Minimum Stiffener for Stability

[Ripley2]

Hello. I have a raised beam connection with a W12X30 end that bears on top of a W16X67 and then a W10x33 column below the W16. Similar to 13th Ed. AISC Manual Fig. 10-31(b). There is a minimal concentrated load of 15k acting on top of the W12. I do not think stiffeners are required for strength because the load is so low. However, I would like to provide full depth stiffeners for rotational stability of the W12 and W16. Rotational stability is required per specification section J and Appendix 6 according to my interpretation. What is the minimum stiffener required for stability? I was thinking of applying Appendix 6 (Stability Bracing for Columns and Beams). Assume a framed structure with W12 and W16 beams. Calculate the required nodal brace system stiffness. Then size the stiffener plate such that is has the same lateral stiffness.

I think the load is so low that it will not matter, but I would just like to know the proper way to do the design. I would guess that I will use a 3/8" full depth stiffener with 3/16" welds as a minimum.

Thenk you very much for any input you can provide!

 

[a2mfk]

I assume no floor or roof or any diaphragm providing top flange bracing for stability?

 

[Ripley2]

Correct, no bracing for the top flange.

 

[ishvaaag]

Now on it would be as per section

J.10 FLANGES AND WEBS WITH CONCENTRATED FORCES
AISC 360-10

 

[Ripley2]

Yes, Section J10 of course. All strength limit states are ok without stiffeners. Section J10-7, Unframed Ends of Beams and Girders, requires there be full depth stiffeners at unframed ends (how big I can only guess). Section J10.8, Additional Stiffener Requirements for Concentrated Forces, says to design the stiffened element as a compression member so that it takes up the additional strength required, but the beams I have provide the required strength without stiffeners. So, the stiffener would be designed for zero force.

Am I correct in thinking that some sort of stability bracing should be provided? Full depth stiffeners would provide rotational stability. Appendix 6 seems to talk about that.

 

[ishvaaag]

Since the W16 will have enough stiffness it is only a matter of stiffeners to rotationally restrain the end of the W12 in its height. Moment if understand well the setup is there zero, so the compressive force in the "free" top flange is scarce, hence we can't use as guide the axial compression in the flange as guide for the required stiffness (and stiffener) (that would contain such transverse movement and notional force).

So a more logical approach would involve a mechanism where the top flange is horizontally displaced respect the bottom flange and then at such predeformed shape of the cross-section check that the section plus the added stiffeners remain stable under the (factored) applied load. This is tantamount to a P-Delta moment atop the height of the stiffeners, plus the axial force (everything factored) and then check the strength of what provided. It was usual that I remember to take 9 times the thickness from the stiffener collaborating, but you can go for more modern web crippling statements of the strength. So really we will be combining in one check both web crippling under the load and LTB prevention.