Mech. Eng. question-Tubing strength

[dean]

Hi

I am building a small crane. I need to know which is stronger from bending. A 7"x7"x.375" square tube or an 8"x8"x.25" square tube. Both in steel and 10' long.

Basically one end is the pivot the other is the load and I will be pushing up in the middle with a cyl.

Thank you
Dean

[vash]

been awhile. if i set bending stress for 100psi for both bars, and solve for max moment, the 8x8 gets 470 in lbs.
the 7x7 sees 530

so the 7x7 is more resistance to bending.

anyone want to check my math.

8x8 is c=4 in
I =, 75.1 in4
S=18.8in3

7x7 is:
c= 3.5in
I=68.7in4
S=19.6in3

[mschuep]

It has been awhile for me too

As vash demonstrated, as long as the steel specification is the same, I think the 7x7x.375 is your stronger section...

However, you might want to look at how you are attaching your section to the pivot...If your weak point is where the connection (i.e. bolts or weld) are, then you might be better off with the 8x8 due to bolt spacing, distance of bolt holes from end of section, or length of weld.

[vash]

dean, can you preload the beam?

weld the ends of the crane closed. drill a hole. run a length of cable inside on the half of the beam that sees the tension loads, and tighten it up with treaded rod. hell, the cable could be the threaded rod. preload it. concrete geeks do it all the time.

[David]

7 x 7 x 3/8

[Amail]

From CosmosExpress:

7x7x3/8 x 10' long, fixed at one end and 1000 lb load at the other end, factor of safety is 4.88

8x8x1/4 x 10' long, fixed at one end and 1000 lb load at the other end, factor of safety is 3.69

7x7x3/8 is stiffer.

[Schumi]

Bending stress is calculated as My/I where M is the moment at the cross section, y is the distance to the neutral axis, and I is the section inertia. All else equal, the section inertia is all that sets the two apart.

The lower the stress the better. Since Inertia is on the bottom of this equation, the greater the inertia, the lower the stresses, the better the beam is in bending.

I'm not going to calculate the inertias but by looking at them they are going to be very close.


This is not accounting for buckling, which may be the more concerning failure mode.

[Amail]

Originally Posted by Schumi

Bending stress is calculated as My/I where M is the moment at the cross section, y is the distance to the neutral axis, and I is the section inertia. All else equal, the section inertia is all that sets the two apart.

The lower the stress the better. Since Inertia is on the bottom of this equation, the greater the inertia, the lower the stresses, the better the beam is in bending.

I'm not going to calculate the inertias but but looking at them they are going to be very close.

If they are some sort of standard steel beam, there are lookup tables for inertias for standard stuff. Or it's easy to calculate section inertia of something simple like that.

I want to actually say the 8x8 has a higher inertia.

That was my gut reaction too, but a quick FEA run on both indicates otherwise.

[Schumi]

did your FEA have square corners? A real roll/extruded steel beam would have those nice rounded edges.

Sharp corners would contribute to stress concentrations which, on the thinner beam, be a bigger problem.


In all honesty both beams would probably be fine for whatever is being done as far as bending strength. Better focus may be to look at what type/sizing of bolts to use as to not yield them and using double shear if possible etc etc etc.

[Amail]

Round corners - I used the wall thickness to define the inside radius of the corner.

Without knowing the load, it's hard to say what the factor of safety would be. I do agree with your concerns of buckling - I think that would be a more likely failure than bending.

[sammyg2]

the tube is stronger with identical materials.

[sammyg2]

Originally Posted by vash

dean, can you preload the beam?

weld the ends of the crane closed. drill a hole. run a length of cable inside on the half of the beam that sees the tension loads, and tighten it up with treaded rod. hell, the cable could be the threaded rod. preload it. concrete geeks do it all the time.

They do that all the time to prove concrete but it's more of a quality control check, a structural or mechanical engineer doesn't have that luxury. They have to use math to make a determination.

[Amail]

Originally Posted by sammyg2

the tube is stronger with identical materials.

10' long, fixed one end and 1000 lbs opposite end

Size F.O.S.
7x7x3/8 3.31
8x8x1/4 3.00

Square tubing is stiffer than round tubing

[vash]

Originally Posted by sammyg2

They do that all the time to prove concrete but it's more of a quality control check, a structural or mechanical engineer doesn't have that luxury. They have to use math to make a determination.

huh? are you talking about determining compressive strength by breaking cylinders?

i am talking about a simple beam in bending. the top (assuming a cantilever beam) of the beam sees tension loading, while the bottom sees compression. i am talking about preloading it. put a compression member into the section experiencing tension.

kinda like taking a stack of book out of a bookcase. you can only successfully do it by pushing the stack of books together.

dean is this a college project?

[vash]

i thought we are working math on square tubing? i just pulled the spec numbers out of my steel manual.

we are working on round tubing now?

[Amail]

I thought square tubing too. Sammy said the tube is stronger using identical materials, I perhaps assumed wrongly that he meant round vs square tube. Looking back, maybe he meant stronger than a beam.

That would mark the very first time - ever - that I was wrong.

...and that would mark the second time.

[vash]

Originally Posted by Amail

That would mark the very first time - ever - that I was wrong.

...and that would mark the second time.

i like that. i am going to use it.

[dean]

Wow thanks for the replys.

It is a crane for my truck. It is going to be square tubing. The tubing does have rounded corners. I also thought that the 7x7x3/8" was stronger because it is much heavier.

My boom is going to have another extention tube in it so I can't do any comp. loading (not that I know what that is).

I am hoping to lift 2000 lbs at 9'.

Thanks again

[vash]

2000 lbs at 9'? simple math, if i was at my office.

let you know monday, if somebody doesnt crank it out first. tow boom = no preload. nevermind.

[Amail]

Quick FEA again - fixed end, 2000 lbs at the other end, 9 ft long, 7x7x3/8" has a factor of safety of 2.78. Your application with the pivot at one end and the reaction force some distance between the pivot and the load will yield a higher factor of safety.