AASHTO-LRFD Flexural Strength for superstructure tutorial is about flexural strength, compression block depth and minimum reinforcement. In design of precast prestressed girder design, it is necessary to check flexural strength of composite section which is girder and deck. At the ultimate limit state, the loads are magnified and nominal resistance is reduced to predefined value, ΦMn>Mu. For tension controlled prestressed concrete sections, Φ is 1.0. Moment resistance of girder with prestressing can be determined using classical concrete theories. Effect of prestressing steel needs to be incorporated into the standard equation. The moment resistance is simply determined by taking the summation of first moments of each force component with respect to the center of compression block of the section. LRFD formulation differs whether compression block stays in deck or not. In order to prevent brittle failure, a minimum number of tension reinforcement should be provided according to AASHTO Article, 5.7.3.3.2. A factored flexural resistance, Mr , at least equal to the lesser of: 1.2 times the cracking moment, Mcr and 1.33 times the factored moment capacity, Mu.
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