Steel Tube Bend/yield strength
 

On Wed, 16 Jan 2008 18:49:56 GMT, Rich Grise wrote:

Crossposted to rec.crafts.metalworking,sci.engr.mech, please manage
followups intellignetly, thanks...

Anyhoo, the boss just asked me to look up OSHA regs on guardrails.

It's fairly straightforward - minimum OD 1 1/4", and able to stand
a minimul load of 200 lbs, from the top or the side. The maximum
spacing for vertical supports is 19".

So, I've got a thing like this to spec:

-------------------------------------------------- - rail
-------- ---------------------- --------------
| | | |
| |----- 19" min ------| | - verticals

36" to 45" high, and it will be welded together.

So I've got all this info, and I'm about to write a summary,
and the boss comes to my cube, and I tell him about the specs, and
he says, "What wall thickness?". I said, "It has to support 200 lbs.
from the top or the side." He says, "OK, you've got to calculate that."

So I've been googling for a couple of hours, and have found all sorts
of specs on steel tube, but nothing that tells me what wall thickness
it needs to support the weight.

From what I've seen of steel tube, it looks like it isn't going to
take very much. :-)

So, can I tell the boss that, say, .065 wall is good enough, or would
you recommend .120 wall? I can imagine sitting on a tube like that,
but I'm only 170 lbs, and I don't think he'll accept "gut feeling" as
an answer. :-)

In order to come up with a real answer you have to analyze the railing
structure as a whole, and it's almost certainly statically
indeterminate. Which means you need to account for the elastic
properties of the verticals as well as the rail, and the type of
joints between the members and the foundation. With some simplifying
assumptions (for example, if the rail is straight and short, ignore
the verticals and treat it as a simple beam) you could produce a
conservative design and just go with it if it's not outlandish.

My daughter, the architect, was ranting on just this subject last
week. The engineering firm her office uses assigned the new guy to
verify a railing design for her, and he was analyzing it to death. I
can't say I blame him if he lacked the experience to apply lessons
learned in earlier to designs to the one in front of him.

--
Ned Simmons

Steel Tube Bend/yield strength
 

Ned Simmons wrote:

snip

In order to come up with a real answer you have to analyze the railing
structure as a whole, and it's almost certainly statically
indeterminate. Which means you need to account for the elastic
properties of the verticals as well as the rail, and the type of
joints between the members and the foundation. With some simplifying
assumptions (for example, if the rail is straight and short, ignore
the verticals and treat it as a simple beam) you could produce a
conservative design and just go with it if it's not outlandish.

Ned is right. The structure is more complicated that you might think.
But I did the calculations based on similar assumptions to those Ned
suggested, and got the following results.

For 1.25" OD mild steel tube with a 0.065" wall, the minimum load that
will result in permanent deformation (not necessarily collapse) of a 19"
span is 491 lbs. With a 0.120" wall that figure rises to 765 lbs.

You can see what I did he
http://www.mythic-beasts.com/~cdt22/handrail.pdf

In reality, the handrail could be several times stronger than this due
to the stiffness of the joints. Or the vertical sections, which I
haven't considered, could be weaker and prove to be the limiting factor.

It's just an estimate. Don't consider it to be an answer in itself. Just
add it to your knowledge and intuition when making a decision.
Incidentally, those figures are provided without warranty. I might have
made a mistake, although I've checked them and the answers seem sensible.

It's also worth noting that handrails are not often damaged by humans.
More often they're damaged by things like prowling forklifts. You're
going to need stronger tubing than this to stand up to a forklift. But
it all depends where your handrail is going to be situated.

My daughter, the architect, was ranting on just this subject last
week. The engineering firm her office uses assigned the new guy to
verify a railing design for her, and he was analyzing it to death. I
can't say I blame him if he lacked the experience to apply lessons
learned in earlier to designs to the one in front of him.

Indeed. If I was designing a handrail I wouldn't do any calculations.
I'd go with my intuition.

Best wishes,

Chris