Continuous Steel Stringers - Bracing Assumption

[EngDM]

Hey all,

For a typical steel channel stringer with welded angles to hold FRP or metal grating, would you typically size these as fully unbraced or rely on the grating to brace it? I'd imagine that since the channels are typically mirrored with the toes facing outwards, that they would buckle in opposing directions. Is this enough to consider it braced, since they'd be working against eachother? Are you then required to check each channel for out of plane bending due to the other channel buckling? The grating connection itself would need to be sized for that brace load, but I've never checked the grating in compression.

I've mostly dealt with shorter stringers where length isn't really a consequence, but for this one I've got a large 8m (32° angle from horizontal) stringer with a cranked mid-height landing that'll get CJP welded. Taking fully unbraced for that length on a channel just tanks my capacity.

I'm thinking it would work similarly to a typical box girder crane, but I'm not sure if those are sized to brace eachother.

 

[jayrod12]

I feel at a minimum you could consider the mid-height landing bracing the stringers. But that may require you to provide something there. But some plan bracing in the landing shouldn't be too costly or onerous to provide. I'm a bit hesitant to use FRP grating as a brace personally since they're only ever connected with those sandwich clips.

 

[EngDM]

I feel at a minimum you could consider the mid-height landing bracing the stringers. But that may require you to provide something there. But some plan bracing in the landing shouldn't be too costly or onerous to provide. I'm a bit hesitant to use FRP grating as a brace personally since they're only ever connected with those sandwich clips.

Agreed on the FRP. If I had typical 1 1/2" bar grating that gets welded, I'm sure this could support the 2% axial/compressive force due to moment. Just not sure how to calc this out with bar grating in compression.

You're right about the mid landing, I have closure channels running from one channel to the other, so at the very least I am mid-height braced.

 

[jayrod12]

I think in terms of checking the grating, it would be a first principles type thing. They are, after all, just rectangular pieces of steel. The cross rods would brace the rectangular bars sufficiently in my mind.

 

[EngDM]

I think in terms of checking the grating, it would be a first principles type thing. They are, after all, just rectangular pieces of steel. The cross rods would brace the rectangular bars sufficiently in my mind.

That's what I was thinking, only the cross rods don't terminate at something solid, so you'd have the bearing bars essentially bracing eachother.

 

[KootK]

would you typically size these as fully unbraced or rely on the grating to brace it?

I'll certainly allow the grating to brace the stringers if I feel good about the robustness of the axial connection between the grating and the support angles on the stringer. I don't always feel good about it as @jayrod12 mentioned.

I'd imagine that since the channels are typically mirrored with the toes facing outwards, that they would buckle in opposing directions. Is this enough to consider it braced, since they'd be working against each other?

I think that the correct model to be envisioning is that of roll bracing.

The grating connection itself would need to be sized for that brace load, but I've never checked the grating in compression.

Per my previous response, I think that you're really working the grating in flexure. I wouldn't sweat the grating itself so long as the tread would be made up of a single piece of grating. It's just the connections of the grating to the stringers that I feel are critical.

That's what I was thinking, only the cross rods don't terminate at something solid, so you'd have the bearing bars essentially bracing eachother.

Again, I see the cross bars as roll bracing, not lateral bracing. Ergo no need for them to terminate at restraint.

 

[EngDM]

I think that the correct model to be envisioning is that of roll bracing.

Never heard this term before, but a quick google image isn't giving a nice visual. Is there another name this goes by?

 

[KootK]

Torsional bracing.