huangyhg

"do not use k-factors if you're running a p-delta/p-sigma an
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
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i don't believe it is as simple as running a second order analysis because computers have been doing that for some time and the k=1 just made it into the steel spec. you need to also account for material inelasticity (which is where the reduced ei comes in - if you don't use this in your analysis you will be underestimating your second order effects which makes the k=1 invalid), and you must also account for material imperfections and erection tolerances (notional loads).
your boss is wrong.
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,
k = 1.0 csa s16.1-94 and forward of course. see 'new canadian provisions for the design of steel beam-columns', kennedy et al, can. j. civil engineering, vol. 17, 1990. in the abstract the following 'for frames dependant on the frame stiffness for lateral stability, no longer is the traditional method, using effective length factors greater than one, allowed.' you are required to do a second order analysis though. which is the point.
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.
the second paper cited contains this statement 'in other words, the function of the notional load is to allow the use of actual length of the column in computing the compressive resistance in sway frames rather than using effective length factors greater than 1.0'. the purpose of notional loads. material imperfections, erection tolerances, residual stresses are accounted for elsewhere.
connect-
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.
structuraleit, your quote:
quote:
and the k=1 just made it into the steel spec.
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.
jae-
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.
but, in order to qualify for k=1 for columns, regardless of lateral system, as questioned in the op, you need to meet more criteria than simply doing a second order analysis.
streit-
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.
well, you can use it, but it is part of the dam. it is just an acceptable alternative to the rigorous second order analysis, but you still need to comply with the other requirements.

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