Steve, go to
www.pwr-tools.com and download their Engineering Power Tools.
The freeware can do this, or pay the $49 for the full version and lots more
features. For 3"x3"x.120" wall square steel tubing with rounded corners,
15.5' (186") long, ends simply supported, load concentrated at the center, I
calculate that a load of 1000 lbs would cause the center to sag 2.6" and the
maximum stress would be 38.9 ksi. Yield strength for structural steel is in
the 35-45 ksi range so 1000 lbs would just cause it to yield. If you want a
factor of 5 safety margin you could lift 200 lbs with each beam, with the
load concentrated at the center. The standard version can do all of that.
Using one of the advanced version features to allow fancier beam loadings,
if your load is concentrated at two points each 3.75' (45") from each end
with 500 lbs at each of the two lift points so the total is still 1000 lbs,
now the deflection at the center is 1.7" and the maximum stress is 18.8 ksi,
which is about half the yield strength. So about 1000 lbs at each point
(2000 lbs total) would just cause the beam to yield, and a factor of 5
safety margin would let you lift 400 lbs total - still less than half the
weight of your boat. Your idea of a truss beam would greatly stiffen things
up, but unfortunately pwr-tools doesn't do truss beams and my references are
at work so I can't estimate how much stiffer, sorry :-). Hope that helps.
-----
Regards,
Carl Ijames
"Steve B" wrote in message
. ..
I want to build an overhead "H" shaped frame for LIGHT lifting. The frame
is for between two containers that are 15'6" apart. They are reasonably
accurately oriented parallel and level their tops.
I was doing a cut schedule layout, and saw a couple of things.
Originally, I was going to put vertical posts coming down and resting on a
plate on concrete. Then I thought that I could span the whole thing with
one piece, and rest it on the top of the containers and make two span
supports.
My end goal is to have two parallel spans perpendicular to the containers,
10' apart so I can lift half my 16' aluminum boat with each span. I could
achieve that with just putting two pieces of 3" x 3" x .120" across there.
The lifting points would be outward from center, about three feet from the
container, so I would not be lifting the heaviest thing at the center of the
span. (One end of the boat would be heavier than the other. I want to be
able to lift the boat and pull the trailer out to work on it at home.)
I want one perpendicular center piece, forming a big H. The long legs will
be 15'6", and the short leg 10' long.
Am I okay to use this idea, and just rest them on the top of the container
on plates I shall make? Or should I put a support pole along the containers
for a total of four support poles?
Should I make any "A" shaped reinforcement along the top of the two spreader
bars to give the center a little more strength? Then put another short
piece between the apexes of the vees to give it more strength?
Does anyone know how to calculate what the lifting strength of the 15'6"
beams would be?
I mainly want this to lift items on to and off my welding table, and to turn
pieces of fencing and gates during manufacture. Total weight, 200# max.
The boat, I would say, with motor, weighs 1,000# max, but the lifting points
would be outboard of center, I'd say the boat is 6-8' wide, so I would have
about 8' divided by two on either side of the boat, putting more strain on
the beam closer to center, not in the middle.
I do know this is for light lifting. I have a lot of experience in rigging,
and know how to do a lot of tricks in lifting.
I personally think that if I put a vee shaped piece on top of each beam,
about 8' wide, and 140 degrees at the apex, that would give me more
strength, and help support the middle, hence less sag and more safety.
The end product would look like a house from top plate up, except for side
wall plates. End plates, vee truss, connector, and peak beam.
Input?
Steve