In the absence of axial forces, in a properly designed beam (that is, a beam for which tension steel yields) the compression region is determined using the condition of equilibrium. Figure below shows three typical cross sections with irregularly shaped compression regions.įortunately, the same principles that govern the behavior of rectangular beams apply more generally to these cases as well. Many reinforced concrete beams have cross sections that are not rectangular. The resultant compression force in the concrete, C, forms a couple with the resultant tension force, T.Įxample: Solution of Maximum Uniformly Distributed Service Live Load That A Beam Can Support Based on Its Flexural Strength The concrete stress distribution may be replaced by an equivalent rectangular distribution with uniform stress 0.85f' c acting over an area ba and creating a compression resultant, C = 0.85f' cba, that acts at distance a/2 from the compression edge.įor bending without axial force applied, equilibrium requires.In a properly designed beam, the tension steel yields thus, T = Asfy.The ultimate strain in concrete is 0.003.Tension stress in the concrete is negligible (that is, all tension is resisted by steel).A complete bond exists between the steel and the concrete that is, the strain in the steel is the same as in the adjacent concrete.Strain varies linearly through the depth of the member.10.2 and 10.3 give the principles governing the flexural strength. Figure below shows a typical cross section of a singly reinforced beam and the notation used.ĪCI Secs. A beam of this sort is referred to as singly reinforced. The simplest case is that of a rectangular beam containing steel in the tension zone only. For most practical designs, ACI specifies the value of φ as 0.9 however, special cases exist for which lower values apply. Mn is the nominal moment strength of the member, Mu is the bending moment caused by the factored loads, and φ is the capacity reduction factor. The basic strength requirement for flexural design is Unless otherwise specified in a problem, flexural members will be referred to as beams here. In the following sections, the ACI 318 provisions for the strength, ductility, serviceability, and constructability of beams are summarized and illustrated. But their behavior in every case is essentially the same. In modern construction, these members may be joists, beams, girders, spandrels, lintels, and other specially named elements. Flexural Design of Reinforced Concrete Beams Courses > Reinforced Concrete Design > Design of Concrete Members > Flexural Design of Reinforced Concrete Beamsįlexural members are slender members that deform primarily by bending moments caused by concentrated couples or transverse forces.
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