Top Loaded S-section, Supported Solely on Bottom Flange

[DickHallorann]

Hi all,

I'm analyzing a steel frame structure that is used to support a crane system. The two S-sections that act as tracks for the crane trolley are two-span continuous, and are fastened with bolts to the top of the columns. This is the only type of support given to them (top flange is completely unbraced). I believe this setup will have an effect on the unsupported length during analyses, but I'm having difficulty being able to define that effect. Galambos' research "Guide to Stability Design Criteria for Metal Structures" addresses this issue, however only in a cantilever system. Has anyone else encountered this type of system or have an idea where to find the appropriate resources needed to solve it?

Thanks

 

[hokie66]

As you described it, it sounds like this violates all the principles of design of crane runways. Is this an existing installation? How did it come to be? Why are you analyzing it now?

 

[BAretired]

Doesn't sound right to me either. Can you introduce some stiffening now?

BA

 

[JStephen]

I'll suggest an approach. Suppose the beam is supported vertically at the bottom, and has a separate lateral support at the top flange. What strength or stiffness is required in that lateral support in order to qualify as a "support" for the top flange? Seems like I have seen recommended that it should be good for 5% of the vertical load, or something along that line. If so, check if a section of the web of the beam can be treated as a vertical cantilever and satisfy this requirement.

S-sections have heavy webs, so it may be more feasable with an S-section than with a wide-flange beam.

Railroad rails are supported only on the bottom flange, so that installation is not unknown.

 

[ToadJones]

THe only crane runway I can imagine not having any lateral support at the columns to the runway girders would be a freestanding runway system. However, these usually have some sort of diaphragm or brace from the web of the girder to the columns (usually A-Frames).
Not having any lateral support for a crane runway beam is a very bad thing. The side thrust lateral loading on the girders from the crane needs to be resisted and taken into the support structure. If there is no top flange bracing, the lateral force from the crane will transfer to the top flange of the girder (weak axis) and cause local bending in the web of the girder. This can cause the web of the girder to crack usually near the web toe of the fillet.

 

[DickHallorann]

That is correct, it is an unstable setup. It was thrown together more than 50 years ago by someone who was not an engineer, and it has been in use this whole time. The frame's purpose is to support the hoists that lift stopgates out of one area in a hydro dam. I am analyzing it as part of a safety inspection.

 

[BAretired]

That is amazing.

BA

 

[DickHallorann]

Yes its quite the interesting situation. Here is a link to a picture of one of the rails. Is there a way to analyze this even?

http://imgur.com/pbtl6

 

[dhengr]

Interestingly enough, the system was probably not built with no engineering involvement, or tech. and mechanical savvy, on that kind of a project. It certainly would appear to show that many of our structures have considerable reserve strength, and don’t always act the way our current codes say they must. A few sketches of the actual conditions would be most helpful if you really want some serious discussion on your problem. Some info. on loads and how they are applied, span lengths, member sizes, etc. etc. would be helpful too. This is probably not a high cycle situation, nor highly dynamic as a crane in a high production operation would be. At the least, you might want to add web stiffener plates over the columns and check the bolt connection at the top of the column. In affect, making the stiffeners and beam an extension of the cantilevered column to take some lateral loading. The beam web, alone, is not a good way to take this kind of loading, although it’s worked for 50 years now.