KL/r < 200 for Bridge Crane Girders?

[StructEngineer90]

The longitudinal loads applied to bridge crane girders from ASCE 7 means that the girder will experience some axial forces. In this case, how important is the KL/r < 200 recommendation?

 

[JoshPlumSE]

I've always thought of the KL/r < 200 as a practical limit on slenderness. If it's more slender than that you m might get it to work on paper, but it's going to be so slender that it's going to look kinda crazy. It's going to be hard to construct. It's going to be easy to damage.

Sure, it can be done. But, I'm not sure it's worth the headache.

 

[BridgeSmith]

Kl/r < 200 is the slenderness limit for tension members (in AASHTO, anyway). The limit for the slenderness of members subjected to compression is far more restrictive (Kl/r > 120 requires special buckling analysis). It's hard for me to imagine a configuration where the girder will not be subject to axial compression as well as tension.

Granted, it's hard for me to imagine a girder (a primary bending member) that would be slender enough for any Kl/r limit to control the size. I would expect lateral torsional buckling to be far more critical than the slenderness limits.

 

[Ron247]

I still use the AISC 9th edition. It has the compression limit of 200 but says it "should not exceed 200". It further states if you do exceed it, you are limited to the Fa equation where your kl/r exceeds Cc.
Tension members are 300 and as I recall that was mostly for the ability to erect them without them acting like cooked spaghetti.

I have never exceeded it except some minor amount like 203. I am not sure about the wording in newer AISC codes.

 

[DCBII]

I'd never recommend exceeding KL/r of 200 for any member in compression, even a member only lightly loaded in compression. It's a user note in the code rather than an outright requirement, but still, that's getting pretty slender. Your bridge girder may take more than just crane side thrust. It may inadvertently act as a strut between the two rails when the building sways.

For tension members AISC recommends L/r <= 300.

 

[canwesteng]

I've definitely exceeded it for lightly loaded monorails - S sections are set up so you almost always fail slenderness in normal applications. For a crane girder you should be using a W section, so I imagine it probably does look a little slender in practice. I'd be more hesitant with top flange loading on a bridge crane girder to be honest, but that's just a judgement call.

 

[glass99]

When we put glass bearing walls into compression to hold the roof up, the slenderness ratio is approx 240-320 depending on how you count (2" thick glass one ply broken, 16ft tall). My TKTS booth in Times Square was built this way in 2008, and a lot of the Apple stuff including the Jobs Auditorium in Cupertino. Also glass fins are about the same, albeit in bending (40ft span, 1.5" thick).