Crane Bridge Girder Torsion and Vibration

[Vilhelm]

One of my clients was modifying a bridge crane. Based on calculations, I recommended the following for each of the two bridge beams: W30x108 with a C15x33.9 channel cap welded to the top flange. The bridge span was 60'-5" fron center to center of rails. The capacity of the hoist was only 15 tons.

I'm confident that the member sizes were structurally adequate for vertical loading (including impact) and for the horizontal/lateral load acting on the bridge girders.

However, after installation, when the trolley is moving across the bridge girders (even with no loading), excessive vibration occurrs in the bridge girders themselves. The movement appears to be primarily torsional and is unacceptable. The bottom flange moves back and forth (cyclically) as the girder twists from one side to the other.

We are having the trolley checked to make sure that it is aligned and riding properly on the rails secured to the top of the girders. It appears unlikely that this is the case, but I feel the need to rule it out.

The issue is not the lack of proper end restraint for the girders as they have a fixed connection (torsionally) to the end trucks.

Has anyone observed this type of dynamic movement, or better yet, have a recommendation for remediation?

One potential thought is to "box" the girders by welding plates on both sides of each girder connecting the top and bottom flanges together, to help resist torsion. But that option may be prohibitively expensive.

Another thought is to install a channel cap on the bottom flange to resist the lateral displacement. But I can't recommend this fix unless I know that it will work.

Since the bottom flange is always in tension, I never imagined that lateral bracing of the bottom flange would be required.

Any help or insight would be greatly appreciated.

 

[paddingtongreen]

Have you checked the straightness and gauge of the rails? If they wander closer than standard, the flanges might try to ride up on the rails and thus push the top flanges of the girders outward.

Something similar might occur if the wheels were set wider than the gauge. A ride up and a slip down, repeated over and over.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

Bridge cranes are almost always made with box girders to overcome the torsional behaviour you described. Design of bridge cranes is a specialist area. Maybe you need to get some help from a crane manufacturer.

 

[rowingengineer]

left field suggestion but have you checked the natural frequency v’s forcing frequency?

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[CTW]

What specification and/or standard was used to design the modification?

 

[graybeach]

I wonder if the W30 gets loaded on only one side of the top flange. That causes global torsion of the beam, and local bending of the web also occurs, which could cause vibrations. The severity of that effect depends on the spacing of the transverse stiffeners. The web of W30x108 is only 0.545" thick and may be overstressed by this local bending.

 

[weab]

Keep in mind that if you decide to go with boxing the girder, all you really need to do is add a plate to one side of the girder. This will create the closed shape that you desire to resist torsion. Though it is true that you no longer have symmetry, I'd think that this would not adversely affect performance.

 

[Vilhelm]

Thanks all for your advice. It was all prudent. Paddingtongreen was the closest to the correct diagnoses. The problem was solved today when the fabricator removed the paint from the rails. The crane was load tested (120%) and ran straight and true, no vibration. I knew something in the trolley/rail system had to be attributing lateral loading to the rails to produce the torsion, because if the load resultant was passing through the shear center no torsion could occur.

I am gratful for the assistance of a streetsmart steel worker.