Variability of the Mechanical Properties of Wrought Iron from Historic American Truss Bridges

The mechanical properties of wrought iron from multiple elements of six late 19th-century truss bridges is evaluated by a program of destructive and nondestructive testing, including hardness testing and tension tests to evaluate the yield stress ($F_y$), tensile strength ($F_u$), and ductility of the material. The yield stress and tensile strengths are found to be in accordance with those published in period reports and in other modern evaluations of the mechanical properties of wrought iron. The main findings of this work come from a statistical analysis of the test results and are (1) that hardness is a poor predictor of yield stress and tensile strength but has some predictive ability for ductility; (2) that there is a statistically significant difference in the distribution of yield stress and tensile strength between material samples from different bridges and, in some cases, between material samples from different members within a single bridge; and (3) that a size effect is present in the material that results in lower yield stress and tensile strength for larger members. These results provide guidance to engineers in the evaluation of historic iron trusses for rehabilitation and suggest that although nondestructive hardness testing is of limited value, a limited program of destructive testing can provide an adequate characterization of the mechanical properties throughout the bridge.