Kiseok Kwak

Prediction of Scour Depth Versus Time for Bridge Piers in the Case of Multi-Flood and Multi-Layer Soil System

A new method called E-SRICOS is proposed to predict the scour depth z versus time t curve around bridge piers. The E-SRICOS method is based on the SRICOS method which was developed in 1999 to predict the scour depth versus time curve around a cylindrical bridge pier for a constant velocity flow and for a uniform soil. The SRICOS method is improved to be applied to real cases that soil is non-homogeneous and velocity varies with time. The new methodologies to handle the multi-flood and the multi-layer soil systems are introduced and included in the computer program called the SRICOS program. This E-SRICOS method with the SRICOS program is verified against full-scale scour measurements.

Lateral response of large-diameter monopiles for offshore wind turbines from centrifuge model tests

Centrifuge tests were carried out to investigate the lateral response of 6-m-diameter monopiles used for offshore wind turbine foundations. Well-instrumented large-diameter model monopiles were tested in dense sands under different end bearing layer conditions and different pile rigidities. A lateral load was applied to monopiles at a level of 33 m above the seabed on a prototype scale to simulate the combination of a high lateral load and significant overturning moment acting on the piles at the seabed level. The test results showed that the measured lateral displacements and moments along the shaft for a given lateral load are much greater than those predicted from p–y analyses using the typically used models of the American Petroleum Institute (API) (1993, “Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms—Working Stress Design,” API Recommended Practice 2A-WSD, 20th ed., API, Washington, DC) and Reese et al. (1974, “Analysis of Laterally Loaded Piles in Sand,” Proceedings of the 6th Offshore Technology Conference, Houston, TX, Paper No. OTC2080, pp. 473–483. In addition, p–y analyses using these two methods underestimated the effect of vertical load on the lateral response of the monopiles, whereas the effect was more pronounced in the centrifuge test results. Monopiles generally support large vertical loads transferred from the self-weight of the tower shaft and wind turbine compartment. Therefore, more studies are required to develop new p–y curves for large-diameter monopiles to replace the existing API and Reese et al. p–y curves, which were developed for small- or medium-diameter driven piles.

A Case Study of LRFD Implementation for Static Bearing Capacity of Driven Steel Pipe Piles in Korea

Resistance factors for static bearing capacity of driven steel pipe piles were calibrated in the framework of reliability theory. In order for the verification of resistance factors and the implementation of the LRFD, a case study on an actual bridge foundation designs using both the ASD and the LRFD methods is comprehensively accomplished. Detailed analysis showed that both of the design concepts gave the design bearing capacities equal to or more than the design loads. The LRFD method, however, could provide the quantitative provability of failure of the pile foundation, whereas the ASD method showed nominal level of safety. ABSTRACT: Resistance factors for static bearing capacity of driven steel pipe piles were calibrated in the framework of reliability theory. In order for the verification of resistance factors and the implementation of the LRFD, a case study on an actual bridge foundation designs using both the ASD and the LRFD methods is comprehensively accomplished. Detailed analysis showed that both of the design concepts gave the design bearing capacities equal to or more than the design loads. The LRFD method, however, could provide the quantitative provability of failure of the pile foundation, whereas the ASD method showed nominal level of safety.

EVALUATION OF RESISTANCE BIAS FACTORS FOR LOAD AND RESISTANCE FACTOR DESIGN OF DRIVEN STEEL PIPE PILES

Resistance bias factors for the two static bearing capacity analysis methods of driven steel pipe piles adopted in the Korean Design Standards for Structure Foundations were evaluated as part of study to develop LRFD codes for foundation structures in South Korea. Of the 2,227 static load test data on driven steel pipe piles collected from all around the country, only 43 were found to be useful for this study. These load test piles were sorted into two cases: pile tip in soil case and pile tip in rock case. Pile bearing capacities were measured from the load-displacement curves by six different failure criteria, and Davissons criterion was found to be most reliable. Statistical analysis of the resistance bias factors was performed, and the results show a significantly high degree of variation in the resistance bias factors.

A probabilistic fragility evaluation method of a RC box tunnel subjected to earthquake loadings

A probabilistic fragility assessment procedure is developed in this paper to predict risks of damage arising from seismic loading to the two-cell RC box tunnel. Especially, the paper focuses on establishing a simplified methodology to derive fragility curves which are an indispensable ingredient of seismic fragility assessment. In consideration of soil-structure interaction (SSI) effect, the ground response acceleration method for buried structure (GRAMBS) is used in the proposed approach to estimate the dynamic response behavior of the structures. In addition, the damage states of tunnels are identified by conducting the pushover analyses and Latin Hypercube sampling (LHS) technique is employed to consider the uncertainties associated with design variables. To illustrate the concepts described, a numerical analysis is conducted and fragility curves are developed for a large set of artificially generated ground motions satisfying a design spectrum. The seismic fragility curves are represented by two-parameter lognormal distribution function and its two parameters, namely the median and log-standard deviation, are estimated using the maximum likelihood estimates (MLE) method.

RELIABILITY ANALYSIS OF STATIC BEARING CAPACITY EVALUATION OF DRIVEN STEEL PIPE PILES USING MCS

As part of Load and Resistance Factor Design implementation effort in South Korea, reliability of the two static bearing capacity analysis methods for driven steel pipe piles adopted in the Korean Standards for Structure Foundations was evaluated. Over 2,000 static load test data on driven steel pipe piles were collected from all around the country. After review of the load-displacement curves, subsurface investigation reports, and other related documents, only 43 load test data were found to be useful for the reliability analysis. These load test piles were sorted into two cases: pile tip on soil case and pile tip on rock case. Resistance bias factor statistics of each pile tip case were evaluated for the two static design methods. Reliability analysis was performed by two types of First Order Reliability Methods, Mean Value First Order Second (MVFOSM) method and Advanced First Order Second (AFOSM) method, and Monte Carlo Simulation (MCS) method. Reliability of the static bearing capacity analysis methods for pile tip on soil case appears to be unacceptably low for safety factors in the range of 3 to 5. Reliability of the static bearing capacity analysis methods for pile tip on rock case is not conclusive. AFOSM method resulted in a little larger reliability indices than MVFOSM method, but the difference was not significant. Reliability indices computed by the MCS method were almost identical to those by the AFOSM method.

Bridge Scour Prioritization and Management System for National Road Bridges

A bridge scour management system is developed to evaluate the vulnerability of bridge piers to scour and to help establish effective disaster measures, taking into account the locality and scour characteristics in Korea. This system is programmed using the techniques of the geographical information system(GIS) for the storage, retrieval, and display of information regarding to bridge scour. The functions of the system are basically divided into 2 parts; prioritization and maintenance. Bridges are initially screened and prioritized for bridge scour inspection and evaluation using basic information which is obtained from the office review. The bridge scour evaluation including site investigation is performed and the vulnerability of bridge piers is categorized into 7 groups taking into account the concise information of the pier-foundation-ground system. The system tabulates and plots all the data and the results. After categorizing all the national road bridges, the compiled and plotted information can be utilized in planning the scour inspection schedule for maintenance and in timely decision-making with regard to remediation of scour critical bridges. A case study of implementing this system which includes the initial screening, scour inspection and evaluation, and categorization of the scour vulnerability of bridge piers is presented.