Report - PEER - University of California, Berkeley

Because the analytical model strongly relies on the stiffness degradation curve,the stiffness decay was assessed through calculating the cycle stiffnesses, K p atrepresentative cycles. Normalized peak-to-peak stiffness - first story drift ratio curvesfor M1 and M3 models are shown in Fig 7. The peak-to-peak stiffnesses werenormalized with respect to the initial stiffness of M1 and M3. Stiffness decay wasobserved at low drift ratios, even before first inclined cracking became apparent. Thisphenomenon is attributed to incipient wall flexural cracking, and perhaps, to somemicro-cracking (invisible to the naked eye) in masonry materials, local loss of mortarbond and adjustment of brick position. After first inclined cracking, but beforereaching strength, the decay increased with drift ratio. At larger drift ratios, K premained nearly constant. At this stage, stiffness decay is associated to cracking andcrushing in masonry walls and RC confinement members. Also shown in the graph isthe stiffness decay trend measured in static cyclic testing (shown with markers only).It is apparent the difference in the rate of stiffness decay among specimens testeddynamically and statically. This difference must be taken into account when revisingthe hysteresis model in the future.4001.2300E200100D = 4,1MD = 2,9UD = 4,1MCBC1.00.8Vb (kN )0-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0Kp / Ko0.6U-1000.4-200-300M-400Drift Drift ratio ratio first (%)Drift angle first story, (%)%EMCBCENVELOPE0.20.00.0 0.5 1.0 1.5 2.0 2.5Drift Drift angle ratio at first story, (%) %Figure 7. Envelope curve of M3, and stiffness degradation curves of M1, M3 andstatic tests (shown with markers).4.3 Comparison between Measured and Calculated ResponseMeasured response from the shaking table tests were compared to calculatedresponses using the abovementioned hysteretic model (still using the stiffness decayrule derived from static tests). For modeling purposes, it was assumed that inelasticbehavior in the structure was concentrated at the ground story and that sheardeformations (shear plastic hinge) would control the response. These features havebeen observed in the field, and were corroborated during the shaking table tests.242