Column Effective Length
Column Effective Length
(OP)
I have an existing column that is just above a column splice location. The lower column extends 4' above the finish floor elevation and is a W14x132. The upper column that is spliced on top of the W14x132 is a W14x53. The floor to floor height is 14'-0". Can the effective length for buckling etc for the upper W14x53 be taken as 10' (14' floor to floor - 4'-0" splice length) since the W14x53 is actually only 10' long from the top of the splice to the top of the next floor. And along those lines I have 24" beams framing into the minor axis of the W14x53 and 16" beams framing into the strong axis of the W14x53. The slab is 6 1/2" thick. As I see it you could reduce the unbraced length for the column even further by the depth of the slab and the clip angle connection of the beams to the columns. Any thoughts would be appreciated as this is an existing condition for W14x53 that is getting extra load and to coverplate the column would be very difficult. Thanks for your thoughts






RE: Column Effective Length
RE: Column Effective Length
I think the best model, assuming the column splice transfers moment, is a single column with K=1 (assuming this is just a gravity column) and different A and I from 0' to 4' and from 4' to 14'.
mkrei, do you have access to SAP or another program that'll do buckling analysis? It should take literally 5 min. to get Pcr from the eigenvalue buckling analysis, divide that by A to get Fe for use in Section E3 of the 13th Ed. Spec. Conservatively use A from the W14x53.
RE: Column Effective Length
RE: Column Effective Length
You *might* be able to do the following. (Do some experimenting to see if RISA gives reasonable results for known problems.)
Create your column in RISA and subdivide it a bunch of times along its length. Give it a tiny initial lateral displacement by modifying the coordinates. Make it a half sine wave. Then apply a vertical load that you're confident is below Pcr and see what lateral displacement you get a midspan using a second order analysis (this is the question -- I don't know what kind of second order analysis RISA does and some types won't work for this). Increase the load and repeat. At just a hair under Pcr, you should have a small deflection, say 0.01". At just a little closer, it'll increase by a lot, say to 10". In some ways, this is actually the best way to use a program to find Pcr. It gives more control than an eigenvalue analysis which finds ALL buckling modes whether you want them or not. Some will be unreasonable. Remember that this gives the elastic load which you use to get Fe and adjust for inelastic buckling using the Spec. equations.
Good luck.
RE: Column Effective Length
I was assuming a pinned splice. Yes, if it is a full moment splice then what 271828 states is correct.
I don't usually use RISA's automatic K generator as it is based on rudementary end conditions that may or may not match what is really happening.
RE: Column Effective Length
RE: Column Effective Length
Thanks
RE: Column Effective Length