LRFD Minimum Flexural Reinforcement Requirements (2019) / Chapter Skim
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6 2.1 Introduction This section presents the current state of knowledge on minimum reinforcement requirements, including philosophical and historical background, factors affecting minimum reinforcement requirements, and the behavior of structural members designed with minimum reinforcement. 2.2 Philosophical Background As previously noted, the main purpose of ensuring minimum longitudinal reinforcement in a reinforced or prestressed concrete member is to provide a sufficient level of protection for the member from failing suddenly in a brittle manner immediately following the formation of first flexural cracks (Figure 2-1)
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8 Figure 2-3. Typical details for post-tensioned test specimen (after Warwaruk et al.
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9 Warwaruk et al.
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10 moment ratio and the percentage of minimum flexural reinforcement. The overstrength moment ratio remains above the conventionally accepted ratio of 1.2Mcr, despite having less than the minimum allowable flexural reinforcement in two of the specimens.
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11 concrete (HSC) beams.
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12 higher. It was noted that the members with low reinforcement ratios underwent large deformation without a sudden decrease in the load-carrying capacity.
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13 shear with the shear keys breaking off. Table 2-3 summarizes the results of these tests.
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14 Aparicio et al.
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15 2.4 Methodologies and Code Approaches In this section, a comparison of current international standards for minimum flexural reinforcement is presented to highlight the philosophical differences in the methodologies and different purposes for their use. This review will aid in the illustration of the varying approaches taken to specify minimum flexural reinforcement.
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16 For prestressed beams with unbonded tendons, the minimum area of bonded reinforcement shall be computed by As,min. = 0.004Act, where Act is the area of the cross section between the flexural tension face and center of gravity of the gross section.
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17 Box 2-1. AASHTO LRFD Article 5.6.3.3 Unless otherwise specified, at any section of a non–compression-controlled flexural component, the amount of prestressed and non-prestressed tensile reinforcement shall be adequate to develop a factored flexural resistance, Mr, at least equal to the lesser of: 1.33 times the factored moment required by the applicable strength load combination specified in Table 3-4.1-1; and ( )
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18 Box 2-2. AASHTO LRFD Article 5.4.2.6 Unless determined by physical tests, the modulus of rupture, fr, for lightweight concrete with specified compressive strengths of up to 10.0 ksi and normal weight concrete with specified strengths up to 15.0 ksi may be taken as 0.24λ√fc′, where λ is the concrete density modification factor as specified in Article 5.4.2.8.
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19 longitudinal reinforcement to prevent brittle failure. When HSC is used, the minimum reinforcement must be provided to satisfy 0.058 (2-5)
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20 discussed above shall also be checked. This second requirement is given by 0.26 ; but not less than 0.0013 (2-10)
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21 2.4.11 Comparison of a Reinforced Concrete Beam Design with Various Codes A comparative study of different codes was carried out by using an inverted-tee reinforced concrete beam cross section, Girder RC1, shown in Figure 2-14. For this study, Girder RC1 reinforcement was designed on the basis of 11 minimum reinforcement equations previously presented in this section.
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22 minimum reinforcement. It is observed that lightly reinforced concrete members can reach their full nominal moment in their post-cracked state and behave in a ductile manner.
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23 2007; Rao et al. 2008; Carpinteri and Corrado 2011)
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24 ductility is implicitly addressed in other parts of specifications for tension-controlled members. A structural member can behave in several ways after it reaches its "yield" point, as illustrated in Figure 2-17.
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