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Flexural Capacity of Fire-Damaged Prestressed Concrete Box Beams

CHAPTER 1. INTRODUCTION

1.1 Introduction

A fiery incident involving a gasoline tanker truck caused significant damage to an adjacent member box-beam bridge in southwestern Connecticut. Although the Connecticut Department of Transportation (ConnDOT) decided to replace the superstructure of the bridge, they were also interested in determining whether the type of damage that these box beams experienced was sufficient to critically impair the structure's ability to serve its intended purpose. To reach this end, ConnDOT coordinated with the Federal Highway Administration’s (FHWA) Turner-Fairbank Highway Research Center to investigate the remaining flexural capacity of the beams in the bridge. Four of the beams were tested to failure and the results are reported herein.

1.2 Incident

On the afternoon of July 12, 2005, a gasoline tanker truck traveling north on U.S. Route 7 near Ridgefield, CT overturned and exploded. The truck came to rest on the east side of the northbound shoulder of the Route 7 bridge over the Norwalk River. The 30,300 liters (8,000 gallons) of burning fuel flowed over the bridge and into the river below. As evidenced by the melting and possible vaporization of the aluminum that was originally part of the tanker truck, the temperature in the immediate vicinity of the truck could have been over 2,467 °C (4,472 °F). Figure 1 shows the remains of the tanker truck on the bridge after the fire.

Photo Credit: Ronald D. Jantzen
Figure 1. Photo. Remains of tanker truck after fire.

1.3 Bridge Structure

The U.S. Route 7 bridge over the Norwalk River is a single span bridge composed of 15 prestressed adjacent box beams. The bridge carries two lanes of traffic and two shoulder lanes, with a relatively shallow clearance over the river. The bridge was constructed in 1957 and spans 14.6 meters (m) (48 feet (ft)). The beams were 0.91-m (36-inches) wide and 0.64-m (25-inches) deep. The beams were topped by a waterproofing membrane and a 160-millimeter (mm)- (6.25-inch-) thick bituminous concrete overlay.

The most recent rating of this bridge was completed in 1999. This load factor analysis was conducted on a representative interior member of the bridge. It concluded that these beams have an inventory rating of 1.33 and an operating rating of 2.23 under HS20 live loading with shear capacity governing in both cases. This analysis also indicated that the ultimate moment capacity of each beam is 1407 kilonewton-meter (kN-m) (1038 kip-ft), corresponding to an inventory rating of 1.42 and an operating rating of 2.63.

1.4 Post-Incident Bridge Condition Evaluation

Visual examination of the bridge structure immediately after the fire found that scaling of the concrete surface had occurred on the bottom flanges of all 15 beams as well as on the exterior faces of the fascia beams. The bridge fascia is shown in Figure 2. The bituminous concrete overlay in the immediate vicinity of the tanker truck was in poor condition, but the membrane was intact. As nondestructive means were not available to quantify the integrity of the prestressing strands, ConnDOT decided to replace all of the beams in the superstructure.

Photo Credit: Ronald D. Jantzen
Figure 2. Photo. Bridge fascia immediately after fire.

1.5 Project Objective

The objective of this project is to determine if the July 12, 2005 incident on the U.S. Route 7 bridge over the Norwalk River caused a significant deterioration of the structural capacity of the bridge. The beams in the structure were numbered 1 through 15 beginning at the eastern fascia beam. Beams 3, 4, 7, and 14 were salvaged from the bridge and transported to the FHWA's Turner-Fairbank Highway Research Center. The condition of the concrete was investigated and full-scale tests were completed on the four beams to determine their remaining flexural capacity.

1.6 Overview of Report

This report is divided into six chapters. Chapter 1 provides background information about the project. Chapter 2 details the experimental program that was implemented. Chapter 3 describes the material characterization testing that was completed. Chapter 4 details that flexural testing of the four box beams. Chapter 5 discusses the results that are presented in the earlier chapters. Chapter 6 presents a summary of the findings of this investigation.