NTRODUCTION OF GEOTECHNICAL PERFORMANCE BASED DESIGN
自1995年世界貿易組織/技術性貿易障礙(WTO/TBT)協定實施以來，WTO會員紛紛引入符合區域性整合乃至全球化標準之設計理念。在這一股改革聲浪中，以結構物性能為設計宗旨所發展出的性能設計(Performance Based Design, PBD)概念可謂大放異彩。本文旨在說明PBD架構及目標，以及此概念與其他設計法(包括LSD、RBD及LRFD等)之關係，並以鄰近日本的Geo-Code 21為例，詳述PBD規範之發展。文末筆者提出國內工程業界如何因應此一世界潮流趨勢之淺見，期能拋磚引玉，吸引更多大地工程領域之專業人士投入這個有趣且具挑戰性的課題。
Due to the implementation of WTO/TBT agreement in 1995, design code tends to be standardized among the WTO members and, further, harmonized with the international standards. It is found that Performance Based Design (PBD), which uses the performance of the structure as the design philosophy, has become the center of the whole standardization and harmonization and has emerged implicitly or explicitly in many countries’ design codes. This paper introduces the concept of PBD, including its origin, structure, relationships with existing design codes, and development around the world. The paper further introduces the development of PBD code in Japan by using a geotechnical design code, namely Geo-Code 21, as an example. The characteristics of Geo-Code 21 are presented, including its future works. Suggestions of our response to such a world trend of design code are provided at the end of the paper.
新世代設計規範之發展是近一、二十年來大地工程重要議題，而近期研究經整合後已可提出一個合理且全備之規範的架構。日本學者Honjo等人(2005)提出了整全設計規範(Comprehensive Design Code)的觀念以清楚扼要的闡明此架構。在這個架構下，合理之大地工程設計規範是以幾個新的設計理念為基礎來發展的；其中包含性能設計(Performance Based Design, PBD)、限度狀態設計(Limit State Design, LSD)與可靠度設計(Reliability Based Design, RBD)等觀念。整全設計規範包含兩大部分，其一為性能需求(performance requirements)之規定，而性能設計理念能有效地架構出此部份之規範，本專輯已另有專文加以介紹。第二部份是關乎大地工程師進行設計檢核(design verification)之規定，學者們認知到較合理之設計理念應為限度狀態設計與可靠度設計，此有別於地工界過去慣用的工作應力設計(Working Stress Design, WSD)方法。本文將針對大地工程限度狀態設計與可靠度設計之理念、基本內涵及其發展背景與現況進行介紹，期能提供國內地工界面對新世紀設計規範議題時能有正確之基本概念。
Development of new generation design code is one of the significant topics in geotechnical society. Recent studies have evolved to suggest an integrated framework for a rational and complete code structure. This is best known by the concept of “comprehensive design code” proposed by Honjo et al. (2005) in the work to unify various Japanese geotechnical design codes and standards. Under this framework, a rational and complete geotechnical design code is developed based on a few new design philosophies, which include performance based design (PBD), limit state design (LSD), and reliability based design (RBD). The comprehensive design code essentially consists of two main components. The first part is the specification of performance requirements, and PBD is the sound concept for devising codes at this level. Relevant discussions on PBD have been given by another paper in this issue. The other part of the comprehensive design code concerns about regulations of design verifications. Researchers have been suggesting that rational approaches for geotechnical design verification are LSD and RBD, rather than the traditional working stress design (WSD) approach. To facilitate local geotechnical engineer to better understand and grasp these two design concepts, background of development and fundamental principals of LSD and RBD are discussed in some details in this paper.
SYSTEM RELIABILITY METHODS IN GEOTECHNICAL ENGINEERING-A REVIEW
近年來由於世界貿易組織(WTO)明文規定各會員國在工程規範訂定時，應當使用性能設計法以減少對自由貿易干擾，因此性能工程逐漸受到各界重視，也成為未來工程設計的趨勢。性能設計法在國內結構工程界已行之有年，但在大地工程界仍處萌芽的階段。大地工程系統之行為相當複雜，主要原因之一為大地工程問題中有極多不確定性，包括模型不確定性、材料性質的不確定性、土石空間分布之不確定性等。如何在這些不確定性下做性能設計是個重要的議題，也就是地工系統可靠度分析。在這篇論文中，我們介紹目前發展較成熟的系統可靠度分析方法，包括一階二次矩法(first order second moment method)、一階可靠度分析(first order reliability method)、蒙地卡羅模擬(Monte Carlo simulation)、重要性取樣法(importance sampling)及子集合模擬法(subset simulation)。並藉由案例分析比較各分析方法之優劣及限制。
This paper reviews the state-of-the-art methodologies for performance reliability analysis. Due to its nature, geotechnical engineering systems are full of uncertainties in their performance, such as material properties, spatial variability, modeling uncertainties, measurement uncertainties, etc. The recent trend is to treat these uncertainties more rigorously, i.e. consider the probability of failure (or reliability) rather than just safety factor. Five well-developed reliability methods, including first order second moment method, first order reliability method, Monte Carlo simulation, importance sampling, and subset simulation, are reviewed, and their capacities of evaluating reliability for geotechnical systems are examined by using three examples.
Since 80s, Europe commission (EC) started to promote a structural design code to harmonize the design practice for members in EC. After two decades of development, a structural Eurocode has recently been published and will soon replace the existing designing codes of the member countries in EC by 2010. Among the ten codes contained in Eurocode, Eurocode 7 is the design code relevant for geotechnical engineering. In principle, Eurocode 7 does not exclude the conventional geotechnical approaches commonly used in practice. The framework of the Eurocode is based on the concept of limit state design (LSD); following the same concept, harmonization in the design of superstructure and foundation design becomes natural. This article introduces the development, principles, as well as general designing concepts of Eurocode7. A few simple designing examples are also included to demonstrate the design approaches in Eurocode 7. The development of Eurocode 7 has encountered numerous challenges before it reaches the current status; the experience is precious for similar code development in other countries in the future.
RECENT DEVELOPMENTS OF AASHTO LOAD AND RESISTANCE FACTOR DESIGN (LRFD) FOR DEEP FOUNDATIONS
載重和阻抗因子設計(Load and Resistance Factor Design, LRFD)已為許多先進國家結構設計規範所採行，該項設計法主要目的係將工程中許多不確定因素加以統計考量，使載重和結構阻抗評估之不確定性能夠適度考慮，進而提昇工程效益；近十年該項設計法已逐漸引入地工設計之相關課題和規範中，其中，樁基礎承載力之評估可為代表。本文旨在介紹AASHTO橋梁樁基礎LRFD近年之發展要點，俾使國人能了解其要點和應用性，以供未來修編規範和設計使用參考。
The load and resistance factor design (LRFD) has already been adopted as design specification in many developed countries. It takes into account the uncertainties in engineering design through rigorous statistic analysis, in which the performance is improved by more precise estimations of loads and structural resistance. For the past decade, this method has become more popular in relevant issues via design codes, representing by the pile foundation. This paper aims to discusses the recent developments of the AASHTO LRFD method for pile capacity, which is hoped to access and facilitate the possible applications of this method.
Load and resistant factor design method (the LRFD method) is an ultimate design method based on reliability analysis. This method, a new trend for axial pile capacity design, can assign pile load factors and soil resistance reduction factors separately according to different level of uncertainty. This paper describes the LRFD method, presents a rational approach for the determination of the resistance and load factors for PC piles embedded in reclaimed soil stratum, and then compares the results with some current design codes.
In Taiwan, the new seismic design codes for buildings and bridges have incorporated a higher level of design earthquake, the earthquake with a return period of 2500 years. It will have great impact to the analysis of soil liquefaction and design of foundation structures. Therefore, a study is conducted to propose a rational design basis for foundation structures of transportation system. Besides, associated methods of analysis and design procedure are provided for engineering applications.
Seismic Performance-Based Design (SPBD) has become the main stream of the civil engineering design codes in earthquake countries since the end of the last century. It will be the future trend of the engineering codes in Taiwan. Although earth embankment is one of the common structures in railway and highway engineering, there have been more studies concerning SPBD for bridge and building structures than for embankment structure. This paper summarizes the concept of SPBD, the performance rank, and the analysis methods used in assessing the seismic stability and deformation of the earth embankment. In particular, a simple two-dimensional numerical method is proposed for analyzing the embankment deformation due to the liquefaction of foundation soils. Some design details for improving the seismic capacity of the earth embankment are also provided.