Arghadeep Laskar

Progressive collapse of a two-story reinforced concrete frame with embedded smart aggregates

This paper reports the experimental and analytical results of a two-story reinforced concrete frame instrumented with innovative piezoceramic-based smart aggregates (SAs) and subjected to a monotonic lateral load up to failure. A finite element model of the frame is developed and analyzed using a computer program called Open system for earthquake engineering simulation (OpenSees). The finite element analysis (FEA) is used to predict the load–deformation curve as well as the development of plastic hinges in the frame. The load–deformation curve predicted from FEA matched well with the experimental results. The sequence of development of plastic hinges in the frame is also studied from the FEA results. The locations of the plastic hinges, as obtained from the analysis, were similar to those observed during the experiment. An SA-based approach is also proposed to evaluate the health status of the concrete frame and identify the development of plastic hinges during the loading procedure. The results of the FEA are used to validate the SA-based approach for detecting the locations and occurrence of the plastic hinges leading to the progressive collapse of the frame. The locations and sequential development of the plastic hinges obtained from the SA-based approach corresponds well with the FEA results. The proposed SA-based approach, thus validated using FEA and experimental results, has a great potential to be applied in the health monitoring of large-scale civil infrastructures.

Seismic isolation of two dimensional periodic foundations

Phononic crystal is now used to control acoustic waves. When the crystal goes to a larger scale, it is called periodic structure. The band gaps of the periodic structure can be reduced to range from 0.5 Hz to 50 Hz. Therefore, the periodic structure has potential applications in seismic wave reflection. In civil engineering, the periodic structure can be served as the foundation of upper structure. This type of foundation consisting of periodic structure is called periodic foundation. When the frequency of seismic waves falls into the band gaps of the periodic foundation, the seismic wave can be blocked. Field experiments of a scaled two dimensional (2D) periodic foundation with an upper structure were conducted to verify the band gap effects. Test results showed the 2D periodic foundation can effectively reduce the response of the upper structure for excitations with frequencies within the frequency band gaps. When the experimental and the finite element analysis results are compared, they agree well with each other, indicating that 2D periodic foundation is a feasible way of reducing seismic vibrations.

Shear Strengths of Prestressed Concrete Beams Part 2: Comparisons with ACI and AASHTO Provisions

In Part 1 of this paper, an equation for the shear strengths of prestressed concrete beams was developed. New formulas were also proposed for maximum shear strength and for the minimum stirrup requirement. Part 2 of this paper compares the proposed shear design method to the shear provisions in the ACI 318-08 and the 2007 AASHTO LRFD Specifications using 148 test beams. The proposed method is shown to be simpler and more accurate than the ACI and AASHTO specifications, despite the fact that the proposed method does not take into account two variables, prestress force and angle of failure crack, that are involved in the ACI and AASHTO shear provisions.

Effect of Steel Fibers on Shear Behavior of Prestressed Concrete Beams

Four full-scale prestressed concrete (PC) I-beams were tested to study the effects of steel fibers on the increase in shear strength and ductility of beams. Beams were cast with no traditional transverse steel reinforcement. To study the effect of steel fibers on shear behavior, all four beams were cast using different amounts of steel fibers. The beams were subjected to concentrated vertical loads up to their maximum shear or moment capacity using four MTS actuators in load and displacement control mode. During the load tests, Linear Voltage Displacement Transducers (LVDTs) were used to measure displacements at several critical points on the web in the end zone of the beams. Several LVDTs were also placed under the beams at the points of loading to measure the actual total and net displacements of the beams. From the load tests, it was observed that the shear capacities of the beams increased significantly due to the addition of steel fibers in concrete. Complete replacement of shear reinforcement with steel fibers also increased the ductility and energy dissipation capacity of the PC I-beams.

Cyclic Softened Membrane Model for Prestressed Concrete

Wall-type or shell-type prestressed concrete structures, such as prestressed concrete I-girders, box girders, nuclear containment vessels, offshore structures, shear walls, etc can be visualized as assemblies of membrane elements. Their behavior can be predicted if the behavior of the membrane elements is thoroughly understood. Since prestressed concrete structures are now widely used, a research project was conducted to investigate the behavior of prestressed concrete elements subjected to shear action using the Universal Panel Tester available at the University of Houston. This paper reports the Softened Membrane Model for Prestressed Concrete (SMM-PC) (Wang, 2006) developed from this study. The SMM-PC generalized the previously developed Softened Membrane Model (SMM) (Zhu, 2000; Hsu and Zhu, 2002) for reinforced concrete and can be used for prestressed as well as reinforced concrete. It is also a rational model like the SMM as both of them satisfy Naviers principles of mechanics of materials (stress equilibrium, strain compatibility and constitutive laws of materials). The new SMM-PC includes the following three new constitutive laws: (1) A constitutive law of concrete in tension that includes the decompression stage. (2) A new prestress factor W p proposed for incorporation into the softening coefficient of the constitutive laws of concrete in compression. (3) A smeared (average) stress-strain relationships of prestressing strands embedded in concrete. In this paper the SMM-PC has also been extended to include cyclic behavior, thereby creating a Cyclic Softened Membrane Model for Prestressed Concrete (CSMM-PC). This has been accomplished by implementing the cyclic behavior of reinforced concrete previously developed at the University of Houston through the Cyclic Softened Membrane Model (CSSM) (Mansour and Hsu, 2005a, b). The CSMM-PC is implemented into a non-linear finite element program based on the framework of Opensees (Fenves, 2005) to predict the behavior of prestressed concrete structures under cyclic loading. The developed program is validated by analyzing a prestressed concrete beam tested under montonic loading, and comparing the analytical results with test data.

Comparative Study of 1D and 2D Simulation Models of Hollow RC Bridge Columns Under Reversed Cyclic Loads

Reinforced concrete (RC) bridge columns can be subjected to large dynamic loads during earthquakes. In order to design these structures, a thorough understanding of their nonlinear behavior is essential. 1D and 2D numerical simulation models are generally used for the analysis of these structures under axial loads and uniaxial bending. The iterative process used in the numerical simulation is cost-sensitive and time-consuming because of the complex constitute relationships of the materials. In this study, two hollow RC bridge columns tested under reversed cyclic loads at the National Centre for Research on Earthquake Engineering (NCREE) Taiwan have been analyzed using both 1D and 2D numerical simulation models. Analysis results from both simulation models such as primary backbone curves, hysteretic loops including pinching effects, and the strength degradation in the post-peak region have been compared and verified with the experimental data.

Periodic Materials-Based 3D Seismic Base Isolators for Nuclear Power Plants

The concept of periodic materials, based on solid state physics theory, is applied to earthquake engineering. The periodic material is a material which possesses distinct characteristics that do not allow waves with certain frequencies to be transmitted through; therefore, this material can be used in structural foundations to block unwanted seismic waves with certain frequencies. The frequency band of periodic material that can filter out waves is called the band gap, and the structural foundation made of periodic material is referred to as the periodic foundation. In designing a periodic foundation, the first step is to match band gaps of the periodic foundation with the natural frequencies of the superstructure. This is an iterative process. Starting with a design of the periodic foundation, the band gaps are identified by performing finite element analyses using ABAQUS. This design process is repeated until the band gaps match natural frequencies of the superstructure, and the field tests of a scaled specimen are conducted to validate the design. This is an on-going research project. Presented in this paper are the preliminary results, which show that the three dimensional periodic foundation is a promising and effective way to mitigate structural damage caused by earthquake excitations.Copyright

Effect of Flexural Ductility on Shear Capacity

The seismic performance of RC bridge piers is a significant issue because the interaction of flexural ductility and shear capacity of such piers with varied amounts of lateral reinforcement is not well established. Several relationships between flexural ductility and shear capacity have been proposed by various researchers in the past. In this paper a parametric study on RC bridge piers is conducted using a nonlinear finite element program, “Simulation of Reinforced Concrete Structures (SRCS),” developed at the University of Houston. SRCS has been previously used to predict the seismic behavior of such piers. The predicted results were compared with the test results obtained from experiments available in literature. Based on the results of the parametric study performed in this paper, a set of new relationships between flexural ductility and shear capacity of high strength RC columns is proposed.