![]() ![]() The capacity reduction factor increases linearly to a maximum value of 0.9 as the tension strain increases from 0.002 to 0.005. However, the code imposes a capacity reduction factor of 0.65 when the strain in the tension steel equals 0.002. With negligible axial force (axial force less than 0.1f'cAg), the strain in the extreme tension steel must exceed 0.004 (that is, approximately twice its yield strain) when the extreme compression edge of the member reaches the ultimate concrete strain of 0.003. 10.3.5 limits the strain in the extreme tension reinforcement at the nominal strength. This provides warning in the event of failure. The maximum limit on the amount of tension steel ensures that the steel yields well before the concrete crushes, so that the beam fails in a gradual, ductile manner and not a sudden, brittle manner. For cantilevered T-beams with the flange in tension, the value of bw used in the expressions is the smaller of either the flange width or twice the actual web width. The code makes an exception to this requirement for slabs and footings, which require minimum temperature and shrinkage steel, and for special cases in which the amount of steel provided in a flexural member is at least one-third greater at every point than required by analysis. The minimum limit ensures that the flexural strength of the reinforced beam is appropriately larger than that of the gross section when it cracks. 10.3.5 and 10.5 limit both the minimum and maximum amount of tension steel that is acceptable in a beam. Other symbols are as defined in figure below.ĪCI Secs. 8.12 limits the effective flange width, be, of such members by the following criteria. Geometric relationships determine the depth of compression region and a summation of moments gives the nominal moment strength of the section.Įxample: Solution of Design Moment Strength of An Irregularly Shaped Beam Sectionįor most cast-in-place floor systems, the slab and beams are cast monolithically and the slab functions as the flange of a T- or L-shaped beam, as shown in Figure below. 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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