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AASHTO-LRFD Deck Design
AASHTO-LRFD Deck Design for superstructure tutorial is about control of rebar spacing, stress check under service loads , calculating distribution and shrinkage-temperature reinforcement. The truck axle loads on the deck are transmitted to girders in the transverse direction. The truck axle loads are moved laterally to produce the moment envelops. Multiple presence factors and dynamic loads that are mentioned in Main Design Loads and Combinations – Live Load are included. Shear and fatigue of the reinforcement need not be investigated. For design of deck, the deck is assumed to be a continuous beam along the girders of the bridge. The dead load that includes deck weight and wearing surface weight, positive and negative moments of deck for a unit width; M = qs2/10 where q is dead load per unit area of deck and s is girder spacing. The live load moments will be obtained from the AASHTO-LRFD Appendix A4-deck slab design table. Girder spacing is used to obtain the positive and negative moments on the deck and interpolation may be used to get the moments of intermediate spacing. To determine the required amount of reinforcement for deck design, both positive and negative moment in the deck should be calculated. The maximum positive moment typically takes place at the middle of the girder spacing. The maximum negative moment takes place at a distance of one-third of the flange width from centerline of girder. Based on past experience, maximum and minimum reinforcement requirements never control the deck design. For the ultimate strength analysis, factored loads will be used. After the reinforcement is obtained with the factored moment combination, control of rebar spacing and stress check under service loads is required. Control of rebar spacing is crucial , since the tension reinforcement distribution controls the flexural cracking. To control rebar spacing exposure factor should be known. The crack width is directly proportional to the γe exposure factor, therefore, if an alternate crack width is required, the γe factor can be adjusted directly. For example a γe factor of 0.5 will result in an approximate crack width of 0.0085 in. In the light of this information, interpolation may be used for an intended crack width or exposure factor. The stress in steel under service loads will be checked to be under the limit of 0.6fy. According to Article 9.7.3, distribution reinforcement will be provided in the perpendicular direction. Reinforcement for shrinkage and temperature stresses is provided near surfaces of concrete exposed to daily temperature changes.