Unbraced Length for Monorail Beam Supported by vertical posts

[machstruk]

Hi everyone,

I have a question about hoist beam and I have a situation that is similar to the one shown in AISC presentation (Design of Underhung Hoist and Crane Girders,

https://www.aisc.org/education/cont...erhung-hoist-and-crane-girders/#.XY90pkZLg2w)

in terms of configuration except that instead of engineered clamps(as shown in the attached picture) the top column post and brace is welded to the hoist beam.

The question is, what unbraced length of hoist beam should be considered for LTB for given configuration? Can the vertical post be considered to provide enough lateral restraint to top flange and that the unbraced length can be considered as distance between two vertical posts ? or should additional bracing (minor axis to hoist beam) be provided and linked back to the floor like done in the above mentioned AISC presentation with the joist system.

There are multiple posts as the hoist beam is continuous and the above mentioned AISC presentation recommends considering hoist beam bottom flange for LTB as it would be in compression where beam is continuous. Would the correct method for determining bottom LTB span be to calculate the distance from post (support) to inflection point and use that length as a cantilever beam with end conditions using the K coefficients shows in the presentation?

I have also attached a paper by Trahair that offers another method to calculate underhung hoists beam strength for various top flange lateral and torsional restraints. For the aforementioned situation, do you think that the vertical post would provide a rotational restraint to the top flange since the attached post has minor axis stiffness which can be effective in preventing rotation of hoist beam top flange.

Thank you in advance.

Best Regards,

https://files.engineering.com/getfi...-48a3-a4d3-ee9a0eccbdd1&file=Figure_Hoist.JPG

https://www.steel.org.au/Resources/...ing-of-monorail-beams_Usyd_R883/download-pdf/

 

[KootK]

1) With a welded post AND a web stiffer, I would say that you could definitely call each post LTB restraint for both sagging and hogging moments. This is, effectively, rotational bracing (roll beam) and the AISC manual has a section that can be used to evaluate the strength and stiffness of the bracing. Appendix 6 or something...

2) Without web stiffeners, there is the potential for an LTB failure mechanism that would be similar to web side way buckling beneath the vertical posts. Unless you can find a way to justify that the this failure mode doesn't present a problem, I'd be inclined to NOT rely on the posts for LTB. obviously, the crane trolley prevents you from being able to have web stiffeners.

3) Do you intend for these posts to actually serve as gravity supports for the monorail beam? If not, I'd be concerned that the connections to the vertical posts may produce that result inadvertently and that there could be undesirable fatigue issues etc associated with that.

 

[WARose]

Would the correct method for determining bottom LTB span be to calculate the distance from post (support) to inflection point and use that length as a cantilever beam with end conditions using the K coefficients shows in the presentation?

I have also attached a paper by Trahair that offers another method to calculate underhung hoists beam strength for various top flange lateral and torsional restraints. For the aforementioned situation, do you think that the vertical post would provide a rotational restraint to the top flange since the attached post has minor axis stiffness which can be effective in preventing rotation of hoist beam top flange.

A lot of what you are asking are things that need to be addressed by calculations. The posts might possibly provide rotational restraint for LTB.....but you have to do the calculations to figure it. (I'd be leery of this situation though. What is (essentially) a hangar providing rotational restraint?)

Another thing with the torsional brace only (with no stiffener), you have to be sure that the web of your girder has sufficient distortional stiffness for it to work that way. (Sort of like using a deck or a slab as a torsional brace. AISC gives guidance there.)

 

[machstruk]

Thanks for the replies KootK and WARose. There are no stiffeners at vertical post/ hanger to hoist beam connection.

I was thinking about this problem and I believe The engineer's reasoning about vertical post providing lateral restrain to top flange of hoist beam can be deemed not-satisfactory because

1) When hoist is between the two vertical posts/ hangers or say near one of the vertical post/ hanger, the lateral thrust by hoist (force perpendicular to length of hoist beam or force in the minor axis of the hanger/ post)will create torsion and would try to rotate/ twist the vertical post at the hoist to post connection. Since the hanger/ post is a w-section, it is not effective is resisting that twist and might rotation. That rotation will cause beam flange to move thus resulting is very limited to no effectiveness. Thus such configuration can't be relied to act as LTB brace point. Whats your take on this?

Some additional information: The hoist rated capacity is big/ actually huge, 60 tons. The hangers length can be considered as 1m. The attached sketch shows the detail and the solution that I am suggesting and there are no stiffeners whatsoever. Fatigue is not a concern as these would be used rarely. I agree with the statement that web need to have enough section distortion capacity if a hanger can act as an effective LTB restraint.

Thank you again and looking forward to your replies.

[URL unfurl="true"]https://files.engineering.com/getfile.aspx?folder=2b0d88b8-ca6c-41aa-a760-3ab8b17bea2d&file=IMG_5791_(2).jpg[/url]

 

[KootK]

I'm still not clear on what the gravity support mechanism is for the crane beam. Is it suspended from these tension hangers or is it supported from regular columns below/beside the beam that you haven't shown us yet? Your answer may have a significant impact on mine.

 

[machstruk]

KootK,

The same hangers/ vertical posts are used for gravity support as well. There are no other items providing support to hoist beam other than what is shown on the sketch in the post above.

Some additional information: At the location where the hangers/ vertical posts are connected with the deck beam, transverse beams are located exactly at hanger-deck joint or in close proximity that provide torsional restraint to deck beam at point of suspension of hangers/ vertical posts. The deck beam support grating.I have updated the previous sketch and the new one can be viewed at:
[URL unfurl="true"]https://files.engineering.com/getfile.aspx?folder=4629d6d9-e254-4083-83b6-e83af50e037f&file=Sketch_R1.jpg[/url]

Thanks.

 

[KootK]

The same hangers/ vertical posts are used for gravity support as well. There are no other items providing support to hoist beam other than what is shown on the sketch in the post above.

Thanks for the info. There is some inherent stability in any beam system where the loads and the support reactions are applied to opposite flanges and directed away from one another. When that is the case, those forces tend to undo any twist that are induced by other effects. The trick is coming up with a method to evaluate that benefit. Certainly, I don't know of an acceptable method for doing that. Lastly, the deeper the beam, the better for this stability effect. And a W36 is a pretty deep beam.

What is the spacing of your vertical hangers?

 

[BridgeSmith]

I'm having trouble envisioning how LTB is even possible with the configuration shown, unless the cantilever off of the left of the photo is long enough to present an issue. With the loading pulling down on the bottom flange and the support pulling up on the top flange, all of the forces are stabilizing the beam in the vertical position, aren't they? Am I missing something?

Rod Smith, P.E.

 

[KootK]

Am I missing something?

I agree that LTB is unlikely but, I think, not impossible. At least not mathematically. Join me in a mental experiment:

1) Replace the beam with a truss with slender chords. HSS3x3 etc.

2) Consider the truss loaded between hangers spaced at, say, 5X truss depth such that you had bottom chord compression at the hangers.

3) Can you say for sure that the compressed bottom chords below the hangers could not buckle laterally? If not, that's effectively LTB.

What is the spacing of your vertical hangers?

I asked this because, the tighter the hanger spacing, the more likely it becomes that you could get a more or less direct strut/tie from the load to the supports at which point it doesn't matter much what buckles where as you'd still have a viable, stable load path.

 

[machstruk]

Thanks for your comments KootK and HotRod10.

Some great comments, however, I think the choice of words by KootK has been excellent when he said:

The trick is coming up with a method to evaluate that benefit.

I do agree that the bottom beam provide stability. The AISC presentation that I have referenced in first post of this topic quantifies that affect. For cantilever type beams, the Cb value can be considered as 1.4 instead of 1 because of bottom flange loading. That is a 40% increase. However LTB can still happen and can't be ruled out.

With reference to the statement "coming up with a method to evaluate the benefit", my research on the subject topic concludes the following approach can be used to evaluate the unbraced length for LTB Limit State.

Trahair, in his paper, "Lateral Buckling of Monorail beams" (download link provided in the first post of this topic), provides a method for analysis and design monorail beam "supported by top flange only" (not compression flange). He considers two boundary conditions at "top flange" of beam (not compression flange). The conditions are lateral restraint (no movement) , and rotational restraint (his study considers "no rotation" of beam top flange for few configurations and for other it considers "rotation allowed" )at beam top flange.

He then evaluates different cases based on configuration of beams (cantilever, overhang etc), and rotational restraint conditions (rotation=0, rotation is allowed) etc.

To quote Trahair:

The double overhanging monorail beam shown in Fig. 3b(c4) has lateral restraints
only at the two (interior) supports, and therefore has no apparent torsional restraint.
While it is unlikely that such a monorail would ever be used in practice, it
nevertheless has theoretical interest because it is able to resist lateral buckling, (as
does the monorail beam shown in Fig. 3a(b2) and discussed earlier). This
resistance arises from the bottom flange loads, which will exert restoring torques if
the beam twists. For equilibrium, the resultant of these loads must act through a
point midway between the two supports, as is the case when neither load displaces
laterally.

Based on his research (He has done FEA modelling for all different cases), the takeaway I have is that monorail beam LTB can be satisfied by considering unbraced length of monorail beam as distance between top flange supports, if I meet Lateral and Rotational restraint requirements at beam top flange are satisfied and the requirements are that no lateral translation and no rotation should be allowed at top flange monorail beam support location. I would also like to mention that Trahair's work does consider the contribution of positive effects of beam bottom flange.

Now, with reference to the support arrangement that I have for the subject topic, since W Sections are used as hangers to support the monorail beam (Please see sketch attached with this post for W section orientation), and considering what Trahair is recommending, I think that the W sections posts don't act as effective LTB Restraint because they won't restrict:

1) Translation of top flange of monorail beam since the posts are approx 1.0 m long and suspending from deck.
2) The won't restrict the rotation of monorail beam top flange. The W Section Post has minor axis stiffness and it can offer some rotational restraint for monorail beam top flange, however, the hoist can produce torque/ twisting at the monorail beam top flange connection with hanger support location (as shown in my sketch), for which the W section has no stiffness and it can cause the post at that location to rotate in the plane of the flange, however, that can result in movement of top flange and perhaps rotation of top flange perpendicular to web (hard to quantify effects of deformation compatibility), or other things hard to quantify but I hope you guys are getting what I am trying to present.

My suggestion is that it is much robust put in a brace perpendicular to monorail beam at all post support locations like the one show in the attached sketch.

Additional Info: The posts are spaced at 1.25m since the load is very high.

The updated sketch can be viewed at:

https://files.engineering.com/getfi...b7d-47b4-8139-dfb7aea68ad7&file=Sketch_R2.pdf

Trahair's paper:

https://www.steel.org.au/Resources/...ing-of-monorail-beams_Usyd_R883/download-pdf/

Thank you very much for taking the time to read this post.

 

[machstruk]

So what do you think about the situation.