Unbraced length/analysis of bent/kinked columns
Unbraced length/analysis of bent/kinked columns
(OP)
Is this correct: I have kinked columns, for example the column extends 2m and then kinks at 10 degrees and continues another 2m. This is intentional, not the result of a failure. To analyze the columns I am breaking it down into smaller segments (say 0.25m) and running a p-delta analysis. Using this approach I should be able to consider the unbraced length to be my segment length (0.25m) when checking capacity. Is that right?






RE: Unbraced length/analysis of bent/kinked columns
I would not even rely on the kink for any lateral support at the 10 degree figure. You still have the orthoginal direction to consider. Depending on the external lateral support, I would look to the full length of the column as the lateral unsupported length.
Mike McCann
MMC Engineering
RE: Unbraced length/analysis of bent/kinked columns
I believe that using the full length for Lb and using AISC equations is giving me a reduction based on secondary effects. By running a p-delta, and with an initial deformation (which is provided by my kink), I should already be accomplishing the same thing. Does that not make sense?
RE: Unbraced length/analysis of bent/kinked columns
If the col is modeled with the kink, then the program will account for the resulting induced moment.
I would use the ASD 89...no P-DELTA in the program and get the resulting axial and moment forces...then use the fomulas in that code that account for incidental P-delta affects.
What KL to use is an engineering judgement call based on col fixity and controling unsupported length of the col.
RE: Unbraced length/analysis of bent/kinked columns
As a "to keep in mind" comment: Don't forget to follow the horizontal loads that are caused by the kinked / diagonal columns through the structure (e.g. horizontal shear at the base plate, horizontal shear at the top-of-column connection, following that horizontal force through the structure). It's just something that can be overlooked at times.
Unless material costs are a major concern, I'd certainly consider a K = 1.0 and the full length of the column as your unbraced length (unless there are braces intersecting the column). P-Delta analysis or not, I wouldn't consider the kink as a restraint in-of-itself.
MJB
"We shape our buildings, thereafter they shape us." -WSC
RE: Unbraced length/analysis of bent/kinked columns
RE: Unbraced length/analysis of bent/kinked columns
RE: Unbraced length/analysis of bent/kinked columns
However I find suspicious AISC does not state explicitly use actual length between braces for flexural strength determination with the same clarity that use K=1 (even for this, you can use segment length should be clarified) in direct analysis context.
It is surmised that the inclusion of P-Deltas, initial imperfections, and material stiffness adjustment reveal the augmented solicitations that stability requirements may ask for.
This means that the structure at the stable final status at the factored level is showing all the augmented values of both XYZ displacements and rotations about such axes, upon which no further displacement or rotation may be attained under the set of standing factored loads, stability requirements included.
This SHOULD mean that in the same way that we can use K=1 and segment length for the checks since the nodes are linearly fixed at the final status and showing the stability enlarged moments at ends of segments, the nodes are to be also rotationally fixed and showing the enlarged moments and rotations at such ends, and we SHOULD be able to resource to in-segment behaviour, i.e., again, segment length, to ascertain the expected flexural capacity of some member that will have such (final, stable) solicitations at ends of segments.
Since however what WillisV says may very well be the current understanding of the issue for flexural strength in direct analysis context -in spite of the faible clarity about the matter in the code, simply letting the design fall in the ordinary statement of Lb, without any further precision- I will try to find document or tutorial or whatever preferably within AISC docs to clear the actual standing.
RE: Unbraced length/analysis of bent/kinked columns
If I were desperate to get the column to work, then I would consider using segment length for flexural buckling length (as WillisV suggests). But, I'd want to be very diligent in reviewing my design calcs and my analysis results. I might even consider adding a slight lateral load (similar to notional load) at those segment joints just to introduce some extra out-of-straightness for the P-Delta analysis.
RE: Unbraced length/analysis of bent/kinked columns
I've been looking for references for this and it is laid out fairly clearly in Eurocode 3 in case anyone ever comes up with this problem in the future. Thanks for the inputs.
RE: Unbraced length/analysis of bent/kinked columns