length of beam and method of supporting said beam are pretty big
factors , a four inch i beanm will carry that load if properly supported
i use a program called beam boy for sizing beams at work , it is
free-ware , just google beam-boy
i personally use the moment and bending stresses , and a safety
factor of 3 to four , with 36000 psi being a standard for most beams you
dont want to get over 9 to 11 k in the bending and moments , you will
see deflections of less that 1/16 inch for a properly designed beam ,
while anything over 1/8 inch you will see the bending and moment
stresses edgeing up into unacceptable levels
On a cantilever beam hoist you want to keep the deflection to a minumum
because with a heavy load it will want to roll down hill in the
direction of the negative deflection. It will take off by itself when
you hoist a load, and that can get exciting.
Every cantilever hoist that I have used never had any restraints or
locks for horizontal motion. Most of the heavier ones ( 2 and 3 tons)
have the cantilefer arm with a secondary turnbuckle rod between about
2/3 the was out on the horizontal and back to the upright above the
attachpoint of the horizontal beam forming about a 30 degree triangle.
i was pointedly being really simple , there really is no point in "P.E."
engineering in a hoist to unload a 1/2 or 1 ton pick up , some basics
will get you all the way home , while 1.67 is a standard in A.I.S.C.
work , there is not a crane, hoist or any other lifting device built
using that small of a safety factor , as shifting or otherwise unstable
load can load that system well over its rated factor very quick .
example would be an overhead crane rated for 70 tons is built to
a safety factor of four . and i have personally seen a 35 ton injection
mold break an eye bolt and bring the whole thing to the floor .
safer is heavier and more expensive
The question might be better stated as "why would a 70 ton overhead
crane with a safety factor of four fail when a 35 ton weight snaps off
one mounting point?"
I have a guess... the loss of a lifting point unbalanced the load,
causing forces at an angle to the crane.
Steve B wrote:
I Googled for building codes (this is in the UK and there are some fairly
clear and simple national regulations). I also looked for more general-purpose
architectural stuff relating to things like "acceptable beam deflection".
IIRC the number that popped out was a deflection of 1/400th of the span was
acceptable for a roof beam. This worked out at 0.81" for my beam. I designed
for 0.4" when supporting the entire roof load. Effectively this was a 4:1 over
design. I made no separate allowance for snow loading, since the worst snow
loading in this area is two or three inches a couple of times in the last 50
When the walls were built and the roof beam was up, but without the roof on
it, the beam was noticeably springy if you jumped up and down on it. Once the
roof was fixed on to the walls and beam, the whole lot became pretty well
rigid in feel when walking over it. More so than a typical wooden floor in a
house. Roof and walls are both 6" thick SIP's.
If you are designing a building you use deflection as primary criteria
because what goes in the building needs a stable structure and you want to
avoid floors having a bouncy feel. For cranes you use the moment and
bending stress.as primary and deflection can be greater.
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
I just downloaded Beamboy and played with it a bit and it works very
well, as far as it goes.
But anyone using it needs to understand that while it may yield good
data on stress and deflection, that's not sufficient to insure a safe
design. The most obvious missing factor re the case at hand, a trolley
beam, is lateral stability of the beam. This is usually not an issue if
the beam is built into a structure like a roof or floor system, but is a
real concern with a beam that lacks intermediate lateral bracing.
Just a heads up to warn against getting lulled into a false sense of
security by a neat piece of software.
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