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.
@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.
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.
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.
@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.
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.
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.
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.
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:
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.
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.
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.
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.
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.
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.
