Engineering question

[Anonymous-0]

With load-bearing from above, is channel stronger with the legs pointing down or up? TIA

 

[ddl]

chinese or american steel?

 

[dpendzic]

the ultimate load on a channel is greater with the flanges down. that way they are in tension, --they tend to buckle when in compression.

 

[Dusty MI]

(quoted from post at 13:43:04 05/19/12) the ultimate load on a channel is greater with the flanges down. that way they are in tension, --they tend to buckle when in compression.

Not an engineer but experience is, steel will stretch quicker that it will compress/buckle.

Dusty

 

[dpendzic]

The modulus of elasticity for steel is the same for compression or tension so the movement is the same.
the thin flanges being unsupported in their height will reach the compressive buckling stress quickly.
If they were supported laterally then buckling wouldn't be a concern and of course a lot depends on the channel section itself--if its a heavy channel with short flanges then buckling is not a problem. In bridge I beams the top flange in compression is always the critical part if its not supported laterally---thats one of the reasons that the concrete deck is usually tied into the top flange .

 

[Anonymous-0]

This is from experience, not anything scientific, if you hit algore in the head with a channel iron, with the legs down, you will kill him. If you hit him with the channel iron legs up, you'll likely just bend good steel!!

 

[frdmnn]

if you try to bend structural c-channel in an arc 90 degrees on a curve machine, their is a hard way, and an easy way. it bends easier with the webs pointing to the outside of the curve, so you are stretching them instead of compressing them.

 

[MarkB_MI]

Speaking in general, the stiffness of a beam is proportion to the moment of inertia of its cross-sectional area around the bend axis, or what is more properly called the "second moment of area". I'm not a structural engineer, but I believe a channel is actually stiffest when it's on its side. That is, if the load is from above, the "legs" are horizontal. If you orient it in the weaker direction as you suggest, I don't think it makes any difference whether the legs are up or down.

Of course none of this takes into account deformation. If the beam is unsupported, it could be subject to forces that would change its shape. You don't describe how you're going to use it, so it's difficult to guess how the beam might deform and become weaker.
Second moment of area

 

[Fergienewbee]

In construction projects, it depends on the use; I have seen them installed both ways. To form the edge of a cantilevered roof overhang I have usually seen them with the legs in or out. I've seen them suspended with threaded rod with the legs down so wood blocking can be bolted inside the legs for attaching drywall or other materials. It would also depend on the thickness of the web--the vertical part--and the legs. I thin web is subject to crippling.

Larry

 

[Ken Macfarlane]

If spanning between supports, legs down is stronger, buts they are both completely terrible compared to using the channel on edge.

 

[dpendzic]

Right on MarkB--We structural engineers actually use a property called section modulus which is derived from the moment of inertia and the point in question distance to the neutral access.

Lets take a typical channel--say C6X13.

The stress in the extreme point is the bending moment divided by the section modulus.

the section modulus in the weak direction is 0.642 in cube. this is if the load is applied in the direction of the flanges.

If the channel has a load applied perpendicular to the flanges the section modulus in that direction is 5.80 inches cube.
That is 9 times stronger in that direction.

 

[Loosehandle]

Ha ha good one!

 

[big fred]

Experience or wishful thinking?

 

[Tims A]

First off lets contract this out to a company half way around the planet. You know where the alleged 4 or 5 thousand engineers went to an unknown university an make 1/4 what I do. Now lets get some lawyers involved to manage the contract, billing, contract changes, renegotiate the pricing and responsibilities. Hmmm were now going to need someone to manage the business cause we can't communicate well with these highly educated folks, that"s our fault. Projects late, that's ok cause the work force is scalable, we"ll add a hundred more bodies and get er done. Didn"t get what you wanted? Weeel submit a fully specified engineering drawing and they will get after it for you. Someone stole your design, your attorney's can take that up with the local government. Yadda Yadda Yadda, people getting retention bonuses much greater than my yearly salary have this all figured out. Oh I feel better now!
Tim

 

[Dusty MI]

You feel better!!!!!!
Not me, I'm retired and have been feeling good, now you bring back memories from my years of working and have got me wrought-up. :&gt

Dusty