sealing steel tubes

You'd be better filling them with sealant.

The only problem with sealing them as you propose, is that the humidity in the internal air will still cause rust to form - except having sealed them you won't be able to inspect the internal surfaces for rust.

Thanks. But what kind of sealant? i find Acrylic sealant, polyurethan sealant, silicone sealant and so on. Regarding what you said about internal air, i agree but anyway it is worse if i leave the tubes open. after some time i hope the water in the internal air will disappear by combining with the internal metal into some oxide.

Do not seal it, put on a rain cap. This link will give you a few ideas.

i cannot put on a rain cap as tubes are bolted to the main structure of the tower thanks to square pieces of steel inserted at the top and end of each tube. the square piece has a hole in it for the bolt.

Does your tower have a roof plate? If so, I would make a hollow bolt with my drill press.

There is no roof plate. the guyed tower is mounted on a tilting structure that allow it to be flexible and to move back and forwerd when the wind is strong. the 72 tubes i want to seal are the frame (braces) of the tower. some are horizontal and some are oblique. it makes a Z shape.

I would not recommend sealing these. they are designed to drain the water from the mast. There should be weeping holes at the bottom. If they have become plugged, they should be cleared.

A number of problems may arise from sealing these, such a ice buildup in the winter. The added weight could compromise the structure in high winds.

If you still must seal them, the polyurathan sealant would be your best bet. It is the more aesthetic, and designed for metal. The epoxy will breakdown in sunlight unless coated, and the coal-tar is both ugly, toxic, and hazardous to the environment.

ok i will try the polyurathan sealant and test its behavior and performance. The 60m high tower is installed on the atlantic coast where temperature is from 10 to 30 °Celsius. Environnement is quite agressive with strong winds, sand from the desert, salt and humidity from sea. Thanks

Please note, this is not my field so take this advice with a professional level of scepticism.

I would think that a marine type expanding Urethane foam would do the job well. You will need to do a little calculating to figure out the volume needs, but it seems like it would have many advantages.

1. The expanding foam will creep into all the nooks and crannies, eliminating the places where water/corrosion could form. It will also fill all the holes that you didn't know were there.

2. It is naturally water repellant and weather resilient.

3. It is easy to apply. Just plug all the holes and pour the appropriate amount in from the top.

4. It is structural. The foam will actually stiffen the structure and make it stronger.

You can find some info about the foam I am talking about here;

http://www.uscomposites.com/foam.html

Good luck on your project.

-Doug

Thanks. Yes it seems the polyurethan foam most suitable for this job. I will do some feedback as soon as i try it.

Great, let me know how it works. I have used it on other projects to both stiffen and create waterproof spaces with good results.

If anyone has any criticism of this approach I would also like to know. I am sure there are people out there that know a lot more about this than I do. Any feedback would be appreciated, especially before msidate does any major work.

-Doug

Foam may deteriorate rapidly if exposed to sunlight

I agree, but that shouldn't be too big an issue inside steel tubes. An end covering may be a prudent precaution.

-Doug

The expanding polyurethane foam would (presuming you use enough) completely fill the tubes. This might not be such a good idea, since they are intended to sway with the wind. Consult your structural engineer first. Polyurethane caps might be a better solution, likely also cheaper. In any event, apply either one SAFELY!

The foam is flexible enough to still give a little bit. It also has a positive effect of dampening any higher frequency resonance in the structure. I don't think that would be a negative issue with using the foam.

There is the issue of how the weight of the foam may affect the sway characteristics of the structure. If the added structural strength isn't enough to support the additional weight there may be issues. Especially in situations where the structure is exposed to a lot of stresses.

My gut says that it will improve the situation, but the advice of a good engineer is very prudent. My gut has been wrong about both food and engineering. :-)

-Doug

Hi msidate,

i have no much idea of the steel tubes used for the purpose.........though i would like to contribute the following:

1. The purpose of sealing/coating required is to protect inside of steel tube, I would suggest not to seal the tube. Because undrain moisture of the tube will cause rapid rusting and difficult to inspect. Moreover, it should be provided weep hole at bottom for drain off and eggress of moisture, and for free circulation of air.

2. If the protection of the tube inside is of high importance and more strength is required such as in case of structural bracings and columns; I would suggest grouting/filling of light weight cement concrete(to reduce the wt. and have more strength). It will be called "concrete filled steel tubes"(CFST). The advantages of CFST members are: Rusting of inside tube is completely protected; high buckling & crushing strength are achieved; recomended for heavily loaded columns and structural frames, and in siesmic zone due to its high strength and load carrying capacity.

I suggest use a heat shrink material as a cap and incorporate a vent tube fixed at a downward angle, so to not allow rain to fall into the open end.