Crane Camber, Logic and Significance! 4

[Gourile]

To all,
Cranes are conventionally manufatured with a camber. Some of the crane design codes as CMAA70 give direct regulation for the calculation of the magnitude of this camber,i.e. dead plus half capacity deflection. Others as UK design code don't mention it and some others, like FEM, have regulations of not having sag. The question is: what is the significance of the camber from scientific point of view, what happens if such a camber is ommited or even reversed to a sag? And would a sag affects long term operation and capacity of a crane?
Thanks in forward for your response.

 

[Ron]

One important aspect of camber (in the case of cranes, a reverse camber) is that they are visual indicators of load. As an example, if you have designed a camber equivalent to some percentage of the load capacity of the crane, as it is loaded you can see the camber reduce, thus giving you a visual indicator of loading. This is not as prominent with sag as you can actually produce more and more sag without having a visual "stop point or check point". The check point for camber is when it becomes flat or straight again. With sag, you just get more sag!

From a stress distribution point, camber can influence the buckling characteristics and can provide a means of redistribution of tensile and compressive forces/stresses in bending.

Ron

 

[ehcet]

Ron! I do not agree with you. Try to see from 25-30 ft
beloww the crane that the bridge deflect under load.
You can not!!!! Camber is required that a fully loaded
trolley is able to travel on the positiv camber or in
case of loaded crane on the horizontal negativ camber.
If you have the usual 60-100 ft. span you can calculate
the snagging from the loaded girder from positiv camber to
negativ. You will be surprised!
Ernie

 

[Gourile]

Ernie!
This is kind of a question that no one could answere it satisfactorily to me. Someone just called it a matter of aesthetics. A very old expert relates this to the breakes of the trolly to perform properly between two limits of the poaitive camber and negative sag. Some codes just ignore it and some others pay extra attention. Thanks anyways.

 

[Hagan]

If, by "camber" you mean the parabolic curve normally on the bottom half of the girder only, I have the answer. If you are talking about something else, I have no clue!!!!

Sorry, I know nothing about crane design, but the curved shape is calculated to give a beam of uniform stress throughout it's length..... am I on the wrong track here??

Rich

 

[ehcet]

Hagan!
The parabolic curve is either made by heating up the
girder center and then cooling it down so it deforms, or
by cutting the web plates in a parabolic form.To my experience there is no uniform stress along the girder.
Ernie

 

Hey guys,
Cranes are used by people. The "aesthetic" thing isn't - it's human factors. ( Looking over a truck-crane's boom is radically different from looking up at a tower cranes. )Ever seen a load "bounce" on a crane? They can act like fishing poles with streatchy lines, but hopefully with greatly minimized deflections...

Any operator can kill any crane, and a crane that's easy to use will be a far better faster one. Remember the pully loads are significantly offset to the center-line of the various beams - that's nearly universal. It gets worse with jib-cranes and secondary lines too.

Get your boots muddy and go see working cranes and watch them. Talk to operators, or at leas tell them you'd like to talk with them about crane design. (Remember, they typically work at least 10-12 hours in a "careful/stressfull" capacity, but would typically want to be helpful.) Go to the Forman first!!! PHOTOGRAPH the cranes and lay-out lines of force on the photos. You'll begin to understand a lot!

If the legal/insurance code you operate under requires something - guess what - it's likely to be WAY too expenive to get around it. People are conservative with TONS above their heads, near expensive buildings... Best to work with it if you must - but still be creative.

LTD

 

[Hagan]

Ernie,

If the curved along the beam (girder) is not to ensure uniform stress under load along it's length, then what is it for?

The use of such shapes is common practice in beam design for this reason. It allows you to use less material and therefore increases the load carrying capacity of the beam (because you have removed mass from the area where it is causing the biggest moment load and not contibuting to strength) This is well documented in Machinery's Handbook.

I guess the structure of a crane girder is a lot more complicated than a simple cantilever beam but I bet the principles are the same!!

Rich

 

[svc430]

Camber on overhead crane girders does not signigicantly affect the strength of the girder section. Its only purpose is to control deflection of the girder relative to the horizontal and, like your old expert points out, to limit the potential of the trolley to drift towards the center when lifting a load.

In general, cranes built to U.S. standards will have a girder camber equal to the dead load deflection plus one half the live load deflection (live load deflection includes the weight of the trolley). In sizing the girder section, the affect of this camber from a strength standpoint is not considered; that is to say, these girders are not designed as an arch.

Over time, the original positive girder camber will decrease, perhaps even becoming negative. It is at this point that trolley drift can become a problem. I've seen some Owners actually install trolley brakes or shim the trolley rails to compensate for this effect, still using the crane for many years after. This is not to say that loss of camber shouldn't be a concern. Particularly if such loss is rather sudden. A good structural inspection may reveal damage that is causing this loss of camber, for example, a fracture through the bottom cover plate of the girder section. In general, a crane girder that has lost its camber should be monitored more closely in the future. The overriding concern here is whether or not such loss of camber is associated with structural damage.

As to the amount of negative camber (sag) that is permitted, I've never seen any codes or standards that actually specify this (probably never will either). I have seen girders with as much as 3/4 of an inch negative camber without any apparent structural damage and still being used at full capacity. On the other hand, I remember one crane of riveted construction that actually had 6" of negative camber that was still in use; although a closer inspection revealed the bottom cover plate fractured complete across its width (that crane was mercifully taken out of service and then scrapped out).

 

[RLJ]

Wouldn't having a positive camber (upward arch) also allow for a positive/compressive stress loading of the upper girder plate and negative/tensional loading in the bottom girder plate? Both upper and lower plates would be in tension if the arch were flat to negative.

In addition, Hagan, yes the "fish belly" design does allow for a uniform section modulus across the girder. However, for modern designs of box grider cranes it is simpler and faster to design a uniform section. You will note that even with the fish belly design the grider will still have a positive camber.

Modern welded box style crane girders are built inverted on a frame that uses presses to form the camber. Cranes using I-Beam construction have the mill roll the beam with the required camber.

I also agree that if you have a girder crane that has lost it's camber you should have it professional evaluated before use. RLJ s-)