MODERN TIMBER BRIDGES IN MISSISSIPPI: AN EXAMINATION OF CRITICAL ISSUES

This study examines two critical issues concerning the development of a modern timber bridge manufacturing industry in Mississippi: (1) site-specific cost comparisons of modern timber bridges and concrete bridges; and (2) causes of deterioration of old style timber bridges. The intent of the study is to provide critical information needed to develop a rationale for changing current policy to allow modern timber bridges to compete for Mississippi State Aid bridge funds. Obtaining acceptable cost data for modern timber bridge structures built in Mississippi proved to be a difficult task, and no reliable conclusions regarding direct cost comparisons between concrete structures and modern timber bridge structures could be determined. However, data from an Alabama bridge replacement site indicated that there would have essentially been no differences in material and labor costs if the bridge had been replaced with either a modern timber bridge, or a precast concrete bridge. Installation by county crews, quick installation time, and elimination of intermediate piers were cited as reasons for selecting a modern timber bridge for this specific site. There are related bridge construction costs that are due to the disruption encountered with bridge outage and traffic detours. These costs should be considered in the decision-making process for repairing or replacing a bridge structure. In a Mississippi case study presented, the public user costs associated with a bridge outage were determined to be $639 per day. Concerning bridge deterioration, both wood and concrete bridge components were adversely affected by rainwater penetrating bridge structures. Water contributes to both spoiling of concrete and decay of wood. Substantial deterioration of wood piling was the result of either damage incurred during installation or inadequate preservative treatments. Many of the creosote-treated wooden piles remained sound after 30 to 40 years of service. The extent of the deterioration of the bridges inspected indicates that a maintenance program must be developed for rural roadway bridges in Mississippi that includes procedures for inspecting wooden components and periodic application of remedial treatment, as well as proper procedures for wooden pile installation. The common complaint that treated timber is inadequate as a structural material is refuted by these findings. However, proper treatment and installation is required.