Structure, architecture and aesthetics
Be it building or bridge, and regardless of equipment or decorative elements to be added, the definition of the structure signifies an architectonic form is created. Evidently, equilibrium and resistance govern the structure, but the resulting structure articulates an architectonic concept; and there is no doubt that the aesthetic value of that structure refers to its architectonic form.
The design process of a building or bridge is the development of a manifold course of decisions and options aiming at a multi-objective optimization. Talent in design is displayed in the weight given to each factor and in the definition of the multi-objective optimization criterion, but art comes in the quite irrational and subjective synthesis of so many factors and objectives.
Structural Engineers are good at weighing the design factors and at balancing the optimization multi-objectives but fill uncomfortable at the irrational and subjective parts of the synthesis. On the contrary, Architects are very able at the irrational and subjective parts of the synthesis and understand better dimensions and proportions in space. Moreover, since Architects know little of the structural designing factors, they feel free at a more speculative design approach.
But aesthetics synthesizes a variety of factors, most of them subjective in nature. Thus, aesthetics can be neither a design factor nor an explicit component in the design multi-purpose objective of building or bridge.