## allowable deflection in lifting beams

## allowable deflection in lifting beams

(OP)

I am very familiar with allowable stresses in beams, however I am not familiar with the allowable deflections in trolley beams used for lifting.

Someone gave me the following guidelines

L/602 - single girder crane/monorail

L/888 - Double girder crane

L/400 - Jib

However I think these are highly subjective to the application at hand.

Are there code issues that restrict the allowable load on lifting beams due to deflection before the maximum allowable stress is reached?

thank you,

weron4u

Someone gave me the following guidelines

L/602 - single girder crane/monorail

L/888 - Double girder crane

L/400 - Jib

However I think these are highly subjective to the application at hand.

Are there code issues that restrict the allowable load on lifting beams due to deflection before the maximum allowable stress is reached?

thank you,

weron4u

## RE: allowable deflection in lifting beams

- The lesser of L/500 and 60mm for simply supported or continuous spans

- L/300 for cantilever spans

As for a lifting beams, I've never seen special criteria. Instead normal deflection limits would probably apply:

- L/250 for simply supported or continuous spans

- L/125 for cantilever spans

## RE: allowable deflection in lifting beams

## RE: allowable deflection in lifting beams

## RE: allowable deflection in lifting beams

The deflection limits above are only to assess serviceability of structures, i.e. floor bounce, wheel travel, etc.

## RE: allowable deflection in lifting beams

It's essentially like bending a paper clip back and forth until it breaks. This happens because you bend it too far. Try repeatedly bending it less than L/500 and see if it breaks and I bet it won't.

## RE: allowable deflection in lifting beams

Weron4u- FWIW Grainboundry movement only occurs during creep- Ie stress at temperatures greater than some percent of the melting temperatures. (often assumed ot be 50%, but I dont want to assume thats true for all metals all cases) It really doesnt happen at room temp in steel.

As far a fatigue is concerned there are lots of theroys as to mechanisms and crack propagation. I'm going to figure that in any beam you would never exceed the yeild point of the material. Thsi is the point where the elastic strength of the material is exceeded and there is permanent deformation. To use your paperclip example: take your paperclip and bend it, not too far, till just before it takes a permanent bend. You could do this for a very very long time with out breaking the clip.

There is a value for most steels called the endurance limit it is the amount of bending stress below wich there will be no failure even when the cycle count exceeds 10million.

(I am not a civil so I dont understand structures)

Nick

I love materials science!

## RE: allowable deflection in lifting beams

Thank you for clarifying my misconception on grain boundary movement. However I still believe that there is some phenomenon present when it comes to large deflections. I believe that some beams can theoretically be stressed below the endurance limit and still fail due to excessive bending. Repeated bending past a certain limit causes some type of premature failure, even though it is under the endurance limit. The steel may not be overstressed in its Mc/I calculations, however it must have stress concentrations that cause it to deform due to the large deflection. If you look at a beam that has large deflections in it, there is deformation, even though it is theoretically stressed under its endurance limit. This usually happens in the compression flange. Keep in mind I am referring to beams that are properly laterally and torsionally braced.

This is where the deflection limits come into play. How far can a beam be bent (L/?) and not be subjected to these stress concentrations that cause it to deform outside its normal plane of bending.

## RE: allowable deflection in lifting beams

## RE: allowable deflection in lifting beams

## RE: allowable deflection in lifting beams

Lifting beams are not unique or complex structural elements. If you design to code compliance for strength, with due consideration for normal deflection limits, you will have a suitable lifting beam that could be used hundreds of times. Normal deflection limits for serviceability design (say L/250 for beams and L/125 for cantilevers) are only just identifiable to the human eye and could not be considered excessive.

While it doesn’t exactly match your situation, you might be interested in a paper entitled “Stability of I-Beams Under Self-Weight Lifting” (Dux and Kitipornchai, Steel Construction, Vol 23 No 2, 1989, Australian Institute of Steel Construction).

## RE: allowable deflection in lifting beams

nick

## RE: allowable deflection in lifting beams

AISC Quaterly Journal had an excellent article on the design of lifting beams. It is probably 10 years old or better. I can't put my hands on the copy, but they can probably help you. If I find it, I will post details.

Finally Lifting beams are subject to a lot of unanticipated loads, so you should be conservative in your design.

Good Luck!

## RE: allowable deflection in lifting beams

JOHMUE

## RE: allowable deflection in lifting beams

## RE: allowable deflection in lifting beams

When using CMAA #74 to calculate the concentrated stresses due to trolley wheels, do you add that stress to the stress in the tension flange, or is it treated separately? You stated above that they shoule be considered separately, but are they then combined? I don't see how the localized stress from the wheels would not be combined with the bending stress.

## RE: allowable deflection in lifting beams

## RE: allowable deflection in lifting beams

## RE: allowable deflection in lifting beams

B30.20 refers to "below the hook" lifting devices for attaching loads to a hoist. This unfortunately does not cover the hoisting beam itself.

Weron4u

## RE: allowable deflection in lifting beams