Differential Deflection of Crane Runway Beams 1

[StructuraHereToLearn]

Hello all,

I was hoping someone can shed some light on the topic of differential deflection of a crane runway beam for me. I have a situation where the runway beam on one side of the building is spanning 28' and on the other it is supported by a 130' clear span truss. These runway beams are about 90' apart from each other. I have seen some info stating that the differential deflection for a bridge beam span of 90' should be no more than 1/4". For the one side, the truss, the mid-span is where I am seeing the max deflection and at this point the adjacent runway beam is supported at a column. The way that I am seeing this is that at this point I can only allow my truss to deflect 1/4".

Any thoughts on this? Am I being to conservative with the 1/4" maximum?

Thanks in advance!

 

[racookpe1978]

1/4 inch? Or 1/4 of one degree (out of level) for the crane girder difference from side to side?

 

[StructuraHereToLearn]

1/4" is what I have seen.

 

[paddingtongreen]

Start by being real, you cannot limit a 130' truss to 1/4" deflection. The limit, from the crane's point of view, is controlled by the its ability to climb a hill. This is sufficiently special that the crane manufacturer must be involved in the decision. If it is too early to go to just one, survey several. Normally, the manufacturer expects the rails to be level and only worries about going up hill on his bridge girder, you are going to add quite a wrinkle to his design process.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[StructuraHereToLearn]

Paddington,
Thanks for the comments. I realize that attempting to limit the truss deflection to 1/4" is unreasonable but I wanted to get someone else's take on the differential deflection issue. Oddly enough I spoke in detail with the crane manufacturer's rep and they were no help. What you have stated makes sense and is much appreciated.

 

[Brad805]

This seems like it could be a case where it would be best if the crane supplier's scope of works includes the crane rail. Companies like Kone are more than capable to supply and install both. At this point I suspect the most important aspect for you is to know the height requirements. We have a related problem in many of our buildings and we request quotes early on to determine the height requirements. Suppliers will provide sections with their proposals that are sufficient for us to complete our job.

 

[FLCraneBuilder]

Start with - What is the anticipated deflection of the truss under load? For discussion purposes, use the AISC limit (for crane runways) of L/600 as a ball park. That will yield in the realm of 2-1/2" of deflection in the truss.

Next assumption - assume crane is in center of truss , and there is a column on opposite side directly across from truss ..... Crane would be "tilted" 2-1/2"..

I suspect this would cause the crane to "Creep" towards the lower end (of tilt) as it rolled - until the wheels flange out - thereby creating the REAL problem that will be encountered - wheel flange wear. This is a real problem and costly to repair when it happens...

Possible Solutions:
1) Obviously determine the real deflections and variances in elevation.
2) Possibly set the runways directly opposite the truss center lower so that the cranes "tilt" will be reduced.
3) Put some hefty side guide rollers adjacent to the wheels on the side where the wheels are expected flange out, but than you've got lateral forces into the rail to deal with.
Note - I do not know how to calc this force, but I see its existence
4) This may be an application where tapered tread wheels will resolve the problem. There are still a few people out there who know how to apply them. And probably not Kone
5) The capacity and duty cycle of the crane will have a significant effect on this. - is this crane used 10 times a year or 10 times an hour 24 hrs a day?
6) It should be noted that crane's usually have no problem gong up hill a bit, so long as both sides go uphill at the same rate. I say this because I suspect the truss will be cambered quite a bit (which is creating another aspect of this problem). CMAA limits runway elevation variance to a fairly small amount, but we have successfully gone uphill 7" in 37 ft on a fairly heavy service cranes. I am working with he CMAA folks to revise this limit...