Xilin Lu

Numerical Analysis of Tall Buildings Considering Dynamic Soil-Structure Interaction

Three-dimensional finite element analysis in time domain on dynamic soil-pile-structure interaction of a practical engineering is carried out in this paper. General-purpose finite element program ANSYS is used in the analysis. Commonly used equivalent linearity model is chosen as constitutive relation of soil. Viscous boundary of soil is implemented in ANSYS program. The influences of parameters, such as soil property, excitation, the rigidity of structure and buried depth, on dynamic characteristics, seismic response and interaction effect of SSI system are discussed.

NEW STRUCTURAL SYSTEM FOR EARTHQUAKE RESILIENT DESIGN

The concept of self-centering structure and replaceable structural member or fuse is a new approach of earthquake resilient structural design. The new structural system is not only capable of preventing from failure of the structure and life safety of the occupants during earthquakes, but also quickly restoring its basic function in a short time following earthquakes. Restoring the original position through self-weight or pre-stressing forces, this kind of structures is defined as a rocking or self-centering structure. The replaceable elements of the other new kind of structural system, which includes coupling beams, energy dissipation devices, and rubber bearings and so on, are designed to be inelastic or failure, like a fuse, and to protect the main structure from damage in the region of high seismicity. The authors of this paper will report three different tests on new earthquake resilient structures, which are shaking table tests on a self-centering RC frame, quasi-static cyclic tests on RC wall with replaceable coupling beams and the ones with replaceable foot parts. All of the three new kind structural systems present an efficient resilient ability under severe horizontal forces in the tests.

Seismic Fragility Assessment of RC Moment-Resisting Frames Designed According to the Current Chinese Seismic Design Code

Abstract Seismic fragility was assessed for 72 RC moment-resisting frame building structures designed according to the current Chinese seismic design code for buildings, taking into account the uncertainty of the structural material strength and earthquake ground motions. The site soil type, the number of stories, and the seismic protection intensity were considered to be the main design variables of the reference structures. Fragilities for four damage levels, i.e., fully operational, operational, repairable, and collapse prevention, are developed in this study. The global seismic damage index, which reflects the effects of individual structural components, and the maximum inter-story drift ratio, which is closely related to the seismic damage of structural and non-structural components, was employed as the damage identifiers. For each frame structure, the probability of exceeding each damage level in an earthquake with a specified PGA was determined by conducting nonlinear time history analysis. Fragility curves for the four damage levels were derived by regression analysis using the nonlinear least-squares method. The structural reliability of RC frames against earthquakes was examined using the developed fragility curves. The results indicate that seismic performance objectives for RC frame structures designed in accordance with the current Chinese code can be achieved with good reliability.

Shaking Table Model Test on the Dynamic Soil-Structure Interaction System

Abstract Shaking table model tests on dynamic Soil-Structure Interaction (SSI) system are described in this paper. In the design and manufacture of the test modals, the similitude formulas and similitude factors of all physical quantities are studied. Through controlling the ratio between diameter of the container and plan size of the structure, a flexible container is designed and manufactured in order to minimize the box effect resulting from the boundary conditions. The simulation design of the soil boundary is proved to be extremely effective in the present SSI tests. Nine specimens including five pile foundations, three box foundations and one fixed base are designed and made. A single column with mass block at its top and 12-story cast-in-place R.C. frame model are used as superstructure, and Shanghai soft soil is employed as the model soil. Finally, some important findings from the present tests are concluded.

Numerical Analysis of Dynamic Soil-Box Foundation-Structure Interaction System

abstract Three-dimensional finite element analysis on soil-box foundation-structure interaction system is carried out. General-purpose finite element program ANSYS is used in the analysis. Commonly used equivalent linearity model is chosen as constitutive relation of soil, and the changing-status nonlinearity of soil-structure interface is considered by surface-to-surface contact element. The computational model and analysis method is verified through comparison study between the calculation and the shaking table test results, and issues drawn from the computational analysis are consistent with the test results. The calculations demonstrate that sliding and separation occur between the foundation and the soil, and it shows that great error will occur when the material nonlinearity of soil and the changing-status nonlinearity of soil-structure interface are not considered in calculation of soft soil. Finally, some important findings from the calculations are concluded.

SHAKING TABLE TESTS ON A COMPLEX HIGH-RISE STRUCTURE WITH TWO TOWERS AND LAPPING TRANSFER COLUMNS

In recent decades, structural engineering tends to progress toward more novel high-rise structures under the requirement of realistic functions and architectural aesthetics. The complex high-rise building structure in this study has two towers with lapping transfer columns. The lapping transfer columns, considering aesthetic requirement in elevation, lead to a complex system of vertical force transfer. The large irregularity in elevation, according to Chinese code, needs a detailed study. A 1/15-scaled model of the structure was tested on the shaking table to evaluate its seismic performance. During the tests, the model was subjected to earthquake inputs representing frequent, basic, rare, and extremely rare earthquakes. The results of shaking table test in terms of the global and local responses as well as the dynamic properties are presented. The tests demonstrate that the designed structural system satisfies the pre-defined performance objectives and the lapping transfer columns are capable of coordinating the bi-level stories to resist lateral forces even under extremely strong earthquakes. To better control seismic damages of the building, some suggestions for improving the design of this structure are also put forward at last.

Experimental Study and Numerical Simulation of the Reinforced Concrete Walls with Different Stirrup in the Boundary Element

Abstract Reinforced concrete shear wall is one of the main lateral resistance members in tall buildings. A further understanding of shear walls is believed important to the seismic design. To day, it is recognized that boundary elements will contribute to the seismic behavior of shear walls, though the extent still remains vague and evidence is seldom given by relative experiment. Further-more, the nonlinear capacity evaluation method of shear walls is also a difficulty, especially considering the boundary elements. This paper aims to extend the relevant existing studies with the following two programs: (1) The experimental study of shear walls with different stirrup ratios in boundary areas; (2) A numerical simulation study of the three specimens. A finite element program SAPCAD is applied to simulate the seismic behavior of the shear wall specimens with different boundary condition. The experimental study and the numerical simulation in this paper indicate that: (1) A properly distributed boundary element may be helpful to avoid local crush and brittle failure; (2) The lateral load-carrying capacity could be influenced by stirrup bars; (3) The cyclical load simulation results drawn by SAPCAD agree with the test results. And some of the stirrup effect could be evaluated by this kind of simulation.

Maximum Displacement Profiles of Reinforced Concrete Frames

Abstract New expressions to estimate maximum seismic lateral displacement profiles of regular plane RC frames at three different damage levels, slight, moderate, and severe damage, are developed in this study for the purpose of displacement-based seismic design. These expressions relating the maximum floor displacements with the maximum inter-story drift ratio over the height and the main structural characteristic of the frame are based on statistical analysis of the results of hundreds of nonlinear time history analysis conducted on a set of 25 plane RC frames subjected to a set of 16 physical accelerograms containing different frequency spectrum. The fundamental period, column-to-beam strength ratio, and damage level were identified as the main structural characteristic having significant effects on the maximum displacement profiles. A case study was conducted on a 12-story RC frame model tested on the shaking table before and shows good agreement between the estimated profiles and test results. The developed profiles are independent of sections and reinforcement of the structure so that they can be used as the starting design variables in displacement-based seismic design.

Numerical Simulation of Dynamic PSSI System Considering Liquefaction

Abstract Dynamic soil-structure interaction considering liquefaction is a very important topic in the field of earthquake engineering. A three-dimensional numerical analysis of pile-soil-structure interaction (PSSI) system in liquefiable site is carried out. General finite differential program FLAC3D is adopted in the analysis. Boundary condition of the model, soil nonlinearity, nonlinearity on the soil-structure interface and pore water pressure build-up process are taken into account in the calculation model. The computational model and analysis method is verified through comparison study between the calculation and the shaking table test results. The distribution of the pore pressure ratio in sand soil and the acceleration of PSSI system are disscussed in this paper. At last, some important findings are concluded.

Parameter Identification and Numerical Analysis of Shaking Table Tests on Liquefiable Soil-Structure-Interaction

Using the parameter identification method of analysis on the test records of soil acceleration and pore water pressure from the shaking table tests for dynamic liquefiable soil-pile-structure interaction system, the dynamic properties of soil are obtained. Based on the recognized soil parameters, numerical simulation of liquefiable soil-pile-structure interaction test has been carried out. The results of the comparision of acceleration response and pore water pressure obtained from numerical simulation and tests show that the rule drawn from the numerical simulation is agreed well with those from the tests, though there are some disparities in quantity. So the reliability of parameter identification and numerical simulation technology in shaking table tests is validated. The result in this dissertation can be referred for future similar research.