Abbreviations AASHTO"American Association of State Highway and Transportation Officials CHBDC"Canadian Highway Bridge Design Code GDF"girder distribution factor LRFD"load and resistance factor design OHBDC"Ontario Highway Bridge Design Code Structural reliability as applied to highway bridges Andrzej S Nowak and Maria M Szerszen University of Michigan, USA Summary The paper presents the application of reliability methods in the development of a load and resistance factor design (LRFD) bridge codes. Structural performance is measured in terms of the reliability index. Load and resistance models are summarized. An important step is the selection of the target reliability index and calculation of load and resistance factors. Load and resistance factors are derived so that the reliability of bridges designed using the proposed provisions is at the predefined target level. Prog. Struct. Engng Mater. 2000; 2:218 d 224 Introduction The structural reliability can be applied in the design of new bridges and evaluation of existing ones. A new generation of design codes is based on probabilistic models of loads and resistance. Examples include: AASHTO LRFD code[1] OHNDC[2], Eurocode[3] and CHBDC[4]. In general, reliability-based design can be more efficient. It makes it easier to achieve either  for a given cost, design a more reliable structure, or  for a given reliability, design a more economical structure. Reliability can be considered as a rational evaluation criterion. It provides a good basis for the decision about repair, rehabilitation or replacement. Deterministic approach is based on analysis of individual components. A structure can be condemned when a nominal value of load exceeds the nominal load-carrying capacity. But, in most cases, a structure is a system of components. Furthermore, when a component reaches its ultimate capacity, it is not necessarily eliminated from the structure. It continues to resist the load but additional loads are distributed to other components. System reliability provides a methodology to establish the relationship between the reliability of an element and reliability of a system. The modern reliability analysis methods have been developed since the late 1960s. They are based on theory of probability and statistics. However, current approach to safety in the design and construction is a result of an evolution which took many centuries. The practical applications of the reliability analysis were not possible until the pioneering work of Cornell, Lind, and Ang in the end of 1960s and early 1970s. Cornell proposed a second-moment reliability index in 1969. Hasofer and Lind formulated a definition of format-invariant reliability index[5]. An efficient numerical procedure was formulated for calculation of the reliability index by Rackwitz & Fiessler[6]. Other important contributions were made by Veneziano, Rosenblueth, Esteva, Turkstra, Moses, and Ang. Their work was further improved by Der Kiuregian, Frangopol, Fujino, Furuta, Yao, Brown, Aayub, Blockley, Stubbs and Mathieu. The developed theoretical work has been presented in books as for example by Thoft-Christensen & Baker[7], Augusti, Baratta & Ciascati[8], Madsen et al[9] Ang & Tang[10], Melchers [11], and Thoft-Christensen & Murotsu[12]. By the end of 1970s, the reliability methods reached a degree of maturity and they are now available for applications. In the coming years, one can expect a further acceleration in the development of analytical methods to model the behavior of structural systems. The real change can be expected by focusing on 218 Copyright ^ 2000 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2000; 2:218d224