Flexure: Behavior and Nominal Strength of Beam Sections
4-1 INTRODUCTION
In this chapter, the stress-strain relationships for concrete and reinforcement from Chapter 3 are used to develop an understanding of the flexural behavior of rectangular beam sections. The effect of changes in material and section properties on the flexure behavior (moment versus curvature relationship) of beam sections will be presented. A good understanding of how changes in these primary design variables affect section behavior will be important for making good design decisions concerning material and section properties, as will be covered in the next chapter.
After gaining a good understanding of the entire range of flexural behavior, a general procedure will be developed to evaluate the nominal flexural strength, $M_n$, for a variety of beam sections. Simplifications for modeling material properties, which correspond to the ACI Code definitions for nominal strength, will be presented. Emphasis will be placed on developing a fundamental approach that can be applied to any beam or slab section.
In Chapter 11, the section analysis procedures developed in this chapter will be extended to sections subjected to combined bending and axial load to permit the analysis and design of column sections.
Most reinforced concrete structures can be subdivided into beams and slabs, which are subjected primarily to flexure (bending), and columns, which are subjected to axial compression and bending. Typical examples of flexural members are the slab and beams shown in Fig. 4-1. The load, $P$, applied at Point $A$ is carried by the strip of slab shown shaded. The end reactions due to the load $P$ and the weight of the slab strip load the beams at $B$ and $C$. The beams, in turn, carry the slab reactions and their own weight to the columns at $D$, $E$, $F$, and $G$. The beam reactions normally cause axial load and bending in the columns. The slab in Fig. 4-1 is assumed to transfer loads in one direction and hence is called a one-way slab. The design of such slabs will be discussed in the next chapter. If there were no beams in the floor system shown in Fig. 4-1, the slab would carry the load in two directions. Such a slab is referred to as a two-way slab. The design of such slabs will be discussed in Chapter 13.