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lateral torsional buckling length for monorail
3

lateral torsional buckling length for monorail

lateral torsional buckling length for monorail

(OP)
Lb is L /4 here right not the total Length.
Just want to be sure.  

RE: lateral torsional buckling length for monorail

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.

RE: lateral torsional buckling length for monorail

(OP)
thanks a2mfk anything i can to here to make Lb = L/4

RE: lateral torsional buckling length for monorail

(OP)
sigh nobody is answering. maybe i should stop posting here already.

RE: lateral torsional buckling length for monorail

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.

RE: lateral torsional buckling length for monorail

I believe that normally crane runways are done as single span. If this was the case then your buckling length would be l/4.

 

RE: lateral torsional buckling length for monorail

OP:

"i wonder why the auther said Lb is usually L in the comment section of simply supported."

Bootlegend gave you your answer.   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: lateral torsional buckling length for monorail

(OP)
monorail is around 50 feet. so i need intermediate supports.

i've been reading about a thread regarding continuous beam if it's considered braced at point of inflection.  

RE: lateral torsional buckling length for monorail

OK...  So break the monorail at the intermediate supports...

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: lateral torsional buckling length for monorail

(OP)
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"

RE: lateral torsional buckling length for monorail

delagina,

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.

RE: lateral torsional buckling length for monorail

(OP)
i changed it to welded connection stub column to top flange monorail and end plate moment connection stub column to bottom flange supporting beam

will that help?
 

RE: lateral torsional buckling length for monorail

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.

RE: lateral torsional buckling length for monorail

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.

RE: lateral torsional buckling length for monorail

(OP)
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.

i find this hard to justify to a non-engineer.

RE: lateral torsional buckling length for monorail

(OP)
bootleleg,

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.  

RE: lateral torsional buckling length for monorail

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.

RE: lateral torsional buckling length for monorail

(OP)
thanks bootleleg,

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.

RE: lateral torsional buckling length for monorail

Your drawing appears to have one span of L/4 and two spans of 3/8L.  Is that an error or am I missing something?

It seems to me that, because the load is hanging from the bottom flange, the unbraced length of the top flange cannot be worse than 3/8L.

BA

RE: lateral torsional buckling length for monorail

BAretired,

That looks like a decent guesstimate of the other spans.  I agree that the top flange is connected at each hanger, but it isn't prevented from twist or relative movement from the bottom flange anywhere along the length.  Think of it this way:  If the hangers were columns supporting the beam from below, what would Lb be then?  It would be unstable as detailed assuming pinned column to beam connections  .You would definitely need full depth stiffeners or bracing of some sort at each end as a minimum.

The question is does the situation change when the beam is hanging and is loaded from the bottom flange.  To me it seems that the beam is in the position of lowest potential energy, so how would it laterally buckle or twist to a position of greater potential energy?  I have usually just worked my way around the problem rather than do what the OP is trying to do.

I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.

RE: lateral torsional buckling length for monorail

@bootlegend, I tend to agree, but think of the load in the middle span, there is energy in the adjacent spans, that is where the bottom flange is in compression, those spans would like to move to a lower energy position. If they did, the rotation at the supports would allow the middle span to deflect more, but slightly sideways, reducing the total energy. And without having to go through a higher energy position to get there.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: lateral torsional buckling length for monorail

(OP)
@BA retired.

total span is divided into 4 spans but not equally. it depends on the location of existing beam support. i just draw it like that but that, but the question was Lb, if it's one span or total span.

RE: lateral torsional buckling length for monorail

Full depth stiffeners would be nice, but maybe they would interfere with the rollers on the underslung crab.  Partial  depth stiffeners would be better than nothing.

BA

RE: lateral torsional buckling length for monorail

(OP)
can you explain how the stiffeners will help in reducing Lb in this case.

thanks,

RE: lateral torsional buckling length for monorail

@paddingtongreen
Yes, I agree with the situation you've described.

@BAretired
I was just pointing out that if this was a typical beam (forget the trolley) you would need some bracing/stifffeners.

@delagina
Picture a typical beam crossing a column.  If stiffeners are provided and sized appropriately and the beam bottom flange is attached sufficiently to the column, the beam will be unable to twist that support.  Are you working with AISC code? If so read the 13th edition section J10.7 on pg 16.1-120 concerning unframed ends of beams and girders.
 

I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.

RE: lateral torsional buckling length for monorail

(OP)
@bootlegend, thanks.

yes i use AISC.

if i can do the full depth stiffener, does it solve the twisting problem so i can use Lb as one span.

because you said in one of your posts you said my case is relying only web stiffness. does having full depth stiffener on each span support help with this.

RE: lateral torsional buckling length for monorail

If the beam is prevented from rotating at each hanger, Lb cannot be more than the length between stiffeners for either flange.  The lateral bracing system must be capable of resisting 2% of the axial force in the flange according to code.

BA

RE: lateral torsional buckling length for monorail

delagina,

What the web stiffeners do in this case is create a cantilever to resist the lateral buckling force. In this case the cantilever is fixed at the top with a pair of bolts that provide the force couple.

RE: lateral torsional buckling length for monorail

You could just make it of separate spans, with just enough clearance to handle the maximum temperature, environmental not spike. This could be adjusted at installation with friction bolts.

There was a thread on monorail splices, I don't know if it will help:
http://www.eng-tips.com/viewthread.cfm?qid=282933&page=2

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: lateral torsional buckling length for monorail

In the existing monorail, the top flange is braced at every hanger, so Lb is the distance between hangers.  

The bottom flange is not laterally braced anywhere other than by the web.  If all spans were simple spans, they would work similarly to a lifting beam suspended from the ends and loaded below the bottom flange within the span, a stable arrangement without any lateral bracing because gravity prevents the ends from rotating.

Because the monorail is continuous, negative moments try  to form at each end of the loaded span.  They tend to compress the bottom flange in the two adjacent spans as noted by paddingtongreen.  If they buckle, there are no consequences other than to reduce the negative moment.

This suggests that if each span is capable of carrying the load as a simple span, the neighboring lower flanges can buckle without consequence. When the load moves to the next span, the same argument holds true.

I believe that is why the original monorail performed satisfactorily.  Lower flanges in the unloaded spans may have buckled on a regular basis, but it had no noticeable effect because the loaded span was stable.

BA

RE: lateral torsional buckling length for monorail

BAretired,

yes possible, but not a very good situation for fatigue e.t.c.

RE: lateral torsional buckling length for monorail

csd72,

I agree and I am not recommending the existing detail, but it is useful to know why things work.

BA

RE: lateral torsional buckling length for monorail

(OP)
thanks, as i said this will be used once every year or few years so it should be ok.

RE: lateral torsional buckling length for monorail

By my code (AS4100) if the tension flange is restrained for translation and twist, the compression flange is then 'partially restrained'.
For bottom flange loading  lb is then equal to the distance between restraints, and the original monorail is a legitimate design (by AS4100).
Fatigue analysis is a separate exercise, but doesn't seem to be an issue here.
Either AISC is conservative, or AS4100 non-conservative.

RE: lateral torsional buckling length for monorail

Out of plane it looks unstable. Like a swing.  

RE: lateral torsional buckling length for monorail

I'd like to call a 20 second timeout here if I may....

delagina- if you have not designed crane runway girders before or even if you have, I highly suggest ordering or downloading CMAA (Crane Mfr of America) Spec #74, it should have been my first response to your post.

Also, this is a good thread:
http://www.eng-tips.com/viewthread.cfm?qid=44818&page=605


Maybe you are on top of everything and just have this cont. girder issue, so I won't go into it, but much of it is not typical AISC building design stuff. Last but not least is the local bending of the flanges from the wheel loads which are some really fun equations (headache time unless you love mechanics of materials) and combined stresses of local bottom flange bending and global bending.

Also, crane customers typically order a crane capacity well over what they need. So just because the other crane monorail girder is working fine, doesn't mean it would not fail if loaded to capacity. Lots of existing buildings don't meet code but still stand up. That's a road you can understand (like BA explained) but you don't want to go down with laypeople.



  

RE: lateral torsional buckling length for monorail

I think you said this will only be used occasionally but use caution with cont. crane girders:
http://www.aisc.org/DynamicTaxonomyFAQs.aspx?id=1648

I have attached a sketch of what I think the solution would be, more out of my own curiosity if I am right than anything. Buckling of an adjacent unloaded span seems so unlikely but... Did I do this right guys?

It also occurs to me that since your buckling will occur at or near mid-span (I think), you could use a reduced moment, but I could not find AISC explanation on this. Not a common phenomenon to say the least.

Using the unbraced beam moment charts, it seems for 5 ton and a 50 ft length divided into 4(?) spans you will have a lot of options if you used M2=Mmax and say L2=Lu<15ft.

 

RE: lateral torsional buckling length for monorail

(OP)
thanks a2mfk,

i'm using a good excel program which also checks bottom flange bending based on simplified and CMAA (see my previous posts above). i also have a copy of CMAA 74.

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