Holes in hollow sections to drain water

[bugbus]

I have some square hollow section posts that are situated outside and exposed to rain and occasional flood waters.

The posts are entirely sealed apart from a small galvanising vent/drain hole near the top. This hole is on the vertical face of the post and drilled in a horizontal direction prior to galvanising. There are larger vent/drain holes through the baseplate also for galvanising purposes, but since the baseplate sits on a grout pad these no longer provide any drainage to the post if water were to get inside.

The client is concerned that the post could fill up with water, which could create a corrosion problem inside. Note that the inside of the post is well galvanised.

Would it be best to try to plug the small hole at the top of the post to prevent any water getting in in the first place, or alternatively drill an additional drain hole at the base of the post and keep both holes open to allow the water to drain out and dry up?

My feeling is that the first option is futile, and water will find a way in no matter how well it is plugged. Second option feels better to me, with appropriate repair of the new holes by zinc rich paint protection.

 

[dhengr]

Gusmurr:
After the fact, solution #2 is the only practical choice. Find the location/side where [( -P/A) + ( -Mc/I)] is smallest for the hole. Make the hole large enough so you can properly paint it and clean it out, from time to time. The zinc rich paint is about the best you can do, after the fact. You will drill slightly above the base pl. because the fillet weld to the base pl. will likely push the drill up there. You might use an end mill to drill the hole right on top of the base pl. if you have someone with the right equipment. Could you drill/core through the grout (from two sides?) to meet the hole in the base pl. from the galvanizing? When you have a closed section you should usually put a working drain hole at the low spots even when you don’t think you need them. Water will likely get in there whether you think so or not.

 

[HTURKAK]

I would prefer plugging the top hole..

If it is new construction , may filling with concrete be an option ?

 

[bugbus]

If it is new construction , may filling with concrete be an option ?

No, the posts are already bolted down and no access to fill it apart from the small hole at the top. There is no chance the client would want to start cutting new access hole for concreting the insides.

Note that the posts will be submerged every few years when it floods. Whatever plugging would have to be pretty watertight.

Could you drill/core through the grout (from two sides?) to meet the hole in the base pl. from the galvanizing?

Unlikely. Practically it would be difficult to do in this case and I don't believe the client would entertain it.

When you have a closed section you should usually put a working drain hole at the low spots even when you don’t think you need them.

Agreed. Unfortunately this was based on a standard road authority design without ample checking, and no allowance was made for a lower drain hole.

 

[Liam Nesson]

For hollow sections its pretty standard to have weep holes at lown points. Google freezing splits galvanised shs columns to see the exspansion forces of frozen water

 

[Settingsun]

Freezing not usually an issue in Australia at places where things get built. I'd seal it if in saltwater environment but draining should be fine in freshwater away from coast.