Lifting Analysis

Published on June 2016 | Categories: Documents | Downloads: 28 | Comments: 0 | Views: 1120
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lifting analysis I noticed that you haven't gotten any responses to this post. If you still have a question, you may want to consider posting some additional detail, perhaps some drawings, sketches, etc. It sounds like you are describing a relatively specific application, so this additional material may help to generate some responses from others who are not as conversant with your application and terminology. Cheers, Chris

You have over complicated the problem. Take the lifting structure and put four pin supports at the pick points, at the bottom end of the sling, then apply a vertical load at the hook. This will simulate the system adequately though it will not take into acount any movement which I suggest you treat as factors to increase the load. From you picture it is apparent that you have four pock points on your structure. My methed would allow you to also elliminate one support so that you can see the effect on the balance of your lifting structure if one point lets go. Again it will not give you the movement but it will tell you what the forces will be in the members. Though it is nice to put the member properties in the model they are not relevent in this case as all you are interested in are the forces and not the stresses. From the forces you can select the proper chains or slings for the job. Hence you can use any section properties for the elements, in this case I suggest using a round section though you could use any.

Hi , I have some queries on this 1. Shall i use any value for the spring? or only 0.05 kips/in (as mentioned by Nihesh) 2. is that the spring value is based on what load i apply on the system? regards

Murali

Hello, Make sure that your cog of lifting and framing is on same vertical plane. this is becouse your lifting fromthe lower elevation, due to any jerks or any lateral load may create high moment at the bast or may your structure get foul with sling which is not recommended. answer to your question -make all the slings tension only members. -define two spring supports at the down of structure diagonally. -keep main lifting hook support as a pinned support.

thanks pawan

hi all, thanks for advice... i have already try your way muh... it is possible result, also for the reaction... do you have any idea about lifting point at the beam? so, the structure need spreader to lift. i mean is about the cord connected structure and the spreader , because it is intersect the member of the structure. it is ok?

Dear Nihesh,

What will happen if consider the cords as tension only members and use release but FY supports with KFY of 0.5. Make sure you have one support in each of the translational direction For me I am using small spring constant at the Y direction and release all other degree of fredoom at the lifting points. Considering tension only member for cords. This will hold the structure stable and provide reasonable results. If someone have other view then I will be happy to know more. hi all,... i am new in staad and also new in lifting analysis... could somebody give me advise in this :

1. in lifting point, should i add support joint ? 2. for sling property, what property should i use so i can get sling force?

thanks

regard raka

Lifting analysis and rotation There is no bearing, there are two plates (padeyes) on the gangway 219 dia member, there is another plate (padeye) at the platform. The gangway is joined such that the two plates are on either side of the platform plate. There is a a simple pin (or a bolt) connecting these plates. The winch operating speed is not an issue since it is a mechanical winch - manually operated - which requires an effort of 8 lbs per 1000 lbs of lift. They have tried to lift the gangway at an angle perpendicularto the gangway itself, they are able to lift it easily with a force of the order of 20kN. The current sling is a 62-deg to horizontal. Wonder if this 30-deg difference causes such a huge difference in forces. Thanks Subra

I couldn't tell much about the pin and bearing detail based on the initial drawings you posted, except to see that the hole was slightly oversized for the pin.

I also don't have a real good sense for how much rotational resistance could possibly be generated based on different pin/bearing configurations, but I suppose you could make some estimates and try to incorporate the effects by assigning a rotational spring constant to the supports that represent the pin/bearings. But you are correct that the non-linear analysis suggestion was not for the purpose of trying to account for the friction at the bearings. I was suggesting the non-linear analysis to see if it would help you to learn more about how the cable sag and loss of stiffness might affect the required cable pull force. The more I think about the scenario, the more I think that it might be mostly due to the inertial forces of trying to accelerate the gangway to the open position. Is there a way to control the startup speed of the winch so that it starts EXTREMELY slowly? That would minimize the acceleration, and therefore minimize the inertial effects to the point where it would be nearly the same as the static equilibrium case.

Chris

Thanks Chris I also guessed that horizontal component of tension is preventing the gangway from rotation. The winch is a Mechanical Winch of 10,000lbs SWL, i.e. 45kN. I will try nonlinear cable type analysis. But I doubt whether it will account for the friction at the bearing ends.

THanks Subra

Subra, I studied the model a little bit, and I agree that 32 kN looks like the magnitude that STAAD predicts (based on ideal geometry) to equilibrate the Selfweight and applied Dead Load only.

Assuming that the load cells were properly calibrated and all that good stuff, my guess would be that the difference might be due to sag in the cable and friction at the bearings. The sagged profile of the cable means that the geometry is not exactly as it has been idealized in the model, and if the sag is significant enough, it actually changes the line of action of the force to be more nearly parallel to the long axis of the bridge, which means more tension is required in order to rotate the bridge about its bearings. Along the same lines, I wonder if the component of cable tension that acts along the long axis of the bridge is increasing the friction of the bridge at its pins/bearings, adding to the required force to lift it. The other thing that comes to mind is inertial effects. It may only take about 32 kN to keep the bridge in stable equilibrium, but F=Ma says that it takes an additional amount of force to move or accelerate the bridge. Finally, depending on where the load cell was placed, it may have been on the motor side of the pulley, and it may have been capturing the effects of some losses due to that as well. You might consider trying a Non-Linear cable type analysis to see if you can incorporate the sag effects in the cables. Note that they can be defined by using the Define button in the Properties dialog, instead of using pipe sections. Hope this helps. Cheers, Chris

Hello I have analysed a Gangway Structure fixed to a platform, using STAAD. The Gangway is connected to structure at one end using a pin which allows for rotation and is being lifted with a winch and wire rope at the far end. The gangway sketch and the corresponding model simulation is attached. After installation the site personnel are experiencing high force in the winch and hence unable to fold the gang way. The same gangway was lifted vertically and there was not problem in folding....the effort used was to the tune of 20kN. the reaction received in the rope in current position is 32kN only.

The force being experienced at the rope, as measured from load cells, is more than 45 kN. One possible reason is structure is not behaving as modeled / envisaged. Can any one help ? STAAD Model and Sketch are attached. Thanks in advance Subra

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