Slip Critical Joint finish 1

[Lutfi]

Here is a simple one for those of you like to design structural steel:

How do you achieve slip critical joint in a hot dip galvanized steel finish?

I like to get input from all of you?

 

[Qshake]

Lufti,

I'm not sure that you can. I'm sure others will post here with relevant experience to help out. However, in my opinion, the hot-dip galvanizing will flake under the stress of the connection thereby eliminating the true benefit of the critical connection.

 

[structuresguy]

Hi Lutfi,

Not having my green book, err should I say my dark blue book now I guess, with me right now, I can't remember exactly, but there is a table in the RCSC spec at the back which lists the finishes acceptable for slip critical surfaces. I can't remember if it says galvanized is OK, but definitely inorganic zinc will work. In fact, with IO zinc you can get Class II (?) surface which will give you higher allowable loads than in the slip critical tables, which are based on Class I (?) surface. I think it also covers any further surface prep after coating to achieve desired results, like a light wire brushing, or something like that. With the IO zinc higher load capacity, and no taping of joint areas like regular primer, you get a double benefit, if corrosion is a major concern. Plus the IO zinc makes any finish painting much easier than hot dip.

 

[structuresguy]

hi again,

I got the bolting spec, and hot dip gets you a Class C surface, which requires hand wire brushing. The others in my previious post should be Class A instead of Class 2, and Class B not Class 1. The Class A surface has a coefficient of friction of 0.33, the Class B surface gets 0.5, and Class C 0.35

However, if says that roughened hot-dip surfaces may show continual slip under prolonged loading, and that the proper ammount of roughening is important. A hot dip surface before roughening has coef. of friction if only 0.19.

So the drawback I see from HD galv is it takes 3 operations: fabrication, shiped out for dipping, then back to the fabricator for rougheing the joints. Whereas with something like an IO zinc primer, that can be done by a competent fabricator all at once. I have seen you mention that you are in FLorida, I know Met-Con in Cocoa has an automated beam punch/saw/shot-blast machine. The beams come out ready to be painted. Pretty neat stuff. Plus they have lots of experience with IO zinc from working out at the Cape.

 

[dbuzz]

The Australian standard AS4100 "Steel structures" (Table C9.3.3.2) gives the following indicative slip factors for hot-dipped galvanised surfaces:

Hot-dipped galvanised
Clean as-galvanised 0.18
Lightly abrasive blasted 0.30 - 0.40

These factors are used in calculating the shear capacity of a bolt (or bolt group) for the serviceability limit state.

 

[CWIC]

Structuresguy has provided the answer for galvanized surfaces of slip critical connections.

It is important to ensure the contractor does not use power wire-brushing methods as this makes for a very "slippery" surface finish.

It did happen on one of my projects 8 years ago on a processing plant stucture. The results were somewhat obvious, when they flew the steel in subsections, the joints did slip under self-weight. It became apparent to the contractor when the members did not fit-up correctly, slippage was noted at almost all of the joints which were power brushed.

 

[Lutfi]

Your input has been very helpful. Thanks to all of the respondants.

Lutfi