i beam size

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

Second that comment. I used beam boy to size the 27ft long beam that holds up my workshop roof.

Mark Rand RTFM

Anyone have any luck w/ this download? The various primary/secondary download sites (at geocities, prodigy, or Getze's page) ain't workin.

How did you determine what an "acceptable" deflection would be? Building codes?

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

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 years.

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.

Regards Mark Rand RTFM

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.

The AISC standard uses a factor of safety of 1.67 in most cases, which=20 results in a working stress of 21.6 ksi for A36 steel.

That's a huge oversimplification. A deflection much greater than 1/8" is=20 acceptable in a long beam, and a very short beam may fail, probably in=20 shear, before it deflects 1/16".

Ned Simmons

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.

Ned Simmons

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.

John

True enough, but what's "a minimum" in that context? The load needs to be pinned, held by cable, etc. to restrain the horizontal motion.

Bill

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.

John

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

SNIP

SNIP

Ohhhh....what happened? Care to elaborate on that a bit??????

Take care.

Brian Lawson, Bothwell, Ontario.

Looks like an eye bolt broke, and the whole thing came down on the floor.

HTH.

Steve

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:

two words , dynamic loading

also not quite sure what the safety factor on that crane was , the beam was rivetted instead of welded , quite old