Reinforced concrete slabs are one of the most widely used structural elements. In many structures, in addition to providing a versatile and economical method of supporting gravity loads, the slab also forms an integral portion of the structural frame to resist lateral?forces.

In spite of their widespread use, there has never been a universally accepted method of proportioning all slab systems, owing to the complexity of formulating a theoretical analysis for the support conditions commonly used in buildings. Although "exact" analyses can be obtained for arbitrary geometry and material response using numerical techniques such as finite difference and finite element methods, the time and cost of preparing input and interpreting output make these techniques unsuitable for design office use except for very exceptional cases.?Fortunately most slab systems encountered in?practice do not require a rigorous analysis, and simplified design procedures may be used.

Structural reinforced concrete slabs are classified by the way they are supported. Slabs supported such that they can bend essentially in one direction only are referred to as one-way slabs; that is, the load is basically carried in one direction. Slabs supported by isolated supports (columns) arranged in more or less regular rows that permit the slab to deflect in two orthogonal directions are known as two-way slabs.?Slabs with supports that are random in size and location are classified as irregular slabs and may exhibit both one-way and two-way behavior. Such slabs require special attention.

Slabs that carry load by two-way action but without the use of beams are one of the most efficient structural systems. Such slabs are economical, since they can be constructed with minimum field labor resulting from the use of simple formwork and reinforcing steel arrangements. In addition these slabs provide high flexibility in column layout, and require the least story height for a specified clear headroom; and in many cases the ceiling finish, if required, can be applied directly to the slab soffit, thereby eliminating costly hung ceilings. For slabs with longer spans or heavy industrial loading, drop panels and/or column capitals may be used.

In certain cases it is advantageous to stiffen the slab by providing beams between columns. Such beams will decrease slab deflections and so permit longer spans with thinner slab sections. The use of beams greatly reduces the problems of shear and moment transfer between columns and slabs. In addition, beams frequently provide an economical solution for unusual loading conditions such as a large stationary concentrated load or line loads.

As the span length of slabs increases, the self weight of the solid slab becomes significant. The influence of self weight may be reduced considerably by forming cavities in the slab soffit using standard pan forms. When the cavities form continuous ribs in one direction only, the slab is referred to as a joist slab. When the cavities form continuous ribs in two directions the slab is referred to as a waffle slab.

Solid one-way slabs are generally economical for spans below 14 ft. On the other hand, one-way joist slabs have been used competitively for spans exceeding 40 ft with the thickness of slab between ribs varying from 3 to 4.5 in., depending on the required fire rating. Solid two-way slabs without beams are generally most efficient for spans up to 25 ft, which can be increased to 28-30 ft with the use?of drop panels. Waffle slabs are used frequently in the 30-40 ft span range. The effects of self weight and deflection problems associated with longer spans can also be overcome by the use of post-tensioning techniques.

The steps in the design of any slab system may be summarized as follows:

(1) Choose the slab thickness and the dimensions of auxiliary stiffening elements, if any, based on experience and code limitations.

(2) Obtain a moment field that is in equilibrium with the loading and compatible with the supports.

(3) Check the shear, torsion, and shear-moment transter requirements.

(4) Select the reinforcement.

The many different design procedures that have been proposed differ from each other essentially only in the second of these steps.

To compensate for the lack of a ready means of analyzing two-way slab systems, most building codes contain detailed rules for carrying out the above design steps for the simple case of a building slab supported only on columns forming essentially rectangular panels of approximately equal spans and supporting only uniformly distributed gravity loading.

Reinforced concrete slabs are generally underreinforced and possess considerable ductility, allowing considerable?moment?redistribution within the slab. This permits using inelastic design procedures for determining moment fields for irregular geometry where code procedures based on approximating elastic behavior cannot be used.

Owing to the inherent ductility of reinforced concrete slabs, all design procedures will provide sufficient strength if the equations of equilibrium are satisfied. However, even regular slabs may have?serviceability problems due to excessive deflections and cracking unless sufficient care is given to these items. It is these two serviceability factors that provide the criteria on which most slab designs are ultimately judged, and their importance in the design procedure cannot be overemphasized.