Powder Coating And welding 3

[NewbieStruct]

I am a new structural engineer.I am reviewing some comments on shop drawings. We have a steel beam to steel column welded connection. The architect has told me that he cannot do that since the beams are powder coated. So you cannot weld powder coated members?

 

[MintJulep]

Powder coat is essentially plastic.

If you try to weld steel coated in plastic two things will happen.

1. The plastic will melt.
2. The melted plastic may contaminate the weld.

 

[NewbieStruct]

So basically, you cannot weld when you have powder coats involved?

I thought, you can scrape the powder coat or leave the area where the weld is w/o any powder coat. Weld the members and then spray paint at the weld.

 

[Guest102023]

3. It will stink like hades.
4. Plastics are a nutritious source of both hydrogen and carbon.

 

[NewbieStruct]

So the answer is no.

I talked to some engineers here. They told me in the shop, the area where the weld is too be applied is left w/o any coat. It is welded on field and then spray painted.

 

[Guest102023]

That would be the recommended practise. Weld away.

 

[MadMango]

Spray painting is not the same as powder coating. I think the architect is concerns about corrosion/color match of the welded and spray painted area as compared with the rest of the powder coated areas. You can grid-off the powder coat, but you have to ensure it is removed from the entire HAZ to bare metal.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[NewbieStruct]

So weld away from the powder coat.

So I should let the architect know that they basically do not powder coat around the area where the members are present.

 

[MiketheEngineer]

True all above plus it may give off hazardous fumes. Same with galvanizing. Can be done with proper ventilation, air masks, etc.
but not usually worth it.

 

[MintJulep]

Have you ever had one end of a piece of metal heated and noticed that the other end also gets warm?

Depending on a whole bunch of things some distance away from the welding site will get hot. It may get hot enough to affect the powder coat in various ways - none of them good.

So how far away do you need to not apply powder coat?

It's not exactly clear from your posts what the roles or relationship between your company and the architect are. However it is clear that communication between the two of you hasn't happened in any meaningful fashion yet. Find out what his specific concerns are so that you can address them.

 

[connectegr]

A structural beam and column, "powder coated"? Why? Even if you bolt these connections, the holes size will be reduced by the thickness of the powder coating. Reaming in the holes will remove the coating and any intended corrosion protection. Is this a corrosion issue or an AESS issue? Powder coating is an expensive process and is rarely used for structural steel members. How are they protecting their expensive painted surfaces everywhere else?

Typically paint is blocked in welded areas and repaired/painted in the field after welding and inspection. During steel erection, there are always painted areas that require repair.

As mentioned above. There is a difference in powder coating and painted steel.

http://www.FerrellEngineering.com

 

[NewbieStruct]

Well, the connection was designed by our company. It is a curved tube steel beam (in elevation) to a steel tube column. The connections were approved by the architect during the design phase. Now, during the shop drawing phase we get the welds red flagged by the architect.

 

[connectegr]

Why powder coated? Why not a typical primer/paint system, which can be done in the shop for most of the members and in the field for the rest. Even if you bolt the connections, the final assembly will probably require field painting.

http://www.FerrellEngineering.com

 

[NewbieStruct]

It is for looks and corrosion resistant. The one thing to note is we do have internal beams along the same frame that need not be powder coated.

 

[connectegr]

You may want to look at other painting systems or options. As I mentioned powder coating is an expensive process and is often limited in the size members than can be used. I have only seen it used for miscellaneous metal components such and stair treads and handrail. The coatings also offer textures which are useful in these applications. But for structural components which are not directly effected by wear, a two or three part paint system might be better and be easier on your erection needs.

http://www.FerrellEngineering.com

 

[racookpe1978]

The other thing not being addressed is the "physical thickness" and the physical properties (compression, wear, cutting, resilience)of powder coating layer. Compared to the very stiff, very thin (a few mils) thickness of a primer + paint, or the slightly thicker primer + 2-part epoxy coating, a powder coat finish can be 30 to 45 mils thick. Sometimes, but not often, even more if the coating isn't carefully applied or drips before hardening.

When compressed by the bolt force between a plate and the beam, this thick of a plastic layer will act more like a soft rubber gasket or sponge and deform and compress. There will be less - but not zero - friction force between the two structural items; and so the load between the two will carried by the shear force of the bolts compared to the high steel-to-steel clamping force usually found in structural joints.

Don't be surprised if assembly and sliding (or wedging) forces damage the powder coat either. Crane lifting? Forklifts? Truck shipping and simple pallet movement and part-to-part vibration and scrapes need to be avoided. Hate to say it, but powder coating really is a very fragile finish compared to what steel erectors usually use - which is primered and painted raw steel that they expect will "always be refinished."

Usually, I find that 1/2 - 3/4 inch of a powder coat finish is completely destroyed (MUST be destroyed!) by the heat of the welding. As mentioned above, at least this much needs to be cleaned/removed before welding to avoid contaminating the weld with the melting/burning plastic. Beyond that, about 3/4 to 1 inch of the plastic will be heat-damaged, softened, distorted or discolored. You can buy commercially "overspray" or powder coat look-alike repair "paints" but they are difficult to use, and look like what they actually are: a shaky-looking repair job.

Typical sequence: Build it and fit it and tack it in a jig. Weld up completely and inspect (dye penetrant or mag particle, visual inspection at a minimum.) Move to the powder coat spray booth/oven, or ship off site to the powder coat contractor. Coat it, return it, attach rest of the parts, ship it.

 

[berkshire]

NewbieStruct
I am presuming that this is an appearance critical " Looking Good" piece at the front of a building, or in an area everybody can see.
If you have not made the part yet, mask off at least an inch to 1 1/2" from the weld joint. If you have made the part and powder coated it already, use tombstone tape masking, and sandblast the powder coating away from the weld area. Disc grinding will not get all of the resin out of the pits in the steel unless the surface is extremely smooth.
If you do, have to weld on the stuff, 7014 iron powder rods lay down best against the contamination, if you can handle the reduction in strength.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[connectegr]

berkshire
I like the quote in your signature. My best/favorite professor use to say "real life is open book"

http://www.FerrellEngineering.com

 

[MintJulep]

the connection was designed by our company....The connections were approved by the architect during the design phase. Now, during the shop drawing phase we get the welds red flagged by the architect.

Don't ever go there again.

The connections were designed by your company.

That means that your company bears the full responsibility for designing them correctly.

More importantly, your company bears the full responsibility if the connection fails.

The architect presumably hires your company because it has expertise in structural design.

Now he is forced to spend his time and budget to review your work in detail because you have demonstrated that you are not capable of reviewing it yourself.

You're lucky that he caught your error now and told you. Otherwise it would be much more costly for you to fix later.

You should thank the architect for catching this, apologize profusely for your error and hope that he comes back to your firm for more design work in the future.

 

[hokie66]

MJ,
That's too harsh. I agree with the architect that powder coated members can't be connected by welding, but how would NewbieStruct have known that the structural steel was to be powder coated? That is certainly an unusual and inappropriate practice. And who else is going to design the connections, if not the structural engineer?