Dr. Maureira-Carsalade, Nelson

The most used global sensitivity analysis (GSA) method is based on variance. This is performed using Monte Carlo Sampling (MCS) or Latin Hypercube Sampling (LHS). It requires a large sample to obtain accurate estimates. Density-based methods, such as the GSA PAWN, have been developed to reduce the sample size without compromising the result. PAWN is simpler than other methods because it uses cumulative density functions (CDF) instead of probability density. This method has been widely used in areas such as environmental engineering with very good results, reducing computation time. However, its use in structural engineering is incipient. The PAWN method was used to classify the design variables of the isolation system in relation to their sensitivity, and in relation to the seismic response of industrial storage racks. The above was analyzed in terms of the effectiveness of each variable to reduce the seismic demand using a novel base isolation kinematic device (BIKD). Racks with different combinations of their structural parameters such as the number of storage levels, the height between them, and isolation period, among others, were studied. The dimensions of the racks were chosen to match those that would later be experimentally tested on shaking table. An earthquake whose response spectrum matched the design spectrum of current Chilean regulations, was considered as seismic forcing. The maximum base shear load, the displacement of the top level of storage and the floor drift were considered as target responses to be studied. Fixed base racks (FBR), as reference, and base-isolated racks (BIR) were analyzed. The results showed the effectiveness of using the BIKD system in reducing all three-target responses up to one order of magnitude. Additionally, it was determined that the parameters that have the greatest influence on the response correspond to the number of storage levels and the height between them, both for FBR and BIR.

Base isolation is an efficient strategy for protecting structures, especially in countries with high seismic risk, such as Chile. This paper presents the conceptual model, mathematical model, experimental validation and numerical analysis of a roller type base isolation device that aims to solve problems of limited tensile strength (compared to its compressive strength) and lateral instability of all types of rubber bearing isolators when faced with elevated axial load. The conceptual model describes the device’s components and operation. The mathematical model establishes its constitutive law based on the equilibrium equations formulated considering large lateral displacements. Experimental tests were run on a shake-table with a load frame to simulate the isolator’s interaction with the superstructure, considering a combination of the device’s design parameters, in order to identify their effect. In the numerical analysis, six simple frame buildings were modelled and subjected to a seismic record using the proposed roller isolator. Error parameters were obtained between the numerical predictions and the experimental results in each loading and unloading cycle, varying between 1.6% and 5.1% for dissipated energy and 4.0% to 17.7% for the magnitude of force. The proposed device worked as a seismic isolator, reducing the structure’s response in a magnitude order in relation to the building fixed on its base.

This article proposes a geometrically nonlinear co-rotational model aimed to characterize the mechanical behavior of elastomeric seismic isolators. The model is able to capture the axial and lateral coupling in both axial directions, i.e. compression and tension of the isolator. Also reproduces the instability the loads in tension as well as in compression, and provides theoretical evidence of the non-symmetric behavior of the isolator in these two directions. To validate model results, a quasistatic analysis was performed on a typical isolator with many different shape factors. From the parametric analysis performed, it is observed that buckling loads are higher in tension than in compression. However, as the shape factor of the isolator increases, the behavior in compression and tension becomes symmetric. It becomes apparent that significant differences in normal stresses and strains under tensile and compressives loads are observed for axial loads smaller than 10% of the nominal buckling load. The example presented shows that lateral displacements of about ±25% of isolator radius and tension forces up to 10% of the buckling load are possible without inducing cavitation in the rubber. Accuracy of the model was also tested against finite element model results and experimental data showing satisfactory results. Furthermore, a response-history analysis of an isolated structure is presented and compared for two isolator models: the two-spring model and the model proposed herein. Finally, material nonlinearity was introduced in the dynamic analysis using a Bouc-Wen type element in parallel with the isolator. The responses are similar between models; however, significant differences occur locally in the isolator for high axial loads and/or large lateral displacements.