Welded Highway and Railway Bridges
|Publication Date:||1 January 1966|
It is intended that these Specifications shali be suitable for the application of shielded metal-arc welding and submerged arc welding in the design, fabrication, field assembling and repair of highway and railway bridges. Other welding processes are not covered in detail but provisions are made for their use.
These Specifications do not treat of such design considerations as arrangement of parts, live loadings and distribution thereof and computation of stresses. It is expected that appropriate general specifications, such as the American Association of State Highway Officials Speciíications for Highway Bridges or the American Railway Engineering Association Specifications for Steel Railway Bridges, will be prescribed by the Purchaser and will control in all matters that are not affected by the use of welding.
In the case of old bridges, material of questionable weldability may have been used (including wrought iron or high strength structural silicon or nickel steeh). Accordingly, it is advisable in making important repairs to an old structure to obtain samples of the material and make laboratory tests to determine the proper welding technique and weld values.
Work should be so detailed that the maximum amount of welding may be done in the flat position. Vertical and overhead welds are more a c u l t to make but, when properly made, they have equal strength to welds made in the flat position. Their use is, therefore, not penalized by a reduction of &e allowable unit stress but they may substantially increase the cost of the work.
In establishing suitable unit stresses for design, the Specifications adhere as closely as practicable to published findings of the Weldin? Research CounciI on the fatigue strength of welded joints. Fatigue failure is determined by three factors: the maximum unit stress, the range between maximum and minimum, and the number of applications of such cyclical variation. When the second and third factors are both large, for a given member of a given bridge, the maximum or fatigue failure stress is now known to be below the yield point found in a static test and consequently will control the design. When either the second or third factor is small, the fatip?ie failure stress is above the static yield point and should not be invoked to control the design.
Fatigue testing has demonstrated that any sudden discontinuity of section and stress path is a factor adversely affecting the strength of members subjected to cyclical loading. Gradual rather than sudden transitions of sections should be employed and, for the same reason, groove welds are preferable to fillet welds.
Comments or inquiries on these Specifications are welcome. They should be addressed to: Secretary, AWS Structural Welding Committee, AMERICANW ELDINGSO CIETYU, nited Engineering Center, 345 East 47th Street, New York, N. Y. 10017.