How can I calculate how much weight a steel gantry crane can hold.  I
am building one out of 6" x 4" .25" wall low carbon steel rectangular
tubing.  The horizontal section will be 9 feet across and rest atop a
vertical 6x4 tub on each side.  I want to know how much weight can be
suspended from the center of the gantry.

I have no idea if I've provided enough information so please let me
know if this is unclear.

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Clarification of Question by 2500hd-ga on 25 Apr 2006 15:33 PDT

My apologies for the typo - there are 6x4" rectangular TUBE steel for
the vertical posts.

It will be similar in design to this one:
http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=6970&productId=200276024&R=200276024

The steel is marked as follows:  "B6M5165 A500 GR B HSC"

Further, would I be better off using a standard W or S Beam for the
horizontal beam as in the link above or can I just use the recangular
tubing for similar strength?

if humans or animals will be under this loaded crane, You need to add
a safety factor.

If the movement along the crane is in any way rapid, Figuring dynamic
forces should also be done.

Does the use of tubs mean it is floating?

Was there a number associated with the steel,  like 1008 or 6061-T6
It might be printed on the tube or the invoice.

My apologies for the typo - there are 6x4" rectangular TUBE steel for
the vertical posts.

It will be similar in design to this one:
http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=6970&productId=200276024&R=200276024

I am picking up the steel today and will give the exact specs on it.

Further, would I be better off using a standard W or S Beam for the
horizontal beam as in the link above or can I just use the recangular
tubing for similar strength?

Assuming a yield strength of 30,000 psi, which is conservative, 4800
pounds can be suspended from the middle of the beam without undue
deflection.  That's if you mount the beam with the 6 inch side
vertical.

I'm surprised redhoss didn't answer this question.

Found a reference listing the minimum yield strength of A500 grade B
rectangular tubing at 46,000 psi.  Which means you can hang 7300
pounds from the middle of your beam.