Well, the wood joists on necessarily going to be resting on the bottom flange. Usually the top of joist is same of the top of beam and beam is much deeper than the joists.
Negative moment where continuous over a support, as well as uplift. If I a good detail to brace the bottom flange, it would result in some significant steel savings.
If you could tolerate it architecturally, you could install 2X4 or greater diagonal wood braces at 4 to 8 feet on center, each side of the beam, nailed off to the floor joists and nailed to a welded-on tab plate at the bottom.
An alternate would be to attach an inverted channel to the bottom flange of the beam.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
The same detail is used a lot in crane rail top flange stiffenings.
The effect in this case is to lower the neutral axis, effectively lowering the stress to the bottom flange, and increasing the allowable unbraced length.
The older codes such as the 1949 AISC Steel Design Manual has a table on page 117 of various sections with different channel sections showing the revised section properties.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
Bottom flange restraint at the support can be achieved by providing continuity between column and top flange. Typically this is achieved by 4 bolts or welding to top of column and web stiffeners to beam.
The trouble with the added channel is the addition of steel and fabrication costs, it may be cheaper just to use a bigger beam.
I can't see why the 2x4 brace option doesn't work as well with pre-engineered wood floor trusses. What's the problem?
Well, if the detail is going to economical the plates will need to be shop welded to the beam, thus setting the location of the braces. The locations of the pre-engineered trusses will need to accommodate this.
I guess I could specifically set the location of the braces ahead of times so the steel fabricator and truss supplier are on the same page.
Are there any special design considerations for the truss or joist that the brace is connecting to? I guess they should be able to safely handle the point load requirements for bracing?
If wood floor joists are dimension lumber, the top of joist should be higher than the top of steel beam to allow for shrinkage of the joist. Placing them flush will result in a continuous hump in the deck over each beam.
It is preferable to place joists or truss shoes on a wood nailer on top of the beam. This provides lateral bracing to the top flange. The bottom flange could be braced using diagonal braces, but the connection from brace to steel beam gets messy.
If the underside of wood truss is about the same as the underside of steel beam, you could use bottom chord extensions attached to nailers on the bottom flange of the beam.
Bracing members placed each side of the beam and fastened to the joist/truss will change the performance of the joist/truss from a simple span to a continuous span because the brace prevents free rotation of the end of the member.
I tend to agree with those who suggested increasing the beam size to function adequately without bottom flange bracing.
I worry about this too. The attached detail is one that I used in the past. The situation was a little different though. I was dealing with a simple span beam and was trying to address torsion resulting from eccentric loading.
Be careful to ensure that, whatever system you go with, the wood bracing and connections are stiff enough to truly brace the steel beam. Brace strength really isn't the issue. This stil comes down to judgement in many situation though.
I love the idea of making the column continuous through the beam (stiffeners, bolts, etc.). Unfortunately, that system has failed in the past and I know of no good way to assign a capacity.
That's an interesting detail that I may use in the future. I guess that strap can take a little compression for that small length. Also, how easy it for them to field punch holes in the web of a beam??
I'm not so sure about the compression loads Abusement. I was kinda hoping that you'd have framing on both sides of the steel member so that you could use tension only bracing on each side.
I was out in the field reviewing another company's work a while back and saw something neat. They welded a continuous angle to the web of the steel beam for the wood framing to rest on. The angle leg pointed downwards. They then coped the wood framing a bit to get it above the top of the steel to allow for shrinkage.
Depending on the thickness of the angle, perhaps you could drill a screw up into the wood framing from below to provide lateral support to the beam? Alternately, maybe something like a Simpson hurricane tie could be used to connect the angle to the bottom of the wood member?
