cantilever system extended 8 ft past a ledge 1

[tazzfire05]

at my job we were wanting to do a cantilever system and we wanted to extend 200lbs out approximately 8ft.. i know every foot it extends the heavier and heavier it will become.. i was wondering at 8 ft how heavy will it seem? i calculated about 1400lbs.. would anyone know what the weight will be?

 

[MiketheEngineer]

200 lbs times 8 feet = 1,600 ft-lbs

But it still weighs only 200 lbs. This is Statics 101 - are you a student??

 

[moon161]

The structure weight may increase, but the weight of what you're supporting does not change. The moment in the cantilevered beam due the load will increase directly with the length.

Are you comfortable with statics & strength of materials?

 

[tazzfire05]

no im not a student, i just haven't done physics in about 3 years. I came across 1600lbs as well.. (1400 was a typo sorry). it just seemed 1600 ft lbs was too easy to find.
im reading about the strength of materials and statics from machinery's handbook. I wanted to use 4"x4"x1/4" tubing to extend the 200lbs. The formulas they had in the book were throwing me off. Since i had to find moment of inertia and the elastic modulus

 

[IRstuff]

Sounds to me that you need to get someone who'd done this sort of design before; an industrial environment is not the place to experiment or learn on the job. You can and should shadow whoever you hire to do the job to learn what to do the next time.

Just knowing the moment is just the begining of the design process.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[rb1957]

"was too easy to find" ... yes, sometimes the answers are straight-forward.

google "bending stresses" ... you should find an expression My/I
M = your moment, 1600 ft.lbs = 1600*12 in.lbs
y = extreme fiber distance (from the neutral axis) = 2"
I = 2nd moment of area of your cross-section (or moment of inertia) = (4^4-3.5^4)/12 = 8.8in4
stress = (1600*12)*2/8.8 = 4400 psi ... very small

 

[TheTick]

Not a student. Probably should be.

 

[msquared48]

Needs to investigate the concept of leverage...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

Check deflection...

 

[rb1957]

true, but at that low a stress level, shouldn't be a problem ...

 

[KiwiMace]

< 1/4" by my reckoning.

 

[rb1957]

how much will the tube weigh ? 4in2*100in*0.3 = 120 lbs ...
sizeable compared to the 200 lbs load ...
M = 200*8+120*4 = 1600*480 = 2080 ft.lbs ... 25% increase ...
stress goes to <6000 psi ... still very small

 

[KENAT]

Depending where you are located there may be building regulations and rules or similar relating to this.

First, they may require the design to be done, or at least checked/approved, by a suitably qualified individual, such as a PE in the appropriate specialization.

Second, there may be rules relating to safety factors/margins, load cases (static V dynamic V what happens in an earthquake...) and other design aspects you need to consider.

Third, even if you can safely size the beam, sizing/designing it's connection to whatever structure it's mounted on may be another matter.

I can testify to this first hand, a few years back I mounted a punch bag to a brick wall via the appropriate bracket for its size, I think I even chose to mount it on a buttress part of the wall thinking it would be more rigid etc. Sadly I hadn't taken into account the strength of the wall, or rather the bonding of the individual bricks via the mortar - especially on the top course.

Low and behold about the second or third time I used it I managed to punch it off the wall, or more accurately I suppose I managed to punch the top brick the bracket was mounted on out of the wall. I'd like to think it was due to my immense power and high work rate etc. but I'm pretty sure it was because I didn't adequately allow for the loads on the top unbound brick.

So, it may be worth contracting an appropriate professional who should be familiar with all the relevant codes etc. to at least give what ever you come up with a once over.

That, or at least make it stout out of things you know about and check your/your companies liability insurance to see if you're covered.

Good luck.

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