Taejun Cho

Risk assessments of long-span bridges considering life-cycle cost concept and near-fault ground motion effect

The influence of the near-fault ground motion on the response of long-span bridges must be considered as a critical factor for seismic design because the response indicates different aspects from existing earthquake characteristics. Also, it is important to note that the safety index for the risk assessment of long-span bridges is determined based on the minimum expected life-cycle cost E(LCC). In this study, earthquake characteristics are analyzed by creating elastic and inelastic response spectrums with actual measurement records (Chi-Chi earthquake records) and then the numerical analysis of the long-span bridge in Namhae, Korea is performed according to the increase and reduction of the member stiffness based on the standard design., the reliability evaluation of the long-span bridge considering aleatory uncertainties is performed on the basis of the combined results of static analysis and seismic response analysis. Also, the minimum LCC is estimated based on failure probabilities by the different alternative design. Because of epistemic uncertainties, the results of reliability evaluation and the LCC of optimal design are selected as random variables; the safety index, failure probability and expected minimum LCC are re-evaluated with regard to critical percentage values for a risk-averse design of the long-span bridge, and are presented graphically using cumulative percentages. It is, therefore, expected that this study will provide the basic information for the risk assessment and optimal design method in performing seismic design of the long-span bridge considering earthquake characteristics.

Fatigue reliability analysis for the crack propagation compared with LRFD specification

In this study, a fatigue reliability is assessed taking into account the uncertainties in load and resistance of a steel specimen, in which fatigue crack propagates. The results of the deterministic fatigue crack propagation has been compared with AASHTO LRFD specification. A response surface method (RSM) combined with an advanced first order second moment method has been applied in order to assess the probability of the remaining life of the specimen under cyclic load as a function of crack length. For composing limit state functions, the stress ranges, stress intensity factor and the remaining life was selected as input random variables for the developed fatigue-reliability analysis program. The choice of Bayesian Belief Nets (BBNs) as a tool for assessing uncertainties in resistant component of a limit state function allows an extended probabilistic assessment for the resultant fatigue life of a target specimen, in terms of resistant components of stress range, stress intensity factor, and material property. Additionally, fragility curve models are proposed to compare the probabilistic fatigue life as describing the probability of a structure to suffer a given damage level when it is subject to a given fatigue life. The proposed integrated stochastic models can significantly contribute to the design and maintenance of infra-structures, demonstrated in the numerical example in terms of damage index with the probability of exceedance the considered damage levels.

A Preliminary Design for Hybrid Building System with Progressive Collapse Prevention Means

In this study, we propose an innovative lateral force distribution building system between tall buildings by utilizing the difference of moment of inertia, resulting the reduction of lateral displacement and the lateral forces in terms of an alternative for the dense human and increased cost of lands in highly integrated city area. A successive collapse prevention means by providing additional bearing plate between connections is proposed. In addition to that, a more economical vibration reduction is expected due to the suggested tuned mass damper on the surface of spacial structure. In the considered verification examples, reduced drifts at the top location of the building systems are validated against static wind pressure loads and static earthquake loads. The suggested hybrid building system will improve the safety and reliability of the new or existing building system in terms of more than 30% reduced drift and vibration through the development of convergence of tall buildings and spatial structures.

Determination of Optimum Glass Transition Temperature of Acrylic Acid Ester Copolymer to Improve Performance of Cement Matrixes

This study was conducted to determine an appropriate glass transition temperature of a polymer to improve performance of the polymer cement mortar and concrete. The glass transition temperature of the polymer, which was synthesized as a redispersible acrylic acid ester polymer, and physical properties of polymer cement mortar were evaluated. The polymer showed a lower glass transition temperature as the ratio of unsaturated hydrophobic ethylene monomer was increased. A decrease in glass transition temperature decreased the tensile strength, compressive strength, flexural strength, and volume change; increased the elongation, liquidity of polymer cement mortar, and adhesion; and delayed condensation. Scanning electron microscope and energy-dispersive x-ray spectroscopy results confirmed that a polymer film formed inside the cement matrix made mostly of carbon around pores and cracks inside the cement paste. A glass transition temperature of 0 to –11°C (32 to 12.2°F) showed the best results. These results can be used for developing novel construction materials.

Development of Deterioration Prediction Model and Reliability Model for the Cyclic Freeze-Thaw of Concrete Structures

The initiation and growth processes of cyclic ice body in porous systems are affected by the thermo-physical and mass transport properties, as well as gradients of temperature and chemical potentials. Furthermore, the diffusivity of deicing chemicals shows significantly higher value under cyclic freeze-thaw conditions. Consequently, the disintegration of concrete structures is aggra- vated at marine environments, higher altitudes, and northern areas. However, the properties of cyclic freeze-thaw with crack growth and the deterioration by the accumulated damages are hard to identify in tests. In order to predict the accumulated damages by cyclic freeze-thaw, a regression analysis by the response surface method (RSM) is used. The important parameters for cyclic freeze-thaw- deterioration of concrete structures, such as water to cement ratio, entrained air pores, and the number of cycles of freezing and thaw- ing, are used to compose the limit state function. The regression equation fitted to the important deterioration criteria, such as accu- mulated plastic deformation, relative dynamic modulus, or equivalent plastic deformations, were used as the probabilistic evaluations of performance for the degraded structural resistance. The predicted results of relative dynamic modulus and residual strains after 300 cycles of freeze-thaw show very good agreements with the experimental results. The RSM result can be used to predict the probability of occurrence for designer specified critical values. Therefore, it is possible to evaluate the life cycle management of concrete structures considering the accumulated damages due to the cyclic freeze-thaw using the proposed prediction method.

Crack Width Prediction in Concrete Bridges Considering Bond Resistances affected by Corrosion

The current design for crack width control in concrete bridges is incomplete in analytical models. As one of the important serviceability limit states, the crack width be considered with the quantitative prediction of the initiation and propagation of corrosion and corrosion-induced cracking. A serviceability limit state of cracking can be affected by the combined effects of bond, slip, cracking, and corrosion of the reinforcing elements. Considering life span of concrete bridges, an improved prediction of crack width affected by time-dependent general corrosion has been proposed for the crack control design. The developed corrosion models and crack width prediction equation can be used for the design and the maintenance of prestressed and non-prestressed reinforcements by varying time, w/c, cover depth, and geometries of the sections. It can also be used as the rational criteria for the maintenance of existing concrete bridges and the prediction of remaining life of concrete structures.