Abstract: A system of structural steel modular units useful in constructing mid-rise building between 3-8 stories high. Modular units are in the shape of rectangular boxes and the height of the units define a single story of the mid-rise building. Modular units are configured to be stackable onto each other such that they can couple to both modular units positioned directly above and below and also to couple to laterally adjoining modular units. The modular units can include cantilevered extensions to define hallways or other desirable features such as balconies. The systems can also utilize a post tension system that includes a plurality of post tension members anchored to a foundation and are coupled to a damper system positioned on the roof of the building.