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42 simplified proposed provisions is somewhat different from current LRFD Sectional Design Model. Therefore, design- that in the Standard specifications in that it can also be used ers and owners switching from the AASHTO standard for partially prestressed members and it produces somewhat method to the LRFD specifications may be required lower estimates of Vc. The larger difference between the to include additional shear reinforcement. The justifica- methods is that the proposed simplified provisions introduce tion for this additional required reinforcement is now the use of a variable angle truss model. discussed. Minimum shear reinforcement is provided to ensure that a member will be able to continue to provide the calculated 3.6.3 LRFD Sectional Design Model LRFD concrete contribution to shear resistance after diagonal Modified Sectional Design Model cracking has developed and progressed. The larger the (CSA Method) amount and the closer the spacing of the shear reinforcement, The CSA Method was developed to provide a simplified the smaller are the crack spacings and crack widths. Provided design approach for the entire class of members for which crack widths are kept sufficiently small, it is considered that the LRFD Sectional Design Model was developed. The shear stresses can be transmitted across cracks. It is this inter- equations for and replace the use of more complicated face shear transfer (or aggregate interlock) that contributes tables and the new expression for x reduces the dependence significantly to the concrete contribution to shear resistance on the angle Theta making the design process non-iterative. at the ultimate limit state and effectively eliminates the depth These changes greatly simplify the design process. effect on shear strength that occurs for beams without shear Although evaluating the capacity of a structure is simplified reinforcement. Minimum shear reinforcement requirements by the CSA method, it is still iterative as x is a function of consist of three components: the angle . (i) A minimum required strength (v fy) of shear rein- forcement, 3.6.4 LRFD Sectional Design Model LRFD (ii) Rules for the spacing of the shear reinforcement, and Proposed Simplified Provisions (iii) Rules for when it is necessary to use minimum shear (Modified Standard) reinforcement This will be a more significant switch than going from the In traditional U.S. design practice and in the AASHTO LRFD Sectional Design Model to CSA Method because it is Standard Specifications, the minimum required strength of returning to the more traditional approach of calculating Vc shear reinforcement is v fy = 50 psi, and the maximum spac- from the diagonal cracking strength and using an approach ing of reinforcement is d/2. This reinforcement is required justified purely by experimental test data. The proposed sim- for most bridge members when Vu > Vc / 2 . The exception plified provisions are not a comprehensive design approach is wide members, including footings and one-way slabs, and thus more limited in what they can be used to design. where minimum shear reinforcement is not required until Since the CSA Method and the proposed simplified provi- Vu > Vc . sions are similarly easy design procedures, the designer is In recent ACI codes and in the AASHTO LRFD specifi- more likely to use the simplified proposed provisions only if cation, the amount of required minimum shear reinforcement the outcome leads to more acceptable levels of required shear has been increased above traditional levels, as shown in Fig- reinforcement. ure 7, and made a function of concrete strength. This increase was made over the concern that with higher strength con- cretes both the calculated concrete contribution to shear 3.7 SAFETY AND ECONOMY resistance is larger and that cracks become smoother, OF STRUCTURES DESIGNED BY SIMPLIFIED PROVISIONS providing less interface shear transfer resistance. The exper- imental test data presented in Figure 20 illustrated that addi- In this section the influences of changes in the minimum tional reinforcement was required. Because the minimum shear reinforcement requirements, the maximum shear stress reinforcement requirement governs over a large percentage limit, and the required amounts of shear reinforcement are of the span in prestressed concrete bridge members, this examined and evaluated using both experimental test data increased requirement is a significant improvement in safety and design case examples. at a cost in economy. 3.7.1 Minimum Shear Reinforcement Requirements 3.7.2 Maximum Shear Design Stress Limit Both change proposals require the use of the same As presented in Section 1.1.3, the LRFD Sectional Design minimum shear reinforcement requirements as does the Model allows the design of members with shear stresses as