What a 3D solve is, and isn't
We've worked our way through nine steps of this pathway, and every one of them ended on a number we could check by hand. All of those checks worked because every analysis made the same three assumptions. The material behaved in a linearly elastic way, the deformations were small by comparison to the size of the structure, and all loads were applied statically. The Beams and frames pathway closed by making sure those three were understood. In this final step, we'll restate them briefly and then look at the limits that apply to 3D alone, which have no counterpart in a 2D model. We'll read step 8's braced building frame again with all of this in mind.
1.0 Linear, static, elastic and first order
Linearly elastic material behaviour means the response is proportional to the load. Double the wind on our frame and every brace force, every reaction and every deflection doubles. The steel never yields, so a brace force is reported whether the section can carry it or not.
Static means the loads are applied slowly and their magnitudes remain constant. There are no inertia forces, so vibration and impact are not captured.
First order means every load acts on the undeflected structure. The columns of our frame carry 2,160 kN of dead load, and they sway a millimetre or two under wind. In reality, that axial load acting through the sway generates a small second-order moment, and a linear solve ignores it. That's acceptable while the deflections really are small, and we'll see in a moment just how small they are here.
Everything we found in the 2D pathway follows from these three, and so does everything in this one. Now let's look at what's new.