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43 large as 0.25f c which is a very substantial increase over the 3.7.4 Evaluation of Change Proposals maximum shear stress permitted in the AASHTO Standard Using Design Cases Examples Specifications. Both change proposals suggest imposing a maximum design stress limit of 0.18 f c. A maximum design In order to evaluate the expected safety and economy for stress lower than that in the current LRFD Sectional Design regions away from supports and for members not well repre- Method is recommended due to the results of shear tests sented in the experimental test database, it is useful to com- on large bulb-tee girders conducted in NCHRP Project 12-56 pare the required strengths of shear reinforcement for a large in which the funneling of diagonal compressive stresses to number of design cases by the four design methods and the support was found to substantially magnify the diagonal Response 2000. These design cases covered some design compressive stresses which then led to compressive failures sections over the length of prestressed and non-prestressed at loads lower than LRFD predicted capacities. members, simple and continuous structures, members with For designers and owners still using the AASHTO Stan- rectangular and I- or T-shaped cross-sections, and members dard Specifications, adoption of either change proposal designed to a different percentage of feasible flexural method will result in a significant increase in the permis- capacity. The required amounts of shear reinforcement are sible shear design stress. This increase enables the same summarized in Table 6. size section to be used to span longer distances or carry If the predictions of Response 2000 are reasonably accu- heavier loads and can result in significant improvements in rate and the design database representative, then a compari- economy. son of the relative amounts of required reinforcement by each For designers and owners using the LRFD Sectional design procedure and Response 2000 is a good measure of Design Model, this change leads to a significant decrease in the accuracy of each design procedure. The relative values of the design shear stress limit. Since this change has been the means of the requirement ratios in Table 6 illustrate that shown by testing to be required, it is a significant improve- the procedures, in order of increasing economy, are the ment in safety. AASHTO Standard Specifications, the CSA (change pro- posal 2), the LRFD Sectional Design Model, and the pro- posed simplified provisions (change proposal 1). Of course 3.7.3 Evaluation of Change Proposals economy and safety must be examined together. For all Using Experimental Test Results methods but the proposed simplified provisions, the actual capacity would be expected to be less than the design The evaluation of the design provisions using experi- strength in about 25 percent of cases. As discussed in Section mental test data, as summarized in Table 5, and more fully 3.5, all methods but the simplified proposed specifications presented in Appendix G, can be used to draw observations were particularly unconservative for continuous members. In on the changes in safety and economy resulting from the only about 6% of the cases are the proposed simplified pro- adoption of these change proposals. Due to the limitations visions found to be unconservative. of experimental test data, this evaluation is restricted to It is also useful to examine where the largest differences commenting on the safety and economy of the regions near are between required amounts of shear reinforcement by the supports for a limited range of member types. different design methods and to assess whether or not these If the test data were representative of bridge girders in differences are justified. For this examination, the results for practice, then a comparison of the means of the strength a selection of the design cases is shown in Table 7 and plot- ratios, as shown in the first row of Table 5, illustrates that the ted in Figure 25. Additionally, a comparison of the required only potentially significant effects are slight decreases in the amounts of shear reinforcement for the eight complete design required amount of shear reinforcement for non-prestressed examples is summarized in Table 8. These results illustrate members if the CSA method is adopted and a modest that some of the largest differences, particularly as a fraction increase in the amount of shear reinforcement required for of each other, are in transition zones (between web and prestressed concrete members if the proposed simplified pro- flexure-shear regions) and in flexure-shear regions; see cases visions are adopted. 3, 4, 7 and 8. In these cases, the AASHTO standard method The relative safety of the provisions can be evaluated by is the least conservative of the approaches and sometimes using the means and standard deviations of the strength ratios quite unconservative if the predictions of Response 2000 are shown in Table 5 to calculate the percentage of cases for accurate. The proposed simplified provisions are always which the measured capacity is expected to be less than the conservative while the CSA method is usually conservative design strength. A resistance factor of 0.9 and a normal dis- relative to the Response 2000 values. tribution are used in calculating the percentages given in the Another area of significant differences is near inflection bottom row of Table 5. The results illustrate that switching points in continuous members. Inflection point regions are from the AASHTO standard method results in a modest a special transition region and the flexural and shear rein- increase in safety for reinforced concrete members. All four forcement detailing requirements for that region as a function methods had a very similar and very acceptable level of of the inclined cracking that can develop in the region have safety for prestressed concrete members. not been adequately researched. The wide variation in shear
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44 TABLE 7 Comparison of required transverse reinforcement Figure 25. Selected design database.