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
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 member, diagonal brace, stiffener, etc.
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
the coefficient Cb is taken as 1.0 for cantilevers where the free end is unbraced. When evaluating Cb for a cantilevered beam, the moment diagram will lead to a value of approximately 2.0 depending on loading conditions. You might be inclined to increase the moment capacity of the member by an equal amount, but this is unconservative and incorrect. Similar to a flagpole problem where K = 2.0, the effective unbraced length is twice the actual length. These two factors cancel each other since Cb would increase the moment capacity and K would decrease it. The proper calculation of the design flexural strength of a cantilever uses the actual length and a Cb coefficient of unity. For cases of restraint to the compression and/or tension flanges at the free end of the cantilever, refer to the SSRC publication Guide to Stability Design Criteria for Metal Structures (
Back to the inflection point question. No - it is not to be considered a bracing point. Current "state of the art" research in the US on this is Yura, "Fundamentals of Beam Bracing" Eng. Journal Q1 2001. In this paper he states:
"The results in Figure 10 show that not only is it incorrect to assume that an inflection point is a brace point but also that bracing requirements for beams with inflection points are greater than cases of single curvature. For other cases of double curvature, such as uniformly loaded beams with end restraint (moments), the observations are similar."
The big helpful thing nowadays to account for this is Cb. Say you have a moment frame girder. I think folks would often use the distance to the IP as Lb, but then use Cb=1.0. On the other hand, if you use Lb=L, but then calc Cb=2.5-3 (or whatever), LTB doesn't kill your beam. The effect is a bit of a wash in that regard.
On a related subject, I hope the next AISC Spec. allows Cb to be determined by analysis as well as the given equation. There are some very nice Cb equations out there. For example, if you have a continually braced top flange, but the bottom flange is in compression for some of the span, you can calc a very high Cb (like 4-5) using one of the Cb equations given by Yura and Helwig in last year's seminar.
