• Figueiredo, Mav
• Jesus, Amp
• Fernandes, Aa
• Ribeiro, As
• De Castro, Pmst

Abstract

The maintenance and safety of existing bridges is a major concern of governmental agencies. In particular, the safety of old riveted highway bridges fabricated and placed into service at the end of the 19th/ beginning of 20th centuries deserve a particular attention, since they were designed taking into account traffic conditions, both in terms of vehicle gross weight and frequency, completely different from those observed currently. In order to assure high safety levels in old riveted steel bridges, highway authorities have to invest heavily in their maintenance and retrofitting. In this context, knowledge of the fatigue behaviour of riveted joints is of paramount importance. The present paper reports research work carried out to characterize the fatigue behaviour of the Portuguese Pinhão riveted highway bridge, designed by Eiffel at the end of 19th century and built between 1903 and 1906. The Pinhão bridge crosses the Douro river and links Pinhão to São João da Pesqueira and Peso da Régua. The bridge has three spans of 68.8 meters each and one span of 10 meters; there is only one deck with 6 meters width, divided in one traffic lane with 4.60 meters width and two sidewalks with 0.675 meters width each. The goal of the paper is to characterize the residual fatigue strength of the bridge. Both traditional S-N approaches and Fracture Mechanics approaches were used. The study is supported by an experimental program for the evaluation of material properties such as tensile strength, toughness and crack growth properties. Also, fatigue tests of riveted joints were carried out. The material and riveted joints were extracted from bridge members. The members removed from the bridge were a diagonal and a bracing from the first span, from Pinhão side. While the bracing is composed by two equal-leg angles, riveted to each other, the diagonal has a rectangular cross section. Several types of specimens were prepared using the material samples removed from the bridge. These specimens were used in chemical and metallographic analyses, hardness measurements, tensile tests, notch toughness tests, fatigue crack propagation tests and fatigue tests of riveted lap joints. The chemical and metallographic analyses revealed that the material of the bridge is a carbon steel with small content of Mn, Si and C. The microstructure is composed, essentially, by ferrite with a small content of perlite. The tensile tests demonstrated a high ductility of the materials with almost null strain hardening. Elongations of 70%, associated to reductions in area of 33%, were observed. The measured mean yield stress and the ultimate tensile strength were 306 and 361 MPa, respectively. The hardness measurements revealed a small scatter, which confirms a good microstructure homogenization. Mean hardnesses of 108 and 116 HV40 were found for the diagonal and bracing materials, respectively. Two types of notch toughness tests were carried out. The Charpy V-notch and the COD tests. Both tests revealed that the materials exhibit very acceptable toughness properties even for the current design requirements, which allows a high tolerance to the presence of defects. Crack growth tests using Compact Tension (CT) specimens were also carried out to characterize the crack propagation law for the materials of the bridge. Two stress ratios were investigated, namely, R = 0.01 and R = 0.5. It was observed that Paris law gives a good description of the crack propagation data (Paris & Erdogan 1973). The global mean crack growth law derived was: da/dN = 3.1961×10 -15·ΔK3.6117 (1) where da/dN = crack propagation rate expressed in mm/cycle; and ΔK = stress intensity factor range expressed in N.mm-1.5. The influence of the stress ratio is small and the materials from the two members present very similar fatigue crack propagation behaviours. Finally, fatigue tests of riveted lap joints were performed which allowed the evaluation of a S-N curve with the following form: logΔσ = 3.3108 - 0.2226 log N (2) where Δσ = stress range in MPa; and N = failure cycles. The observation of the facture surfaces revealed, for some specimens, the existence of initial cracks that nucleated and propagated during the last century. The comparison of the experimental data with the AASHTO (1995) class D S-N curve revealed that the later is conservative. The residual life of the bridge was evaluated using a S-N approach, based on the class D S-N curve, proposed in the AASHTO procedures for riveted joints. Considering a standard vehicle with a gross weight of 300 kN and three axles (RSAEP, 1983) and using information about the daily average traffic flow supplied by the Portuguese highway authorities, the stress spectra at each member was evaluated and the critical one was identified. This analysis demonstrated the safety of the bridge, against fatigue, for a period of 30 years after its rehabilitation. Subsequent analyses, with more precise stress spectra, based on the actual...

Topics

• impedance spectroscopy
• microstructure
• surface
• Carbon
• crack
• strength
• steel
• fatigue
• hardness
• tensile strength
• ductility
• homogenization