This Demonstration shows the computed stresses in a simple 2D model of a truss bridge, where red means more stress. Certain aspects of the model are definitely not realistic. For example, the weight of the trusses is not included. And, of course, the model is in 2D. You can see that when the truss with maximal stress weakens (shown in yellow), the stress spreads out to other parts of the bridge. If you weaken the next truss, then the stress propagates further. And when you weaken yet another truss, then the constraints cannot be satisfied at all anymore—so there is no static equilibrium for the bridge, and the bridge cannot continue to stand. Hover your mouse over the trusses to see the stress.