Stiffness of round bar

The stiffness of all steels (regardless of heat treatment) are so close to one another that it will not make any difference.

It is governed by steel's modulus of elasticity, it's cross section and the load

You can't change the first.

You have to increase the diameter or decrease the weight. Use pipe

Paul K. Dickman

You won't. Heat treatment and alloy have almost no effect on stiffness. To get anything stiffer, you have to go to tungsten or tungsten carbide, or some equally expensive exotic.

=========== This appears to exactly correct.

One thing that you can do to improve stiffness is pre stress the tube or pipe by placing a threaded rod inside it and using this to compress the tube. The more compression stress you can apply without collapsing the tube the stiffer the tube will be.

Unka George (George McDuffee) .............................. The past is a foreign country; they do things differently there. L. P. Hartley (1895-1972), British author. The Go-Between, Prologue (1953).

As long as you're working within the elastic limit of the materials, prestressing won't help; superposition applies.

...

If you can keep it from twisting, bend it upward enough that it settles straight.

jsw

If it's an isotropic material, like steel.

If it's a concrete pipe, then stressing it in compression will certainly strengthen it against the stretch from bending, but I don't know if it'll _stiffen_ it.

Not that Proctologically Anxious, or Existentially Violated, or whatever his name is today, is using a 1" diameter, 7' long concrete tube.

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Which explains those flat bed trailers that have an arch built in.

Wes

It's a tricky question, because it varies from 8 to over 300 GPa. Steel is

It depends primarily on the measurement relative to fiber direction, but also to fiber percentage and type of fiber. There are high-modulus fibers that are brittle, and low-modulus ones that you can tie into a knot.

Typical values for high-performance automotive-type (racecar) epoxy layups, multi-directional (fibers arranged in multiple axes) is around 100 - 180 GPa. But that's per unit volume. Per unit weight, it's *much* stiffer than steel. Plain, individual fibers can run well over 300 GPa.

BTW, regarding your comment about concrete, its modulus in compression is only around 30 GPa. The modulus in tension is almost irrelevant, because the strength is so low. But pre-tensioned concrete is never in tension. You're either adding to the compression load, or reducing it.

Drapery hanging project? Too cheap for a Walmart traverse rod?

Prestressing won't stiffen a concrete tube either.

I didn't think it would. The only kind of stuff that would stiffen under prestress -- that I can think of at least -- would be something like caltrops in jelly, or a big stack of washers.

Dude, it would have to be some hellified drapes to require 1" solid bar, donchathink??

It's ackshooly for gym-type stuff: chinups, equipment/heavy-bag hanging, etc.

Have you noticed how much stiffer some aluminum is than other? Even within, say, the 6061 class?

It seems that plate is much harder/stiffer than bar. Even in my limited experience, if you give me two different pieces of say 6x6" x 1/4 cut from plate and bar, I can immediately tell the diff -- the plate almost has a ring to it.

And seems WAY stiffer.

Is this my imagination, or do you see the same thing? If this is in fact true, why such a diff in aluminums, and not amongst steels?

Also, the 1" bars I use in weight lifting *seem* much stiffer than the 1" HR stuff. More imagination?

It's your imagination. Like steel alloys, aluminum alloy stiffness falls into a narrow range, although it's a bit wider than that for steel -- around

Yup. Until they take a permanent bend, the stiffnesses of various steels (around 200 GPa) and aluminums (62 - 75 GPa) are very close.

Higher strength can give the allusion of greater stiffness. The actual stiffness (Young's modulus) is roughly the same for 1100 grade aluminum and for 6061, but bending a bar of each, the weaker 1100 reaches its elastic limit much sooner. So you never get to feel as much "spring" in it as you do with a same-size bar of 6061. This can feel like greater stiffness. But if you clamped the end of a bar of each in a vise, and hung a weight from the other end of each, you'd see that equal weights cause an equal amount of bend -- until the weaker one takes a permanent bend.

Aluminum-beryllium alloys are much stiffer, however, as are aluminum composites.

GAK! Allusion should be illusion.

Heh, DAT proly woke you up, eh? You proly won't be able to get back to sleep for like 4 more hours!! :)

And yeah, mebbe it is that better "spring" that makes stuff feel like harder, stiffer material. Inneresting....

Where no one needs to grab it, weld some of the same size bar lengthwise under the main bar- something draped over it will still slide past that spot.

Dave

Grind some flats on the end of the bar. Fasten it securiy in place AFTER slightly bending it in an arc to match the arc you get now when it is under load. It should just about be level under load after doing that presuming you can properly manage it's desire to twist with those flats, etc.