Beam Bracing

[NE12345]

I am currently working on a project where I have to make a foot walk on a crane OSHA compliant, to meet the 48" rule from low obstruction the deck has to be lowered below the girder (W24x100) and outrigger (W16x26) by about 4-5 inches. The beams are 47 inches apart centerline to centerline and the distance between runway rails is 47' 6". Initially made drop down foot walk supports every 5' utilizing C channels see attachment for picture. During the calculations of the outrigger with the large span and the distributed loads from the live load and dead loads the outrigger fails in LTB. I am trying to figure out a way to have the foot walk deck supports act as bracing between the 2, I have looked at AISC and from what I can understand they would act as tension flange torsion bracing. So I was thinking of using AISC App 6.3-2 to size the floor channel for the strength and stiffness along with placing full length web stiffeners at each of the foot walk support locations in both the outrigger and girder thereby reducing the unbraced length of the outrigger.

My questions are:
1. Am I using everything correctly?
2. What can I do to brace these beams together enough to reduce the unbraced length?

 

[KootK]

You're on the right track. Moment connect the channels and the connection between the drop down channels and the beams. Provide stiffeners like you said and you'll have LTB bracing via torsional restraint. The proportions look goofy but your AISC checks will take care of that.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Hokie93]

How much additional flexural capacity do you need? Since the load is applied on the bottom (tension) flange and LTB governs, the beam will have additional flexural capacity compared to load applied at the shear center or at the top flange. Equations that include the load height effect are given in the Guide to Stability Design Criteria for Metal Structures, 6th edition, by Ziemian. The topic was also covered in earlier editions of the same reference.

 

[SteelPE]

I have always been told that web stiffeners do not really count as bracing for LTB and to never count on them for LTB. As stated above, you can probably squeeze a little extra capacity out of the system but is it that capacity enough and is it worth it? I might consider just reinforcing the beam as necessary and moving on.

 

[KootK]

Stiffeners by themselves do not count as LTB but stiffeners installed as part of an assemblage that provides rotational restraint absolutely does. Having organically occurring framing members pull double duty as stability bracing for other members has always been a cornerstone of efficient design in steel. This is a bit of an atypical case of course so beam reinforcement might be a competitive alternative.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.