Braced length of free-standing steel portal frame

[bhiggins]

Hi Guys and Gals!

I've been tasked with designing a free-standing steel portal frame. The span is 35', height is 10', and the frame will be supporting a wood cantilevered deck and roof. The frame is not braced laterally where the beam meets column, but I am assuming the beam top flange is braced by the wood deck. The wood deck is inset roughly 4'-6" from the beam end supports. In this situation, what would the bottom flange braced length be since the supports are not braced laterally? I've attached a drawing of the proposed design:

The frame is also designed for wind loads (no seismic).

 

[DCBII]

I would tend to use the length of the columns plus the beam. That's probably a little conservative, but for bending the question is at what point is the compression flange braced against twist of the cross-section. Under reversible lateral load both flanges are at one point or another in compression. This holds true at the beam/column interface, where there is no lateral brace. If the beam wants to twist at the column, the only thing restraining it is the weak axis flexural strength of the column. Think of it as the beam flange applying an out-of-plane point load at the top of the column.

 

[JoshPlumSE]

Gosh, that's a good question. Honestly, I have a hard time believing that I never encountered this situation before. Maybe I did and didn't think deeply enough to recognize the issue.

You could argue that the column moment connection provides some restraint vs twisting. Personally, I think that is a weak argument. Just gut feeling, I'd include half the height of the column on either side. But, limiting it to an absolute max of 2*L_beam.

I have no objection to DCBII approach other than it "feels" too conservative to me.