The simple beams were within a factor of 2 or 3 of just enough, so my gut
says your truss will be plenty - I just can't back it up with numbers
without looking up some stuff. If I get a chance tomorrow at work I'll try.

-----
Regards,
Carl Ijames

"Steve B" wrote in message
. ..

http://www.flickr.com/photos/deserttraveler/

Here's a pencil drawing. It is all 3 x 3. The house like frame would give
it rigidity, and take some of the bending out of the cross pieces by
transferring it into compression at the peak. The peak will keep the thing
from torquing. The D rings will be the lifting points for the boat, but
other than that, there will be three roller D rings on the main 20' beam.
Where it sticks 5' out from the crossmembers, it will be supported by chain
to the peak.

The biggest thing I will lift with this are an aluminum boat with a 40 hp
motor @ the four points, about 3' in from the sides where they rest on top
of the plate on top of the container. After that, the maximum I would lift
would probably never reach 500#, but it would be used for items up to 150#
during construction of the item on the welding table. By your estimation,
Carl, it should be safe for that.

What do you think of this design? I know I could overload this, but I know
the weight I would be lifting, and I'm thinking 300# would be a big lift.
This is not the design that I would like to do, but a compromise. I want to
keep the open space without losing it to supports. I was an Offshore
Petroleum Institute certified rigger, various union OSHA certified courses,
and a crane operator. I know this will do what I want it to do, be an extra
man for me when I need to flip a piece of fence, or lift something up on the
table to work on or weld. I'm just trying to get an idea on the max load,
and then I'll divide by 4 or 5 and live with that.

Steve