Evaluation and Repair of Wrought Iron and - Purdue e-Pubs ...

116signs on or near the bridge that might lead to an estimate of the age and manufacturer ofthe existing bridge. Figure 5.1 is an example of a typical sign that marks the year thebridge was constructed and the manufacturer.Once the manufacturer and age of the bridge is collected, an estimate can be madeof the materials utilized in the existing bridge. Typically in the Midwest, bridges thatwere constructed approximately before the late 1890s consist of historic wrought irontension members with either wrought iron, cast iron, or timber compression members.Bridges constructed approximately after the late 1890’s were primarily constructed usingsteel.Often it is useful to fully clean a piece of metal by removing any surfacecorrosion and investigate its surface appearance to determine if the metal is wrought iron.This is because wrought iron has distinctive slag inclusions that are often present on thesurface of the metal and visible by the naked eye. These inclusions are typically darkerthan the rest of the metal and are elongated in one direction. Figure 5.2 is a photographof a test coupon that shows some of these large surface inclusions.If during the bridge inspection some fractured tension members are found, theappearance of the fractured surface could help to identify if the metal is wrought iron orsteel. Wrought iron’s fractured surfaces that had failed in tension are typically fibrousand jagged in nature unlike typical smooth and angled fractured surfaces of steel.Typically there is very little necking near the fractured surface of wrought iron which iscommon in steel. Figures 4.4 and 4.5 illustrate the differences between the fracturesurface for both wrought iron and steel.If it is determined that the bridge being investigated is most likely historicalwrought iron, the AASHTO Manual for Condition Evaluation of Bridges (1994)recommends using an operating maximum unit stress of 20,000 psi, and an inventorymaximum unit stress of 14,000 psi when determining the load rating for these bridges.