SECTION I. DEVELOPMENT AND DESIGN
PRINCIPLES AND DEFINITIONS
6-2. From the structural engineering viewpoint, the three main elements that support live loads in a building are tension members, compression members, and bending members called beams. Beams command the most study, because the bending stress varies over the beam cross section, instead of being uniformly distributed.
6-3. The concrete's beam strength is increased greatly by embedding steel in the tension area as shown in Figure 6-1. When steel reinforcements in concrete help carry imposed loads, the combination is reinforced concrete.
Figure 6-1. Cross section of a concrete beam showing location of reinforcement in tension
6-4. The concrete's shear strength is about one-third the unit compressive strength, whereas, tensile strength is less than one-half the shear strength. The failure of a concrete slab subjected to a downward concentrated load is due to diagonal tension. However, web reinforcement can prevent beams from failing in diagonal tension.
6-5. The concrete's tensile strength is such a small percentage of the compressive strength that it is ignored in calculations for reinforced-concrete beam. Instead, horizontal steel bars well embedded in the tension area provide tensile resistance.
6-6. Bond Strength is the measure of effective grip between the concrete and the embedded-steel bar. The design theory of reinforced-concrete beam is based on the assumption that a bond develops between the reinforcement and the concrete that prevents relative movement between them as the load is applied. How much bond strength develops depends largely on the area of contact between the two materials. Because of their superior bonding value, bars having a very rough surface (deformed bars or rebars see Figure 6-2) have replaced plain bars as steel reinforcement.
Figure 6-2. Deformed steel-reinforced bars
6-7. A beam subjected to a bending moment deflects because its compression side shortens and its tension side lengthens. Therefore, the weak tension area shown in view 1 of Figure 6-3 must be reinforced with steel as shown in view 2. (View 1 exaggerates possible cracking to show the beam condition if loaded sufficiently.) The concrete in the compression areas usually does not require reinforcement.
Figure 6-3. Concrete beams subjected to vertical load
6-8. Creep or plastic flow is the tendency for loaded concrete members to deform after time. Creep occurs over the whole stress range, rapidly at first, then much more slowly, becoming small or negligible after a year or two. Some experts believe that the first few service loads remove the initial strains on well-designed, reinforced-concrete structures and if they are not overloaded, they will respond elastically. Because of creep, you cannot predict concrete deflection with any satisfactory degree of accuracy using the common deflection formulas. However, you cannot always trace failures to creep, because it usually disappears in well-proportioned structures before too much deflection occurs.
6-9. Homogeneous beams are non-reinforced beams composed of the same material throughout, such as steel or timber beams.
6-10. The neutral axis is the plane in a beam where the bending stress equals zero. This is shown as line N-A in Figure 6-1.
6-11. The design of a reinforced-concrete structure consists mainly of predicting both the position and direction of potential tension cracks in concrete and in preventing the cracks by locating sufficient reinforcing steel across their positions.
6-12. The practical experience of many structural engineers combined with comprehensive tests and investigations conducted at universities and elsewhere, provide the solutions to many common problems in reinforced-concrete design. For most practical designs, engineers refer to standard specifications.
BASIS OF DESIGN
6-13. The expression "to design a beam" means to determine the size of a beam and the materials required to construct a beam that can safely support specified loads under specific conditions of span and stress. Economy in the use of materials and efficiency in strength, spacing, and arrangements of reinforcing steel are the main factors that influence the design. This manual does not attempt to cover how to design reinforced-concrete structures or members because there are many authoritative texts available in this field.
|David L. Heiserman, Editor||
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Revised: June 06, 2015