EVALUATION OF DEFORMATION CAPACITY FOR RC T-SHAPED CANTILEVER WALLS

If RC structural walls are properly designed and proportioned, these walls can behave in a ductile manner. To achieve this goal, the designer should provide adequate strength and deformation capacity of structural walls corresponding to each performance level (e.g. immediate occupancy, life safety and collapse prevention). This study investigates the drift and ductility capacities of T-shaped structural walls on the basis of results from experimental tests and sectional analyses. To determine proper deformation capacities for T-shaped structural walls, structural performances of T-shaped walls were evaluated with several parameters such as longitudinal reinforcement ratio, distribution of longi-tudinal reinforcement, lateral confinement ratio, and axial load ratio. Based on these results, the level of deformation capacity specified in current design provisions (ICBO, UBC 1997), which were expressed as both strain-based damage limit and interstorey drift ratio, were evaluated.     

Testing of Superelastic Recentering Pre-Strained Braces for Seismic Resistant Design

Over the past decade, the use of shape memory alloys (SMAs) in passive control devices has been explored. Nevertheless, some aspects in regards to the cyclic behavior of SMAs and the effect of pre-straining need to be clarified. In this study, small-scale shake table tests have been performed to explore the effectiveness of SMA bracing systems as compared to steel bracing systems. The reduced-scale experimental results imply that SMAs used in braces are more effective in controlling the response of a steel frame compared with a traditional bracing system. A finite element model (FEM) of the frame is developed in order to compare the analytical results with the shake table tests. Further, the effect of pre-straining the SMA braces is evaluated through both experimental and analytical studies. The results show that pre-straining improves the performance of the frame compared to the nonpre-strained case. However, as the level of pre-straining increases above approximately 1.0% to 1.5%, the benefits of pre-straining decrease compared with low-to-moderate pre-strain levels.     

Seismic Site Response Modeling for Three Missouri River Highway Bridges

Three highway bridges spanning the Missouri River flood plain were selected for evaluation of seismic site response for moderate size earthquakes emanating from the New Madrid Seismic Zone (NMSZ) in the Midwestern United States. The NMSZ is known to be capable spawning earthquakes larger than magnitude (M) 7.0, four of which occurred in a three-month period between 1811 and 1812, and the M_w 6.0 earthquake of October 1895 centered near Charleston, Missouri. This study evaluated the likely impacts of long period motion of these historic earthquakes on three long-span highway bridges using geotechnical data obtained from recent investigations. Our results suggest site amplification between 6× and 9×, depending on the magnitude and epicentral distance. We believe that threshold magnitude for serious foundation failure and damage to these bridges is between M_w 6.5 and 6.6. Above these magnitudes widespread liquefaction is predicted, which would effect the peak horizontal acceleration and spectral accelerations, causing the ground motions to be different than predicted. Increase in amplification of the response spectra also should be expected where the periods are higher than 1.0 sec. Therefore, M_w 6.5+ earthquakes at ranges 210–260 km could be expected to engender resonant frequency problems for multiple span bridges and tall buildings (10 to 25 stories) in channel corridors containing 20 to 46 m of unconsolidated sediment.     

Influence of Uncertain Source Parameters on Strong Ground Motion Simulation with the Empirical Green's Function Method

A combined stochastic and Green's function approach was developed to simulate strong ground motions in Southwest Western Australia (SWWA) in a previous study. Although it was demonstrated that adopting the source parameters derived from other regions yielded reasonable simulation of ground motions in SWWA as compared with a few available strong motion records, the effect of source parameter variations on simulated ground motions was not known. This article performs a statistical study of the effects of random fluctuations of the seismic source parameters on simulated strong ground motions. The uncertain source parameters, i.e., stress drop ratio, rupture velocity, and rise time corresponding to the empirical source models are assumed to be the respective mean value of the parameter and normally distributed with an assumed coefficient of variation. The Rosenblueth's point estimate method [Rosenblueth, 1981 Rosenblueth, E. 1981. Two-point estimates in probabilities. Applied Mathematical Modelling, 5: 329–335. [Crossref], [Web of Science ®] , [Google Scholar]] is used to calculate the statistics of the simulate ground motion parameters corresponding to different magnitudes and epicentral distances. The accuracy of the Rosenblueth's point estimate method in estimating the mean and standard deviation of ground motion PGA, PGV, and response spectrum is proven by simulating the ground motions from an ML6.0 and epicentral distance 100 km event with both the Rosenblueth's point estimate method and the Monte Carlo simulation method. A sensitivity analysis is preformed to investigate the effect of random fluctuations of each source parameters on strong ground motion simulation. A coefficient of variation model for ground motion parameters is developed based on the simulated data as a function of the variations of the three source parameters and earthquake magnitude, which can be used in probabilistic predictions of earthquake ground motions with uncertain source parameters.     

Optimum Design Approaches for Improving the Seismic Performance of 3D RC Buildings

In this article, a number of design approaches for 3D reinforced concrete (RC) buildings are formulated in the framework of structural optimization problems and are assessed in terms of their performance under earthquake loading. In particular, three design approaches for RC buildings are considered in this study. In the first, the initial construction cost is considered as the objective function to be minimized. The second one is formulated as a minimization problem of the torsional response, while a combined formulation is also examined as the third design approach. The third approach is considered with two distinctive formulations. According to the first approach, the torsional behavior is minimized by minimizing the eccentricity between the mass and rigidity centers, while the second one is achieved by minimizing the eccentricity between the mass and strength centers. It is shown that the optimized designs obtained according to the minimum eccentricity of the rigidity center behave better in frequent (50/50 hazard level) and occasional (10/50 hazard level) earthquakes, while the designs obtained according to the minimum eccentricity of the strength center formulation was found better in rare (2/50 hazard level) events. Designs obtained through a combined formulation seem to behave equally well in the three hazard levels examined.     

Simple Models for Inelastic Seismic Analysis of Asymmetric Multistory RC Buildings

A simple stick model is presented for the inelastic seismic analysis in 3D of two-way eccentric multistory RC buildings. It has 3 DoFs per floor, point hinges at the ends of the vertical elements connecting floors, elastic story stiffness derived from the corresponding story force-interstory deformation relations of the elastic 3D structure under inverted-triangular floor loading (by torques for torsional stiffness, by horizontal forces for the lateral ones), story yield forces derived from the total resistant shear of the story vertical elements, but no coupling between lateral and torsional inelasticity. It is evaluated on the basis of comparisons of response histories of floor displacements to those from full nonlinear models in 3D of four actual buildings. Alternative locations of the story vertical element with respect to the floor mass center are examined: (a) the floor “center of twist” of the elastic 3D building under inverted-triangular floor torques; (b) the story “effective center of rigidity,” through which application of inverted triangular lateral forces does not induce twisting of floors; (c) the centroid of the secant stiffness of the story vertical members at yielding and (d) the centroid of the lateral force resistance of story vertical elements. Among alternatives (a)–(d), the floor “center of twist” provides the best agreement with floor displacement response-histories from full 3D nonlinear models. This means that the static eccentricity that matters for torsional response may be taken as that of the floor “center of twist.” The center of resistance comes up as the second-best choice.     

Seismic Analysis of Non Proportionally Damped Two-Way Asymmetric Elastic Buildings Under Bi-Directional Seismic Ground Motions

This study investigates the effectiveness of the modal analysis using three-degree-of freedom (3DOF) modal equations of motion to deal with the seismic analysis of two-way asymmetric elastic systems with supplemental damping. The 3DOF modal equations of motion possessing the non proportional damping property enable the two modal translations and one modal rotation to be non proportional in an elastic state. The simple approximation method is to use the single degree-of-freedom (SDOF) modal equations of motion, which are obtained by neglecting the off-diagonal elements of the transformed damping matrix. One, one-story and one, three-story non proportionally damped two-way asymmetric buildings under the excitation of bi-directional seismic ground motions are analyzed. The analytical results are obtained by using the proposed method, noted simple approximation method, and direct integration of the equation of motion. It is seen that the proposed method can significantly improve the accuracy of the analytical results compared with those obtained by using the simple approximation method. Moreover, the proposed method does not substantially increase the computational efforts.     

Evaluation of Approaches to Characterize Seismic Isolation Systems for Design

Current design codes generally use an equivalent linear approach for preliminary design of a seismic isolation system. The equivalent linear approach is based on effective parameters, rather than physical parameters of the system, and may not accurately account for the nonlinearity of the isolation system. This article evaluates an alternative normalized strength characterization against the equivalent linear characterization. Considerations for evaluation include: (1) ability to effectively account for variations in ground motion intensity; (2) ability to effectively describe the energy dissipation capacity of the isolation system; and (3) conducive to developing design equations that can be implemented within a code framework.     

Design Earthquake Ground Motion Prediction for Perth Metropolitan Area with Microtremor Measurements for Site Characterization

Perth is the largest city in Western Australia and home to three-quarters of the state's residents. In recent decades, there have been a lot of earthquake activities just east of Perth in an area known as the South-West Seismic Zone. Previous numerical results of site response analyses based on limited available geology information for PMA indicated that Perth Basin might amplify the bedrock motion by more than 10 times at some frequencies and at some sites. Hence, more detailed studies on site characterization and amplification are necessary. The microtremor method using spatial autocorrelation (SPAC) processing is a useful tool for gaining thickness and shear wave velocity (SWV) of sediments and has been adopted in many previous studies. In this study, the response spectrum of rock site corresponding to the 475-year return period for PMA is defined according to the probabilistic seismic hazard analysis (PSHA) based on the latest ground motion attenuation model of Southwest Western Australia. Site characterization in PMA is performed using two microtremor measurements, namely SPAC technique and H/V method. The clonal selection algorithm (CSA) is introduced to perform direct inversion of SPAC curves to determine the soil profiles of representative PMA sites investigated in this study. Using the simulated bedrock motion as input, the responses of the soil sites are estimated using numerical method based on the shear-wave velocity vs. depth profiles determined from the SPAC technique. The response spectrum of the earthquake ground motion on surface of each site is derived from the numerical results of the site response analysis, and compared with the respective design spectrum defined in the Australian Earthquake Loading Code. The comparison shows that the code spectra are conservative in the short period range, but may slightly underestimate the response spectrum at some long period range.     

An Investigation of Characteristic Periods of Seismic Ground Motions

The design seismic base shear was obtained from the spectral elastic acceleration S_a divided by a system behavior factor R, accounting for ductility and overstrength. The behavior factor is currently taken as a constant for a given type of structures in various codes regardless of structural periods. In fact, the behavior factor is also a spectrum varying with the natural periods of structures. In order to understand the relationship between the spectral values and the corresponding characteristic periods in these two spectra, S_a and R_μ, this article carries out an investigation into the characteristic periods of 370 seismic ground motions from 4 site types. It is found that the periods T_ga at which the peak values appear in the S_a spectra are much less than the periods T _gR at which the R_μ spectra take a maximum value. Two characteristic periods are necessary to determine the seismic action if a more elaborate procedure is required in practice. Statistical study on these two periods is carried out for the 370 records, and results are presented. For site types A–D, the ratio of these two periods has a statistically averaged value of 5.5–6.7.

The maximum input energy S _EI , relative velocity S _v , power density P _SD , and the Fourier amplitude F _S spectra were constructed to determine their characteristic periods, respectively. These four spectra predict similar characteristic periods to T _gR . T _gR is very close to the characteristic period T _gd of the elastic displacement spectra.

Analysis of SDOF systems under combined harmonic excitations shows that the S_a spectrum is more sensitive to high-frequency excitations, while the displacement spectrum is more sensitive to long period excitations. For the elastic-plastic S_a spectra, peak values tend to appear at shorter periods even the amplitudes of the longer periods are greater than that of the shorter period. This provides an explanation on different characteristic periods in the S_a and R_μ spectra.