You have to decide on the number of sections or movements. You also have to decide on a means of relating these movements to create a coherent whole . This process goes on right down to the smallest details, until the construction is as satisfactory as you can make it. The artistic person can create whatever she or he likes, as long as a living can be made from it, or by some other means. Building a bridge is different.
Economy is far more important than in art.
The bridge designer is forced to work with what nature offers, rather than with a blank sheet of paper. The width of the crossing, the type of ground, the depth of water, the range of the tides, and the navigational requirements, for example, are not under the designer's control. But he or she can use art and science to produce the best solution to the problems. And solving the problems may lead to great ideas, just as the search for a line with the right rhyme and rhythm may lead to an idea that a poet might not otherwise have had. At the beginning of Beethoven's quartet Opus 131, or Bart k's Music for Strings, Percussion and Celeste, we seem to be hearing not so much tuneful, foot-tapping music as the building of a great structure.