Bottom Running Crane Runway - Unbraced length? 4

[SeanMD]

I am having trouble determining the proper unbraced length to use for a bottom running crane runway.

I am looking at basically two types: continuous runway with top flange hanger supports at 25ft oc, and 50ft runways (2 span continuous) with top flange hanger supports at 25ft.

Runways are attached/hung from roof framing, and are not "braced" against rotation. Only the top flange is braced, and the web is open to allow the crane bridge truck wheels to pass. Bottom flange is not braced.

Obvious problem is reverse curvature bending, the bottom flange is in compression at the hanger locations.

The way I understand it, LTB brace points really need to prevent rotation to be effective. In this system I have a lateral restraint at the tension/top flange at the hanger location from the roof framing, which is not a brace as it does nothing to keep the beam from rolling.

This is a condition where the trolley wheels at the bottom flange (pulling down on the bottom of the beam) and the hanger loads (pulling up on the top of the beam) will tend to offer resistance to beam rotation, they will help to keep the beam straight.

A Cb modifier of 1.4 could be justified to increase the Cb factor for bottom flange loading, etc, but to apply a Cb factor at all you need to know the unbraced length.

I want to say it is 25ft, but am not sure. A colleague suggested considering inflection points, and I know that practice is no longer approved. Interestingly the 2017 AISC web webinar "Design of Underhung Hoist and Crane Girders" suggested: Must consider LTB of girder bottom flange at supports. Treat
similar as a cantilevered condition. Must locate inflection
point

Obviously the system works, as it is very common in industrial facilities. Can anyone point me to a reference on how to determine the correct unbraced length?

 

[Ingenuity]

Not a direct solution to your unbraced length question, but by way of reference for an alternative method:

"Lateral buckling of monorail beams" by N.S.Trahair in Engineered Structures; Volume 30, Issue 11, 2008 pp 3213-3218: Link

It is a fee-based download.​

Trahair covers two-span continuous monorails (bottom-running cranes), as well as single span, cantilevers, and overhanging (cantilever with back-span).

His approach is essentially an elastic buckling analysis, using simple closed-form approximations (backed up with FE simulations). He does NOT consider distortional buckling in this paper. It has a worked example of a 2-span, top flange supported, bottom flange loaded, monorail, of WF (I) section. It has a Australian code reference (Trahair is a retired prof from Uni of Sydney) but his approach is somewhat 'universal' for EC, AISC etc codes:

The graphic symbol legend is:

 

[SlideRuleEra]

The University of Sidney, who sponsored the paper Ingenuity referenced, has made it available for free download from their website:

"Lateral Buckling of Monorail Beams", by N.S. Trahair

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dik]

I did a long cantilever monorail a couple of years back... and used the provisions of Eurocode to design it... Current codes, here, do not make allowance for load location relative to the shear centre. Thanks for the info Ingenuity and SRE.

Dik

 

[SeanMD]

Thank you Ingenuity and SRE, that is an excellent reference and will go on my bookshelf.

For this problem, I neglected to add that these are very long runways, total crane runway lengths exceed 1000ft. So I cannot account for any torsional resistance at the extreme ends, as it would be ineffective in the middle.

Sean

 

[Ingenuity]

You are welcome.

Trahair also has a paper entitled: "Lateral-Distortional Buckling of Monorails " - free PDF here: Link - that you may be able to 'play with' to account for a (slender?) web without end restraints.

That is a long crane runway!

 

[WARose]

Two excellent papers that deal with this are:

'Distortion Buckling of Steel Beams', Structural Engineering Report No. 185, University of Alberta Department of Civil Engineering, By: Essa & Kennedy, April 1993.

'Buckling of Suspended I-Beams', ASCE Journal of Structural Engineering, Vol.116, No. 7, July, 1990, by: Dux & Kitipornchai.

 

[Ingenuity]

I did a long cantilever monorail a couple of years back... and used the provisions of Eurocode to design it...

dik: A cantilever to Eurocode probably had Nethercot (and maybe Trahair) as the primary author/s of the code provisions.