Ho-Seong Mha

Fatigue reliability assessment of an existing steel railroad bridge

Systematic procedures to assess the reliability level and remaining fatigue life of an existing steel railroad bridge are demonstrated by examining critical bridge members. Critical members are selected among those members experiencing the maximum stress ranges higher than the corresponding endurance levels over two million cycles, due to the maximum loading condition. Applied loads on the bridge vary randomly, and this results from the uncertain nature of the input loading conditions, such as traffic schedules, passenger volumes, and so on. Only passenger volumes are assumed to be random, which are determined based upon the provided data and on-site visual observations. The processes to evaluate the remaining fatigue life are classified into three categories according to the given assumed loading models: (1) simplified procedure; (2) probabilistic procedure; (3) deterministic procedure. Stress time histories induced by the moving subway trains are obtained by using the Finite Element analysis, which is calibrated based on the field measurements. The coincidence rates of two trains at the same location are also considered and are simulated based upon the provided traffic schedules. Some members with peculiar details due to improper manufacturing have been found. A repairing method to improve the fatigue strength of the members is proposed. Corresponding fatigue tests have been conducted to verify the proper category for the members with peculiar details and the repaired members. Various illustrated results obtained by the proposed procedures are presented for comparison. It is found that the simple deterministic procedure may be adopted to get quite consistent information on the remaining fatigue life of a bridge with acceptable accuracy.

Environmental impact assessment and eco-friendly decision-making in civil structures

This study develops two useful procedures in performing an environmental-impact assessment. One is the advanced life-cycle assessment (LCA) method, which effectively tracks the flow of materials and considers the recycling and demolition of a civil structure. The other is an eco-friendly decision-making procedure, which may effectively apply when determining the prototype of a civil structure. The advanced LCA method differs from traditional LCA procedure, as it classifies the input material prior to the impact assessment. Classification work is performed to establish independent life-cycle stages for each material. The processes of recycling and demolition are appropriately added to the life-cycle stages. The impact assessment is performed separately for the materials, and results are aggregated at the end of the analysis. The eco-friendly decision-making procedure enables designers to choose an economical, and environmentally friendly, alternative during the planning phase of the construction project. This procedure rationally amalgamates economical value and environmental effects into a single indicator. The life cycle cost (LCC) of a structure can be analysed by using conventional LCC tools, whereas the environmental impact is estimated by LCA. The results from LCC and LCA are then integrated by using either a CO2 conversion method or an analytical hierarchy process (AHP). The CO2 conversion method presents the result as a monetary value, whereas the AHP presents the result as a non-dimensional value. A practical example using a steel box girder bridge and a pre-stressed concrete (PSC) box-girder bridge is also given in order to aid the understanding of the presented procedure.

Dynamic Behaviors of the Curved Steel Tunnel Lining due to Wind Loads by Passing Vehicles

Dynamic behaviors of the tunnel linings of curved tunnels with various curvatures are investigated to examine the effect of wind loads due to passing vehicles. In the case without backfill, the responses of the tunnel lining should be considered to examine the clearance of the lining. A steel tunnel lining is selected to see the influence of the wind load upon the tunnel lining more clearly. The wind pressure upon the lining is simplified into the pressure and suction while the vehicle passing the loading positions. As the radius of curvature decreases, the response decreases, showing that the strength against the deformation is found to increase since the asymmetry of the deformation shape is reduced. It is found that the responses increase as the passing vehicle speed increases.

Analysis of Support Reactions of RC Slab Bridges with Various Skews

The support reactions of two-span RC slab bridges with various skews are studied through the time history analysis for moving vehicles. Dynamic behaviors of bridges are evaluated by using the commercial FEM code and the bridge is modelled using shell elements. The analysis method is verified by comparing measured values and analysis results. Parametric analysis are performed for various skew angles. The bearing below the end slab with an obtuse angle shows the largest reaction forces. As the skew angle is reduced(a skew angle of means a straight bridge without skew in this study), the distribution of reaction force is growing uneven and the largest reaction force is increased. In case of the model having skew angles under , the bearing located near the bearing with the largest reaction force shows the uplift force. Thus, it is concluded that the bearing in RC slab bridges with skew should be selected carefully by considering the variation of reaction force and uplift force.

A Study on the Seismic Performance of Energy-Dissipating Sacrificial Devices for Steel Plate Ginder Bridges

A new Energy-Dissipating Sacrificial Device (EDSD) is developed for steel plate girders, which can effectively dissipate the energy stored in the structures during seismic actions. To verify the performance of the EDSD, various seismic responses of a sample bridge with the EDSD are analyzed in terms of energy, member forces and deformation. The full scale model tests are conducted to certify the performance of the EDSD when it is applied on existing bridges. Using the improved hysteretic model of the sacrificial member, the seismic analysis for an example bridge is performed. The results show that the proposed EDSD under seismic excitations can significantly decrease the energy stored in the bridge structures and reduce the relative displacements of each superstructure to the ground. The EDSD is also found to function as a structural fuse under strong ground motions, sacrificing itself to absorb the excessive energy. Consequently, economical enhancement of the seismic performance of bridges can be achieved by employing the newly developed energy-dissipating sacrificial device.

Seismic Behaviors of a Bridge System in the Stochastic Perspectives

Semi-analytical methodology to examine the dynamic responses of a bridge is developed via the joint probability density function. The evolution of joint probability density function is evaluated by the semi-analytical procedure developed. The joint probability function of the bridge responses can be obtained by solving the path-integral solution of the Fokker-Planet equation corresponding to the stochastic differential equations of the system. The response characteristics are observed from the joint probability density function and the boundary of the envelope of the probability density function can provide the maxima ol the bridge responses.

Dynamic Responses of Multi-Span Simply Supported Bridges under Bi-Directional Seismic Excitations

A Seismic analysis procedure of bi-directional brideg motions is developed by using mechanical bridge model. A three-dimensional mechanical model can consider major phenomena under bi-directional seismic excitations, such as nonlinear pier motion under biaxial bending, pounding and bearing damage due to the rotaion of the superstructure, etc. The analyses utilizing the uni-directional and the bi-directional bridge model for the 3-span simply supported bridge are then performed. The seismic responses in two cases are examined and compared by investigating the relative displacements of each superstructure to both ground and adjacent superstructures and the restoring forces of RC pier. The analysis using either the uni-directional model or bi-directional model is acceptable for estimating the displacement responses of a bridge, but the bi-directional analysis is found to give more conservative results for resisting forces of RC piers. To make general conclusions, therefore, the analysis using the bi-directional bridge model should be performed in evaluating the seismic safety of bridges.