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52 Figure 5-12. Contours for max principle strains at BC deflection of 0.03 inches (displ 25). the regional moment calculation in the sloping exterior webs. equal to one stirrup spacing. The stirrups (and other rebar) are Though not studied in depth, there was no apparent effect on modeled explicitly (unlike in two dimensions, where the stir- the behavior of vertical webs caused by super-elevation so it rup is smeared). This allows introduction of the out-of-page was not included in the single cell section that was studied. compression due to prestress. Using this model framework, The variables that may significantly influence local behav- geometry variabilities were introduced directly into the ior are as follows: models--e.g., one web can have one thickness, and another · Web depth, have a different thickness. Also, by using this model prototype, · Web thickness, the effects of web slope are included and can be compared. · Web slope, Webs A and D have different slopes and can be compared with · Cover thickness, B and C, which are vertical. Webs A and D demonstrate the · Number and configuration of tendon ducts, differences related to web sloping away from the radius of · Number and configuration of duct ties, curvature versus sloping toward the radius of curvature. (Two · Stirrups, and additional cases were later added with vertical-web exterior · Concrete material properties, especially assumed tensile webs to provide additional comparisons.) strength. The tendon duct arrangements and local reinforcements are shown in Figure 5-17. In configuration Type 4, analyses A last variable, Pt/R, is evaluated by the analyses producing were run with (4b) and without the center web ties (4a). This Pt/R versus deformation curves. So each analysis includes the refers to the two rebar ties in the middle of the group of ducts. full range of Pt/R up to failure. The analysis matrix to study In configuration Type 3, separation of the ducts by 11/2 inches the parameters for the multi-cell series is shown in Table 5-1. was found to provide increased resistance to lateral tendon breakout. Further separation may provide even higher resist- ance, but it was the opinion of the project team that enforc- Model Prototype: Three-Cell Cast-In-Place ing even larger separations between the ducts begins to pose Box Girder a very significant limitation on the effectiveness of the pre- A multi-cell, cast-in-place box configuration was used as stressing. Designers need to be allowed some flexibility in shown in Figure 5-15. The basic model (shown in Figure 5-16) duct placement in order to achieve a range of vertical posi- uses 3-D elements and a slice, with out-of-page thickness tions of prestressing within the webs, for typical designs.