Nylon Shims between bolted plates

[snowmachine88]

I am modifying a typical detail for a decorative railing design. All of the locations where plates are bolted together have nylon shims between the plates.

What is the purpose of the nylon shims? Is it to prevent moment transfer between plates so that the railing and connection can be designed as simply supported? Is it to aid in thermal movement?

How do I properly size the shims? Is it based on the hole or slot dimension on the plates? Is it based on the plate dimensions?

The typical details can be seen here:

http://www.dot.state.mn.us/bridge/pdf/cadd/files/bdetailspart2/bridgedetailsmanualpart2.pdf

On page 99.

Thank you!

 

[tbuelna]

I did a quick scan thru the document you linked. It looks like the nylon spacers are part of an electrical bonding system. The nylon spacers might provide some electrical insulation of the handrail from the rest of the guardrail structure in the event of a lightning strike. It would probably help to read thru the drawing notes again. The drawing callouts and general notes were not easy to interpret.

 

[MintJulep]

They are just bearings to let the railings float as the bridge moves.

I doubt that the size was engineered originally, beyond the availability of nylon stock in 1.5" wide x 1/4" thick strips.

 

[racookpe1978]

Nylon spacers like that also prevent scratches and scrapes at the joint between the painted/coated steel pieces: The nylon won't rust and won't cause scratches in the mating parts, so they won't be as likely to rust. The nylon spacers won't be as strong as steel, but they are only in compression, so the compression strength across the joints will be "enough" to work for handrails and railings.

Actually insulating parts of the railing from other parts of the bridge is almost certainly to fail, if only because the bolts going through each piece will be almost impossible to keep all insulated.