Hot Dip Galvanizing Draining/Venting

[ToadJones]

Does anyone know of a good reference for how to detail vent/drain holes for galvanizing closed shapes like HSS or dodecagonal poles?
Is this something I might consult the galvanizer on?

 

[Gumpmaster]

If it's not critical, cover it in a general note and let the detailer figure it out. Sometimes they'll detail a 3" hole in a 4" tube, which you can just change on the shop drawings. Most of the time, the detailer gets it right and it's one less thing to put on the design details.

If location and size are critical, put it in the design details. I would consult a galvanizer for general parameters.

 

[ToadJones]

Gump-
I'm handling the detailing on this one as well.
I don't know how common it is, but in our office, we usually handle our own detailing as engineers, especially on things where details are critical to performance. I have wondered for a long time how common this is.

 

[j19]

See ASTM A 385. It gives recommendations on how big the drain holes should be as well as where to locate them.

 

[doct9960]

ToadJones,

Lots of info on the AGA website...

http://www.galvanizeit.org/aga/desi...n-considerations/venting-drainage-fabrication

 

[ToadJones]

doct9960-

Thanks. I was looking all over that site earlier and couldn't find this!!
Thanks

Toad

 

[hokie66]

You should be aware that if the galvaniser doesn't think your hole are sufficient, he will probably make his own.

 

[JedClampett]

On the same subject, here's a question that I think of whenever I'm designing galvanized tubes.
Let's say I have a 4 x 4 tube with a cap on one side and a base plate on the other. Venting is sufficient. How much zinc ends up in the tube? It seems that while the tube is being rotated to drain the zinc out the vent holes, a lot of it will be solidifying inside the tube. And I'm guessing the galvanizer doesn't like to waste zinc inside of members.

 

[Ltwine]

I am not aware the process of galvanizing, so colud be wrong.

If a tube is completely sealed by cap and welds, why galvanize the interior? The vent holes are to release the pressure, and the process should be quick enough to avoid significant amount of material from getting inside of the tube. I guess the name "hot DIP (not submerge) galv.." suggests that.

 

[hokie66]

One of the primary advantages of galvanizing is that the interior is protected. Internal corrosion often occurs in painted sections that are supposedly sealed. Hot dipping does require submerging, not just a quick dunk.

ToadJones,
Another reference on vent hole requirement, from Australia.

http://www.ingal.com.au/FAQ/faq13.html

 

[Ltwine]

Shall rename to "hot boiled/cooked galvanize" to closely reflect the fact

 

[asixth]

The AISC has a freepub on galvanizing:

http://www.aisc.org/UploadedContent/2009NASCCSessions/D9/default.htm

Best thing would be to speak to a galvanizer about vent holes, if they don't think the tube is sufficiently vented, then I doubt they would put it in their bath.

 

[ToadJones]

If you dont allow for venting, closed shapes cant literally rupture due to pressure on the inside.

 

[ToadJones]

Jed-
This is precisely why I want to make sure that holes are located properly.
I think those well versed in this area know the manner in which items are submergered and brought back out of the bath. They have a good idea of where the zinc needs to drain out and they are sure to locate the holes to mitigate the zinc puddling inside.

 

[paintballJim]

Toad,
Placement is also very dependant on how the item is dipped (ie. which end goes in first and at what angle). When we had some semi-trailers dipped the galvanizer came out and we discussed the angle things would go into the tank and he marked where he wanted the holes. If your project is for a sufficent number of parts that might be a good option.

Also in your earlier reply I think you intended to say that parts CAN explode when dipped rather than cant.

 

[ToadJones]

Indeed that was a typo!!

Yes, I was assuming that the hole placement depended on how the item was dipped. I am going meet with the fabricator and galvanizer prior to wrapping up the details.

Thanks

 

[graybeach]

Toad,
I think Gump was referring to the detailing fabricators do, not engineering details. I agree with his answer. The fabricator and galvanizer will likely have different ideas about where the holes should go depending on the specific dipping procedures. We recently procurred some galvanized hand rail. We showed holes where we thought they should be but included a note stating that the sizes and locations could be adjusted subject to owner approval.

 

[transmissiontowers]

For your 12 sided poles, it depends on how big the pole is. If it is big enough for a man to stand inside like some of our transmission poles, cut a large hole and seal weld the inside shaft to BP weld. If you don't seal the inside, when you acid dip the pole, the acid can stay in the crack (and you don't ever want acid in your crack). When the shaft is dipped in the zinc vat the acid will boil out and can stop the zinc from coating the crack.

If the pole is too small to get inside and seal weld, we make the hole in the BP the same size as the shaft and have it completely open so the zinc can wash through. This does make for a thicker BP, but you never have to worry about water building up inside.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[ToadJones]

Tranny-towers-
Does a full-pen weld mitigate the acid-in-your-crack problem?
The pole is not big enough to stand up in and was hoping to avoid having the pole nested in the base plate.

 

[transmissiontowers]

The BP to shaft weld is always a full penetration with 100% UT verification. On smaller poles a backing bar is sometimes used but we always want the bar seal welded to prevent acid entrapment. Sometimes a sacrificial backing tape is used without problem. You need a hole in the BP large enough for a welder to get his arm and head in to do the weld. The hole in the BP is often key hole shaped or diamond shaped to allow the zinc to drain. Another option is to have a 2" hole put in just inside the shaft weld to drain the inside.

The pole fabricator usually has a method they follow to allow for good galvanizing drain.

The other problem with poles is the way they are set on the bolts. Do not let them grout the base. Water will build up inside the shaft and rust the anchor bolts or the pole. I saw a bus column in a substation with 6 feet of water inside because there was no drain or a clogged drain hole with grout. Keep the distance between the top of concrete and the bottom of the base plate under 2 bolt diameters so you don't have to consider bending in the bolts. Look at ASCE 113 and ASCE 48 for some equations and further explanations. (I helped write the one in ASCE 113).

_____________________________________
I have been called "A storehouse of worthless information" many times.