Mobile Crane Loading

[TS59401]

Hello,
Recently we have been asked if we should be including a horizontal reaction (similar to the design of bridge cranes) to our analysis of work bridges. We do look at wind loading on the structure and equipment, but have not looked at a horizontal load related to a crane pick. Has anyone else seen this before or implemented a horizontal reaction? If so, what percentages or factors are you using?

I have not been able to find any code recommendations for this situation either.

Thanks in advance.
Tom

 

[JStephen]

You might try running this by a crane manufacturer or supplier. Usually, the crane capacities assume essentially vertical lifting.
If a crane was rigged up as a dragline, that would potentially get a lot higher loading, but that's not a normal crane pick, either.

 

[TS59401]

Thanks JStephen; not a bad idea. Just to clarify we are just (at lease so far) considering vertical lifts and pile driving.

 

[dhengr]

TS59401:
Rigging training, crane lifting instructions and crane operating instructions, do instruct that the hook be over the C.G. of the load so that there can not be (should not be) any appreciable sliding or swinging of the load. Either of these are very dangerous and generally uncontrollable during the lifting operation, so they are not encouraged. Careless swinging of the load could also induce some lateral loading at the crane base, and there can be some nominal wind loading on the lifted load face area before they shut the lifting operation down. Truth-be-told, I’m sure that crane and crane boom designers do include a small lateral loading component to protect their own design and equipment. But, this is not some fixed percentage of the lifted load since 1 or 2% of the 100ton cap’y. is quite different than 1 or 2% of a 10ton nominal lift. And, they don’t advertise this lateral design loading or allow you to use this up indiscriminately. Then, remember that for your bridge to see this lateral load it must first go through the tip of the boom, which might be a boom loading or crane stability issue. I would consider some small lateral loading on your bridge for these contingencies, but then just wrap it into your wind loading/lateral loading factors. I would rationalize that at max. wind loading, lifting has likely been suspended, so they don’t all occur, at their max., at the same time. Obviously, the crane and boom sitting stationary, on your bridge, might see a max. design wind loading, but you say you’ve already considered that. And, I suggest that normal secondary lateral lifting loads might be covered by that max. wind loading condition. Thus, you haven’t ignored these loadings, but you have made a rational engineering judgement of how they may all work (be additive) at the same time.

 

[TS59401]

All good points dhengr and I tend to agree. I have one follow up question that you can probably help me with:

ASCE 7-10 suggests crane runway beams (stationary bridge cranes) be designed for 20% of the rated capacity + hoist + trolley as a lateral force. I think the reviewer is using this as the basis of their question. My initial thought is that this lateral force (described by ASCE 7) is dealing with accelerating the trolley after the load is picked and not the act of picking a load eccentrically. If I can explain that we don't need to use 20% as a lateral load, then I probably can show that the 1% or 2% is less than the design wind load.

As I am writing this; I bet that I can show 20% of the pick weight, applied horizontally at the boom tip, would cause crane instability. Whatever the lateral forces are - they cannot be enough to tip the crane.

 

[Lomarandil]

I do think that most mobile cranes on work bridges and trestles do generate traction and braking forces equal to a few percent of machine weight (depending on tracks vs tires, surface condition, how many flaggers are on duty, etc).

But I never designed a working crane for lateral load during a pick.

----
just call me Lo.

 

[TS59401]

Thanks Lo. That makes me feel better, since I have not ever included a lateral load for a mobile crane either.

 

[Guest090822]

I’ve done a lot of construction engineering-never have I considered lateral forces while set up on a bridge with a crane.

1. OSHA limits the wind speed for operating the crane. The resulting wind load pressure will be very small and insignificant.

2. Breaking forces? Most cranes when set up are going what 5 mph or less? I’m struggling to see any significant forces.

I’ve had situations where we had to lower crane booms because of bad weather/high winds, but I just don’t see how any type of wind loads are enough to overcome the dead load of crane once all the counter weights are on?

I don’t think you have anything to worry about.

 

[canwesteng]

Yeah, imo if you already have design wind load on the bridge and the crane, then you are covered since you can't lift during wind load events. You could just backcalc how much the delta is between your 25kph lifting wind and the design wind, and if that's 20% of the vertical your basically set to satisfy the reviewer with little work.