2. Detailed Survey of the Extent of Corrosion on Eyebars Chains and Stiffening Trusses
A detailed examination of all recovered portions of the stiffening trusses and eyebars chains was conducted by the firm of Modjeski and Masters to determine what influence surface corrosion may have had upon the load carrying capacity of the eyebars chain, stiffening trusses, towers, and chain bent posts. Reference 15, "Corrosion Survey of the Silver Bridge (No. 1663)," is a complete documentation of this work. The diagrams indicating the locations at which detailed examinations were made are keyed to both the conventional panel point designations and the piece identification numbers painted on the member fragments by the Federal Highway Administration team which conducted the work at the reassembly site. Thickness measurements were made with calipers to determine the extent of corrosion at selected locations in the top chord members and on all bottom chord pieces where there was measurable corrosion. Similar measurements were made on approximately 13 percent of the diagonal and vertical members of the stiffening trusses selected on the random basis. Similar measurements were made on a large number of eyebars. The summary of this report reads as follows:
"The degree and location of corrosion observed on the primary truss members of the Silver Bridge did not significantly alter the ability of the individual members to carry the amount and type of stress to which they were subjected. Measurable corrosion was found on parts of many primary members below the roadway level. This metal loss appears to be the result of corrosive attack by roadway drainage. The corrosion was sufficiently advanced to indicate a need for replacement of some lower chord secondary material, principally lacing bars and stay plates.
"Pitting corrosion was observed on eyebars heads and pin hole surfaces. The maximum metal losses caused by corrosion amounted to less than three percent of the eyebars head, by conservative estimate. The pitting on pin hole surfaces removed substantially less than one percent of the critical cross-section through the pin hole.
"The corroded portions of the eyebars heads and pin holes were not accessible for maintenance of any type while the structure was standing. The eyebars heads between the abutments and chain bent posts were below the roadway level, and therefore, subject to corrosive attack by roadway drainage. The vast majority of the eyebars heads were located between Panel Points 0 and 58, above the roadway. Corrosion pitting of these eyebars is related to atmospheric conditions, but could not be related to roadway drainage."