Dynamic and Probabilistic Seismic Performance Assessment of Precast Prestressed Rcfs Incorporating Slab Influence Through Three-Dimensional Spatial Model

The dynamic and probabilistic seismic performances of precast prestressed RCFs are assessed in this paper, and the slab influence in the overall structural behavior is considered during the process. The threedimensional spatial model is established to provide the numerical basis, and the slab is modelled through L-/T-section beam-slab fiber-sections considering the effective width and centroid positions. The adopted model is verified with the experimental data, and the slab influence in hysteresis curves is investigated by parametric study. Then, two groups of precast prestressed RCFs are well designed to evaluate the slab influence in dynamic responses through seismic excitations, and the modal analysis, roof displacement analysis, maximum and residual drift ratio analysis are conducted for discussion. Moreover, the incremental dynamic analysis and fragility analysis are also conducted to investigate the probabilistic performance of precast prestressed RCFs with or without slabs. In general, different demand parameters may result in the variability of analyzing results, and ignoring the slab influence may underestimate the structural capacity under the frequent earthquakes (i.e., elastic stage) and overestimate the structural capacity under the rare earthquakes (i.e., plastic stage). In a sense, the research proves the significance of slabs in the seismic performance of dry-connected precast prestressed RCFs, and meanwhile provides the reference for the further explorations of slab factors in precast concrete structures.

CHARACTERIZATION OF ENERGY DISSIPATIVE CUSHIONS MADE OF Ni-Ti SHAPE MEMORY ALLOY

Earthquake-resistant design of structures requires dissipating seismic energy by deformations of structural members or additional fuse elements. Owing to its easy-to-produce, plug-and-play, high equivalent damping ratio, and large displacement capacity characteristics, energy dissipative steel cushions were found to be an efficient candidate for this purpose. However, similar to other conventional metallic dampers, residual displacement after a strong shaking is the most notable drawback of the steel cushions. In this work, cushions produced from Ni-Ti shape memory alloy are evaluated numerically by experimentally verified finite element models to assess their impact on the performance of earthquake-resistant structures. Furthermore, a reinforced concrete testing frame is retrofitted with energy dissipative steel and Ni-Ti cushions. Performance of the frames (e.g. dissipated energy by the cushions, hysteretic energy to input energy ratio, maximum drift, and residual drift) with different types of cushions are evaluated by nonlinear response history analyses. The numerical results showed that the steel cushions are effective to reduce peak responses, while Ni-Ti cushions are more favorable to reduce residual drifts and deformations. Hence, a hybrid system, employing the steel and shape memory alloy cushions together, is proposed to reach optimal seismic performance.

Risk-adjusted Design Basis Earthquake’s Adequacy for use in Seismic Design Codes

In most buildings’ seismic design codes design basis peak ground acceleration (PGADBE) is provided by employing a uniform-hazard approach. However, a new trend in updating seismic codes is to adopt a risk-informed method to estimate the PGADBE so-called risk-adjusted design basis peak ground acceleration (PGARDBE). An attempt is made here to examine the adequacy of the PGARDBE to fulfill the assumptions made in seismic codes for converting the maximum considered earthquake’s (MCE) intensity to PGADBE. To this end, the performance of regular intermediate steel moment frames (IMF) is assessed in terms of collapse margin (CMR) and residual drift ratios in the event of MCE and design basis earthquake (DBE), respectively. The PGARDBEs are computed for Karaj County, Iran. A set of 96 index archetypes of regular IMF are designed considering four design parameters, which include the number of stories (2, 3, 6, 9, 12, and 15), span lengths (4 and 8 meters), occupancies (residential and commercial), and seismic demands (0.15, 0.25, 0.35 and 0.45g). The PGADBE prescribed by Standard No. 2800 for Karaj neither meets the assumed acceptance criteria nor stands on the safe side. Meanwhile, PGARDBE fulfills the acceptance criteria but does not necessarily satisfy the implicit assumption made in codes that the code-conforming buildings have at least a CMR of 1.5 if the MCE occurs. This emphasizes that the PGARDBE should not be used without examining the CMR fulfillment. The results recommend that a lower limit need to be set on PGARDBEs, which is found to be 0.35g for Karaj. Outcomes also reveal that the code-conforming buildings designed with the proposed PGARDBE can fulfill both repairability and life safety performances at the DBE and MCE, respectively. These buildings also have a high chance to be even considered as repairable ones at the seismic demand of MCE. Furthermore, regardless of the employed method for estimating PGADBE, various relationships between design parameters with different performance indicators such as CMR, residual drift ratio, ductility demand, imposed drift ratio, and building’s normalized weight are presented. These relationships can be used to evaluate the buildings’ safety factor against collapse and repairability, justification of using IMF in regions with high seismicity, level of structural and nonstructural damage as well as the economic consequence of changes in PGADBE. The presented relationships provide a multi-criteria decision-making tool to decide on the optimum PGADBE leading to an affordable alternative and tolerable damage.

Numerical Investigation of Residual Displacement of Rocking Self-Centering Columns Under Cyclic Loading

Well-designed rocking self-centering (RSC) columns are capable of achieving small residual displacement. However, few studies conducted the quantitative analysis for the residual displacement of RSC columns. The residual displacement is the product of the struggle between the self-centering (SC) capacity and the energy dissipation (ED) capacity. In this study, a SC factor and an ED parameter were defined to reflect the SC and ED capacity of the RSC column, respectively. The influence of eight common design parameters on the SC factor and the ED parameter was explored using factorial analysis. Parametric analysis was performed to investigate the tendency of the SC factor and the ED parameter with the increase of maximum drift. According to the results of the parametric analysis, the effect of the SC factor and the ED parameter on the distribution of the residual drift was researched statistically. A simplified formula was proposed to calculate the upper limit of the residual drift. What is more, a set of predictive regression formulas was established to estimate the actual residual drift, these regression formulas have an applicable condition that the ED parameter should be larger than 0.75. When the ED parameter was less than 0.75, the residual drift is approximate to zero.

Influence of Maxwell Stiffness in Damage Control and Analysis of Structures with Added Viscous Dampers

Viscous damping systems are often implemented in structures to reduce seismic damage. The stiffness of these elements is dominated by the most flexible part of the set including brace extender, auxiliary mounting elements and damping unit. Existing experimental data are used in this study to show that the actual stiffness of the set is about 25% to 50% of the value generally adopted in current engineering practice, which is based solely on the brace extender. A numerical study shows that this reduction has large implications for several variables related to damage control: residual drift ratio, storey acceleration and plastic strain energy dissipated by the frame members. Other variables, such as member forces and rotations, can experience large variations, particularly for non-linear dampers and high damping levels, especially in the top part of the building and more conspicuously for moderate earthquake intensities. In the absence of accurate data, Maxwell stiffness for analysis based on brace extender properties should be substantially reduced, with recommended factors between 0.25 and 0.50. Given the scarcity of experimental data, these results should be considered preliminary.