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When using weathering steel (COR-TEN) on a project, what sort of bolts should I be using?

I have yet to see fixings made of weather steel; presumably because the way it uses a sacrificial layer to form a protective rust coating makes it unsuitable for precision applications (such as bolt threads; maintaining the structural area, etc.).

I am also aware that mixing different metals; even various steel alloys, can lead to accelerated corrosion due to anodic/cathodic behaviour.

So what type of bolts should be specified with weathering steel?

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For structural applications (in the US), the most common bolt for weathering steel is ASTM A 325 Type 3.

Type 1 is a plain steel bolt that can be galvanized, but in this situation the zinc in the galvanizing will quickly be used trying to protect the rest of the structure.

Update for British bolts

Interestingly, the only option for UK seems to be to get these same bolts in metric (M24) size. References:

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  • $\begingroup$ Interestingly ASTM A 325 Type 3 are weathering steel bolts according to Portland Bolt. However as they're American they don't come in EC/UK sizes- what to do outside the ASTM A 325 zone is probably another question though! $\endgroup$ – thomasmichaelwallace Feb 5 '15 at 22:10
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WR grade bolts are available in the UK in 1/4 inch imperial size HSFG (when imported from America) and also in M24 TCB's, both of which are used regularly in bridge steelwork in the UK.

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I assumed you'd use normal bolts, but use a nylon washer to electrically insulate the bolt from the weathering steel (in order to avoid bi-metallic corrosion).

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    $\begingroup$ That would definitely work- although I'm not sure the HA/Network Rail would accept the maintenance risk- considering that they struggle to fit/maintain/inspect their existing assets in any case... $\endgroup$ – thomasmichaelwallace Feb 6 '15 at 23:53
  • $\begingroup$ A nylon washer would adversely affect pretension and slip resistance which are often an important part of a structural design. $\endgroup$ – Ethan48 May 6 '15 at 13:22
  • $\begingroup$ @Ethan48 - that's only applicable if you go for High Strength Friction Grip bolts. It is perfectly possible to design bolts which work in bearing instead - these are colloquially known in the UK as "black bolts". $\endgroup$ – AndyT May 6 '15 at 13:56
  • $\begingroup$ Sure. At least here in the US, Corten steel is only manufactured in fairly heavy shapes so it would usually be used in a fairly structural application. Even bolts designed for bearing here ('snug tight') are most often high strength bolts and covered by the RCSC spec once you're dealing with steel of that size. Perhaps that's not a common practice in the UK. $\endgroup$ – Ethan48 May 6 '15 at 14:15
  • $\begingroup$ @Ethan48 - thanks for the further info. I'll admit I can't comment on common practice in the UK - early on in my career I worked on a few steel bridges, and distinctly remember designing both black bolts and HSFG bolts. But I had no input into the decision on which bolts, and I haven't designed anything bolted in years - it's been mainly concrete bridges with one welded steel bridge. $\endgroup$ – AndyT May 6 '15 at 14:20
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Re-iterating some of above as manufacturer I can advise that Weathering Steel TCBs are generally available in M24 diameter only, with the following lengths: 70, 80, 90, 100, 105, 110, 120, 125, 130, 140, 150, 160, 170, 180, 190, 200, 220, and 240 mm.

CSK & alternative diameters can be manufactured bespoke - dependant on quantity. Galv / Zinc Flake / GK Coatings in WS structures are susceptible to bi-metallic corrosion.

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