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Unbraced length of girders supporting a monorail

Unbraced length of girders supporting a monorail

Unbraced length of girders supporting a monorail

(OP)
I'm working on a frame right now that has 4 monorails on it. There are girders with S section monorails bolted to the bottom flange of the girders. I have these modeled as fixed moment connections but my question is about the unbraced length of the girder. Right now I am assuming the beam is unbraced across the entire length, even though the bottom flange is "brace" by the top flange of the monorail. Does this seem like a reasonable approach?

RE: Unbraced length of girders supporting a monorail

It's reasonable if:

1) You're confident that the connection between beams can couple monorail flexure to girder torsion and;
2) You evaluate the stiffness of the monorail bracing scheme for it's suitability as bracing. AISC has checks for that.

Note: I'm not especially well versed in industrial work.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Unbraced length of girders supporting a monorail

Sure, you can design it that way but since the monorails will impart transverse and lateral loads to the girders, you might want to provide horizontal bracing and struts (a load path) to bring those forces to the vertical bracing lines and to the ground. As a result, the girders most likely will have a shorter unbraced length depending how you detail the bracing.

RE: Unbraced length of girders supporting a monorail

(OP)
jike, right now I have some horizontal bracing at the column to girder connections. Is that what you are referring to? Adding more of this along the entire length? Or are you suggesting adding a braced connection at the actual intersection of the girder and monorail. The maximum axial force in the girders is 4.7 kips. The design margin on the moments is also pretty high. Based on this, I'll check. but I'm assuming the shear connections can transfer this load into the horizontal and vertical bracing. The max monorail weights are 3.3k, with a max horizontal acceleration of .31g.

RE: Unbraced length of girders supporting a monorail

I apologize....I did not see the horizontal bracing. I assume the bracing is at the top flange of the girder. I would probably add some stiffeners at each monorail connection.

The girder unbraced length should be 38 to 40 and 40 to 41.

RE: Unbraced length of girders supporting a monorail

(OP)
That's what I have for the unbraced lengths right now. I've added horizontal bracing in each bay along the longer section of the frame. I trying to balance economical vs. excessive horz bracing. I like the idea of having a continuous horz brace system. I've attached a screenshot of revised version. This will def take out more of the axial load in the girders that has to transfer through shear connection, but like I said earlier the connection should be able to take the interaction of shear and axial with these loads.
I'm also modeling it defining all braces as truss members as opposed to tension only because I get instabilities due to "leaning of structure" because eccentric loading. I get some failures of 2L4x4 angles under compression. adding more braces my help that but then again, not saving any money because I'm adding more connections and steel.

RE: Unbraced length of girders supporting a monorail

I'm curious Jike, why do you not feel that the monorail beams could be LTB bracing for the girders? The stiffeners that you recommended would satisfy my requirement for a suitable connection.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Unbraced length of girders supporting a monorail

(OP)
I will definitely add stiffeners. This is the connection currently. I might model the monorails as pinned to each girder, as this would take some of the torsion out of the girders. Right now there is a splice plate for moment transfer. I did think that the bolts would brace the bottom of the girders, but not sure about the top flange in compression.

RE: Unbraced length of girders supporting a monorail

Nice. Yeah, left to my own devices, I'd call that LTB bracing. Even more so because the dominant load will be applied at the bottom flange. I don't think that the monorail beam will directly (nodally) brace either flange. What it will do is provide rotational restraint to the entire section. And that's just what you want if LTB is your concern. You're probably thinking the same thing -- just a matter of semantics.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Unbraced length of girders supporting a monorail

(OP)
Regarding the fixing of the monorails, when I made them pinned between all girders, my max lateral deflection goes up .9 in under seismic LC. Max girder deflection goes from about .137" to .4". Instead of beefing up the girders, I might go back to the moment connection. In reality there is not a big difference I don't think. Either way, torsion will be induced by a load on the center of the monorail due to the vertical reaction. The bracing forces don't change much. What's your thoughts on this?

RE: Unbraced length of girders supporting a monorail

I don't like this last bit MF. Technically, there's nothing wrong with what you've proposed. I just don't care for the idea of using monorails as part of your primary lateral load path. Besides, if you proceed with that strategy, you'll have to deal with girder torsion design, girder torsion stiffness issues for drift, and awkward connections all over the place. It could turn into an accounting nightmare and time suck for you. There's also a pretty good chance that you'll screw something up do to the complexity of the load path. I know I would. Also, if building modifications are proposed in the future, it's unlikely that anyone will clue in to the fact that you've used the monorails as part of your primary lateral system.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Unbraced length of girders supporting a monorail

Now that I think of it, future renovations might also be a good reason to not use the monorails as bracing. For this kind of building, how likely is it that the monorail configuration will be re jigged in the future? I suppose that's a discussion to have with the owner.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Unbraced length of girders supporting a monorail

(OP)
I had similar thoughts. So I'm planning on just keeping the connections as fixed and designing the connections and splice for the reactions. It's also a stiffer structure then. I'm not sure about the monorail locations moving, still coordinating with building engineer.

RE: Unbraced length of girders supporting a monorail

Just so we're clear, I'm recommending the opposite mF. I don't think that you should count on fixity between girders and monorails to help stiffen and stabilize your building as a whole.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Unbraced length of girders supporting a monorail

(OP)
Oh, when you said thinking the opposite when I mentioned modeling the monorail pinned, I thought that's what you meant. My fault. I think I'm just over thinking this with the monorail connections. If they're fixed you basically have a horizontal moment frame taking some of the load.

RE: Unbraced length of girders supporting a monorail

Monorail would need to be continuous for me to count on it to brace the girder. I would also require a brace taking longitudinal force out of monorail rather than depending on the torsional strength of the girder.

RE: Unbraced length of girders supporting a monorail

(OP)
I agree that taking the longitudinal force out of the monorail with a brace is the best way to go. I've attached a picture of the frame. Right now the horizontal bracing at the top is between panel points to take the load back to the vertical bracing. If I change the bracing around a lot and brace at the intersection of the girder and monorail I end up getting a lot of bracing. I'm debating having the horz angle bracing bolted to the top flange of the monorails as it intersects it to engage the brace when the monorail is engaged longitudinally. This will provide bracing for the horizontal brace also so I can reduce the KL/r. The thing is this will induce some bending into the brace. I can do this or rework the bracing. Another thing I found when I reworked the bracing to have some of it connect at the girder/monorail intersection is that the lateral deflections under seismic loads went up considerably. THanks

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