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George Zucas

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Edit: Sorry about the vague title, it was intended to be complex beam system boundary conditions but somehow it turned out like this.

Hello,

I am trying to learn complex beam system designs and I sometimes struggle to assign boundary conditions. For example I am trying to design the lifting system in the picture (only one of them). I made it the same for the most part; 12 I-beams attached to each other and diagonal pipes inbetween at the sides. Now I'm defining boundary conditions. There are a few options in my mind and they all give vastly different results. Here are some of them:

I made it such that the I-beams under the structure carry all the load (distributed over the length) and fixed the upper four corners. While this give plausible results, this creates only vertical forces and therefore the diagonals carry little to no load. This doesn't take into account the forces created by the angled ropes. So I don't think this is a good model.

Then I modeled the ropes as beams, disregarding their stress values since in this model their job is to transfer the load only. I don't worry about the ropes at the moment. Then I fixed the system in the hook of the crane. Since the hook only prevents translation I made this end as free in terms of moments. No translation allowed but that is all right? The ropes are free to rotate. This gives me some insane values for combined stress in the beams, hundreds of thousands of MPas. If I fix the top end completely ( no translation, no rotation) then it gives plausible results. Though I don't know how correct this is, there shouldn't be any moment resistance there.

Third, I only designed the beams and instead of ropes, I inserted appropriate forces instead ( removed the distributed load). But then where to fix the system? If I fix the lower corners then the beams under the huge structure carry barely any load since the system is supported at the corners.

As you see, I am at a loss. How do you approach to assigning these conditions? How should I proceed?

Any clue to lead me to the right path is appreciated. Also is there any good book that goes into detail about these systems? In school we pretty much only analyzed very simple 1-2 beam systems with pinned supports. My textbook is the same, same exercises over and over.

Hello,

I am trying to learn complex beam system designs and I sometimes struggle to assign boundary conditions. For example I am trying to design the lifting system in the picture (only one of them). I made it the same for the most part; 12 I-beams attached to each other and diagonal pipes inbetween at the sides. Now I'm defining boundary conditions. There are a few options in my mind and they all give vastly different results. Here are some of them:

I made it such that the I-beams under the structure carry all the load (distributed over the length) and fixed the upper four corners. While this give plausible results, this creates only vertical forces and therefore the diagonals carry little to no load. This doesn't take into account the forces created by the angled ropes. So I don't think this is a good model.

Then I modeled the ropes as beams, disregarding their stress values since in this model their job is to transfer the load only. I don't worry about the ropes at the moment. Then I fixed the system in the hook of the crane. Since the hook only prevents translation I made this end as free in terms of moments. No translation allowed but that is all right? The ropes are free to rotate. This gives me some insane values for combined stress in the beams, hundreds of thousands of MPas. If I fix the top end completely ( no translation, no rotation) then it gives plausible results. Though I don't know how correct this is, there shouldn't be any moment resistance there.

Third, I only designed the beams and instead of ropes, I inserted appropriate forces instead ( removed the distributed load). But then where to fix the system? If I fix the lower corners then the beams under the huge structure carry barely any load since the system is supported at the corners.

As you see, I am at a loss. How do you approach to assigning these conditions? How should I proceed?

Any clue to lead me to the right path is appreciated. Also is there any good book that goes into detail about these systems? In school we pretty much only analyzed very simple 1-2 beam systems with pinned supports. My textbook is the same, same exercises over and over.

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