Restraing Compression Flange LTB 1

[ahmedhegazi]

Dear All

I got a composite floor slab(steel and concrete slab) where they are connected toghter using shear studs and metal decking, the problem is i have got a cantilever beam nearly 5 meters ,allowable stress considering lateral torsional buckling is exceeding allowable , i have a though to reduce LTB by providing stiffener at mid span perpendicular to web and welded to both bottom and top flanges(Both sides of the web) , i believe by this i am preventing the buckling shape of unrestrained compression flange ,anyone has got input in this?!

thanks in advance

 

[ash060]

What about deflection? With a 15' cantilever that would probably be a major issue, and being that the concrete is on the tension flange, you are really not getting any composite action.

The stiffeners are fine, but you still have to provide some kind of kicker to resist the twisting, stiffeners alone won't do it.

What kind of backspan do you have?

 

[kelowna]

If I understood what you wrote, I do not think the stiffener proposed will have any effect on LTB. You need to restraint the compression flange, connecting it to the top flange through a stiffener does not help.

 

[JAE]

I think AISC recommends bracing the TENSION flange on a cantilever. I'm away from my office right now but there is, I believe, a section in the AISC specification that states this.

 

[ash060]

There is a minimum requirement for the tension flange at the end of the cantilever, but along the length the compression flange can be braced as far as I know.

In his case the tension flange is braced along the length

 

[BAretired]

How long is the backspan? You probably need to brace the bottom flange of beam at the column, in the cantilever and in the backspan.

A sketch would help.

BA

 

[apsix]

If the top flange is restrained against twist then the web stiffeners will provide bottom flange retraint.
I believe that the composite slab will provide adequate twist restraint.
See attached for AS4100 (Aust) recommendations.

 

[BAretired]

I agree that the composite slab will provide some restraint against twisting. If the restraint is adequate, then stiffeners may be adequate to brace the bottom flange.

The test on what is adequate is that the displacement of the bottom flange, [Δ]_b under the required brace force does not exceed the assumed initial misalignment, [Δ]₀.

The required brace force is 2% or 2.5% of the compression in the flange to be braced, depending on the code used unless a more detailed analysis is carried out.



BA

 

[ahmedhegazi]

First i would like to thank aspix for his valuable attachment,the case shown in Fig5.4.2.1 a is what i am planning to do .

More clarification to my question is that i have back span of composite beam(10 M) the back span and cantilever are supported on main beam.

I agree with ash060 that deflection will govern due to rotation/deflection of main beam

For calculating thickness of stiffner , i would check it under bending moment(2.5%Force in flange/2*H web)

Thanks all for your input.