Crane runway beam problem, advices needed

[fkd]

Hi all,


I'm facing an annoying problem, so I will ask you for advices concerning the solution that I'm intenting to use, the problem is that due to assembling errors a crane (40t) runway beam that I've designed was placed 65mm out of the desired axis, the beam is a welded W with the following dimensions:

d=600mm
bf=200mm
tf=19mm
tw=10mm

The upper flange is restrained by a horizontal plane truss, the lower flange is free, restrained only at the support points that are 5700mm away, by now you can guess that the problem is the torsional moment originated by the undesired excentricit.

Now the worst part:

- they can't relocate the beam
- they can't transform the beam in a box section

So I'm intenting to use this solution: make a system using the horizontal plane truss plus a new truss connecting the lower flange to that truss so I'll have a space truss system that will be connected at the ends to the collums of the building tranfering the torsion to them and restraing the hole system.

I'll check the flanges against bending and will add stiffners at the connection points of the new truss to the lower flange and maybe aditional stiffners between them.


Any advices will be very much apreciated,

Thanks in advance,

Fred

 

[gb156]

Fred,

If I understand your situation properly, it sounds like your solution could be adequate.

Some things to consider which may already be obvious.....

Kicking a member from the bottom flange up to the horizontal truss at the top flange will reduce the torsion in the beam at the location of the kickers, but not necessarily between the kickers. Check the beam for torsion between the kickers.

The horizontal truss at the top flange should be checked for the new additional loading.

The beam flange needs to be checked for local bending (as you already noted), but because the load is moving, adding stiffeners may not prevent the beam flange from failing between the stiffeners. Essentially, you should probably check the flange with the stiffeners as a plate with 3 sides fixed and one side free.

Hope this helps! Good luck!

 

[visitor]

Provide some cross bracings in the vertical plane, connecting the top and bottom horizontal trusses. Then your
new space truss will be suitable for taking the torsion.
I hope the column is strong enough to take the addition
bending moment due to the torsion in the crane beam. Ypu can check that also.

By this you can make the crane girder to take the vertical loads and the space truss to take the torsion. I faced a simialr situation. But it was not a crane beam. There was a
bracket to take the precast fascia. The design was initially done by some body and fabricated. When they started erection they faced the problem. Then I provided a
box section adjacent to the existing steel beam and connected the bothe memebrs at suitable intervals. Now the structure is perfectly allright and standing there for more than 3 years.

 

[fkd]

Thank you both for the advices.

There are some missing issues as follows:

- the system has only one horizontal plane truss restraining the top flange,

- the crane's manufacturer will add an addittional plate over the top flange to acomodate the rail, a very heavy one TR57 (I don't know if this code means anything for you, it's our local standart).

- the max (the excentricity varies along the beam) torsional moment is 43kN/m (impact and load factors included)

 

[svc430]

I agree with gb156. You need to consider the twisting of the beam section between your panel points on the truss. Depending on your wheel spacing, the torsion on your runway beam may not be reduced that much by adding kickers.

I’m guessing you are going to have a difficult time getting the top flange to take the eccentric bending of a wheel load applied that far out from the web. You might consider adding a vertical plate under the top flange directly under the rail center, spanning (bending) between vertical stiffeners to take the local wheel load. This would have to run at each panel the full length of the beam. The depth of this plate could approach half the girder depth or more depending on your wheel load. In crane girder design, this would be analogous to the internal diaphragms of a box section, sized to take the trolley wheel load over to the web plates (eliminating/minimizing the local bending of the top plate between the webs). Another approach might be to consider the crane rail section itself helping to resist the wheel load in bending between your vertical stiffeners. Here again, in the design of crane girder box sections, the rail is checked for bending between the internal diaphragms of the box section (assuming no contribution of the top plate). I’m referring to U.S. crane standards here (CMAA No. 70 and AISE Technical Report No. 6) for cases where the trolley rail is centered between the web plates of the box girder section. A more complicated analysis would be to consider the actual bending stress in each member taking load between the vertical stiffeners (top flange and crane rail). This could be done by equating deflections of each member (crane rail bending/deflecting between vertical stiffeners and top flange bending/deflecting as a cantilever beam out from the web or as a plate supported on 3 sides), then solving for the load in each.

I’m wondering just what size of crane rail you have. It would seem to me that with a 65mm eccentricity, your crane rail base is going to overhang the top flange of the beam (you will only have 35mm from rail center to edge of flange based on the dimensions you’ve given). If this is the case, you’re probably going to have to add on to the width of your top flange in order to provide for clamping the crane rail to the beam. That’s a lot of welding and grinding. At some point here, the people making the decisions may have to re-think whether all this work is worth it over reworking the beam to get it in the correct location.

Best of luck!!

 

[fkd]

SVC430,

your considerations are right, but don't we can consider that the rails base will transfer the vertical load to the web of the girder by means of a flexural moment inside the rail (that is not designed for this)?

P.S. This can be a new issue concerning the moments acting on the girders top flange, don't you agree?

 

[fkd]

OOps, I'm wrong the rail isn't restrained agaist rotation...........

 

[svc430]

fkd,

Actually, you are correct regarding the additional moments in the girder top flange. Testing on crane runway beams have shown additional stresses in the top flange that result from application of the local crane wheel load. AISE Technical Report No. 13 (Design Guide for Mill Buildings) requires calculation of the local flange bending stress that results from the crane wheel load, which then must be added directly to the overall (vertical) bending stress in the top flange. The equation for this local bending stress is somewhat simple and depends on the moment of inertia of both the rail and flange (among other things).

This equation assumes, of course, that the rail is centered over the web plate of the beam. AISE No. 13 allows for a maximum eccentricity of the rail equal to 3/4 of the web thickness. In your particular case, the resulting local bending stress of the top flange will greatly exceed that calculated from the AISE equation (which, as presented in that standard, does not include any variable for rail eccentricity).

I would be very interested in reading a post describing your final solution to this problem.

Best regards,
svc430

 

[jrmack]

fkd,
The problem you have described, a 65mm eccentricity on a W shape beam with a 200mm flange for a 40 tonne crane is ugly. What would concern me most, is if I came up with some bandaid fix to this problem, I might overlook some aspect that was not already mentioned. Fatigue would be a concern for me. Surely something could be done to correct the alignment.
good luck, jrm.