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OCR for page 63
63 inforcing by both reducing element size and congestion of 3.3.4 Moment Redistribution reinforcement. Analytical formulations were used to establish the strain limit for which negative moments at the internal supports of 3.3.2 Fatigue continuous beams can be redistributed. This strain limit needs to be verified experimentally by testing continuous beams. Limited available data indicate that the fatigue limit of higher strength, and particularly micro-composite alloy steel, may be markedly improved over that of conventional black 3.3.5 Control of Flexural Cracking steel. A study to establish reliable S-N relationships for differ- and Corrosion ent grades of reinforcing steel is recommended. Such a study must consider full-section bars (not coupons) and include a The current provisions in the AASHTO specifications for range of bar sizes. maximum spacing of reinforcement for Class 1 exposure are based on an assumed crack width of 0.017 in. A Class 2 expo- sure corresponds with a crack width of 0.013 in. At the same 3.3.3 Shear Friction time, there appears to be little or no correlation between As discussed in Section 2.6 and in Zeno (2009), the basis for crack width and corrosion. The current equation for maxi- current shear friction design methodology is entirely empirical mum spacing requires that the tensile stress in steel rein- and does not represent the actual observed behavior. While the forcement at the service limit state be calculated. For a beam, current design approach is calibrated for the use of steel hav- this is relatively simple. However, for a bridge deck, it is more ing yield strength less than 60 ksi, it is shown to be inadequate complicated because of arching action and two-dimensional for other cases (both higher and lower yield strengths). It is rec- load distribution. Research is needed to address the issue ommended that an extensive study be undertaken to establish of control of cracking by distribution of reinforcement and a more rational design basis for establishing shear friction its impact on corrosion of reinforcement. The research capacity. Such a study will also support the understanding of should include all types and grades of corrosion-resistant shear capacity in general. reinforcement.