Elasticity theory depends on guessing the way in which a structure works, and the loads it will be subjected to, and designing it to carry those loads in the assumed manner. This ensures it is safe if the structure is well understood, but it may not be safe if the structure carries the loads in a different manner. Therefore, it gives an upper bound on the collapse load (the load at which it collapses will not be higher than that calculated). Elastic design is sensitive to deformation of the structure, and only works for small deflections.