Crane Runway Beam - Lateral Deflection

[m1ke99]

I'm evaluating an existing building for an increased crane capacity. Client wants to go from 30 Ton to 35 Ton. Overhead double girder, top running.

Everything is looking okay, except for lateral deflection on one of the crane runway beams. The runway beam on one side is on the exterior gridline of the building and laterally braced at the columns. The other runway beam is connected with a horizontal truss to the adjacent runway beam and therefore much stiffer for lateral deflection.

Is it okay to assume the lateral (side thrust) loads will be distributed to the stiffer runway beam. Or, does each runway beam need to meet the deflection criteria separately?

 

[Ron247]

I guess I am missing the Bridge Girder issue. I see no way the lifted load lateral can travel anywhere except along the bridge girder. After all, that is near the start of the load path and you cannot get to the end truck without traveling along the bridge girder from the lifted load. I think the disagreement is which if any direction can you rely on the load to go as it travels from the bridge girder to the end truck. I can run to the right with the load but the force moves the left girder or it can move the right girder in the same direction. It depends on which cheek is first to touch the rail, where my lifted load is when I stop traveling along the bridge girder etc.

 

[TehMightyEngineer]

It depends on which cheek is first to touch the rail, where my lifted load is when I stop traveling along the bridge girder etc.

This would make sense to me. I've seen gantry cranes under test loading shift suddenly when traveled until the cheek of the wheels were suitably bearing against the rail so I know that the friction on the rails isn't generally enough to hold the skewing and other lateral loads of the crane in place. Also, make sure mill wrongs don't drop washers onto the rail under 125% test loading of modified gantry cranes; they make a great wheel chock apparently.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[canwesteng]

I'd also question where your deflection criteria is coming from. From the crane manufacturer? Because if the concern is wheels binding at L/400 I think that is insanely conservative.

 

[WARose]

I think this paper illustrates very well what I am talking about:

http://www.ein.org.pl/sites/default/files/2018-01-12.pdf

In particular see Fig.11 and Table 1. Looking at that closely, you'll note one wheel in physical contact and the rest aren't. You'll also note (in Table 1) the forces are not equal per side.

The paper also provides real world outcomes. See Fig. 2 & 3 and note the uneven wear on the wheels and (in Fig 2b) the damage on the inside rim. Fig. 3 also shows rail wear (on one side only) which would contribute to the problem over time.

The paper also notes that even in a snug fit, this issue doesn't go away:

In case of constant contact between the wheel flange and the
same rail head, the crane travels in a straight line without any „waddling“.
During a longer period, this leads to wearing of only one rail
head side and its corresponding wheel flange.

 

[jayrod12]

I don't think anyone may be arguing your maintenance issue. But I would tend to be from the camp that still doesn't see how that issue is improved, or worsened by providing 100% (or at least the majority) of the lateral support on one side only. Based on the very last paragraph quoted, even in a perfect world, there's still the same maintenance issue.

 

[KootK]

I think this paper illustrates very well what I am talking about

The paper does not discuss lateral wheel loads caused by lateral runway deflections. Rather, it discusses plain old skewing issues that arise from crane issues rather than runway issues. That version of the skewing problem is independent of runway lateral deflection. In fact, one could argue that the more flexible beam would actually improve skewing related damage because the beam would offer up less resistance to the torque induced by the skewed trucks.

 

[CANPRO]

The way I'm reading this, 100% of the lateral load will tend to go to one side regardless of the comparative stiffness between the two sides - just due to misalignment, standard tolerances, etc...

So there is a chance that 100% of the lateral load could go to the "soft side". If that happens, one of two things could happen - if the "slop" in the crane assembly exceeds the deflection limit, the "stiff side" won't have a chance to engage until the "soft side" exceeds its allowable deflection. Or, if the "slop" is less than the allowable deflection, the "stiff side" should engage and restrain the entire crane assembly from exceeding allowable deflections.

With that said, having one side significantly less stiff than the other side may exacerbate the problems noted by WARose.

 

[WARose]

[blue](jayrod)[/blue]

I don't think anyone may be arguing your maintenance issue. But I would tend to be from the camp that still doesn't see how that issue is improved, or worsened by providing 100% (or at least the majority) of the lateral support on one side only.

I'm not talking maintenance.....I'm talking forces applied. See the pic I've drawn below.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1551465717/tips/railway_btim0m.pdf[/url]

If that "gap" is greater than the lateral deflection of the runway beam (without the backing truss).....there isn't going to be any help from the other side.

 

[WARose]

[blue](Kootk)[/blue]

The paper does not discuss lateral wheel loads caused by lateral runway deflections. Rather, it discusses plain old skewing issues that arise from crane issues rather than runway issues.

It's the same thing. CMAA's design manual includes a lateral force calculation from skew that you should include (should it control) in your rail beam design.

 

[KootK]

@CANPRO: I would argue that the slop is present in the system regardless of whether or not one of the beams is over-flexible. So if the crane can handle the slop on a normally designed runway, it shouldn't be a problem for this situation either.

 

[CANPRO]

The conversation started about how the loads are resisted and how the comparative stiffness of the rails effects the resistance. The conversation has shifted slightly to the source of those lateral loads. I think both need to be discussed together - its great to say the stiff side takes all of the lateral load, but as WARose is trying to illustrate, its more than just a function of the stiffness of the rails, you need to consider how that load is generated. You don't necessarily get to choose which rail takes the lateral load - if after installation the wheels are hard against the "soft rail", well that rail is going to take some lateral until the "stiff side" is engaged.

 

[CANPRO]

kootk, I agree that the slop and associated maintenance issues are there regardless of how stiff the beams are. See my post at 18:40 - I think it would only become an issue if your total slop is greater than the allowable deflection.

 

[KootK]

It's the same thing. CMAA's design manual includes a lateral force calculation from skew that you should include (should it control) in your rail beam design.

I think both need to be discussed together

There are two sources of load but only one of them is related to the lateral deflection requirement. And, as far as I can see in CMAA, it's not the skewing version. You'll have to pardon my crappy version of CMAA. I lost access the the latest stuff when I stepped out on my own.

You can tell from the whole span/wheelbase business that the skewing loads are intended to be a localized phenomenon resulting from the applied torque.

You'll not that the lateral deflection requirement is based on a percentage of the vertical load rather than skewing.

 

[KootK]

This is like the engineering equivalent of a cute kitten video: Link

 

[WARose]

[blue](Kootk)[/blue]

You'll not[e] that the lateral deflection requirement is based on a percentage of the vertical load rather than skewing.

Again: same thing. More weight.....more force from impact.

AISC Design Guide 7 says this:

13.2 Side Thrust
Horizontal forces exist in crane loadings due to a number of
factors including:

1. Runway misalignment
2. Crane skew
3. Trolley acceleration
4. Trolley braking
5. Crane steering

ASCE has cooked all this into a single number for design (i.e. 20%).

 

[KootK]

Again: same thing. More weight.....more force from impact.

I completely agree that skewing is a source of lateral load. However, it appears that you're assuming that skewing also plays into the lateral deflection requirements. And I disagree with that assumption. If you can back it up, please do.

The language on skewing forces always seems to be like the CMAA blurb posted above: "forces tending to skew the structure". Just like the clip below. What does an L/400 deflection requirement even mean in the context of a torque like that?

 

[KootK]

I think it would only become an issue if your total slop is greater than the allowable deflection.

I don't think that the slop should even enter into the deflection requirement. I believe that the lateral deflection requirement is aimed at accomplishing two things:

1) Preventing excess lateral curvature in the rail that might mess with wheel travel and;

2) Mitigating dynamic effects of lateral loads imposed by the crane.

All cranes have to be designed and constructed such that they can deal with their inherent slop. I believe that it is the deflection occurring after the slop is taken up that is the issue and the spirit of the L/400 limit. Other than the two issues mentioned above, does a crane really care if it shifts 4' to the left? I'd say not as it's just rigid body motion. And, according to code, you could permit 4' of lateral deflection so long as you had a 1600 ft long runway beam. Probably a truss.

 

[jayrod12]

Probably a truss

Made me laugh out loud.

 

[KootK]

Given that the new crane is 35 ton and they've been using a 30 ton without issue, I don't foresee a huge probability of a maintenance catastrophe coming to pass here.

Furthermore, the OP hasn't said the years this thing has been in operation and it's level of usage (Class A, B, etc).

@OP: can you enlighten us on this aspect of your problem when you get back to the office? Given that we're having a Battle Royale to the death on your behalf here, I sure hope that you're not skulking behind a bush someplace waiting to see who the last woman standing is.

 

[WARose]

[blue](Kootk)[/blue]

However, it appears that you're assuming that skewing also plays into the lateral deflection requirements. And I disagree with that assumption. If you can back it up, please do.

I already have have. AISC DG 7 calls it a source for lateral/horizontal forces and gives deflection criteria for the same. You may want to divorce the two.....but you just can't do it. AISC doesn't put out design guides for machines (AFAIK). They do it for steel.

An older AISC Journal article says this:

Lateral forces can be caused by mishandling of loads,
misalignment of the runway, crane skew, and seismic loads.
For design purposes this force is considered to act at the top
of the rail and perpendicular to the rail. The AISC formula
for determining lateral force is as follows:
Lateral force at each rail = 0.10 × (lifted loads + trolley
weight). Other organizations, notably the AISE Technical
Reports 6 and 13, have similar rules, and these should be
examined as the case may dictate. Also see Table 5.
Lateral deflection should be limited to about L/400.

'Tips for Avoiding Crane Runway Problems', by: DAVID T. RICKER, 4th quarter 1982.

[blue](Kootk)[/blue]

The language on skewing forces always seems to be like the CMAA blurb posted above: "forces tending to skew the structure". Just like the clip below. What does an L/400 deflection requirement even mean in the context of a torque like that?

I've drawn that/explained it too. It's very possible that one side could be engaged while the other isn't.

At the end of the day, you have to defend the design. My way would sound a whole lot better in a deposition.