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Re: Crane Runway Bracing Vibration

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Gary,

I do not consider myself an expert on vibrations but I do have some thoughts on the subject.

It seems to me that if you have out-of-plane vibration you must also have some out-of-plane forcing function. It might be a good idea to try to find that forcing function. Some possibilities include:

1.) Uneven drive wheel forces which would also cause racking of the bridge, hence, horizontal reactions of the runway beams. If this happens I would expect unanticipated deflections at the top of the columns. It would also mean that the problem is mechanical rather than structural.

2.) Eccentricity of bracing loading could cause bending of the bracing members, hence, there would be a tendency for horizontal deflection at the cross-over point. If this happens I would expect vibrations when the crane is in other bays as well.

3.) Some form of resonance may be working here; this would result in large deflections from small out-of-plane forces. Adding structural members adds mass and stiffness together; it does not necessarily change the k/m ratio in the most efficient way. You could test for this by laying some sandbags over one of the bracing cross-over points.

Those are my thoughts for now, Gary. Like you, I have never encountered this before and I've worked on a few hundred cranes.

Regards,

Daryl
___________________________________________________________________________

On 06/06/2014 5:46 AM, Gary L. Hodgson and Assoc. wrote:
Fellow listers,
I have a problem that I have never run into before. I designed a free standing crane rwy for a 30 ton EOT crane. The column centres are approx 28.5 ft, with only 5 bays and the columns are braced about the weak axis by X-bracing in one bay (at centre of rwy, both sides) consisting of L6x4x5/16 with the long leg horizontal, connected by a bolt where the 2 angles cross. When the crane starts up and starts travelling down-aisle, the bracing vibrates to the annoyance of the customer and his workers. The vibration is out of the plane of the X-bracing. This does not happen if the crane starts elsewhere, and just passes over the braced bay. First, I had the existing bracing pulled tighter - did'n't work. Next, I added reinforcing angles i.e, boxed the angles but that did not solve the problem. My next attempt will be to add a horiz strut between the columns and then connect that to the X-bracing with a short length of angle to the centre of the X-bracing so it will prevent or decrease the amount of out-of -plane motion.
Can any of the members suggest another solution?
Gary

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