Once the size of the cross-section of a beam has been determined based on serviceability and strength requirements, the required area of flexural reinforcement, As, is calculated by setting the required flexural strength, Mu, equal to the design flexural strength, Mn. The size and number of reinforcing bars must be chosen to (1) provide an area of reinforcement equal to or greater than the amount that is required, and (2) satisfy the minimum and maximum spacing requirements in ACI 318-14, Building Code Requirements for Structural Concrete and Commentary.
where fs is the calculated stress in the flexural reinforcement closest to the tension face of the section due to service loads and cc is the least distance from the surface of the reinforcement to the tension face of the member. It is permitted to assume that fs = 2fy/3 where fy is the specified yield strength of the reinforcement. Table 1 contains values of the minimum number of bars required in a single layer for various beam widths based on Grade 60 reinforcement (fs = 40,000 psi), cc = 2 inches (1.5-inch cover plus the diameter of a #4 stirrup), and the overall longitudinal reinforcing bar diameter (approximate diameter to the outside deformations of the bar), which is given in Table 2.
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ACI 318-14, Section 9.7.3, contains the requirements for the development of reinforcing bars in beams. For beams subjected to uniformly distributed gravity loads where the shape of the moment diagram is known, the development lengths in Figure 1 can be used. These recommended details include the requirements for structural integrity reinforcement in ACI 318-14, Section 9.7.7, and can be used for beams that have been designed using the approximate bending moment coefficients in ACI Table 6.5.2. The Notes in Figure 1 are as follows:
Lapping of continuous bottom bars at supports often presents congestion and installation problems. For example, it is common to splice all the bottom bars over the columns away from the section of maximum positive reinforcement, as shown in Figure 2a. This arrangement is the simplest to detail and is most suitable where the beams are wider than the columns. However, it can result in congestion in the beam-column joints. One way to circumvent this issue is to use the detail in Figure 2b: splice bars are provided in the joint, which are spliced to the bottom bars on both sides of the joint. This arrangement works very well with preassembled beam cages because no bottom bars pass through the column during installation. Even though this arrangement increases the amount of reinforcing steel that is required, the cost of the additional material may be more than offset by the savings in labor and other costs; it may be the most cost-effective arrangement in certain situations. 2ff7e9595c
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