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

117The inventory maximum unit stress should be used if the structure has corrosion damageor cracking which might lead to a lower load capacity. The operating maximum unitstress should be used if the bridge has been recently rehabilitated and in good condition.AASHTO also states that, “coupon tests should also be performed to confirm materialproperties used in the rating.”The unit stress values outlined in the AASHTO Manual for Condition Evaluationof Bridges were compared to the resulting strength data found during this research study.One standard deviation below the mean yield strength found from investigating thehistorical data is about 30,000 psi, which is also roughly equal to the average yieldstrength found from the experimental testing completed. When using this value for amaximum yield stress and requiring that the allowable stress to be equal to 0.6 * Fy, theallowable stress for wrought iron would be 18,000 psi. This number is comparable to theoperating maximum stress provided by AASHTO.Even if the exact year and manufacturer of an existing historic bridge are known,it is still beneficial to undertake material testing. This enables the engineer to have abetter understanding of the material properties of the members in the bridge. Somematerial tests that are beneficial are chemical analysis testing, charpy impact testing andtensile coupon testing. From these results the chemical content and strength of thematerial can be determined.In these tests, there are some defining results that help determine if the materialstested are historic wrought iron. The results from a chemical analysis of historicalwrought iron, when compared with structural steel, would show a relatively low carboncontent (C2.0%) would indicate thatthe material is cast iron (Aston, 1941). An excess of silicon along with phosphorous andsulfur is also typically found from a chemical analysis of wrought iron. The presence ofsilicon is especially important in determining if the material is wrought iron, because thedefining slag in wrought iron consists mainly of this element. Also, the tensile coupon