Moment frame bracing with steel joist
Moment frame bracing with steel joist
(OP)
I have a steel moment frame. The beam in the frame supports steel joists connected to the top flange only. Can I rely on the joists to brace the beam for the axial force it carries, or is my unbraced length the entire length of the beam?






RE: Moment frame bracing with steel joist
http://www.FerrellEngineering.com
RE: Moment frame bracing with steel joist
BA
RE: Moment frame bracing with steel joist
recent, extensive thread on the subject, with a great accompanying technical article link...
Like BA and connect are alluding to, in a moment frame your bottom flange will also be in compression, so you in effect will have two unbraced length checks to do. Your bottom flange may very well control your design and like BA said you will need diagonal kickers to brace it.
Always consider up-sizing the steel member to avoid bracing, steel tonnage vs bracing and stiffeners is very often more economical.
RE: Moment frame bracing with steel joist
If your joist connections can transfer axial (as connect pointed out), you can design your brace as a lateral stiff spring that must satisfy a minimum stiffness & a minimum strength to overcome 1% of the axial in the main member.
If the brace can satisfy these limits, then your main member under compression can be considered to have a smaller unbraced length.
Unfortunately, as BA & a2mfk pointed out, in a moment frame you will need to check both the top/bottom flanges...
RE: Moment frame bracing with steel joist
This also would hold true with negative bending on the beam for lateral torsional buckling.
One thing to consider to brace the bottom flange would be a kicker member (i.e. angles) to brace the bottom flange back up to the diaphragm. This is typical in the pre-engineered metal building industry to brace the bottom flange of the bents back to the purlin framing.
Nick Deal, PE, SE
Michael Brady Inc.
http://www.michaelbradyinc.com
RE: Moment frame bracing with steel joist
To address the OP, though, I don't think a joist connected to the top flange only braces the beam against compression. More importantly, it doesn't brace it for negative bending and that will be more of a problem than the axial bracing.
RE: Moment frame bracing with steel joist
If the member is subjected to large axial forces for some situations like conveyor belts with equipment loads, If I were the designer, I would rely on those secondary members in the bracing system as lateral restraints, and making sure a clear and simply load path of the bracing system from the major loads to the ground supports.
P.S. If beam subjected to large hogging bending moment, which bottom flange subject to compression bending stress, you could consider "fly-brace" to shorten your effective length in bending.
RE: Moment frame bracing with steel joist
RE: Moment frame bracing with steel joist
For lateral-torsional buckling (flexural), then yes, I would say that the joists provide restraint to the top flange. I believe I have had this debate with SEIT in the past regarding whether this detail provides lateral restraint or not.
RE: Moment frame bracing with steel joist
Have I ever said that this detail provides lateral restraint for axial load in the past (with only one flange braced)? If so, I must have had too much eggnog before I started typing.
RE: Moment frame bracing with steel joist