calculation of load distribution on multi-lifting points with multiple snatch blocks 4

[Truc Do]

Dear all,

Please help me to calculate load distribution on a multi-lift points when tilting pre-cast panel using multiple snatch-blocks as illustrated in the attached photo.
Thank you very much in deed.
Bets regards,

Truc Do

 

[phamENG]

I'm having the same problem - image file won't open. Rather than attaching it at the bottom, use the image icon (with a camera) in the top bar of the reply pane. That lets you embed the image directly into your post.

This is usually not a place to try to sharpen your pencil. Being a little extra conservative on these types of calcs is usually a good idea.

Never mind. I see now that you double posted.

 

[Truc Do]

Dear PhamENG,

It is not double posted. Multiple fixed lifting points are differrent from multiple snatch block lifting points.
Regards,

Truc Do

 

[phamENG]

Sorry - very similar topics. I see now this is for precast and the other is for a truss.

There isn't anywhere near enough information here to go with, but in general I'd start with 1/2 of the weight of the panel for each lift point, all hardware and the connections having a factor of safety of 5.

This may seem extreme, and it is, but for good reason. There are a lot of variables here and it's entirely plausible that through wear and tear on the rigging equipment, slight variations in length from the manufacturer, an error on the part of the rigger or crane operator, or a hundred other little things you could end up with all the weight on just 2 attachment points. Given the consequences of failure, it's a good idea to be extra conservative.

Certain lifting devices - spreader bars, lifting beams, load compensators, etc. - can help mitigate this. I imagine your arrangement might warrant some reduction, but how much I don't know. I also question the use of blocks at every juncture. It creates an awfully unstable arrangement. I'd say the first set should be static slings, and then a system of blocks could help balance the load.

Again, all this is just shooting in the dark because I don't know the specifics.

 

[Lomarandil]

PhamEng's approach works, and can be a good starting point. Rarely do you save a lot of time and money by optimizing rigging.

That said, snatch blocks do pretty much prevent the weight from being split between only 2 attachment points (as can happen with fixed length rigging). The load won't be evenly distributed between all the points either, but somewhere in between.

Instability can be addressed through using fixed length slings, but use of a tagline from the ground or auxiliary line ("whip line) from the crane is more common for tripping operations.

If a more detailed solution is warranted, start from the basics.
[ul]
[li]Find the CG of the pick, and how it will relate to the load geometry as the load is tripped up (at least 15 degree increments, if not smaller). The crane hook will always be aligned with this CG.[/li]
[li]Next, geometry (sling lengths and rigging points) will suggest the angles of each sling at any given rotation. For a two-level rigging problem like this, you'll need to start with a guess at the locations of the middle-level blocks.[/li]
[li]With rolling blocks, the total load in any sling has to be constant, which eliminates one variable. You will often need to assume a weight distribution between the rigging points to eliminate another variable.[/li]
[li]Check statics (sum of vertical and horizontal forces) for each of your components. Any imbalance means the assumed sling angles (e.g. block locations) are not correct. Adjust your assumptions and check again.[/li]
[li]Once you have a working solution for each of your rotation increments, you'll want to check that your weight distribution assumption was reasonable. Otherwise, adjust and iterate again.[/li]
[/ul]

You'll draw a lot of sketches and geometry to start, but this is a problem that definitely benefits from a spreadsheet or MathCAD solution as you get closer to the end.

----
just call me Lo.

 

[robyengIT]

some info on lifting beam design

 

[dhengr]

Truc Do:
That’s a rigging scheme for lifting long tilt-up panels which are not fully reinforced for the bending stresses due to lifting at fewer & wider spaced lifting points. It allows the sling lengths to adjust as the panel tilts/rotates up, without radically and suddenly changing the lifting point loads. Stability requires that the left end (lower end) be weighted or held down during the lifting and tilting process. That might also be Lo’s tag lines or whip line. The wire rope forces at a & b or equal, and those at c & d are also equal to each other, but may be different than those at a & b. And, of course, the wire rope forces btwn. the top snatch block and the lower two snatch blocks are equal to each other. I agree with Lo’s approach to the problem and with PhamENG’s want for a fairly large factor of safety and concern about system stability during lifting.

 

[HTURKAK]

The set up shown on the picture requires six rolling blocks. Apparently , the lower end of the panel ( left end ) weighted on the ground which also pointed out by Mr dhengr as a requirement and a pink star for his comment.

In this case, the max. load distribution will be at the starting of the rotation ( essentially horizontal) and the C.G. of the panel will not coincide with the crane hook and you may calculate the forces using the rules of simple statics.

If you provide a sketch showing the lifting points configuration, weight and dimensions of the panel , you may get more help.

I just want to remind also, rotating the panel around one side touching the ground , will develop very high corner bearing stresses . In order to protect the corner line, you may use a steel angle and sand bed.

 

[Lomarandil]

Good clarification -- if the panel is tripped with one edge on the ground ("tripping in the dirt"), my statement about the crane hook and pick CG aligning is not true. That only holds for single crane picks with the load in the air.

More precisely, the CG of the pick will be aligned with the net center of support.

----
just call me Lo.

 

[rb1957]

I don't know much about lifting but that RH sketch scares me (if those are just rollers in the lifts). It seems very unstable and subject to movement.

If the lower lifts crossed (so that the left one picked up "c" instead of "b", and the right one "b" … that may be slightly better.



another day in paradise, or is paradise one day closer ?

 

[Lomarandil]

That's the whole point RB, when you are tripping the load from horizontal to vertical.

Not shown is another element (tagline, whip line or the ground) used to control that movement.

----
just call me Lo.

 

[rb1957]

sounds fraught with peril

another day in paradise, or is paradise one day closer ?