What Constitutes Beam Bracing

The two common bracing forms that I would expect for this:

1) Horizontal bridging lines if there are a bunch of beams in a row requiring bracing.

2) Single X-brace beside the beam if there's only one beam needing the bracing.

3) Roll beams. Short beams connecting the beam to be braced to one of it's neighbors. In this instance the connection between the roll beams and the beam to be braced are envisioned as moment connections coupling strong axis flexure in the roll beams to torsion in the beam to be braced.

SteelPE said:

If this is an option, what would this bridging need to be anchored to at each end…

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Usually:

1) terminate the bridging at a CIP/CMU wall, steel post, or something of the sort that is thought to provide axial restraint to the bridging line.

2) install cross bracing intermittently along the bridging line to axially brace the bridging against the roof deck.

SteelPE said:

..or would the simple act of tying all of the beam together at the bottom flag solve the LTB issue?

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This is not normally done. Rather, it's thought that each braced beam contributes more stability demand to the bridging line along it's length since, theoretically, they could all buckle in the same manner (CCW or CW) and accumulate demand. Whether or not that's statistically probable is, of course, a matter for debate / stochastic analysis.

If you had purlins on top I’d normally do a simple fly V brace from bottom flange to purlin to resoove my uplift LTB woes.

In this case it doesn’t sound like you have purlins supporting the metal deck? So perhaps a line of herringbone bridging along the entire length fixed back to a column each end?

Have you a sketch showing the layout?

Good call with the fly bracing which is, effectively, another form of roll beam bracing with the moment connection that I mentioned formed by the triangulation of the fly brace. It doesn't sound as though we've got joists sitting on these beams but, when there are such joists, it's common practice in North America to run little angles from the bottom flanges of the girders up to the top chords of the joists. It's pretty much the same thing as fly bracing.

Interested in this topic specially beams "braced" by channel/wide flange diaphragm without horizontal brace.

Say you have a steel platform on grating 20' in length and the 20' beam has intermediate beams every 5', would that be considered braced at 5' without horizontal bracing?

Looking at some publications of this topic like in bridge girders design, I don't need the horizontal brace. The channel/wide flange "diaphragm" may need stiffeners though so it's full depth of the girder/beam it's bracing.

Kootk, on roll beam bracing, if the beam is say W16 and the roll beams are W8x18, is that considered braced or need stiffeners for the "moment connection coupling" to work.

AskTooMuch said:

Say you have a steel platform on grating 20' in length and the 20' beam has intermediate beams every 5', would that be considered braced at 5' without horizontal bracing?

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Yeah, it would be considered as bracing in my book so long as you tended to the details including, but not limited, to:

1) Depth of roll beam at least 60% of depth of girder. AISC has something to say about this but I don't recall the specific value.

2) Stiffness of the roll beam and whatever supports it in accordance with AISC appendix 7.

3) Roll beam to girder connection nominally a moment connection. We normally consider full depth double angles, shear tabs, etc sufficient.

KootK said:

2) install cross bracing intermittently along the bridging line to axially brace the bridging against the roof deck.

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I talked to another engineer about this and he seemed to agree with this statement (even before you said it). My only challenge to this would be from a open-web steel joist standpoint. I don't ever recall there being intermittent cross bracing on these systems. It seems they just anchor the bottom chord bridging to the top of the last structural member and call it a day.

FYI, this whole exercise is resulting from my steel fabricator clients telling me that open-web steel joists, which used to have a lead time of 6 weeks, having a lead time of around 10-12 months. I am starting to hear this from multiple clients. So my clients are looking for an alternative solution..... So we are looking at using steel beams spaced at 10'-0" o.c. with 3" steel decking. I am not exactly happy about it, but the numbers seem to work.

SteelPE - all the OWSJ systems I've designed have either had cross bridging or horizontal bridging with an X one joist bay off of the wall. The all have to tie into the wall or some other competent structural member to continue the load path. For really wide roof systems, I've had to put intermediate X's in to keep the cumulative bracing force down to reasonable levels for the little 1.5" and 2" angles. These serve for erection stability for floor systems and bottom chord compression bracing for roof systems (though they usually require an extra line or two to achieve it).

With spans that wide between beams, you probably need to either a) choose a beam size that precludes LTB altogether (nice, wide flange) or b) run beams of the same depth but as light as you can get them between at mid span or 3rd points and anchor them to the wall at each end and tie the diaphragm into them. That'll restrain your beams and potentially keep total weight of steel down, though it could add a good amount of fab time for all the connections.

You gotta coordinate with the OWSJ guys a bit. I've seen them use the bottom chord of beams as tie off for the bridging on occasion. That's fine in some circumstances but not when you're meaning for that same bridging to brace you beam.

If we're talking about beams as infill framing without OWSJ, I like bottom flange bridging with a cross brace every fourth spacing or so.

phanENG said:

With spans that wide between beams, you probably need to either a) choose a beam size that precludes LTB altogether (nice, wide flange) or b) run beams of the same depth but as light as you can get them between at mid span or 3rd points and anchor them to the wall at each end and tie the diaphragm into them. That'll restrain your beams and potentially keep total weight of steel down, though it could add a good amount of fab time for all the connections.

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Why couldn't you just run horizontal angle bridging from bottom of beam to bottom of beam with anchorage of the lightest W section occasionally along the length of the bridging run?

KootK said:

If we're talking about beams as infill framing without OWSJ, I like bottom flange bridging with a cross brace every fourth spacing or so.

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Yes, this is what we are talking about...... but why doesn't the joist manufacturer require the same cross brace?

SteelPE said:

Yes, this is what we are talking about...... but why doesn't the joist manufacturer require the same cross brace?

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They do. SJI specs give similar bracing rules for their products as we've been discussing here. If the single angle kicker that you mentioned works in compression as well as tension, and the load path is complete, that can replace the cross bracing.

SteelPE said:

Why couldn't you just run horizontal angle bridging from bottom of beam to bottom of beam with anchorage of the lightest W section occasionally along the length of the bridging run?

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It's possible. I suppose if you keep it as tension only, it may work out okay - L/r<300 puts you into an L2.5x2.5 at least. That's not too bad. Compression will work, too, but the angle will get a lot bigger - L5x5 or so depending on the required brace force and stiffness.