Nature provides engineers with models that are both aesthetic and practical. The catenary curve of a spider's web, the shape automatically taken up by a chain or string when not pulled tight,is a simple and satisfying from that suggests the line of least resistance. it forms the basis of the suspension bridge principle,as beautifully displayed in brooklyn Bridge. The use of twisted wire rope cable here creates a lighter kind of suspension bridge in which physics and aesthetics combine to form a parallel to nature. Le Corbusier called the bridge 'full of native sap'. More rigid bridge structures look like skeletons, although freedom of movement is not part of the design brief. Bones depend on the additional strengthening of muscles, which hold the structure together and provide the means for its movement, while bridge like South Grand Island are constructed from a series of boxes and parallelograms. Like bone, steel has some strength when bent,but is more reliable in compression.