Unbraced length for Continuous Beam

[studentin9]

Hey all,

I wanted to reach out about a question for continuous beams and where to place bracing. Reading into AISC steel manual, I see they have a formula for Cb that relates to an unbraced length (C-F1-5) for a continuous beam but I can not find code describing how far away bracing should be placed from the middle supports where negative moment is the highest. The only information that I can find is that the inflection point from positive to negative moment is not a bracing point. I have heard that most just placed at locations of joists past the inflection point into the positive moment area, but is there a specific equation/code that gives the location at which bracing near the center supports gives the most moment capacity. I am currently dealing with a continuous beam that has one middle support that causes so much negative moment the beam is failing at 200% over moment capacity. I would greatly appreciate some direction into the standards for unbraced distance from that middle support.

 

[DaveAtkins]

Normally, I do not brace the bottom flange and assume Lb is equal to the entire length of the larger span. I choose a beam which is wider and heavier than what would be required for a fully braced beam.

If you would rather use a lighter beam, brace the bottom flange at every joist where negative moment can occur (it might be the entire length of both spans). Then assume the unbraced length is the joist spacing.

DaveAtkins

 

[studentin9]

DaveAtkins,

Thank you for your input. For this specific problem, the beams are an part of an existing structure and we are placing new beams and columns that simply lie under the existing beam structure bottom flange. These are really weak beams (W12x14's) and are failing extremely with negative moment.

 

[CDLD]

Agree with Dave.
If you need to squeeze more capacity out of an existing beam, you can add bottom flange braces or increase your Cb factor.
See AISC commentary, section F1.

There is a formula that allows you to make use of top flange support for negative bending.

 

[SWComposites]

If the existing beam is failing that bad in buckling, using braces may not be appropriate; probably should look at reinforcing the beam itself.

A sketch of the configuration would really help. How are the new beams attached to the existing?

 

[BridgeSmith]

The location of the bracing is chosen by the designer based on what is needed to minimize the reduction in capacity due to lateral torsional buckling (LTB). For me designing bridge girders, it's usually a matter of trying various bracing locations to determine the largest distance from the pier to the brace point before LTB becomes significant. After 20 years designing bridge girders, I have a pretty good idea where to start for the rolled W shapes and plate girders we use, but not for a W12; I would guess somewhat less than 12' would be where a W12 beam would start to lose significant capacity, but that's just a WAG.

 

[EngDM]

There is a formula that allows you to make use of top flange support for negative bending.

Not OP, but I will need to look for this when working with existing cantilever beams from the 80's to get them to work under existing loads (assuming Lb is the clearspan usually has them failing).

Ty for reference.

 

[CDLD]

@ ENGDM, exactly - it's great for that situation.

Make sure you have torsional support at the support points.

 

[JLNJ]

The AISC Commentary has quite a bit of guidance on unbraced lengths and continuous beams.

 

[human909]

THIS is an excellent video on the subject:

Applications of AISC Appendix 6 Bracing Concepts
Appendix 6 of the AISC Specification provides guidance on the stability design requirements for bracing systems. This session provides an overview of the background behind these requirements and a discussion of the many factors that affect the stiffness and strength behavior of bracing. Examples of effective and innovative applications of the provisions towards various bracing systems are provided.

https://www.aisc.org/education/cont...ons-of-aisc-appendix-6-bracing-concepts-n11a/

(HINT: Skip the first part on columns if you wish and jump straight to our mate Joseph Yura on beams.)


Oh and I absolutely brace the bottom flange on a continuous beam. Not only do you have a negative moment by you have a destabilising load from the column below.

 

[studentin9]

Thank you all for your responses, you have been a great help to figuring this out

 

[lexpatrie]

Ideally, there's (if there's a column under the beams) there's a stiffener where the beam bears on the column. If it's not there you're in a much different stability situation (Station Square, Burnaby, etc. Take a look at the AISC FAQ here on this forum. These collapses pertain to cantilever framing but if your framing situation has the beam landing on the column there's a stability concern if there's no stiffener even if it's not cantilever framing.)

There are ways to eek out more capacity from beams using "more detailed" analysis, but there needs to be appropriately substantiated bracing.