allowable deflection in lifting beams 1

[weron4u]

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

 

[dbuzz]

Australian standard AS 1418 specifies the following deflection limits for crane runway girders and monorails:
- 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

 

[chicopee]

I have not encountered deflection as a criteria to lifting beam design. Instead, I have used working stresses w/ a factor of safety of five as a criteria for bending ,shear and crushing. Where you have concentrated loads on the beams from the fasteners, be sure to reinforce the flanges to webb with welded gussets plates. The large factor of safety is warranted because of the abuse that theses beams are subjected during handling. With such large factor of safety deflection becomes a moot subject at least for the types of lifting beams we were fabricating.

 

[weron4u]

The only problem there chicopee is that when you are designing a 20ton lifting beam, it is not feasible to assign a factor of safety of 5 so that it theoretically holds 100tons. The cost of steel would be outrageous.

 

[dbuzz]

I agree that a factor of safety of five is excessive. Just design to the normal linit state method, including a dynamic factor to account for snatch load.

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

 

[weron4u]

What I am concerned about is the movement of the grain boundaries on the microscopic level. When a beam deflects past a certain point, I believe that it is no longer just stressing the beam, but that the grain boundaries are slipping and breaking as well. And if you bend past a certain point enough times, those microscopic cracks will propogate into larger cracks and eventually mitigate to the surface, causing failure below the fatigue stress.

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.

 

[NickE]

Whoa! hold on a minute here...

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!

 

[weron4u]

Nick,

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.

 

[chicopee]

weron4u- you will find out that a safety factor of five may not be as outlandish as you think. With the proposed safety factor, you will be probably around a safety factor of 3. Depending on the application of the lifting beams, don ot discount the possible severe abuse.

 

[chicopee]

WERON4U-I MADE THE MISTAKE OF MISINFORMING YOU ABOUT THE SAFETY FACTOR OF FIVE ON BEAM CALCULATIONS; UPON REVIEWINGS SOME OF MY PAST CALCULATIONS I HAD SELECTED FACTORS OF TWO. THE SAFETY FACTOR OF FIVE CAME ABOUT DUE TO THE SELECTION OF CROSBY FITTINGS USED OF THE BEAMS. SORRY ABOUT THE MISTAKE.

 

[dbuzz]

The "phenomenon" you are talking about re: “repeated bending” is called fatigue. Most design codes address it in one way or another. It is usually only critical when there is stress reversal (i.e. the stress in an element changes from tensile to compressive).

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).

 

[NickE]

Thanks dbuzz, for adding that note about fatigue. I tried to explain that phenomenon to weron4u, however I think I didn't do a good job. There may be a confusion between endurance limit (the stress level in reversed bending to cause failure in 10million cycles) and elastic limit (the stress level required to cause permanent deformation.

nick

 

[DRC1]

Beams are not subject to stresses. Stress are a theoretical construct to simplify design. Beams are subject only to strain, but using stresses greatly simplifies the engineering. ( I had a lot of trouble with this the first few time this was explained to me but I gradually came to see the point) Thus be controlling the stress you really are limiting the strains, which deals with your concern for deflection. However if you stay within the recomended guide lines for crane runways you should be okay. Most rigging is designed to a factor of safety of 4-5 against ultimate not allowable loads. Also you will want to talk to the trolley manufacturer about allowable deflections they require as that will probably be your driving criteria.
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!

 

[johmue]

If you want to be sure buy the CMAA #74 for single girder bridge beams and it give design specifications for hoisting. A simple factor does not take into account localized flange bending due to the trolley wheel loads. This standard takes into account both deflections and stresses. For example top running runway design should have deflections less than L/400 for lateral deflectios based upon 10% of maximimun wheel loads and verticle deflections less than L/600. Stress and localized stress must be considered separately. The bridge design is much more detailed to account for Dead Loads, Hoist Loads, Inertia Forces from drives, Wind loads for outdoor cranes, Collision forces, Torsional forces from stopping and starting bridge motors and so on. I wouldn't take the chance of applying a simple factor of 2 if you do I wouldn't stand underneath it.JOHMUE

 

[unclesyd]

As an adder to johume's post make sure you have radio control not the standard pendent type.

 

[weron4u]

johmue,

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.

 

[drgauf]

Yes, there are codes for this. However they involve mathmatics for you to utilize them,... and for you to ask this question shows some education needed....

 

[RiggitTony]

For lifing beams you need to refer to ASME B30.20 This will give you the design criteria required.

 

[weron4u]

Riggit Tony,

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

 

[RiggitTony]

Refer to Crane Manufactruers Association of America CMAA Specification #74 and #70 for design criteria for monorail and double girder cranes.