U channel and squire tube which one is strong

8 cm wide by 3 cm U Channel and 3 x 3 inch x 2 mm tube

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8 cm wide by 3 cm U Channel and 3 x 3 inch x 2 mm tube
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That's a fairly complex engineering question (??)
Apart from in pure tension, the load limit is usually about when the section will buckle - go unstable - or exceed elastic bending (smallish) and plastic bend by large amounts to collapse.
Normally, when the service is not pure tension, closed sections - SHS's - Structural Hollow Section - are much stiffer for the same amount of material and will give a much higher load bearing.
The technical breakthrough of being able to economically manufacture large amounts of Structural Hollow Section from good-specification steel has been a transformation. Other advantages with SHS's are eliminating rust-traps, with hermetically-sealed internal volumes (no corrosion) and smooth external sections advantaging paint systems to give good protection against corrosion for long low-maintenance service. Hence the return (?) of truss bridges.
Complex matters.
You'll be wanting to study Second Moment of Area and the beam and column calculations / equations. The Euler column and the Euler-Bernoulli beam (both derived around the 1750's - about 250 years ago) which serve well for most applications of beams and columns.
Regards, Rich Smith
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writes:

When I built a log splitter, sawmill and a hydraulic bucket loader for my tractor I welded every joint that wouldn't have to be taken apart to store or modify them. However structural steel design manuals say to avoid field welding whenever possible, due to high cost. They are more neutral about shop welding versus bolting. Why would field welding be prohibitably expensive? Heavy construction equipment is almost entirely welded.
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Big difference between commercial and hobby, in all practical / real senses.
That contention, "field welding expensive", would be true for typical commercial cases. Commercially, you use MIG (GMAW) in a workshop, and SMAW on-site
* in a well-set-up fab-shop MIG (GMAW) is vastly faster than stick (SMAW) applied in the same situation
* they'd be talking about bolted steel connections for buildings - "rattle-gun" (impact wrench) a few bolts, rather than weld (SMAW) (noting that at the ends of beams, where the bolts are, you only have a small shear force, with all the serious big beam bending stresses far away in the mid-length of the beam)
Hence, commercially, due to processes used and the majority application, the statement is true.
In a hobby workshop, at best you still have a single-phase electric power and you cannot pull those 15kW from the mains which makes fabshop MIG so productive. Most MIG's are transformer and something like 50% efficient, whereas many SMAW sets now are inverters and high-90's percent efficient - so those 3.12kW (British 240V 13A max) give almost twice the bang-per-buck and even up the productivity. No loss of productivity outdoors with stick, which is one of the few processes which is in reality rather tolerant of wind and rain. Then you are going to have much more trouble making bolted joints that in a well-set-up commercial shop, with all your marking tools, benches, ironworker for punching holes, etc, etc, etc.
In summary - it's no wonder you see a different picture where for your home fabs. welding is vastly easier and quicker.
It all makes complete sense - be assured of that.
Regards, Rich Smith
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writes:

So MIG indoors but stick outside. I though flux-core could stand a breeze too. Does the time the crane spends holding the beam in position figure in?
For reference, I do have a milling machine to locate and drill gusset plate and beam end hole patterns, a 1 ton crane to lift steel, and my welding and plasma cutting circuit is 240V, 100A which is half the panel's capacity. I'm equipped to make and test prototype robotic and aerospace components when I'm not sure what I want without seeing (and modifying) the mental concept. The boss told me my parts looked like they came from a Norden bombsight
The sawmill etc were retirement projects.
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That's a lot of experience! Need to only say what I can reasonably comment.
Gassless FCAW (Flux-Cored Arc Welding) can be used outdoors, yes. Never met it - seen it in a welder testing ("Coding") centre once but not watched what its like, running.
With shielding gas FCAW - not outside.
Crane time - yes, I would reckon - all times and use of resources add.
I think I have said as much as my experience permits.
Best wishes, Rich S
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On 1/21/2020 1:31 PM, Richard Smith wrote:

works well outdoors as long as the wind isn't too bad . It does burn hotter than solid wire , probably because the flux consumes the oxygen in the weld zone (I think ...) . It does spatter more , and you do have a little flux to clean off the weld , but the flux is usually pretty soft and easy to remove - the spatter is harder to get off . The main reason I use it is because it does burn hotter and I've been doing repairs to thicker sections , right at the limits of my Lincoln 110V Weldpak unit . If I need more power , I use either the 225A (AC only) Tombstone or the TIG (AC/DC 250 amps) welder in stick mode .

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Snag
Yes , I'm old
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writes:

Thanks. I've acquired a heap of galvanized tubing that might become an upgrade to my 50' antenna mast, and was wondering if I'd missed a reason why welding on a structure was discouraged, since it's how ships are built.
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* you'd burn the galv away around the weld - and the weld never has any galv (spray with zinc-based paint - which will need periodically reapiring / re-applying)
* you are not supposed to weld over galv. Zink toxicity & disturbs arc (arc goes a lilac colour) & could affect weld strength and fusion
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writes:

We went over this a while ago, and I asked you what you paint on the weld because the brush-on zinc-rich paint I have lets rust bleed through after a year or three, even though I sandblasted the area clean first. I then sprayed on waxy LPS-3 which kept the rust from expanding, but it seems to need some existing rust to soak into or it washes off.
The goop that does last outdoors is Ox-Gard, for aluminium electrical connections. The element and feed connections on my antennas remain at a few milliOhms for many years after scrubbing them and quickly applying it. I measure the resistance with a voltmeter while 1.00A flows through the joint, 1mV = 1 milliOhm. I had to drill out the rivets and install aluminium screws and nuts.
Our digital TV reception is much better than the old analog, and TVs aren't taxed in the USA, however almost everyone prefers to pay $150/month and up for cable. Antenna reception is pretty much a do-it-yourself project with no repairmen to call. This British digital receiver with the spectrum analyzer program is a great aid in aiming the antenna to minimize multipath. https://www.sdrplay.com/rsp1a/
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On 22/01/2020 22:42, Jim Wilkins wrote:

I used some Rustoleum aerosol cold zinc spray on some outdoor galvanised steel I had to weld and after cleaning and applying 5 years on no sign of any rust.
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On 01/23/2020 05:00 AM, Gunner Asch wrote:

ayup, oilytown Taft, home of the quadracentennial Oildorado Festival, the atmosphere lays down this protective oily grime onto purt' near everything in sight.

Ayup.
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On Wed, 22 Jan 2020 17:42:17 -0500
<snip>

I'm just beginning to work with an RTL-SDR I picked up some time ago:
https://www.rtl-sdr.com/wp-content/uploads/2018/02/RTL-SDR-Blog-V3-Datasheet.pdf
So far I've been pleased with it, works better than I thought it would. Sure could have used this back when I was still working as a two-way tech...
Already considering a HackRF but the one you linked to looks pretty good too. You could have used an RTL-SDR for your antenna job for ~$30. So I figure you are using the RSP1A for other stuff too...
Happy with it, caveats?
Using Linux nowadays, so I have to check for software compatibility. Looks like the RSP1A is probably supported.
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Leon Fisk
Grand Rapids MI
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wrote:

I lived in the world of high-end precision measurement long enough that I want at least 12 bits of accuracy; the RSP1A has 14. My portable DVM resolves to 1mV in 22.000V. Back in the early 80's I went to the trouble of designing and building a 4-1/2 digit multimeter because I couldn't buy one.
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On Sat, 25 Jan 2020 10:42:12 -0500
<snip>

I noticed that in its specs...
Early on I concerned myself with minor differences in voltages and other bits of minutia. I soon learned this rarely had anything to do with my need to fix something. Watch the relative values and go for the likely failures. Like the old quote said, "round up the usual suspects".
Thanks for the explanation :)
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Leon Fisk
Grand Rapids MI
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wrote:

I agree that repairs don't need it, but R&D requires not only high accuracy but NIST-traceable calibration. I like it for hobby use because it shows trends rapidly.
Inaccurate measurements and other poor lab technique have led to false claims of room-temperature fusion etc.
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Laser'ing is really the great thing - assume it's gone even more that way in the States? Avoid having to debur punched holes, flatten plates again, etc. Holes all there laser'ed. Get a pallet-load of plates with identities "etched" with defocussed laser beam.
I've made big-ish platforms with bolted connections not needed for any structural reason, solely so the broken-apart structure will fit on a 3~1/2 tonne flatbed truck.
Regards, Rich Smith
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Gunner - you are almost certainly right. We say "lasered", but it is probably plasma-cut. Transfer CAD files to profiler and the CNC cutting machine makes them to-drawing.
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Hi again Gunner, and anyone else who wants to join in...
So this thread - it's more of analysing structural performance - strength / stiffness / load-bearing.
Something I find really exasperating here, in the UK. Is the same in the US? You opinion? I think that with * CNC plasma / laser cutting * press-braking with a lot of software guidance * high-strength tough thin plate * highly-controlled welding - even if manual (GMAW processes) * CRUCIALLY - Finite Element Analysis modelling easily done you can make much higher performing structural assemblies from plate, not assemblages of sections - various angles, box-sections, etc. - for much nigher-performing steel fabrications. Much stiffer, much more load-bearing to weight, well-predicted fatigue resistance at high cyclic loads, etc. Fairly-much - make in welded steel (cheap) for ad-hoc machine-chassis, etc., to overall design strategy of riveted aluminum aircraft sub-assemblies (expensive). Finite Element Analysis enables you to know under design loads the stresses, deflections and likely fatigue resistance of the proposed design which the fabricator "details" to the overall specification of the component. The thinking is so conservative here and there seems to be not a single person in any engineering / leadership (none of that - is "management") role with whom you can talk the absolutely obvious. I spent about 30 days busting my brain around how to use a Finite Element Analysis package, and went from zero to being show the falacies in shoddy work with no effort put in by contracted-in engineering consultants. If you know FEA at all - "shell elements" enable you to model thin plate structures very readily and economically. It is very difficult to make a design for a single component which will take more than a minute of a current personal computer's time to solve.
I did a web-page about this concept http://www.weldsmith.co.uk/skills/fea/1706_thinplt_str/170601_thinplt_laser-fold-stl.html
It's so exasperating that what is obviously and readily done by someone working "on the tools" is invisible by reason of unfamiliarity to most in "leadership"...
It seems that there is a "lazy" assumption that progress is only being made in "leading" endeavours like computing, bio-whatever and so on, and no-one but those on-the-tools can see there's similar levels of advancement possible in "traditional" (sic.) endeavours, as the overall technological advancement lifts the "baseline" of what is readily possible.
Thoughts?
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