huangyhg

beam inflection point
for continuous or cantilever beams can the inflection point be used as a lateral bracing point? asd wording seems not to allow it while lrfd does. thanks

the new 2005 aisc spec. appendix 6 specifically forbids the ip to be used as a braced point.
when you think about it, there is actually nothing special about an inflection point that could provide restraint.
if you take into account the stress reversal along the segment length, you should get reasonable capacities in this region.
ok, with that in mind. what sort of unbraced lengths do you use if you are designing continuous beam lines with cantilevers? i can't assume its braced at the column.
i did a little reading in my salmon and johnson steel structures book. they talk about how inflection point could be used as a braced point and give explanation. but follow up by saying they do not suggest it.
if the beam is not braced by the column, then are you going to design the column as a flagpole because than the top of the column would not be braced either.
i would make sure to make the connection at the column to brace the beam, or you might have problems with the column design.
i would think you can consider it braced at the column. if you beams rides over top of your column you should be providing a stiffener in the beam. if you have a composite slab over the beam, then your stiffener will brace it.
also, please note that in the cantilever portion of the beam, it is more critical to brace the top (tension) flange.
if you are providing a moment connection through the column, then the beam would be braced by virtue of the shear connection (assuming it is located properly).
why could you not assume the beam was braced at the column?
if the top flange is not restrained, i have used web stiffeners to secure the top flange to the bottom flange, which in turn is secured to the support.
i would use twice the length of the cantilever as the unbraced length.
drc1-
it is recommended to use the actual length of the cantilever as the unbraced length (provided it is not braced) with a cb=1.0. it is very conservative to use lb=2*actual length.
i will try to find the reference.
don't just "assume" it is braced at the column without taking positive steps to detail that brace whether it be an intersecting
here is a q&a from aisc's website regarding the unbraced length of the cantilever. they recommend using the actual unbraced length with a cb=1.0 as noted in my previous post.
unbraceed length of cantilever
question
07/01/2004
i was wondering what the laterally unbraced length value lb is for a cantilever? my intuition tells me that i should use twice the actual length of the cantilever for lb, but i don’t see any provisions for it in chapter f or appendix f of the specification. does limiting the cb value to 1.0 for cantilevers provide all that is needed, and then i would just use the actual length of the cantilever for lb?
question sent to aisc’s steel solutions center


answer(s)
in section f1.2a of the 1999 lrfd specification (a free download from
sergio zoruba, ph.d.
american institute of steel construction

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