Design Philosophies for Highway Bridges Introduction Limit States Philosophy of Safety Introduction • Allowable Stress Design • Load Factor Design • Probability- and Reliability-Based Design • The Probabilistic Basis of the LRFD Specifications John M. Kulicki Modjeski and Masters, Inc. Design Objectives Safety • Serviceability • Constructibility Introduction Several bridge design specifications will be referred to repeatedly herein. In order to simplify the references, the “Standard Specifications” means the AASHTO Standard Specifications for Highway Bridges [1], and the sixteenth edition will be referenced unless otherwise stated. The “LRFD Specifications” means the AASHTO LRFD Bridge Design Specifications [2], and the first edition will be referenced,. | Kulicki . Desgn Philosophies for Highway Bridges. Bridge Engineering Handbook. Ed. Wai-Fah Chen and Lian Duan Boca Raton CRC Press 2000 5 Design Philosophies for Highway Bridges John M. Kulicki Modjeski and Masters Inc. Introduction Limit States Philosophy of Safety Introduction Allowable Stress Design Load Factor Design Probability- and Reliability-Based Design The Probabilistic Basis of the LRFD Specifications Design Objectives Safety Serviceability Constructibility Introduction Several bridge design specifications will be referred to repeatedly herein. In order to simplify the references the Standard Specifications means the AASHTO Standard Specifications for Highway Bridges 1 and the sixteenth edition will be referenced unless otherwise stated. The LRFD Specifications means the AASHTO LRFD Bridge Design Specifications 2 and the first edition will be referenced unless otherwise stated. This latter document was developed in the period 1988 to 1993 when statistically based probability methods were available and which became the basis of quantifying safety. Because this is a more modern philosophy than either the load factor design method or the allowable stress design method both of which are available in the Standard Specifications and neither of which have a mathematical basis for establishing safety much of the chapter will deal primarily with the LRFD Specifications. There are many issues that make up a design philosophy for example the expected service life of a structure the degree to which future maintenance should be assumed to preserve the original resistance of the structure or should be assumed to be relatively nonexistent the ways brittle behavior can be avoided how much redundancy and ductility are needed the degree to which analysis is expected to represent accurately the force effects actually experienced by the structure the extent to which loads are thought to be understood and predictable the degree to which the designers .
