Li Guoping, Zhang X
Overview on the Development of Design Method for Concrete Structure
Since the reinforced concrete materials are used in the civil engineering, the design method for reinforced concrete structure has been developed and improved continuously. At the end of the 19th century and the beginning of the 20th century, the Allowable Stress Design (ASD) method based on the elastic theory and adopting a single safety factor was the earliest design method for reinforced concrete structure. However, as this design method only takes the safety problem of materials in the service stage into account, the plastic behavior of reinforced concrete materials fails to be reflected. As a result, it is impossible to reflect the stress state and resistance when the cross section failed.
In the 1930s, a new approach- Ultimate Strength Design (USD) method, which took into account the plastic behavior reflected in the failure stage of the reinforced concrete materials and the single safety factor, was developed on the basis of allowable stress method. This method considers the safety of structure in the failure stage of material, but the safety factors used are still determined by the engineering experience and subjective judgment.
In the 1950s, with the deepened researches on the variability of materials and loads, the Limit State Design (LSD) method, which replaced the single safety factor with three semi-empirical and semi-probabilistic partial factors, saying the load, material and working condition, was initially taking shape. This design method, which is developed on the basis of Allowable Stress Method (for materials) and ultimate strength design method, not only takes the safety and serviceability of structures into account, but also preliminarily considers random elements of these performances.
From 1970s, the reliability theory based on the probability theory and mathematical statistics was gradually put into practice in the field of civil engineering. The Limit State Design method is gradually shaped into the reliability-based limit state design method by being developed into the approximate-probabilistic design stage at present from the semi-probabilistic design stage in the beginning. However, although the limit stage design method is the commonly-used method at present, the recognition for the structural performance in the field of civil engineering has been already beyond the perspective focusing on the safety and serviceability.
The Performance-based Design (PBD), which initiated in 1960s and achieved great development in 1990s, gradually enters the practical and promotional stage. This method takes the safety, serviceability, sustainability and other performances of structure as the design objects and integrates the durability with other relevant performances, so as to deepen and expand the concept of whole-life design for structure. Like the laws of development of previous design methods, the performance-based design not only combines the limit state design, but also transforms the requirements of the theoretical basis in the existing specifications concerning the utility, durability and other performances from the qualitative indicators into the quantitative or partially quantitative indicators. Fig. 1 shows the correlations among aforesaid design methods.
Definition and Development of Performance-based Design
Definition of Performance-based Design
From the perspective of general definition, performance refers to the capability of finishing something; in ISO 6420, performance was defined as a behavior related to the product and users (1980). At present, most definitions for the structural performance of civil engineering are qualitative and there is no clear method that uses more quantitative indicators to ensure the performance. The performance-based design mainly refers to establishing the purpose-oriented quantitative performance indicators according to requirements of environment, function, users, etc.; analyzing the requirements of construction, service, management and maintenance before proposing the performance verification methods and the comprehensive measures ensuring performance objectives to be achieved; and adopting reasonable and consistent technical standards, acceptable technical means and methods to evaluate the design objectives in a quantitative manner.
Development of Performance-based Design
In 1963, Nordiska komitten for byggbestammelser (NKB) began to make consistent the building codes. The performance-based design approach was firstly adopted in the building construction industry by five member counties in Northern Europe. A 5-stage framework for performance-based design developed to unify specifications applied in different countries was established in 1978 and was called NKB Level System. The International Organization for Standardization (ISO) also enacted corresponding criteria for the preparation of building codes for performance-based design in 1980 and 1984. After that, the NKB framework was used as a common infrastructure for building codes in many countries. The UK, New Zealand and Australia respectively revised their own building codes according to the performance-based design approach in 1991, 1995 and 1996. Nearly at the same time, the United Nations Economic Commission for Europe (ECE) conducted the NKB framework-based research for the purpose of the publication of Model Provisions for Building Regulations. In 1996, the performance-based design method was also introduced into the building seismic resistance performance design by Structural Engineers Association of California (SEAC). In addition, Canada and other countries were also making researches on the performance-based design. In 1998, Japan revised the details of Building Standard Laws according to the performance-based design. A new technical committee for defining “performance of a civil engineering facility” was approved by the Structural Engineering Institution (SEI) of American Society of Civil Engineers (ASCE) during 1999 Structures Congress. At present, the performance-based design is only reflected in the seismic and fire-resistant design specifications. The research on the performance-based design of infrastructures is just started and the shift of structural design to the performance-based direction is also at the beginning. The Concrete Committee of Japanese Society of Civil Engineers engaged in this work from 1995, completed the performance-based Standard Specifications for Concrete Structures in 2002, published the English edition of this specification in 2005 and finally revised the performance-based Standard Specifications for Concrete Structures in 2007. The committee also has a plan to compile Asian model codes. The fib Model Code 2010 and Eurocode 2 planned to be completed in 2015 are both codes revised to be in line with the performance-based trend.
Features of Performance-based Design and Current Situation of Development of Specifications in China
Features of Performance-based Design
If the civil engineering structure is regarded as an industrial product, the prescript-based design is a kind of prescriptive-procedure-oriented design; in contrast, the design required to give reasonable and proper quantitative performance index and corresponding characteristics is product-oriented, that is to say, the design is for practical and deliverable products instead of raw materials. Compared with the product-oriented design, the performance-based design for civil engineering structure has following features:
(1) The primary issue to be considered in performance-based design is to determine the environmental feature, construction capacity, service demands, management and maintenance requirement and other basic references; and the prerequisite of the design lies in clearly specifying the users of structural system and the performance object, especially, independent technical innovation is allowed and even encouraged. Finally, the most suitable structural plan can be defined by reasonable and unified standards and acceptable technical means and methods.
(2) The structure with the performance-based design should be built on the basis of interacting mechanism among design, construction, operation, maintenance and other aspects. Ensure the object of performance can be achieved by synthesizing control measures and verification methods of each aspect. If the work of each aspect said above is undertaken by many individual institutions, the structural performance can only be submitted in a minimum or unsecured manner.
(3) The structure with the performance-based design should be provided with a warranty for performance simultaneously and the prerequisite for developing the structure towards secured and performance-based direction is improving laws, contract mechanism and indemnity for risks imposed by the long-term performance guarantee.
It is obvious that the essential condition of performance-based design is to ensure that the structural performance can meet requirements; and the performance-based design is interrelated to and dependent on the construction, operation, management and maintenance, life-cycle detection and laws, agreements, contract and delivery mechanism, etc. rather than an isolated problem. Moreover, it is of more social and economic meanings.
Current Situation of Specification Development in China
In recent decade, most design standards and specifications for civil engineering structures in China have been revised into the new versions of reliability-based limit state design method. The design specifications for highway concrete bridges are shifted from design method based on a semi-empirical and semi-probabilistic theory (1985) to the reliability-based limit state method (2004). However, as it is very difficult to carry out data statistics and technical analysis, many basic problems remain to be solved. Moreover, the development of specifications in China lags over 10 years behind the development pace of foreign equivalent specifications, Confronting the developing trend of performance-based specifications in advanced countries and the laggard situation in China, it is urgent to start the research on the performance-based design and form a new generation of specification system. Otherwise, China will be still in the laggard state and it will be difficult for China to fulfill the dream of becoming a technological country.
At present, Tongji University cooperating with CCCC Highway Consultants Co., Ltd. has drafted the initial plan and started the prophase preparations about relevant research.
Preliminary Plan for Research on Performance-based Specifications
Based on the objective description about structural performance, a series of basic research have been carried out, so that the idea about the general rules, sub-volumes for design, sub-volumes for construction, management and maintenance, volumes for check and assessment on the specifications for performance-based concrete bridge has been taking into shape, and the research tasks are proposed in following stages (Fig. 2):
Fig. 2 Plan for Research on Performance-based Specifications
In order to carry out the above plans, we will arrange related technicians and experts to track the dynamic state of advanced performance-based research, make great efforts to research related basic theories, and organize and attend the following international and domestic conferences and academic activities:
(1) Cooperating with Ministry of Transport and Permanent International Association of Road Congresses (PIARC) to organize and attend the “International Seminar on Performance Design and Risk Management of Long-span Highway Bridges” to be held in China in 2012.
(2) Attend the annual meeting of International Association for Bridge and Structural Engineering (IABSE) and discuss the performance-based specification system and related basic research.
(3) Discuss the performance-based design during the “Sino-American Bridge Technology Seminar” jointly held by the Ministry of Transport and American Federal Highway Administration.
(4) Disseminate the ideas of performance-based specifications in various international and domestic bridge seminars.
Based on the above plans and activities, we are striving to propose Recommended Performance-based Specifications(Guidelines) for Concrete Bridges first in the industry during 2016 to 2020. The Recommended Specifications will then be put into test and amended before being applied as formal executive specifications.
Performance-based design represents the development trend of bridge design methods. The Society of Civil Engineers in some advanced countries (like Japan) have issued the relevant specifications and continue to amend them. Such specifications as fib Model Code and Eurocode 2 will be revised to be in line with the performance-based design trend. Because there are various defects and problems in Chinese bridges and bridge specification system, the above plans are made to call the attention of the related administrations, scientists and engineering technicians in the bridge engineering field, to have them make efforts together to support and participate in relevant works, and to get rid of backwardness as far as possible in the development of the design methods, with the purpose of changing China from a country just with a large number of bridges into a country with advanced bridge technologies.
(Authors are from Bridge Engineering Department of Tongji University and CCCC Highway Consultants Co., Ltd.)