44. Analysis iteration 2 - Adding props and stay cables

Questions and discussion for this lecture live here. Fire away by hitting Reply below :fire:

I watched lectures 38 to 44, but I am still very confused about the “initial starting point” and the “elastic deflected shape”. Could you please help explain this a bit further?

  1. I’ve understood that we should play with Blender parameter to get the starting point shape, in such a way that after analyzing it to get the elastic deflected shape under selfweight - the two shapes should be very close to one another. Am I right at this point?

  2. Once we have established the deflected shape under selfweight, and now we know all the axial forces in the cables. What is its relationship with the construction process? In constructing this cable net structure, should we construct the structure profile based on the initial starting point (in Blender) or the already deformed shape? Do we need to apply pretension in the cables during construction or do we just construct the net based on the elastic deformed shape, then the stress in the cable naturally appears and same as the magnitude from the analysis?

Thank you.

Hi Tien,

Yes, we would expect the initial shape in obtained from the Blender simulation to be quite close to the shape obtained from our iterative stiffness method analysis. The role of the initial shape funding in Blender is simply to help us find a stable geometric configuration to start our iterative analysis with…one that gives us the greatest likelihood of successfully converging.

The construction process is actually quite a complex topic in itself and beyond what I aim to cover in the course, where our focus is only on implementing the iterative equilibrium analysis. Having said that - we can think of the converged structure under self-weight as a snapshot of the structure in equilibrium. Under it’s own weight, and it this shape (i.e. with appropriate cable lengths to allow the structure to take up this shape) - our analysis tells us (an estimate of) the theoretical forces in the cables.

The question as to how we get the structure into this shape (the construction process and sequence) is actually not one we address in the course. However, one could image hoisting and tensioning the structure until it takes up this particular shape. In doing so, one would expect the tension in the cables to be that predicted by our analysis (allowing for all of the usual discrepancies between theoretical analysis and real-world behaviour).

Thanks, Sean. I think now I get it: the ultimate purpose of “form-finding” is to find “an initial form” that is geometrically stable so that the FEA solver could converge to an equilibrium. Tien.

Yes exactly - you have it now :+1: