Design of Steel Structures for Fatigue

Fatigue considerations have been included in specifications for the design of steel structures for many years, usually in the form of decreased design stresses in the case of stress reversal. However, with the wider use of welding and high strength steels, fatigue has become more critical and fatigue provisions have become more comprehensive. The purpose of the paper is to analyze the application of current fatigue provisions to structural design. The three most common specifications relating to steel design, AASHO, AISC, and AREA are analyzed. Particular emphasis is placed on 1965 AASHO criteria as amended by the 1966-1967 ballot in relation to the design of both simple and continuous girder bridges. AASHO provisions follow the same basic relationships as the older fatigue criteria of AWS. However, AASHO provisions are more comprehensive, accounting for the strength of steel, type of stress, magnitude of stress ratio, and type of road and loading. Included are all allowable AASHO fatigue stresses for yield strengths from 36 ksi to 50 ksi, a flow chart for AASHO fatigue provisions, and two design examples using AASHO fatigue provisions.