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59 this specimen was underpredicted due to the conservative C estimate of experienced maximum concrete strain. This V Tested Recommended agreement indicates that the prediction methods shown in the M Detail Detail attachments to this report are adequate for the implementation V of the conventional hybrid detail. For the concrete filled pipe < ld and dual shell specimens, the prediction methods used provide good agreement with the observed response up to a lateral drift T ratio of 2.0%. Above this level of lateral demand, the observed response had continual reduction in lateral capacity due to grout bedding layer degradation. These details still provided Figure 2.90. Girder shear slip mechanism. acceptable lateral response up to a 5% drift ratio when the lat- eral capacity approached 80% of the maximum recorded capacity. Within the realm of design demands, the prediction the inadequately anchored shear reinforcement in the girder, methodology is reasonable and conservative. Future work is which cannot develop the required shear within the deck. The required to verify the benefits of modifications to the grout use of headed reinforcing bars is expected to greatly reduce the bedding layer for improving performance of the second and observed shear slip by fully anchoring the shear reinforcement third hybrid specimens. within the reinforced concrete deck. As shown in Figure 2.90, the observed horizontal cracking Nonlinear Time History Analyses between the deck and girder provided a length of embedded shear reinforcement that was less than the required develop- In presenting a new structural system for use in seismic ment length. Therefore, although the shear strength of the sys- regions, the potential implications of realized displacement tem was maintained, there was continued slip of the bar during demands during strong ground shaking must be investigated. A repeated cycles of testing. To mitigate this issue, the use of well- series of nonlinear time history analyses were conducted on a anchored shear reinforcement in the deck is recommended. calibrated model to determine the level of displacement ampli- fication in inelastic systems as compared to similar elastic sys- tems. The results from the conventional hybrid specimen test 2.4 Analytical Results were used to calibrate a lumped plasticity model for dynamic 2.4.1 Nonintegral Hybrid Connections analysis. A comparison between the recorded experimental results and the calibrated model is shown in Figure 2.91. A series of analyses was conducted on the hybrid specimens The calibrated model was developed in the analysis package in order to assess the adequacy of these systems for implemen- RUAUMOKO (32) by combining a modified Takeda model tation in seismic regions. The first set of analyses relates to the and a bilinear elastic model. The input parameters were based adequacy of the presented simplified and complete prediction on the response predictions, including the relative contribution methodologies that are discussed in more detail in Tobolski of the post-tensioning and conventional reinforcement, and 2010 (5). The second set of analyses relates to the investigation then finetuned based on quasi-static simulations in the analysis of the potential inelastic displacement demands for hybrid model. systems. The nonlinear time history analyses were performed for records developed for Site Class B, C, and D. A total of 30 ground motions recorded from California earthquakes were Analysis Prediction Methodologies modified using the wavelet modification program WAVGEN The design and implementation of hybrid systems relies (33). Each record was modified to be consistent with a speci- heavily on the ability to predict the response of these systems. fied design spectrum developed in accordance with the 2009 The lateral force-displacement response of the hybrid mem- LRFD SGS (1). The records were manipulated and developed bers is provided in Figure 2.67, Figure 2.75 and Figure 2.80. for each of the site classes, resulting in a total of 90 spectrum Each of these figures also includes the lateral force-displacement compatible records. The resulting response spectra for each envelope prediction and the predicted nominal yield demand site class, in addition to the actual modified response spectra, using the simplified procedure. For the conventional hybrid are shown in Figure 2.92. Review of Figure 2.92 indicates specimen, both the complete and simple prediction methods that the achieved mean response spectrum for each site class provide very good agreement with the recorded response from matches well with the target spectrum with variability between experimental testing. The lateral displacement capacity for actual time history records.
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60 Drift Ratio, % -3 -2 -1 0 1 2 3 100 75 50 Lateral Force, kips 25 0 -25 -50 -75 Experimental Analytical -100 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Displacement, inches Figure 2.91. Comparison of analytical and experimental hysteresis (5). Nonlinear time history analyses were conducted using the calibrated lumped plasticity model for periods ranging from 0.1 sec to 3.0 sec at 0.1 sec intervals. Specified viscous damping was equal to 5% using tangent stiffness damping. The analyses were performed for inelastic force reduction factors ranging from 2 to 6 for single degree-of-freedom systems. The initial runs were performed with elastic response in order to deter- mine the expected yield force in the system. Considering the multiple site classes, earthquakes, and force reduction factors, a total of 13,500 analyses were performed. The results of the analysis for Site Class D are presented in Figure 2.93, with individual "x" marks representing a single inelastic displacement modification factor from a specific earthquake. The line labeled "HYB Mean" represents the mean response parameters over a range of periods. Additionally, a plot titled "EPT Mean" is presented that represents the results from a similar series of analyses conducted on elastic-plastic single degree-of-freedom oscillators. The hatched region on the plots represents the region in which the experienced dis- placement demand results in ductility values in excess of the maximum code limit of 6. Results from these analyses indi- cate that the hybrid systems investigated have displacement demands similar to those of more conventional systems. Thus, these systems are not expected to experience displace- ment demands significantly greater than those experienced by CIP or emulative systems, and provisions published in the code for these systems can be used for hybrid systems with a similar level of safety. For all systems, the overall trend observed is that the mean inelastic displacement factor approaches unity as the period approaches infinity. This trend agrees with the Figure 2.92. Acceleration response spectra (5). commonly accepted equal displacement principle (34).
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61 10 10 Inelastic displacement factor, CR R = 2 - Site Class D R = 3 - Site Class D Inelastic displacement factor, CR 8 HYB Mean 8 HYB Mean EPT Mean EPT Mean D > 6 D > 6 6 6 4 4 2 2 0 0 0.1 0.2 0.3 0.5 0.7 1 2 3 0.1 0.2 0.3 0.5 0.7 1 2 3 Period, seconds Period, seconds 10 10 Inelastic displacement factor, CR Inelastic displacement factor, CR R = 4 - Site Class D R = 6 - Site Class D HYB Mean 8 HYB Mean 8 EPT Mean EPT Mean D > 6 D > 6 6 6 4 4 2 2 0 0 0.1 0.2 0.3 0.5 0.7 1 2 3 0.1 0.2 0.3 0.5 0.7 1 2 3 Period, seconds Period, seconds Figure 2.93. Hybrid system inelastic displacement modification factor (5).