Why leave a brether hole when welding structural tubing?
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    Default Why leave a brether hole when welding structural tubing?

    The title says it all.
    I know there's a practice of leaving a breather hole at the lowest part of a welded structural tubing for venting out moisture. What I wanting to know is which is the better from a longevity point of view. Welding a structural tubing with a vent hole, or welding it with no vent hole and pressure testing?

    The tubing in in this question is 6x6x.25 square tubing, 12' long sections vertical mount to cement pad. The interior of the tubing will not be coated, just paint on the outside with all season weather contact. These are exposed beams.

    Thanks all.

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    Many times this rule is for stuff that is hot dipped galvanized.

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    Quote Originally Posted by Gos View Post
    The title says it all.
    I know there's a practice of leaving a breather hole at the lowest part of a welded structural tubing for venting out moisture. What I wanting to know is which is the better from a longevity point of view. Welding a structural tubing with a vent hole, or welding it with no vent hole and pressure testing?

    The tubing in in this question is 6x6x.25 square tubing, 12' long sections vertical mount to cement pad. The interior of the tubing will not be coated, just paint on the outside with all season weather contact. These are exposed beams.

    Thanks all.
    I replaced the lower 12" of about 6 6x6x.25 vertical box tubes on a building a few years ago. All rusted through from the outside. No rust inside. It was from salt aplication on walkways and driveways to control snow and ice buildup. The building was only about 20 years old at the time.

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    OT, but FWIW, a carnival ride collapsed and killed someone in Ohio this year. The cause was structural tubing rusting from the inside, it looked perfect on the outside.

    Ohio State Fair ride death caused by corrosion, says ride maker - CNN

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    Ya same here we cut up tons of old square and rect tube, almost always looked like new inside.
    That said I have seen plenty of tube severely rusted out inside because moisture was allowed to enter somewhere in the structure.

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    Quote Originally Posted by Mud View Post
    OT, but FWIW, a carnival ride collapsed and killed someone in Ohio this year. The cause was structural tubing rusting from the inside, it looked perfect on the outside.

    Ohio State Fair ride death caused by corrosion, says ride maker - CNN
    On that one I would have drilled small holes at each intersection, then
    pressurized the whole frame with nitrogen, put a pressure gage
    where you can check it (seen this done before, not my idea)

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    Quote Originally Posted by digger doug View Post
    On that one I would have drilled small holes at each intersection, then
    pressurized the whole frame with nitrogen, put a pressure gage
    where you can check it (seen this done before, not my idea)
    I'v done this on road race motorcycles, works great.

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    Oxygen sealed in a completely closed tube would be quickly consumed by initial corrosion. After this corrosion will stop. An older practise on ships was to throw burning rags or newspapers into uncoated void spaces, after which they would be sealed airtight. With the oxygen burned off, corrosion stopped.

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    Quote Originally Posted by moonlight machine View Post
    I'v done this on road race motorcycles, works great.
    Yup, first time I heard of it was on an early model british helicopter,
    I read that the gage was right next to the pilots seat for easy viewing.

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    Quote Originally Posted by Gos View Post
    The title says it all.
    I know there's a practice of leaving a breather hole at the lowest part of a welded structural tubing for venting out moisture. What I wanting to know is which is the better from a longevity point of view. Welding a structural tubing with a vent hole, or welding it with no vent hole and pressure testing?

    The tubing in in this question is 6x6x.25 square tubing, 12' long sections vertical mount to cement pad. The interior of the tubing will not be coated, just paint on the outside with all season weather contact. These are exposed beams.

    Thanks all.
    In our structural steel division we make HSS columns for structures weekly. For columns that are not being hot dip galvanized I have never seen a vent hole requirement nor seen the need to add them. Most have baseplates and will be below grade, leveled , grouted and covered with concrete about 8" to a foot high.

    Two things we are careful about. First is to make sure the steel tube has no moisture inside when welding so it's blown out with compressed air if needed. Second is to make sure there are no pin holes or gaps in the welds.

    Never had an issue and I would guess the structural steel engineers have not either since it's never addressed except for a few who want the section buried in concrete coated with roofing tar.

    I should add that a good zinc-rich primer will go a long way to keeping the rust at bay.

    Walter

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    Thanks to everyone for the quick responses.
    I'll TIG the roots then continue with 7018 just to be "for sure for sure".
    Later,
    Gos

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    The only time I've ever left a drainage/vent hole at the bottom of a weldment is if there's a chance that moisture can get into it, such as a telescoping leg, or something with an intermittent weld. If the welding is 100%, then there's no reason to leave a vent/drainage hole.

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    I worked for an out fit that built construction site elevators, could be made as tall as the owner wanted as long as he had the sections to do it. These were jig welded with thick wall rectangular steel tubes for the four corners and 2" square tubes for the struts with a rack gear on the one corner leg for the elevator gear box. A couple of the square struts were getting water in them and swelling up in freezing temperatures so they added a drain hole for those 2 only. How or why those 2 got the water in them is not understandable but that 1/8" hole fixed the problem.
    I always have a drain hole in anything I build for out doors use since that experience and have never had that type of problem.

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    Water get's in the smallest of places....

    FWIW I like drilling (as opposed to torching) a hole at least 1/2" dia.
    as I have seen smaller ones rust shut.
    Attached Thumbnails Attached Thumbnails gedc0238.jpg  

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    I think sealed-up is best, unless galvanizing when you need holes big enough to ensure galvanizing inside as well. Pressurized even better because cracks will be revealed right away

    I seem to recall being told that welded tubular aircraft frames used to be filled with tung oil and drained to prevent internal corrosion...of course that would require strategically-located vents...Anyone know any details on this?

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    Quote Originally Posted by magneticanomaly View Post
    I think sealed-up is best, unless galvanizing when you need holes big enough to ensure galvanizing inside as well. Pressurized even better because cracks will be revealed right away

    I seem to recall being told that welded tubular aircraft frames used to be filled with tung oil and drained to prevent internal corrosion...of course that would require strategically-located vents...Anyone know any details on this?
    Your post made me search ( as I was curious about how repairs were made on an oil preserved structure ) and I found this ;

    Corrosion Protection Method for Welded Steel Tube Structures - Aircraft engineering - Eng-Tips

    One advantage of drilling breather holes in TIG welded tube structures for motorsport ( and probably aircraft as well ) is the elimination of blow out at the end of a weld with cluster joints or a chambered area, which is mandatory for any components to be heat treated.

    Plus it's far easier and quicker to just plug weld one breather hole if the component is to be sealed, rather than waiting for the part to cool before finish welding a joint with a small unwelded area.

    More here; anti corrosion oil for tubular aircraft structures - Google Search

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    Quote Originally Posted by magneticanomaly View Post
    unless galvanizing when you need holes big enough to ensure galvanizing inside as well.
    The holes in parts to be galvanized is not so they get galvanized inside. It is so there is no risk of them blowing up when heated.

    Below is from a design guide from the American Galvanizers Association:
    "The primary reason for vent and drain holes is to allow air
    to be evacuated, permitting the object to be completely
    immersed into cleaning solutions and molten zinc.
    Proper hole sizing and location make it safer to galvanize
    and provide an optimal finish. The secondary reason for
    venting/drainage is to prevent damage to the parts. Any
    pickling solutions or rinse waters that might be trapped in
    a blind or closed joining connection will be converted to
    superheated steam or gas and can develop a pressure of up
    to 3,600 psi (1,100 MPa) when immersed in molten zinc.
    Not only does that pressure have the ability to damage
    to the fabrication being galvanized, but can also put
    galvanizing personnel and equipment at risk."

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    Thinking more about this- It seems to me that aside from galvanizing it would be best to seal the thing up moisture inside or not. I was thinking today about sprinkler lines which from what I have seen are generally less than schedule 40. They sit full of water and last pretty much forever. The reason is that the oxidation uses the oxygen up and then quits. I think in the sealed cavity once the oxygen is used up in oxidizing he steel that it would stop.

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    Quote Originally Posted by Pete Deal View Post
    Thinking more about this- It seems to me that aside from galvanizing it would be best to seal the thing up moisture inside or not. I was thinking today about sprinkler lines which from what I have seen are generally less than schedule 40. They sit full of water and last pretty much forever. The reason is that the oxidation uses the oxygen up and then quits. I think in the sealed cavity once the oxygen is used up in oxidizing he steel that it would stop.
    2 points.
    1. some fire systems are "dry" they use compressed air to sense when a head melts, then the water flows.
    2. water trapped can freeze.

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    Quote Originally Posted by digger doug View Post
    some fire systems are "dry"
    True but the typical in building systems (no freeze risk) that I am familiar with are full of water.

    Quote Originally Posted by digger doug View Post
    water trapped can freeze
    Also true but for the sake of this discussion I would assume someone would not weld the thing up with a bunch of water in it. Dry would be best.


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