Lateral brace at the inflection point.

[Robbiee]

The attached sketch shows a moment frame where the bottom flange of the beam with fixed ends is laterally braced at the inflection points. The 2% of the compression force in the flange is zero because of the location being at the inflection point. What is the force in the brace to be designed for?

 

[jike]

I would design it for 2% of the maximum compression force in the bottom flange. It will most likely turn out that slenderness will control the size of the brace and not the brace force.

 

[Lion06]

I would follow the strength AND stiffness requirements of AISC 360-05 App. 6 using the maximum negative moment.

 

[Stillerz]

Chances are you could design it for 2% of the maximum allowable force in the compression flange due to bending and the load would still be insignificant.

 

[happymam]

I am sorry

 

[Robbiee]

happymam,
You're sorry for?

 

[BAretired]

Inflection points move about with different spans loaded. Is 'x' the location of the inflection point with live load on all spans or with braced span unloaded?

BA

 

[Robbiee]

BA,
The frame is a single span one and the "X" is the inflection point when all gravity loads are applied.

 

[Stillerz]

BA has a point. The inflection point will move with lateral loading. I guess it will just come down to unbraced lengths for compression.
I had the exact same situation in a building jsut recently. Exactly the same thing. I braced it exactly the way you showed and had a concrete floor diaphragm. I would up putting the "kickers" at the third points. Top flange braced continuously by floor.

 

[Robbiee]

Stillerz,
So you sized your braces to take 2% of the max compression in the bottom flange due to negative moment?

 

[cntw1953]

Ailmar:

What was the background/factors that rendered the kickers necessary? Was the top flange continuously attached to a rigid floor system, or adequately restrained against side way movement?

Keep in mind that brace is required to prevent compression flange from pre-mature buckling that could cause instability problems, or to prevent beam from rotation due to externally applied forces (torsion). Do you have such concerns?

Sorry to ask questions over your question, but if you can clearly identify the reason for providing kickers, then you maybe able to answer your own question with confidence.

 

[Stillerz]

I had the entire building in a model.
I needed to cut down the unbraced length of the comp. flange.
I had some L3x3x3/8 angles available (design-fab-build in house).
Checked 2% of compression force, damn near negligible. Calculated full force in the compression flange based on the yield and took 2%. It was still negligible.
Say W16x36 bf=5.5, tf =0.345 A= 1.89 in^2.
Used 50 ksi
50 ksi x 1.898 in^2 = 95 kip
95 kip X 2% = 1.9 k.
Used sinlge angle compression tables to check.
Not even close for my length.
Checked Kl/r...not a prob.
Used the L3x3x3/8 as I determined it would suffice as a braced point for beam.

I might be way off base here....

 

[JrStructuralEng]

I agree, 2% of compressive force at brace point.

Does anyone know if there are single angle compression tables in the CISC? I couldn't find any (or somewhere else?) A quick reference would be helpful.

 

[BAretired]

It should be noted that providing 2% of the compressive force in the braced member is not the only requirement to be satisfied. The displacement must not exceed the assumed initial misalignment of the member being braced, ordinarily taken as 0.002*sp where sp is the distance between braced points. This would include the axial deformation of the brace and any displacement in the end connections and their supports.

BA

 

[JrStructuralEng]

dang BA, can I hire you to coach me??