Beam inflection point
Beam inflection point
(OP)
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
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RE: Beam inflection point
RE: Beam inflection point
If you take into account the stress reversal along the segment length, you should get reasonable capacities in this region.
RE: Beam inflection point
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.
RE: Beam inflection point
I would make sure to make the connection at the column to brace the beam, or you might have problems with the column design.
RE: Beam inflection point
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).
RE: Beam inflection point
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.
RE: Beam inflection point
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.
RE: Beam inflection point
RE: Beam inflection point
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 www.aisc.org/lrfdspec), 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 (www.stabilitycouncil.org).
Sergio Zoruba, Ph.D.
American Institute of Steel Construction
RE: Beam inflection point
"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."
RE: Beam inflection point
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.
RE: Beam inflection point
Thanks
RE: Beam inflection point
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All the specifications should be available there. Not sure if you have to be an AISC member to see these.