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41 methods to the required strength determined by program can designed by the provisions, and the use of provisions for Response 2000. In this table, only the results from design evaluating the condition or strength of existing structures in cases in which all methods required greater than minimum the field. shear reinforcement are considered. From this table, the fol- Two change proposals were presented and current design- lowing observations are made: ers may be using the shear design provisions of either the AASHTO Standard or LRFD specifications. Thus, there are 1. The proposed simplified provisions provided the most four possible changes that a designer can make: conservative estimate of the required amount of shear reinforcement with a mean ratio to the Response 2000 · AASHTO Standard Specifications LRFD Modified requirements of 1.57. It also had the smallest coefficient Sectional Design Method (CSA Method) of variation of the four design methods of 0.23. If a nor- · AASHTO Standard Specifications LRFD Proposed mal distribution of data is assumed and a strength reduc- Simplified Provisions (Modified Standard) tion factor of 0.9 is applied, then it would be expected that · LRFD Sectional Design Model LRFD Modified Sec- in only 6% of cases would sections be under-reinforced tional Design Model (CSA Method) relative to the amount of shear reinforcement required by · LRFD Sectional Design Model LRFD Proposed Response 2000. For each of the six design cases, the pro- Simplified Provisions (Modified Standard) posed simplified provisions were conservative. 2. The AASHTO Standard Specifications had the lowest The effect of each of these changes on shear design is dis- mean reinforcement requirement ratio of 1.26. When cussed below. coupled with a COV of 0.31, this suggests that in 25% of the cases sections would be under-reinforced relative to the amount of shear reinforcement required by Response 3.6.1 AASHTO Standard Specifications LRFD Modified Sectional Design Method 2000. The Standard specifications were found to be par- (CSA Method) ticularly unconservative for the design of the continuous box beam and somewhat unconservative for the design of Differences in the design by the AASHTO Standard Spec- the continuous RC beam and Bulb-Tee girders. ifications and LRFD Sectional Design Model were presented 3. The LRFD Sectional Design Model and CSA methods in Section 1.1.3. Because the CSA Method is only a simpli- had similar mean reinforcement requirement ratios for fication to the LRFD Sectional Design Model, all of these most of the design cases. This was somewhat expected differences remain the same except item vii as the design given that the relationships for and were also procedure by the CSA method is non-iterative. Based on the derived from the MCFT using the longitudinal strain at use of the CSA and AASHTO standard method in preparing mid-depth and similar assumptions as described in Sec- the design examples and in calculating the required amounts tion 2.1.3 and justified in the paper by Bentz (51). The of shear reinforcement in the design database, it is considered CSA method was somewhat less conservative for con- that the CSA method is slightly easier to execute than the tinuous members. For the LRFD Sectional Design AASHTO standard method. It should be reemphasized that Model, the mean reinforcement ratio for all members is the CSA Method enables a section to be designed for a much 1.42 with a COV of 0.37 suggesting that in 21% of higher shear design stress than permitted in the AASHTO cases sections would be under-reinforced relative to the standard Method. Furthermore, the CSA Method is a com- amount of shear reinforcement required by Response prehensive design approach capable of designing a section 2000. For the CSA method, the mean reinforcement for shear also subjected to the actions of axial load, moment, ratio for all members is 1.40 with a COV of 0.48 sug- and prestressing and that it is derived from a complete behav- gesting that in 28% of cases sections would be under- ior model for shear. By contrast the AASHTO standard reinforcement relative to the amount of shear rein- method is an empirical approach for the design of prestressed forcement required by Response 2000. and non-prestressed flexural members justified by a fit of design equations with experimental test data. If the results from Response 2000 are perfectly correct, then only the proposed simplified provisions come close to satisfying the general design philosophy that less than 5% of 3.6.2 AASHTO-Standard Specifications designs are unconservative. LRFD Proposed Simplified Provisions (Modified Standard) 3.6 EFFECT OF CHANGE PROPOSALS Given that many states have not yet switched to using the ON DESIGN PROCESS LRFD Bridge Design Specifications, many designers proba- bly will choose to use the proposed simplified provisions This section is concerned solely with the effect of the because they are more similar in structure to the AASHTO change provisions on design for engineers. It addresses the Standard Specifications than is the Modified LRFD Sectional effort required by the designer, the range of structures that Design Model (CSA Method). The equation for Vcw in the