Certain works of structural engineering design are also works of structural art. Such works can be classified as structural art when they attain excellence in the three areas of efficiency, economy, and elegance, as defined by Prof. David P. Billington of Princeton University. A key part of the concept of structural art is that the structural engineer making the design must exercise his or her creativity and playfulness to create an elegant structure within the constraints imposed by engineering requirements. Structural art is a topic of active scholarly research at several universities in the United States, including Princeton University, University of Massachusetts Amherst, and Roger Williams University, and in other parts of the world such as Spain and Germany. While structural artists often collaborate with architects, the discipline of structural art is based upon engineering rather than architectural design. A recent summary about this topic can be found in a review paper.