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Old 04-04-2022, 05:13 PM   #1
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How calculate max load for cantilevered strut?

I don't know how to figure this out, or even where to look to figure out how to figure it out.

Consider a section of 1-5/8" steel, slotted strut (superstrut) supported only at 1 end (cantilevered), with a length of 2.5 feet. How much weight will it support at it's end?

Not necessarily looking for a fish. A net would be better.

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Old 04-04-2022, 06:06 PM   #2
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Here's a net: https://www.engineeringtoolbox.com/c...ms-d_1848.html (look for "Cantilever Beam - Single Load Calculator").

I'm glad you don't want a fish - my undergraduate degree was in Physics, but that was 33 years ago.
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Old 04-04-2022, 06:11 PM   #3
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Start the video camera for ( you'll be a YouTube star). Buy a piece and screw it to something like a deck, tree, forklift, etc with 2.5ft sticking out and hang weights or crap you have lying around on the end until it snaps. Use only half the load from the experiment as a max load for your project! I'm no mathematician though! Total cost of answer is only $20 or so. Following to find out how real engineers might solve this one..
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Old 04-04-2022, 06:42 PM   #4
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Here's a net: https://www.engineeringtoolbox.com/c...ms-d_1848.html (look for "Cantilever Beam - Single Load Calculator").

I'm glad you don't want a fish - my undergraduate degree was in Physics, but that was 33 years ago.

Perfect. Muchas Gracias!
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Old 04-04-2022, 06:50 PM   #5
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Start the video camera for ( you'll be a YouTube star). Buy a piece and screw it to something like a deck, tree, forklift, etc with 2.5ft sticking out and hang weights or crap you have lying around on the end until it snaps. Use only half the load from the experiment as a max load for your project! I'm no mathematician though! Total cost of answer is only $20 or so. Following to find out how real engineers might solve this one..
Truly not a bad idea to be honest. I'm just too lazy right now to want to go outside.
Sometimes simple is best.

A couple days ago I was trying to figure out how far in from the edge of the bus I could mount lights & cameras on our roof rack to allow them to see/illuminate what I needed them to. Simple problem with a flat roof, but the curve made things complicated. I probably spent an hour measuring things & creating rough drawings before I realized if I just stood by the side of the bus and looked up, my line-of-sight pretty much told me what I needed to know
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Old 04-04-2022, 09:31 PM   #6
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Just another consideration is static versus dynamic loading-- what might work just fine on a motionless bus-- might not work on a bucking bronco bus going down the highway over bumps. The dynamic loads might be several times what the static load was.
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Old 04-04-2022, 09:57 PM   #7
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Just another consideration is static versus dynamic loading-- what might work just fine on a motionless bus-- might not work on a bucking bronco bus going down the highway over bumps. The dynamic loads might be several times what the static load was.
You sound like you know what you're talking about That's easy to tell from my vantage point, as I do not!

Considering the problem at hand (loads on our roof rack) is there a multiplier you would consider to provide a reasonable safety margin?

FWIW I was figuring on building something that would support my body weight (Fat as F00Bar I believe is the technical term) when parked, and maybe 1/3 of that in luggage when moving. Don't know if that's a reasonable expectation. Defintions: FAF == 250lbs. That's not one strut section cantilvered... multiples spaced between 28 - 56" apart (span of 1-2 windows).

Our hope is to keep everything as inboard & low as possible. But we have a short bus, and limitied storage, so I'm trying to look for ways to carry more 'stuff' when necessary.
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Old 04-04-2022, 10:53 PM   #8
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if you are within the weight of the unistrut... then....

imo, i'd go just shy of half the distance of the span.

if i had 2 beams, 24" apart, then i'd span the beams with no more than 4' unistrut. 1' hangover on each side. (a bit less than) half of the span.

there were lots off data sheets available for a google search on "unstrut deflection" i think the above example was rated for 1260 lbs from their chart.

but, with 2 posts and a beam, one post supports half the beam. it will just as easily support a cantilever going half the distance the other direction.

i have no degree in physics or engineering. this is my "line of sight" guide to overthinking your cantilever. you are limited to half the span, get an engineer's help to exceed that number.


https://www.google.com/search?q=How+...hrome&ie=UTF-8
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Old 04-04-2022, 11:08 PM   #9
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Just another consideration is static versus dynamic loading-- what might work just fine on a motionless bus-- might not work on a bucking bronco bus going down the highway over bumps. The dynamic loads might be several times what the static load was.
RVIA uses an 8X dynamic loading factor in its recommendations for propane tank storage racks in an RV, so a rack designed to hold a 40 lb. tank should be capable of supporting a 320 lb. static load in all directions. Lights and cameras are probably less of a big deal in terms of the consequences if they break loose, so a smaller loading factor would probably be appropriate.
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Old 04-04-2022, 11:13 PM   #10
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there were lots off data sheets available for a google search on "unstrut deflection" i think the above example was rated for 1260 lbs from their chart.
Yep. I looked at those. But it was my (perhaps erroneous?) understanding that was for a span that was supported at both ends? And I didn't know if I could extrapolate anything meaningful from it for a cantilever situation (I like using that word since I just learned it! ).

We're thinking alike. I'm just trying to not over-engineer (or more accurately, over-jack-o-all-trades, masta of none) any more than I have to. As it is that's probably exactly what I'll do, at the cost of 6MPG. On the plus side, if the earth's gravitational field ever weakens, I'll be laughing while the egg-heads float off into the ether
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Old 04-04-2022, 11:16 PM   #11
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RVIA uses an 8X dynamic loading factor in its recommendations for propane tank storage racks in an RV, so a rack designed to hold a 40 lb. tank should be capable of supporting a 320 lb. static load in all directions. Lights and cameras are probably less of a big deal in terms of the consequences if they break loose, so a smaller loading factor would probably be appropriate.

I really do owe you now.
Would a 5-gallon container of ospho + a few abrasive discs make us even?
Seriously - thanks That gives me a frame of reference so I can only over-engineer things by a factor of 4.
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