Mon Sep 27 08:53:47 1999
From: Dave K

Hi Bill and the group!

As Dan noted, good references for the structural properties of steel
include the Machinery's Handbook.  Another good reference for dimensional
references is to go to a local steel supplier (my local is Chicago Tube and
Iron) and beg/buy a copy of their product catalog.  Each secton contains a
brief explanation of the metalurgical properties, as well as the
specification for common manufacturing process.

There are many considerations when selecting structural metals, and one
which is most frequently misused is pipe... people use pipe to build things
which really should be using mechanical tubing.  Here's the difference:

Pipe is identified by OD and Schedule.  Schedule 20 being common, 40 being
stouter, 40, 80, and higher.  As the 'Schedule' gets higher, the wall
thickness also rises.  Pipe is intended to be threaded for fittings, or
notched for Victaulic-style fittings.  The load imposed on pipe is usually
only internal pressure and whatever stresses are required to hang the pipe
over a given span, and to thread on the fittings.  Pipe is usually welded,
with a seam cleaned and finished so that it imposes very little flow
restriction.  Because of these factors, pipe tends to be made from a very
narrow selection of alloys that don't handle mechanical stresses that one
would see in say, a roll-bar.

Mechanical tubing, however, lives in a different world:  It is used to
support loads and transfer force.  The alloys are more selectable... 1010
(mild) steel, 1020 (not so mild) steel, 4130 (serious Chromium-Molybdenum
alloyed steel), etc.  as well as aluminum 3000, 5000 series aluminum for
soft applications, 6000 for medium strength... a common structural aluminum
is 6061, and in many cases, it recieves a T6 hardening process to make a
very stout structural unit.  

Keep in mind that an alloy is rated by strength in Ksi (aka Thousands of
Pounds per Square Inch).  Mild steel is usually in the 28,000psi realm,
while slightly better milds are just over 30... while some exotic  ($$)
steels approach the 50ksi mark.  Typically, as the Ksi figure gets higher,
you'll have a tougher time fabricating with it.  And for 4wd fab purposes,
it usually doesn't pay to try using anything other than mild steels, as
you'll be spending lots of money and time just to loose a few pounds... and
by the time you've mastered it, you'll be welding up aircraft landing gear.

Mechanical tubing is dimensioned very differently from pipe.  Mechanical
tubing is labeled by Outside Diameter and Wall Thickness, and information
tables from your local supplier will usually have ID.  The manufacture
process usually starts out as some sort of flat sheet, which is wrapped
into a circle, then welded.  For many types of tubing, the wrap is done by
a sleeve-die that the metal is forced into, but in applications where
higher-strengths and greater quality-control (hardness, straightness, etc)
are required, the preferred method of forming the tubing is drawing it
(pulling it) over a shaping mandrel... as the strip is pulled over a shape,
it conforms to it, to yield a very straight tube with consistant ID.

ASTM specification (like A513) describes what the internal weldment finish,
straightness, wall consisancy, etc.,  must be within (as well as a bunch of
other things that we usually don't get too concerned about).  The weldment
is important to the tubing though-  if you're gonna fit one piece of tubing
inside another, they need to fit.  If you're engineering an 'interferance
fit' (where the internal tube is slightly larger than the external tube)
then that weldment is an area of very serious concern-  too much weldment
will cause the fit to be too tight... likewise, an improper weldment method
(ERW, etc) would turn out to be too weak for the force of interferance,
causing the tubing to 'rip' open along the seam.

As for structural channel, beams, etc., it's all very similar, actually
easier to deal with than tubing.  To calculate structural loading
properties of tubing, I and H-beams, channel, angle, etc, in beam or
columnar applications, the Machinery's Handbook is your best friend-  it
has an entire section called "Section Modulus"... this is the armload of
formulae that will tell you just how much that piece of metal will hold.

And if you hate math, look for a piece of software called Engineering Power
Tools (EPTools).  It's out there somewhere... it has all the section and
elastic modulus formulae in it, and will simplify your efforts greatly.

Remember:  Materials have no strength:  SHAPES have strength.

DK  :-)
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