Design rollers that will support end reaction of girder - temporary

[black79ta]

I am designing falsework to support a RR bridge span. Deadload only. the span is 121' and I want to allow for thermal expansion with using 3/8" steel rods under one girder end. This is temporary, about 2 months. grease wont work, the coef. of friction (observed from previous work) is about .2, I don't want that much side loading into the falsework. hence using roller rods. another option is PTFE pads, I would rather use the rods since it is more cost effective. the load into each girder end is 55k. i would use 15 rods (8" long) under each end. The unit load is 430 lbs per inch of rod. I was studying Hertz info in contact loading and have come up with very high contact load pressures. I am wondering how one would approach this design wise.

thanks in advance.

 

[bridgebuster]

You need larger diameter rods. 3/8" rod doesn't have much contact area.

 

[gwynn]

I don't have a theoretical approach to solve the issue off the top of my head, but I would probably just specify Hilman rollers if you are looking to keep the friction that low. Most of the contractors I deal with have a few 37.5 ton or 50 ton capacity rollers in inventory. Even if they don't, the 37.5 ton rollers are around $600 each so they are not a huge expense item. If you are working for the contractor I would give them a shout and see if they have anything suitable.

 

[black79ta]

Thanks for the replies. I increased the rod diameter and it still was not working. I am going to use ptfe bearing pads/#8 mirror finish stainless steel.

 

[bridgebuster]

Sounds like a good idea. The rods have to be maybe 3 inches in diameter.