Galvanized Faying Surface - without wire brushing?!?

[connectegr]

Any suggestions?

A project with completed erection, with many connections designed as Class A slip critical. The project is galvanized, so the coating is not Class A certified and the coatings are not blocked. What can be done if the erector failed to wire brush the faying surfaces?

The required slip coefficient is 0.33, for Class A. I think galvanized surfaces without wire brushing are approx. 0.15.

Some of the connections have OVS holes, so "bearing" connections are not an option. And they are not overly conservative to account for the difference in mu values.

http://www.FerrellEngineering.com

 

[nutte]

Is it safe to assume this is the same project with the unknown bolt pretension? Hopefully that erector has closed up shop.

Can you remove some members and go through RCSC's testing procedures to determine what your actual slip coefficient is? Maybe it's closer to 0.33 than you expect.

You've probably thought of the obvious fixes, like welding the connections, replacing the bolts with higher grade ones, and reaming holes and putting in bigger bolts.

 

[connectegr]

nutte...
All of your suggestions are on the table. And yes both of these issues are on the same project. Luckily we are only consulting on the possible solutions. There other issues, but these are the most difficult.

Bigger bolts will require reviewing 100's of connection calculations for the changes in net section.

Welding is an option. But is never fun with galvanizing.

I had not considered testing the actual faying surface without wire brushing. I have read that the galvanized surfaces are rather generally "slick".

http://www.FerrellEngineering.com

 

[ToadJones]

To my knowledge the galv'd services actually have to be wire brushed by hand, not machine.
I have always wondered how effective it really is.
Some of the recent galvanized projects I have done the galvanizing was really rough in spots and smooth in others. Some spots super shiny and others very dull.
It seems to me that the galv roughness depends on the thickness of the material.
I might tend to agree with Nutte about a possible test.

 

[rscassar]

I had this problem once, galvanized steel with tensioned bolt connections and they just install it with no steelwork preparation or verify that the bolts are pretensioned.

I got them to provide flange plates and weld the connection together. Since then, whenever shop drawings are submitted I always make the special not to the fabricator to ensure he fully understands what needs to happen with surface preparation and installation.

 

[rscassar]

I had this problem once, galvanized steel with tensioned bolt connections and they just installed[/color red] it with no steelwork preparation or verification[/color red] that the bolts are pretensioned.

I got them to provide flange plates and weld the connection together. Since then, whenever shop drawings are submitted I always make the special note[/color red] to the fabricator to ensure he fully understands what needs to happen with surface preparation and installation.

 

[csd72]

I read an australian article that tested galvanised surfaces and found that they were more like 0.3 rather than the conservative 0.15 - I will see if I can find it.

Is there any way you can carry out a test on the exposed surfaces to check this?

 

[Ron]

Keep in mind that hot-dipped galvanizing (zinc) is a soft, malleable material, prone to creep under shear loads. The wire brushing increases the friction coefficient, but does not correct the potential creep issue, particularly with poor pretensioning. Potential increases with galvanizing thickness. Might not be an issue, just consider it.

I'm not so sure I agree with your contention that adding larger bolts will require recalculation of all the connections. Yes, it will stiffen the connections, but it seems to me that you could check a few and make sure that the redistribution of stress doesn't throw a load where you don't want it....as unlikely as that might be given the application. Welding would do the same thing.

My inclination would be to replace the bolts, using plate washers for oversized holes or, preferably, using larger bolts.

As much as it gets poo-pooed, I prefer using a calibrated wrench method of tightening. If you use a direct tension indicator such as a Skidmore-Wilhelm device, along with actual jobsite bolts, you can establish a reasonably consistent and relatively accurate torque value that relates to the actual tension developed in the bolts, without guessing or conversion calcs.