This disclosure presents a new strategy for modular construction: an adjustable module. This disclosure develops a new adjustable module comprised of a four-bar linkage which can form more efficient variable depth structures. Applications can include rapidly erectable bridges, roofs, buildings, and grid shells, among others. This disclosure provides specific detail related to rapidly erectable bridge applications for variable depth arch forms. Existing rapidly erectable, steel panelized bridge systems have limited material efficiency as they are comprised of rigid modules in a girder-type configuration. This disclosure shows that the module can form variable depth three- and two-hinged arches which increase the material efficiency. The module and other arch components can be made of standard rolled steel sections, thereby forming a versatile, kit-of-parts systems.