Slab-on-Grade versus Framed Slab

Abstract

This paper presents comprehensive and practical engineering review (not mathematics or computer science paper) of the observed behavior of the two types of slabs that have been used for the bottom floor slabs, at grade and underground, of buildings in the United States and overseas. The typical design of the slab-on-grade (SOG) built in the United States is described. The design requires only nominal reinforcing steel, but modern day design also requires ground preparation and improvement as necessary, including underslab perforated drainage pipe network embedded in the granular subbase, riser pipes, and a series of pumps. Sealing at all slab isolation joints and waterproofing membrane or water barrier system are provided for water tightness. The framed slab is supported directly on the building framing and on the building foundation. The design can accommodate the soil and underground water pressure and in itself is watertight as the slab is cast monolithically with the structural walls and footings. The behavior of the SOG depends so much on the behavior (soil properties) of the soil strata on which the slab is resting on. It is sensitive to the variation of the soil conditions at and around building foundations, leading to uneven bumpy and cracked slab and leaking basement. These were demonstrated in the report of short- and long-term performances of various projects in Thailand and United States in the past 25 years. The paper refers to various analysis and design techniques that may be used to improve the design of both the SOG and the framed slab for serviceability and economy. It is concluded that both types of slabs may be selected to suit the functions and serviceability requirements of the buildings. The SOG requires less concrete and reinforcement than those for the framed slab counterpart, but when all other factors are considered including additional underslab drainage and pump system, operating, and long-term performance and maintenance, the overall costs of both slabs may not be far apart, yet the performance and integrity of the framed slab will certainly be superior.