Beam deflected 8

[Joss10]

The horizontal beam for the overhead crane showed in the here attached photo is noticed deflected by naked eyes, more probably due to overloading.
Since the deflection is permanent (plastic deformation), shall it be considered rejected or there is some maximun allowable deflection value.
The spam is arounf 10 meters.
Thanks in advance for any input on this point.

 

[rb1957]

bent = failed ... no? ask yourself "would i stand underneath this beam when it's carrying the rated load ?"

it could be "acceptable" but the amount of analysis would be "daunting", IMHO.

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[JStephen]

First, I would check what elastic deflection is, and make sure I'm not just seeing that.
I can't think why the beam would be considered rejected if serviceable otherwise. Structural members can be cold-rolled to a radius and still be used for the design loads.
Just FYI, I can't see any deflection in the photo onscreen.
It may be possible to straighten a member also.

 

[jmen68]

OP are you trying to determine if this beam is structurally sound and in need of replacement? Obviously that would depend on your criteria for rejection. I think that a beam that is noticeably negatively cambered you should reject on a deflection criteria. This looks like a pretty simple overhead crane setup, and if you have a fabrication facility onsite you could size the beam on the crane (to fit your loading plans) and replace it with an off the shelf W or HP shape.

I also have the general rule of if something looks like junk, not always, but it probably is junk. Meaning if you spent the time to determine if the beam is sound, replace it anyway. In my experience things that look in bad shape tend to get noticed and not in a good way.

 

[dcarr82775]

If permanent deflection is grounds for rejecting the beam, millions of cambered beams are in trouble. If the slope doesn't prevent typical operation of the hoist, maybe just test it to the rated load and see what happens

 

[precast78]

First I would check if the posted capacity is correct or not. In the US, the factor of safety of crane like this is 5:1 to yield.

 

[jdgengineer]

I don't believe the 5:1 safety factor to yield is correct. Typically, the 5:1 safety factor is applied to elements below the hook (shackles, cables, etc.) and it is my understanding that it is 5:1 to load test (failure) and not necessarily yield (I may be wrong on this).

The bridge beam is typically designed according to a combination of ASCE 7, CMAA 70 or CMAA 74, and AISE Technical Report 13 (if applicable). AISC Design Guide #7 also has a lot of useful information.

It's my understanding that the bridge beam is typically designed for the full rated load + vertical impact factors + longitudinal tractive forces + lateral thrust forces. These bridge beam design is often times governed by fatigue considerations or deflection requirements (Light-duty crane L/600, Heavy duty crane L/1000). Unloaded the beam should have relatively no deflection as the self-weight of the beam and hoist is typically very small compared to the rated load capacity. Therefore, the fact that this beam has deflected so much is either a sign that the bridge beam was designed improperly or that the beam was significantly overloaded.

I'd run some calculations to check the bridge beam to see how it stands from a code perspective. If you find that the bridge beam is significantly underdesigned it's likely other elements of the crane may be as well...

 

[jdgengineer]

I realized much of what I said directly applies to the runway beam, but can be extrapolated to apply to the bridge beam (or close to). Either way, conclusion is still mainly the same.

 

[BUGGAR]

By the label on it, it looks like a manufactured unit. What does the manufacturer say (if the manufacturer is still available)?

 

[dhengr]

Evaristo10:
I’d check with the crane manufacturer. They should give you good guidance on these matters as regards their crane. The crane actually looks to be in pretty good condition. I would stretch a wire along/above the top of the crane beam and record the deflection without a load, say at the 1/8th or 1/5th points. Then load the beam with 2 and/or 5 tonne loads with the trolley at the center of the beam, and measure deflection again. Then go to the manuf’er. with this info., they will know the grades of the beam materials, etc., and should be able to make some judgements on the condition of their crane. This should not be a difficult problem for you to put some preliminary numbers on too. Remember, when you load a beam like this until yielding starts in the flanges, you will get some permanent deflection, which is what you might see when your beam is unloaded. But, at the same time the plastic mechanism has started and the deflection will grow quickly without much increase in load.

 

[ajh1]

In the US, cranes must be able to show they can lift 25% above the rated load to get their initial inspection permit. I don't have any information on what is an acceptable deflection under that condition, but I would think on what appears to be a non-motorized trolley any significant deflection would make it difficult to maintain the position of the trolley when there is a load present, let alone try to move the trolley laterally when under load.

 

[hokie66]

Agree that "naked eyes" are not good enough to determine what is deflected. To my eye, the top of the beam is level, and the bottom of the beam sags. That could be due to lateral distortion of the bottom flange, which would not warrant rejection. Some measurements are required before assessment.

 

[FLCraneBuilder]

I will take the time to respond to this one

1) Beware of optical illusions - get up there & run a string line, laser or other measuring method. Know the magnitude of the problem
2) This topic is often visited in the CMAA spec writing meetings I participate in. One school of thought is this reverse CAMBER (and we are talking camber) is irrelevant except for serviceability as noted by others above.
3) For this crane, the trolley is a free rolling unit - what we call a push trolley. It will tend to free roll to the center of crane if its excessively cambered. This can be a significant problem if not dangerous - depending on circumstances
4) Thanks to other posters for noting the 5:1 factor is NOT applied to structural members. I get tired to explaining this to our clients engineering staff or the spec writers at various engineering firms
5) While up there measuring camber, closely inspect for other anomalies that might not be otherwise visible... all components from the hoist chain to the beam to the end truck frame to the end truck wheels to the rail to the building. If there is excessive camber, and its thought to be a "new" condition, then all aspects to the crane system should be carefully evaluated.
6) We once found that a main girder had 4" of reverse camber on a 4 month old 5 ton crane.... The owner initially thought our girder design was inadequate. Then we found that a 20,000# coil up-ender (under this crane) had been pulled off its foundations - along with the epoxy anchors that held it in place
7) There are numerous charts out there that tell you what the "minimum" girder size should be on a crane of this type
I suggest a quick check to insure its properly sized.

 

[rb1957]

how would you account for the plastic stresses induced by the (presumably) permanent deformation ?

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[Agent666]

You can work out the residual stresses from the difference in plastic stress distribution and elastic stress distribution. Think of it as the elastic distribution being recovered when unloaded.

 

[zcp]

Seems like you should get actual measurements. Calc in current config. Load test. Then look into derating if needed.

ZCP

http://www.phoenix-engineer.com

 

[rb1957]

could go straight (no pun intended) to testing ... if it can carry rated load*SF then ok

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