts of Direct Analysis Method (reducing the flexural stiffness of the members based on the axial loading) are what allow a typical elastic analysis (with PDelta for 2nd order effects) to approximate the inelastic buckling of the member or frame. It's not perfect, but I'd argue that it's good enough. And, it is relatively simple compared to trying to get the "true" inelastic buckling values for this member or frame.
RE: DAM Appendix 7 Application to Step Columns
Good call. Such a simple concept yet I get it confused time and time again.
I think really the confusion (for me) lies with in the fact of using the B1B2 method which I relate to effective length method. I forgot that B1B2 is just the 2nd order analysis part of the DAM (Add notional loads to account for initial imperfections, 2nd order analysis, and reduce stiffness to account for axial loads  ?)
In my thread I was referring to columns which were pinned and gravity only. In this case K=1 would be conservative (I believe)and basically you would use the Newmark Method (numerical method I believe to be similar to a successive approx.) to find your elastic buckling strength then use the elastic buckling strength equation Pe=pi^2*E/(L/r)^2 to find L/r then from there you would determine which equation AISC E3a or E3b to use.
EIT