The Ribbed slab (UBC 1816) or PTI design method was developed by the
Post-Tensioning Institute from an earlier effort made by the Prestressed
Concrete Institute that had been based on research by the Building
Research Advisory Board of The National Academy of Sciences in the 1950's.
In addition to structural loads, design criteria in the form of
anticipated soil movement is required from the Geotechnical Engineer.
The design is then computer generated resulting in a typical 4 or 5 inch
thick slab that is continuously cast with deepened perimeter and interior
beams placed in a grid pattern within the structures footprint.
Shown is a ribbed slab prior to concrete
The arrangement of the beams and their
depth is determined by the anticipated movement numbers. When
properly constructed the finished slab foundation is quite rigid and if
conservatively designed, the deflections induced by adverse soil behavior
are relatively minor.