Unbraced Length of Continuous Steel Beam
Unbraced Length of Continuous Steel Beam
(OP)
I was wondering what have you all used for unbraced length for continuous steel beams over steel columns with joist bearing on t/beam at 5'-0" o.c.?
Presuming only gravity case, i.e. no uplift. Could we utilize the inflection point? in other words, for the beam with positive moment - Use the joist spacing as the unbraced length?
Your valuable input would be very much appreciated.
Thanks in advance.
Presuming only gravity case, i.e. no uplift. Could we utilize the inflection point? in other words, for the beam with positive moment - Use the joist spacing as the unbraced length?
Your valuable input would be very much appreciated.
Thanks in advance.






RE: Unbraced Length of Continuous Steel Beam
Note that, in reality, you should (but currently can't) be able to use the entire length but then calculate a whopper Cb value because the top flange is more or less continually braced. There are Cb equations out there that take this into account. The Spec. does not currently say anything that allows one to use such an equation. Hopefully, it will be there next time.
RE: Unbraced Length of Continuous Steel Beam
Not according to AISC. Use the full span length and calculate the correct Cb ratio for that span.
Here's a couple of previous threads on the topic:
thread507-189712: Beam inflection point
thread507-148451: Inflection Point Lateral Bracing
RE: Unbraced Length of Continuous Steel Beam
Thanks for your quick response.
RE: Unbraced Length of Continuous Steel Beam
271828 - You mentioned about Yura and Helwig's method in Cd calculation, do you know where could I find such equation? Could you please provide me more information on that?
Thanks.
RE: Unbraced Length of Continuous Steel Beam
DaveAtkins
RE: Unbraced Length of Continuous Steel Beam
RE: Unbraced Length of Continuous Steel Beam
lutein, the Cb equation is in the Yura/Helwig seminar notes. There are various versions of these, but I suspect it's in most or all of them. It *might* be in the latest SSRC Guide also.
RE: Unbraced Length of Continuous Steel Beam
He thought for a moment and then said "yes".
However, two years later, Yura had developed more research and a new Cb formula and an engineer I knew had asked the same question and gotten a "no" for a response from Yura. So the thinking then (about 1985) was that inflection points should not be used as a brace point.
In the past we did use inflection point distance, but added about 20% to that length with Cb = 1.0 so we rationalized that doing it that way was OK...but as 271828 says, "pretty haphazard" with no reasearch or logic to back it up.
RE: Unbraced Length of Continuous Steel Beam
Related to this issue, lutein, are there stiffener plates each side of the beam where it passes over the column? I read an article almost 20 years ago about a roof collapse, due to buckling of a beam flange where it passed over a column. Ever since I read that article, I ALWAYS show stiffener plates above the column in this type of construction (even if the AISC formulas say you don't need stiffener plates). So, if you are reinforcing the roof structure anyway, I would add plates if they are not already there.
DaveAtkins
RE: Unbraced Length of Continuous Steel Beam
Besides, the critical flange for a cantilever can sometimes be the tension flange i.e. the same flange as the critical flange for the span!
RE: Unbraced Length of Continuous Steel Beam
After my research on some textbooks, I will specify on drawings to extend the joist bottom chord to brace the beam at several points - defenitely on top of column, and intermediate.
RE: Unbraced Length of Continuous Steel Beam
RE: Unbraced Length of Continuous Steel Beam
In particular, read App. 6.3 and 6.3.1.
RE: Unbraced Length of Continuous Steel Beam
"Design Guideline for Continuous Beams Supporting Steel Joist Roof Structures" by James Rongoe...available at www.aisc.org to Members...in the ePubs sections