Recently, it became fairly apparent that construction cost of affordable housing – using current building codes and models – is quite high on a unit cost basis. This cost issue reduces the number of units available and makes them prohibitively expensive for many potential buyers. However, current knowledge enables construction of multi-story concrete flat plate buildings of optimum cost. Typically, sixty to eighty percent of structural concrete volume in a multi-story flat plate building goes in slabs. In the U.S., it is typical for multi-story residential buildings to be post-tensioned concrete construction, typically with 8 inch (203 mm) thick slabs spanning up to 30 feet (9.1 m) between columns and/or bearing walls. This framing system layout provides architects with flexibility in room and unit layout on a given floor, and in adjacent stories, since the unit walls are not required to stack, as is the case in economical multi-story wood frame construction. Structural engineers have long known that greater number of columns reduces both the slab spans and the overall cost of construction; shorter spans allow for using thinner slabs.