Should pipe based structures (RHS SHS CHS) be sealed or have their ends left open? 1

[RonOz]

There are many tubular steel structures in the world. Circular, square, rectangular. Combinations of all three. Many of these structures are capped at the ends by welding a plate in place. It's commonly done. Aesthetics is one of the reasons. Another being to protect the internal surfaces from corrosion. Keeping water out will prevent rust.

The issue came up in another forum; one dominated by welders.

The notion was that tubular structures should have their ends left open, or if they are capped then drain holes should made in appropriate locations to allow water to drain from the pipes. The argument is that despite best efforts there will be pinholes in the welds or tiny gaps where the weld was not made properly, and these tiny holes will permit air to enter. Humid air will condense on the inside of the pipes and water will accumulate with the result that corrosion will take place at a faster rate than had the pipes not been sealed. The implied result is a premature failure.

The counter argument was that the welder can do a good job and the pipe is fully sealed, but if not and pinholes remain, then paint will seal any remaining problem areas. Consequently, corrosion will not be a problem as the structure is fully sealed.

What's your opinion on this?

 

[LittleWheels]

Welding tends to heat the trapped air. The final section of a weld tends to blow outwards.

 

[kingnero]

True for doubler plates and similar small air volumes, not for large hollow structures.

Pinholes will suck in moisture. HDG structures are by definition open, to mitigate this. Personally, I don't like fully closed tubular structures, but architects obviously do for esthetic reasons, and you'll find plenty more examples of these than those with open ends.

 

[RonOz]

Hi LittleWheels, I doubt there would be any blowout of metal unless very thin stock was involved. Heavy stock would likely be able to resist the effect of higher air pressure on the inside, and as the material cools the effect would be reversed. There may be a negative air pressure on the inside. Any tendency toward increased air pressure due to the heat of welding will escape through the gap of the parts not yet sealed.

 

[MintJulep]

* It's hard to make things really waterproof. Better to design with the assumption that water will get in, and add a way for it to get out and mitigate corrosion.
* If you really really need "waterproof", then take the time to understand what that really means and how you can ensure that you get it.
* Even if you achieve "waterproof" there will be moisture in the air that's trapped inside. Maybe not a problem for large structures with thick walls. Maybe a problem for small structures with thin walls. Take the time to figure this out, purge with dry somethings, or evacuate if you can't tolerate any moisture inside.
* There are many corrosion inhibiting coatings that can be applied to the inside of hollow sections. It's not that hard to do - if you give consideration to it during design and the fabrication process.
* Welding is not the only way to make the final seal.

 

[LittleWheels]

I accept that my stated concern does not apply on any sort of large scale fabrication (hence ignore my previous comment). As a hobby, I occasionally brazed and welded bicycle frames (wall thickness about 1.0mm) and found it to be a problem. Some frame-builders manage to weld frames with fully sealed tubes (e.g. Fat Chance Cycles) without issue, so obviously it isn't too big a problem for professionals.

 

[RonOz]

It's quite a skill to weld 1 mm thicknesses. I had some experience many many years ago, and recall vividly of allowing the flame to overheat the metal and have it explode in a spray of hot metal in the workbay, leaving a hole where I needed a weld.

 

[RonOz]

My original post hoped to elicit your experiences in civil engineering. My interest was academic only.

The images were picked up from the web in a random search. I think they're beautiful. Do you think they might have, or if you had designed them, would they have included drain holes to allow water to escape? Or would you have them completely sealed?

 

[DrZoidberWoop]

Here's a fun tip. For galvanized hollow sections, roughly larger than 8"x8", be sure to specify cap/base plates be attached after galvanizing. This helps keep the personnel at the galvanizing plant safe from spray caused by trapped air. The 8"x8" size is based on experience/observation, not any sort of analytics.

 

[LittleWheels]

RonOz, brazing is more forgiving on thin tubing than welding in general (less heat needed). TIG welding thin sections is a little easier (still not easy!) than alternative welding methods commonly used in civil engineering.

 

[EdStainless]

I would always advise to cap and drill weep holes.
Even minute openings will result in trapped water inside over time.
As these members heat and cool each day/night there is no avoiding it.
Just be careful that the drain holes are actually at the bottom and that they drain to someplace acceptable.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[r6155]

Never leave it open. Some type of tiny animal can access it.

Regards

 

[RonOz]

I fancy that if a tiny animal can go in then it can leave at its leisure. Though, aren't we referring to holes maybe 2-3mm in diameter? Insects might make a nest inside.

 

[r6155]

Use a 1/4" threaded hole with a plug. You can perform inspection at your convenience.

Regards

 

[CWB1]

Depends on the replacement cost and expected lifespan. On cheap to replace, short-life components the goal is generally to minimize cost. On expensive and/or long-life components the goal usually isnt to drain water but rather provide an easy washout for the dirt and debris that eventually finds its way in - IOW you provide BIG holes, not small ones.

 

[RonOz]

As engineers, does this issue concern you?

When you look up to exposed trusses in a new shopping centre mall or the spans in a multi-lane bridge that you’ve had a hand in building or designing, do you wonder how much corrosion is occurring on the inside of those enclosed structures? And knowing, in some cases, as the structure ages there is no way to inspect them for damage. An accident waiting to happen.

The sealed pipe structures look good, but are they collecting water and rusting from the inside?

Open-ended pipes allow air to circulate, which may become wet and then dry. But have you seen how much condensation forms on cold surfaces? Condensation probably occurs inside those pipes where the air is free to move. Are they rusting faster than the sealed pipes because of the sweet fresh air that gets inside them?

 

[geesaman.d]

Use a 1/4" threaded hole with a plug. You can perform inspection at your convenience.

I work with submerged hollow shafts; we do a 25psi pressure test, then NPT plug that gets seal welded shut. For carbon steel, whatever amount of oxygen that's trapped in there is pretty harmless.

Having small open holes to the interior creates any number of fun problems, wasp and hornet nests for example.

 

[PersonalProfile]

1. I propose generally seal welded - the likelihood of holes etc is relatively low if the weld standards that I am aware of are used, and
2. I interpret the following to state similar, though also the corrosion associated with intentional holes is less than I would have estimated without this reference:

Samples which had holes through the wall of the section (unplugged drilled holes, damaged seam welds, and holes due to external corrosion) all
showed some corrosion adjacent to the hole but elsewhere remained unimpaired, revealing only slight internal corrosion.

There is no possibility of internal corrosion in hollow sections, when these are sealed at both ends, whatever the environment in which they are found.

Ex:

https://www.falconrme.com/wp-conten...ar-steel-vs-channel-steel-research-Part-1.pdf

 

[kingnero]

Question should be linked together with the required corrosion protection system.
No point discussing open or closed, if the client requires HDG or powder coating for example.

 

[r6155]

Several times during manufacturing, a visual inspection of the internal surface of components is necessary.
Modifications may occur during service but it is necessary to check the internal surface before doing so. Therefore, a plugged threaded hole defined in the design specifications is essential.

Regards