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15 may also provide significant shear capacity. Additional com- with traditional approaches, and developing simplified pro- ponents are the vertical component of the force of draped visions may require making conservative assumptions. prestressing strands and the shear transmitted directly to the support by arch action. The relative magnitude of each of these components to the total resistance depends on many Influence of Depth factors but it is generally agreed that the dominant concrete components to shear resistance in beams with transverse A core assumption in the ACI 318 and AASHTO Stan- reinforcement are shear in uncracked compression zones and dard Specifications is that the shear capacity is proportional interface shear transfer. to the depth of the member. This assumption was investi- Although researchers agree on the foregoing mechanisms gated in a landmark study conducted by Shioya et al. (40) of shear resistance, the structure of code provisions and the in which they tested reinforced concrete members that amount of shear reinforcement required by different codes ranged in depth from 4 to 118 inches. All members were for the same design situation vary because of the complexity simply supported, did not contain shear reinforcement, of shear resistance mechanisms, the factors that influence were lightly reinforced in flexure (0.4%), and subjected to these mechanisms, and the different methods used to evalu- a uniformly distributed load. In Figure 14, the normalized ate the contributions of the shear reinforcement. shear stress at failure is plotted versus the depth of the The discussion presents some of the complexities of devel- member. The horizontal line corresponds to the shear oping a model for shear resistance and to show how different strength calculated using the traditional shear design codes have chosen dramatically different approaches. Those expression of the ACI and AASHTO Standard Specifica- approaches have then lead to the development of different tions. The results show that the shear stress at failure infrastructures for design equations and different ways of decreases as the depth of the member increases. Of partic- thinking about shear. For this development of proposed ular concern is that members greater than 36 inches deep AASHTO simplified shear design provisions, primary failed under stresses approximately one-half of the strength resources were underlying models for shear resistance and calculated by these codes of practice. However, although behavior, shear design equations in current national codes of this depth effect is marked for beams without transverse practices, and expressions for calculating shear capacity that reinforcement, available test data show little if any depth are promoted by individual researchers. effect for beams with transverse reinforcement (41). 1.2.4 Factors Influencing Shear Resistance Influence of Concrete Strength Different factors can have surprising effects on shear resis- In traditional U.S. design practice, and in the LRFD tance. Shear is complex, there are potential safety concerns Sectional Design Model, the contribution of the concrete to Figure 13. Mechanism of shear resistance.