Minimum Negative Flexure Concrete Deck Reinforcement
In composite steel girder construction, minimum requirements associated with the longitudinal reinforcement in the concrete deck slab in negative moment regions must be satisfied in accordance with Article 6.10.1.7 of the AASHTO LRFD (7th Edition, 2014). Wherever the longitudinal tensile stress in the concrete deck slab due to either factored construction loads or the Service II load combination exceeds 0.9fr, the total cross-sectional area of the longitudinal reinforcement should not be less than 1% of the total cross-sectional area of the concrete deck slab. For normal-weight concrete, the modulus of rupture, fr, is computed as: fr = 0.24 √f ‘c.
The reinforcement used to satisfy the requirements of Article 6.10.1.7 should not have a yield strength less than 60 ksi, a bar diameter not exceeding 0.75 in. (No. 6 bar), nor a spacing exceeding 12 in. The longitudinal reinforcement should be placed in two layers uniformly distributed across the deck slab width, and two-thirds of the total longitudinal should be placed in the top layer. It should be noted that if longitudinal post tensioning is used in the deck, in precast deck panels for example, the longitudinal post-tensioning can be designed to provide the minimum negative flexural reinforcement required by Article 6.10.1.7.
In previous specifications, the requirement for 1% was limited to regions of negative flexure, which was typically interpreted by designers as the regions between points of dead load contraflexure. In continuous composite steel bridges, the deck can be exposed to significant tensile stresses beyond the points of dead load contraflexure due to moving live loads, especially in long span bridges or when the supports are skewed. Placement of the concrete deck in stages can also produce tensile stresses greater than the rupture strength of the concrete in regions where the concrete deck was previously placed. Thermal and shrinkage strains may also cause increased tensile stresses in the concrete deck slab. The 1% longitudinal steel requirement is intended to address these tensile stress issues and provide sufficient crack control. The 1% requirement of past specifications has proven to be reasonably satisfactory for crack control. In the current specification, this requirement is simply extended to locations where the concrete tensile stress exceeds 0.9fr in an effort to further limit cracking of the concrete deck slab.