Grasshopper

algorithmic modeling for Rhino

Hi, 

I try to modelize a bridge with an arch and some tie rods which are not vertical. But my tie rods are not reacting like I wish. So I made very simple model. A top beam that symbolize the arch, a bottom beam and a tie rod between them. I put some joints to release the rotation of my tie rod. But after I analyzed the model, the deflection of my tie rod is huge, 10m. I did the same analysis (same length, same cross-sections,...) in Robot and I get reasonable deflection, 26 cm. And I really don't understand why.

Does someone know what I do wrong ? 

Thanks

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Hi Gregldr,

the tie rod has a very small bending stiffness compared to its span. This is the reason for the huge deformation under dead weight.

In order to stiffen it you need to prestress the rod and calculate it using second order theory (see attached file). Beam elements with large normal force and small bending stiffness may lead to numerical difficulties in Karamba. Therefore I turned them into truss elements and gave them an initial NII in order to make them initially stable.

Since the bottom and top beam are relatively soft the application of a prestrain in the cable does not lead to a large prestress force because of the deflections in the system.

Best,

Clemens

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