As you have it drawn and I understand it, its a 4 span continuous span, so you will have negative moment at the span supports closes to the load point. Unfortunately that puts your negative flange into compression, and if its a crane monorail running on the bottom flange you cannot brace it. I would think you'd need to:
-check your top compression flange for worse case positive bending moment at L/4
-bottom flange unbraced for L
You may end up with a stockier section which may also be advantageous for the local stress checks you will have to do for the crane wheels which I assume are going to roll on the bottom flange. Of course, depending on your system... Remember for many cases an increased WF section is better than bracing or reinforcing the beam.
First off, does it need to be multiple spans? Can you use a larger beam so that you will only have positive bending? Then you could come up with some bracing for that top flange pretty easily. It all depends on the magnitude of loads and spans.
I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.
i dont want to break the monorail at support. because i will only have 2 bolts as supports per end of monorail at support.
anyway, this what i got from continous beam design per salmon and johnson
"in continous beam, the point of inflection has often been treated as a braced point when design equations did not provide for the effect of moment gradient"
I wouldn't count on that even in a beam that is definitely braced at the support points. What you are looking at is a beam that has essentially no torsional brace points if the bolted connections are considered pinned. In other words it is unstable as a model. You can reference Tanner's "Allowable Bending Stresses for Overhanging Monorail Cranes" from the 1985 AISC journal, but even that example counts on one torsional brace point.
The thing I have questioned at times is whether it matters as long as your trolley/crane is hanging from the bottom flange. As in your case, if the trolley hangs from the bottom flange, where is the beam going to laterally buckle to? It isn't the same case as when you have a compression load on the top flange of a beam that is supported from below. In that case I can see that the beam will roll over. But what happens in your case if the buckling stress is exceeded? I'm not sure. So I have always provided bracing somehow.
I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.
You did not mention the capacity of the monorail or the frequency of use. The detail we have used in the past is:
1. Break the monorail into multiple simple spans.
2. Add a web plate splice to so the load can be shared by the other pair of bolts in the 4 bolt hanger connection.
3. Do a partial pen weld and grind smooth the bottom flange only, so that the trolley does not have to jump the gap between monorail sections.
4. Brace the hangers and/or top flange of the monorail for your desired unbraced length.
Technically your monorail bottom flange is still not restrained from moving relative to the top flange doing what you have done. All you have to count on is the monorail web stiffness out of plane.
I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.
so adding web plate splice and welding the bottom flange of 2 monorails is suppose to have lighter section due to buckling than a continous beam even if both are supported basically the same way.
thanks for that info.
considering this is a 5 ton monorail. frequency of use once every few years, maybe monorail web stiffness out of plane will be ok. lateral force is just 10% of load. thickness of web is 0.4 in.
If you can get hold of the AISC journal article I mentioned above you'll read that the author states that the out of plane stiffness is small, hard to model, and generally unaccounted for. That is why I don't use it, and probably most other don't either. The article is 25 years old, but I don't know of anything else presenting a new argument.
Justifying things to a non-engineer is often times harder than the engineering you are actually doing.
I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.
the thing is there is also an existing continous monorail typical to what i am doing and is doing fine. new monorail i am doing will run parallel to it.
it will only fail if i consider 50 ft Lb.
that makes it harder to justify since the existing is doing fine.
