Repair to Monorail Beam

[ask111]

Is it an acceptable practice, in order to repair an existing monorail beam, to cut and remove part of the deteriorated section of the beam (bottom flange and 4" of the web x 2' long) of an S18 and replace it with a new bottom flange cut from a similar S18 with full penetration welds.

I am supposed to approve/reject this detail as the Representative of the Owner and am not comfortable with this repair. I would like to replace the entire monorail beam which is 72 feet long and quite heavy (5 kips dead weight). The monorail is rated for 10 tons.

 

[kingnero]

If you can get your hands on qualified welders and a qualified procedure, I wouldn't worry about it. A 72 ft beam is worth repairing. I would say for a 15 ft beam, I wouldn't even consider this repair and go for a new one without batting an eye. As always, it depends. What are the costs of both options? Any other things to consider (can you weld at the site, or do you have to take down the beam and remove it from the workshop, can you reach it easily, ...)

http://www.fusionpoint.be

http://be.linkedin.com/in/fusionpoint

 

[canwesteng]

Only issue is fatigue, but full pen wels are the best weld detail for this. Most monorails are used so sparingly that fatigue isn't a concern, but check with the owner to get an idea of number of cycles and take a shot at stress range to get an idea if this is an issue or not.

 

[MotorCity]

In theory I think it would work, but practically speaking I would doubt the quality of surface prep (cutting, beveling, weld access holes etc.) and quality of weld that could be achieved in the field on a beam located XX' above the floor.

 

[weab]

I say go with it. I'd use full pen welds ground smooth everywhere.

 

[dhengr]

Ask111:
What do the monorail manuf’ers. splices look like and how are they located w.r.t. your damaged bot. flg.? Could you replace a shorter length of beam, the original wasn’t installed 75' long, was it? Use the same splice detail on a 5'-10' long new section, paying attention to hanger location w.r.t. splice location as this relates to changing beam moments and shears. Also, with the 2' long infill repair, how does this 2' length compare with the wheel spacing on the crane trolley? The question is..., what length causes the lowest moving wheel load on the repair?

As for the 2' infill repair, who’s repair detail is this, did they shoot the bull, and are now trying to fix it on the cheap? I wouldn’t be afraid to do it, but I’d want to set some of the details and parameters; and pick my best welder and I would tell him what I wanted and then watch him do it. I wouldn’t want to try to write a procedure for this, I’d rather show him and tell him. Then I’d do some NDT after the weld grinding and clean-up. The potential problems that I see are as follows:
1. Go up into the web 6", or so, not just 4" and cut a 1.5" rad. corner, not a square corner, at the reentrant corners. I don’t want the welder starting and stopping in that sharp corner for all the triaxial stresses and residual stresses that could cause. I would like him to be able to smoothly weld through that corner to a cleaner start/stop point. I want the increased depth because at one time or another (one trolley position or another), you might have almost 20kips of loading on that joint/splice.
2. I would want the welder to grind (clean-up) the flame cut edges to be nice clean, blunt butt edges, and good fit-up btwn. the parts. Do the same grinding with any bevels which are flame cut. It might be just as easy to grind all the bevels. About the only bevel (from both sides) which is constant in size is the top longit. web weld. The rest are to an 1/8" land at mid-thickness and wide enough so he can make a good root pass weld with good fusion, and then be able to build out a couple more passes. Then, he should back gouge from the other side to good sound weld metal and do another root pass and a couple more passes. You should know what steel the original beam is made of, and can use the drop for testing if needs be; to check weldability and select the proper filler metal and process.
3. The trickiest welding and grinding area is the ‘k’ area btwn. the web and flg., and it is quite large in those S18's. I would prefer that he does most of the welding from above and down-hand, so this upper bevel grinding should reflect this by being deeper, but still remain as narrow a bevel as allows him to make his root pass weld and get good fusion. The bevels on the bot. of the bot. flg. should be smaller to minimize overhead welding, but they will be contoured too, to about an 1/8" land. Then, good root passes and 2 or 3 short fill passes in the deepest part of the groove (both sides) from above and working out to the shallower groove areas. Then from below, back gouge into sound weld metal and make the root pass across the bot. flg., and a few fill passes in the deeper part of the groove. I might want some pre & post heating in the ‘k’ area to soften things a bit during welding and cooling. Finish the welds and grind smooth, at least on the bot. flg., all faces and edges, and up the ‘k’ region where there could be trolley wheel contact or clearance issues.
4. You will likely get a slight upward hump in the monorail beam in this repair area. And, maybe a slightly different wheel wear pattern right at the welds. Some good UT should show any defects.

You have two primary stress conditions to worry about, namely the beam’s main bending stresses (normal bending stress) and the bot. flg. bending stress (cross flg. bending) due to trolley wheel loadings. Then, you have shear stresses too and they can all be additive in some cases.

 

[dik]

Do you know what the material is? Do you need to do some test coupons? Is the existing welded or rivetted (I assume it is welded, but just curious).

Dik