Dynamic Lift Load Factors 1

[asixth]

Hi guys,

I have an a cage that is being hoisted at manufacturing yard with a lift load of 2,000kg (4,400lbs). I am designing the cage to LSD. What dynamic lift factor should I apply to this load to account for the uncertain dynamic effects?

I have refered to the Crane design code for my region and have found load factors to vary between 1.1 to 2.2, depending on the fundamental frequency of the cage and the lift velocity. My previous company used a 1.35 dynamic lift factor based on this code whilst my current employer uses a 1.5 factor but seems that no-one has ever questioned the use of this factor. I am interested to hear how others have handled this problem?

 

[asixth]

Does anyone have a similar application where they are required to use a dynamic factor for lifting? This could be for cranes or even the pre-engineered building industry.

 

[Ussuri]

We usually have to consider dynamic effects for lifting structures and equipment in the industry I work in. We operate offshore construction vessels and regularly have dynamic lifts in the offshore environment.

The dynamic effects come from the wave motion which induces vessel motion affecting the lifted object. This coupled with the stiffness of the crane and stiffness of the wire will affect the load. In addition, once an object is lowered subsea it will pick up additional forces from hydrodynamic action on the structure.

Not sure that is any use to you though.

 

[Dinosaur]

When concrete tilt-up construction is used I typically see a special multiplier used to account fot eh force necessary to break the concrete free of the formwork. I think it is about ten percent extra.

 

[EngineerTex]

How is your application a dynamic situation?

A code that you can use is API 2C American Petroleum Institute Rules for Offshore Pedestal-Mount Cranes. The dynamic lift factor is 2.0. That may seem high, but if you are dealing with truly dynamic lifting, it is absolutely essential. This is superimposed on top of AISC's allowable stress for the member in question.

Again, I would say that you need to verify that it is actually a dynamic lift.

So, dynamic lifts require an additional factor of 2.0. And that's the answer. Just for emphasis, though, I will give theory and explanation as to why the 2.0 is justified.

Conceptualize the following situation:
1) You have a mass supported from below
2) At the same time, you have a tension member attached to the load from above.
3) All of the slack is removed from the tension member such that loading of the tension member from the mass is impending.

In this scenario, if you instantaneously remove the support from the bottom, the tension member will see a force equal to EXACTLY two times the weight of the mass. There are two (if not more) ways to analyze what happens at the time that the load is transferred from the support to the tension member. The one I like is the analysis of the spring-mass-damper system. Neglecting damping, of course, the system will oscillate for an infinite amount of time with an amplitude that corresponds to a force that is twice the weight of the mass. Damping is not neglected, so you don't get exactly 2.0 times the force in the loading scenario, but you get incredibly close to it. So close, in fact that you will have to consider it to be 2.0.

This is exactly the case that Ussuri is talking about, except that in the case of a heaving supply barge, while the crane winch is taking up slack, the mass may be moving downwards at a speed that is faster than the speed encountered during the natural frequency of the crane-mass system. This is something important for you to consider if you are dealing with truly dynamic lifts, as this will impart an inertial factor that is actually ABOVE the 2.0 dynamic factor normally encountered in a dynamic lift. This is why for heavy lifts, i.e., a lift that is at 80% of crane capacity or more, the lift will not be performed in seas above about a half a meter.

So, in that my experience has been both the design of the offshore cranes and a whole boatload (literally) of equipment that is lifted by these cranes, the 2.0 dynamic factor on top of the AISC allowable load is the design philosophy I have used. The reason that I use API 2C for the lifted items is because I believe that if that's the factor you use for the lifting equipment, it stands to reason that you should use the same factor for the lifted equipment.

If you're designing a cage, I'm sure that you know that the CG may end up anywhere, since you won't necessarily have control over how the users will load the cage. In that case, you can also look at DNV (Det Norske Veritas) 2.7-1 specification for offhsore containers, but I can almost guarantee that that would be way way overkill for a dynamic lift onshore.

What type of dynamic effects are you getting with your lift?

-T


Engineering is not the science behind building. It is the science behind not building.

 

[Ussuri]

EngineerTex

I think we are talking crossed purposes here. I was considering a already suspended load. The vessel motion will generate additional accelerations over and above that of gravity. This will effectively increase and decrease the mass of the lifted object throughout the phase of the vessel motion. For example the DNV Marine Operations Rules for lifting (part 2 chapter 5) gives an offshore DAF of 1.3.

You are talking about snatch/shock loads on the crane due to basically a dropped object. The DAF for a shock load could be massively higher than this, but it is greatly dependent on the stiffness of the lifting system.

My copy of API 2C gives the dynamic coefficient as:

C_v=1+V_r[√](K/(g[×]SWL))

which is dependent on the crane rating, stiffness and velocity. I couldn' see a single value give as 2.0, which clause is it in.

 

[warrenw]

asixth:
I don't know what "designing the cage to LSD" means so what I say may not apply. So for what it is worth;

ASME B30.20-1999 "Below-the-Hook Lifting Devices"
Chapter 20-1 Structural and Mechanical Lifting Devices

"The load bearing structural components of a lifter shall be designed to withstand the stresses imposed by it's rated load plus the weight of the lifter, with a minumum design factor of three, based on yield strength of the material"

 

[EngineerTex]

Ussuri,

I think that you're correct that we're talking about two different things. My experience has been with cranes on platforms that are fixed to the seabed, lifting objects off of heaving barges/boats. From your post, it sounds like you usually deal with cranes mounted on the boat/barge itself.

In API 2C, section 3.2.1, Bottom Supported Structures, paragraph "c" states: "In the absence of a specified Significant Wave Height from the purchaser, offlead, sidelead, and wind forces shall be taken as zero, and the dynamic coefficient shall be taken as 2.0"

I consider this to be pretty reasonable for the specific application, but I still don't know what asixth is going to be doing on land that would cause dynamic effects. I'm assuming that his crane will be stationary and that whatever he's landing his cage on may or may not be moving. That was why I was asking how he had dynamic loads imparted to his cage and/or crane.

And I would agree with you that shock loads can potentially be much higher. Again, that's why heavy lifts are generally not performed during significant wave action.

In the case that the crane itself is experiencing the dynamic effects, then I would agree that your equation is appropriate. But asixth, note that along with the equation that Ussuri posted, the minimum dynamic coefficient is 1.33.

-T



Engineering is not the science behind building. It is the science behind not building.

 

[Ussuri]

Yeah, that makes sense. The cranes I deal with are pedestal mounted on the construction vessels we work with. We dont have the situation of the 'support' ie the vessel, moving away from the load under a falling sea and then having all weight transfer to the wire very quickly. We dont use API 2C as it is not applicable for shipboard cranes (actually we don't design them anyway, we buy them from a supplier like Kenz or Hydramarine).

Our lifts are done in a more controlled manner as the load and the crane are attached to the same thing and hence moving in the same phase.

asixth, BS 2573 gives a variety of impact factors of anywhere between 1.1 to 2.0 depending on your type of crane. These are typically onshore cranes, not offshore as discussed above.

 

[asixth]

Ok, thanks for the replies everybody, they are very insightful.

EngineerTex,

The crane is onshore, I believe what you are referring to when you say dynamic loads are the additional forces acted on the crane by wave action. This is certainly not what I was implying when I referred to a dynamic load factor. For my scenario, the force applied to the lifting frame cannot be considered static, so I was looking for a factor that I could incorporate into the design to account for any magnification of forces due to impact during the lift.

I am interested by your thoughts that the tension seen in the cable when the support is first removed will equal twice the weight of the mass. Can you further explain this theory or redirect me to an appropriate reference.

warrenw,

When I mentioned "LSD", I was implying limit state design. The steel strength reduction factor is 0.9, the live load magnification factor is 1.5, so my factor of safety without including an additional "dynamic/impact load factor" is 1.67. At present, I am inclined to use a dynamic/impact load factor of 1.5, this will give an overall factor of safety of 2.5, this would be a consistent will the responses in this thread that vary between 2 and 3.

Ussari,

Thanks, the impact factors in the British code are consistent with the values set out in the Australian code, I don't particularly want to use the lower end of the scale because it is very hard to impose a "controlled lift" with the guys on-site.

Once again, thanks everyone for the replies.

 

[CTW]

I second warrenw's recommendation. The cage can be considered a below the hook lifting device, thus follow ASME B13.20.