"Do not use K-factors if you're running a p-delta/p-sigma analysis"
"Do not use K-factors if you're running a p-delta/p-sigma analysis"
(OP)
Hello all,
This morning my boss came around the table after reviewing my structural model and states that the reason why we use the effective length factors is to account for p-delta effects. So, if we are to run a p-delta analysis in our structural model, then I can take k = 1. Of course, when my boss stated the above, that was earth shattering for me because forever I've been running a p-delta analysis while using k factors greater than 1. I remember reading a discussion about this on this forum a long time ago, but I can't seem to recall what the verdict was, but I do know there were a lot of confused fellows like me.
I follow the CSA-S16 (Canadian)
Thanks
This morning my boss came around the table after reviewing my structural model and states that the reason why we use the effective length factors is to account for p-delta effects. So, if we are to run a p-delta analysis in our structural model, then I can take k = 1. Of course, when my boss stated the above, that was earth shattering for me because forever I've been running a p-delta analysis while using k factors greater than 1. I remember reading a discussion about this on this forum a long time ago, but I can't seem to recall what the verdict was, but I do know there were a lot of confused fellows like me.
I follow the CSA-S16 (Canadian)
Thanks






RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
K factors account for member fixities other than those in the Euler column (pin-pin w/ no lateral translation).
Your boss is probably referring to the Direct Analysis Method. As StrEIT summarized above, carrying out a RIGOROUS 2nd order analysis is only one part of the DAM. You must account for loss of stiffness, member imperfections and so forth.
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
You might want to read 'Proposed provisions for the design of Steel Beam Columns S16-2001', Can J. Civil Eng. Vol.27, all those recomendations were adopted in CSA S16-01, S16S1-05 and Supplement #1 to it as well.
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
That is not the way it is for AISC. If you don't take care of the inelasticity, erection tolerances, etc. in addition to the notional loads you can't use k=1.0. That is the basis for the DAM which is the only situation in which you can use k=1.0. If you don't use DAM, then you are, by default, using the effective length method (which by definition uses an effective length, kl).
Again, this if for AISC.
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
I think is meant in reference to the use of second order methods in AISC and not to the code in general, right? k=1.0 for columns, struts, etc. has been around since Adam bit the apple.
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
Right, I guess I should have said that k=1 in all regardless of lateral system used just made it into the spec meaning you can now use k=1 where you couldn't before (i.e. sway frames where k is greater than 1). Like you said, k=1 for gravity columns (regardless of lateral system), struts, etc. has always been used.
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
I don't have my spec in front of me, but doesn't the amplified second-order method also allow you to use k=1?
That's the method with the B1 and B2 factors.
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
htt
I watched it is very informative, and has all the information on the DAM
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
k factors (or effective length factors) are meant to account for end conditions that are other than pin-pin, NOT to account for second order effects. You can go through the derviation of buckling loads (even using energy methods - or you can use the more time-consuming differential equations) and you will see that the buckling load is dependent largely on the end support conditions and that the difference between those buckling loads is the effective length of the column (which can be considered the distance between inflection points). Since it is the distance between inflection points that is considered the effective length of the column, it is clear that ACTUAL length needs to be replaced by an EFFECTIVE length when calc'ing column buckling loads. Doing a second order analysis does not negate the need to account for end support conditions UNLESS you account for all the other criteria and variable as outlined in Appendix 7 in AISC 360-05 for the DAM.
Just think about the requirement for the effective length method.........You are still supposed to use a second order analysis, but you ALSO have to use effective lengths of columns.
Doing only a second order analysis DOES NOT, in and of itself, automatically allow the use of k=1 (unless you have a braced frame).
RE: "Do not use K-factors if you're running a p-delta/p-sigma analysis"
Please note that there are cirumstances in which you can use k=1.0 for moment frames in both the second ordeer analysis using B1 and B2 as well as the first order analysis methods, BUT you need to meet several other criteria IN ADDTITION TO the analysis (e.g. drift ratios, see the paper). It's not accurate to say "a second order analysis automatically means k=1.0" for frame columns.