Washer a Faying Surface for Slip-Critical? 3

[Althalus]

The Steel Manual describes Class A, B, & C finishes on the faying surfaces.

A = Uncoated (plain)
B = Coated (painted)
C = Galvanized (spec. Hot Dipped)

From other literature I've gathered that the Class B has a higher coefficient of friction than plain or Galvanized. So that made me wonder about combinations.

We usually have painted steel with galvanized bolts, nuts, and washers. So, this leads me to ask:

If the steel parts are painted (and prepped to a Class B), but the connector parts are all galvanized (Class C) then what coefficient do I use for the friction capacity of the bolt?

I've looked through the commentary and searched elsewhere online. But I can't find anything that says either way.

 

[r13]

Check RCSC sections 3.2 and 3.2.2 as suggested in quote below.

"Faying surfaces in slip-critical joints must meet the requirements in RCSC Specification Sections 3.2 and 3.2.2. RCSC defines a faying surface as “the plane of contact between two plies of a joint.” NOTE that the surfaces under the bolt head, washer and/or nut are not faying surfaces."

 

[Agent666]

As r13 notes, the faying surfaces are just the surfaces of the plies that are in contact, it has nothing to do with the bolt surface coating. So if the plies are painted but you have galvanised bolts it would be classified as Class B provided you meet whatever other requirements there might be on paint type as painted condition could vary considerably. For example is only the primer coat applied, or 2/3 coats of a system, slip performance could be quite different.

You need to look into what coefficient of friction is assumed in your code slip calculations for each class. Only certain coatings may satisfy the default assumed values. For example in my local code (not AISC) a coefficient of 0.35 is used by default unless deemed otherwise via testing. Though you are free to substitute any value in the calculation for slip capacity provided you establish this. To achieve the 0.35 coefficient either the surfaces should be unpainted, or have a certain type of coating (can't recall exactly, but it is either organic or inorganic zinc based coatings, otherwise you need to test the slip achieved).

The assumed coefficient of friction is presumably embodied in the Classes used in AISC, sorry not familiar with this aspect of AISC.

From other literature I've gathered that the Class B has a higher coefficient of friction than plain or Galvanized.

Based on my experience this isn't always the case. As rolled steel would typically perform better with a higher coefficient of friction being achieved, but obviously depends on the coating and surface preparation of steel for the coating application.

Typically faying surfaces are unpainted (masked off) in my experiences as only selected types of paint will actually give me the default coefficient of friction in my code to give the equivalent of uncoated 'as-rolled' steel surfaces.

Otherwise, this coefficient could be determined via test to ensure it falls within whatever the assumptions are in AISC. Most codes have standard test methods for establishing this slip coefficient. In my experience when you test you get a better value than the min code default, provided your tightening method is calibrated and surface preparation is achieved you'll achieve the improved slip performance.