Introduction

There are tens of thousands of iron and steel bridges that are considered historically significant and which remain in service as part of our transportation systems. For bridges with structural redundancy that have been in continuous service, with minor corrosion damage, a visual examination may be all that is required. But where redundancy is insufficient, significant distress or damage is present, or an increase in loading is required, a complete evaluation of the condition of the members as well as structural capacity may be required for continued service.

Because the geometry and sections of the main members are relatively simple to model, material characterization becomes a key issue in the structural analysis of historic bridges.

The determination of material strengths for historic iron and steel bridges remains problematic, particularly with the need to assure ductile behavior. For members and connections that are critical to the support of the structure, special methods may be needed to qualify their integrity.

A rigorous evaluation is necessary because the materials are of unknown quality, the critical fabrications were never qualified by modern standards, loads have increased, and the effects of decay and fatigue may have reduced the capacity. On the other hand, the importance of historic bridges as significant works of history requires that our evaluation methods be the least invasive.

This paper suggests a nondestructive protocol for the evaluation of nineteenth and early twentieth

Figure 1. Upper chord pin-and-eyebar connection in an 1881 Whipple Phoenix Truss. The eyebars and compression sections are wrought iron, the joint blocks are gray cast iron, and the pins are wrought iron on the upper chord and steel on the lower chord.

century bridges built of iron and steel. In addition to visual assessment and structural analysis, the suggested protocol relies primarily on a combination of materials characterization and nondestructive testing. The approach is based largely on available methods; it represents the author's search for a rational basis for evaluating complex structures in which the quality of the materials and their properties are unknown.

It is the thesis of this paper that in the context of historical data and the observed condition of the bridge, microstructure, hardness and chemical analysis are sufficient to characterize the behavior of the material.

The goal is to have an approach that in most cases eliminates the need for physical sampling and testing of bridge members, and where sampling is used can help reduce the ambiguity that comes from a limited sampling program.

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