Cantilever Beam Brace Point

[Arun4567777]

In Cantilever beams, the tension flange is typically braced to prevent Lateral-Torsional Buckling (LTB), unlike Simply Supported (SS) beams where compressiom flange is braced to prevent LTB. LTB occurs when the conpression flange laterally deflects and whole section twists. How in Cantilever beams instability is achieved and why we should brace the tension flange.

 

[driftLimiter]

Link

Here is a discussion about this feature of cantilever beams.

 

[Tomfh]

In a simply supported beam, the compression flange is the critical flange, as it is the flange that buckles furthest.

Bracing this flange provides the most effective stabilisation, similar to bracing the top of a bookcase rather than the bottom being the best way to stop it falling over. The top is the “critical” location where bracing offers a greater resistance against rotational instability, compared to bracing lower down near the centre of rotation.

In a buckling cantilever, the top flange also typically buckles the furthest. Bracing this flange is therefore the most effective location to prevent buckling, regardless of which flange is tension and which is compression.

 

[human909]

I find this contains some of the easiest to follow guidance on cantilever beams:

https://www.steelconstruction.info/images/9/99/AD-408.pdf

 

[dik]

...also depends on the shear centre. Py would try to 'correct' the situation in your sketch.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Arun4567777]

Is it the tension flange that initiates the instability in Cantilever beams. As for a SS beams it is the compression flange that initiates it. How a tension flange buckles.

 

[human909]

Is it the tension flange that initiates the instability in Cantilever beams. As for a SS beams it is the compression flange that initiates it. How a tension flange buckles.

While it is much easier to think of buckling as a compression only phenomenon this is not the case (

https://www.youtube.com/watch?v=EKngs1vvcJU[/URL]]as shown in this video

). In the case of a beam section the section buckles, not just the compression flange.

Think of buckling as a the system trying to achieve a lower energy state. AKA the load centre moving in the direction of the load (generally downwards) in a manner that uses the least amount of energy.

 

[lexpatrie]

Tread carefully here, as the Canadian and the U.S. Steel codes don't treat these the same, in terms of notation. Australian code isn't something I know.

I have some concerns that this is not being explained correctly but lack the time to delve into it.

One does not typically "brace" a tension member.

The issue with brace points is they need to prevent twist or lateral movement, conventionally.

The AISC (steel construction) code issues forum has a few frequently asked questions (written by me....) that might help a little with this discussion.

 

[human909]

One does not typically "brace" a tension member.

No but bracing the tension flange of a cantilever is a good idea and the most effective brace point on a cantilever section.

The issue with brace points is they need to prevent twist or lateral movement, conventionally.

Agreed. So what exactly is the issue?

 

[EngDM]

I quite like the diagrams provided by CISC.

 

[lexpatrie]

That (#) note is NOT correct, if you ask me. How old is that source? That looks pre station square.

Also, this penchant I keep seeing for using a BCX on the steel joist as a bottom flange brace is, to me, unjustified given the section isn't controlled (for one), and for two, strength and stiffness, let alone the goober weld they'll do there. We haven't yet had a collapse of one of these, but the "omitted stiffner" has a long history of being "present" during roof collapses. That's in the AISC forum FAQ I work on periodically.

 

[dik]

I always like girder web stiffeners at cantilevers. It maintains the cross-section in case a hinge forms.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Smoulder]

"Australian code isn't something I know"

Aus code doesn't accept lateral restraint alone for cantilevers in simple hand calc methods. Elastic buckling analysis is needed if you want to use lateral restraint to increase capacity. Cantilever = beam without twist restraint at one end for this discussion.

 

[lexpatrie]

Here's a decent "background" article, Design of Cantilever Beams, March 2005 Modern Steel Construction, Eskildsen. The article refers to quite a few other papers, I would imagine google scholar might assist in tracking down decendents of this work.