A Fatigue Study of Prestressing Strand

This study review the literature which reports on fatigue studies of pre-stressing strand and also presents the results of a series of strand fatigue tests. The fatigue tests compared tests on samples of strand from several manufacturers with tests on a sample of strand to be used in construction of girder fatigue specimens. Strand fatigue data from the literature and from the tests were comvined to form a strand fatigue data base which was analyzed using regression analysis techniques. A stress range vs. fatigue life curve for prestressing strand was developed and was used as the basis for a strand fatigue design equation. The design equation was compared with current AASHTO code provisions for fatigue of structural steel and was found to lie midway between Category A and Category B curves for redundant structures. Category B is recommended for checking the allowable fatigue stresses in uncracked girders. Designing to this category provides an extreme lower bound to all the strand data collected and should ensure no fatigue problems regardless of the size, strength, and relaxation properties of the strand. Girder which are cracked and then subjected to fatigue loads may produce fatigue failures at much lower strand stress ranges than Category B. Recommendations for cracked girders will be developed from the girder tests to be performed as part of the research project. The results also indicate that Category B is satisfactory for designing strand tensions systems, such as cable stays, when considering the life of the strand. The fatigue performance of the socketing or grip system must be evaluated spearately.