Asset Management of Bridges

Proceedings of the 9th New York Bridge Conference, August 21-22, 2017, New York City, USA

Khaled M Mahmoud, Khaled M Mahmoud

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351338011
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Construcción e ingeniería arquitectónica

Maintaining bridges in good condition has extended service life and proven to be more cost effective than allowing degradation to advance, necessitating costlier bridge rehabilitation or replacement projects. Preventive maintenance is therefore an important tool to retard deterioration and sustain the safe operation of bridges. This includes a continuous effort of periodic inspections, condition evaluations and prioritizing repairs accordingly. The above measures define the framework for asset management of bridges.

On August 21-22, 2017, bridge engineering experts from around the world convened at the 9th New York City Bridge Conference to discuss issues of construction, design, inspection, monitoring, preservation and rehabilitation of bridge structures. This volume documents their contributions to the safe operation of bridge assets.

Cable-supported bridges

Chapter 1

Management strategies for suspension bridge main cables

K. Mahmoud

BTC, New York City, USA

W. Hindshaw

Transport Scotland, Glasgow, Scotland, UK

R. McCulloch

Amey, Edinburgh, Scotland, UK

ABSTRACT: There are two recognized methods for the evaluation of the remaining strength of cables based on internal inspection findings and testing results, NCHRP Report 534 Guidelines (Report 534) and the BTC method. Report 534 provides visual-based assessments of the corrsion on wire surface. The Guidelines assign a proportion of the cable cross-section to each of the stage of corrosion observed in the wedge openings. The evaluation of Report 534 then proceeds with the evaluation of remaining strength based on wire sample test results and the hypothized proportions of the stages of corrosion in the cable cross-section. The BTC method provides a reliability-based evaluation of the remaining strength and residual service life of cables. The method includes random sampling without regard to wire appearance, mechanical testing of wire samples, determining the probability of broken and cracked wires, and fracture-based analysis of cracked wires. The probabilistic-based method forecasts the residual life of the cable by predecting the increasing rate of detrioration and strength degradation. The BTC method is published in the latest FHWA Primer for the Inspection and Strength Evaluation of Suspension Bridge Cables. The BTC method is currently being applied alongside Report 534 Guidelines at the Forth Road Bridge in Scotland. The dual application of the two methods provides the bridge owners with high level of confidence in the estimated cable strength.

1 INTRODUCTION

The main cables are the most critical and vulnerable elements of a suspension bridge. Operating in harsh moist environments, cables are susceptible to wire breaks, cracking, and embrittlement. All these phenomena culminate in a reduction of the cable load carrying capacity. It is therefore imperative to the long-term asset management and preservation strategy for the main cables to ensure that the condition of cables in known. Several cable inspection and strength assessment strategies have been used over the years in an attempt to preserve the cable against deterioration. Some of these strategies are focused on the visual assessment of corrosion along the wire surface, whereas other strategies depend on reliability-based techniques and measured mechanical properties.

There are two recognized methods for to assess degradation and estimate the remaining strength of bridge cables Report 534 Guidelines and the BTC method. Report 534 Guidelines rely on the visual appearance of corrosion observed on individual wire surface. The wire sampling employed in the visual-based method assigns a number of wires to be extracted from each corrosion stage. The strength assessment then follows by a hypothesized proportioning of the cable cross-section to the observed corrosion stages along the wedge openings (NCHRP Report 534 2004). This produces cable strengths that are derived by the visual assessment of surface corrosion on wire surface. The BTC method is a patented methodology that employs a reliability-based analysis to estimate the remaining strength and service life of both, parallel wire and helical wire bridge cables (NYSDOT Report C-07-11 2011). It is included in the Federal Highway Administration (FHWA) Primer for the Inspection and Strength Evaluation of Suspension Bridge Cables (FHWA 2012). The BTC method applies to both zinc-coated and bright bridge (non-galvanized) wire. In the BTC method, wires are collected from the wedge openings, utilizing random sampling of individual wires, in each investigated panel. The randomly selected sample is tested to obtain the mechanical properties, including ultimate strength, ultimate elongation, yield strength, Young’s modulus and fracture toughness. The probability of broken wires is estimated based on inspection observation of broken wires, and probability of cracked wires is estimated based on the cracks detected from fractographic examination of wire fracture surfaces. The ultimate strength of cracked wires is determined using fracture toughness criteria. All these data is utilized to assess the remaining strength of the cable in each of the investigated panels. The BTC method employs a probabilistic-based approach to assess the remaining service life of the cable by determining the rate of change of broken and cracked wires detected over a time frame, and available data from previous cable investigations, and measuring the rate of change in effective fracture toughness over the same time frame. The words reliability and probabilistic are used to describe the method of assessing the remaining strength and service life of the cable.

This paper presents a comparative analysis of the evaluation of remaining strength and residual life of bridge cables utilizing Report 534 Guidelines and the BTC method.

2 METHODS FOR THE ASSESSMENT OF CABLE STRENGTH

There are two recognized methods for the evaluation of remaining strength of bridges cables:

• Report 534 Guidelines; depends on the visual assessment of wire surface corrosion, and

• BTC method; employs reliability-based analysis of inspection findings and mechanical properties of sampled wires.

The two methods present two different techniques to the modeling of wire degradation and the cable strength evaluation. This in fact is advantageous to bridge owners, as it would allow owners to make well-informed decisions about cable degradation and future maintenance strategies. The two separate methods give owners confidence as although different, the results provide upper and lower bound envelopes for the cable strength.

The BTC method has been applied alongside Report 534 at the Bronx-Whitestone Bridge, Mid-Hudson Bridge, in New York, USA. Currently, the two methods are being applied at the Forth Road Bridge, in Edinburgh, Scotland. It is noted that in the dual application of the two methods, there is no duplication in the inspection, sampling or testing effort.

2.1 Visual evaluation of wire degradation

The first major cable investigation was conducted by Hopwood and Havens of the numerous breaks of wires in the helical strands of the main suspension cables of General U.S. Grant Bridge over the Ohio River in 1984. In their study, (Hopwood and Havens 1984) first classified the visual corrosion for the galvanized wires in the helical strands into four stages (Hopwood and Havens 1984):

Stage 1:	wire surfaces have a shiny metallic appearance, though some signs of white zinc corrosion product may be visible in spots.
Stage 2:	wire surfaces dull as the zinc corrodes. The wires eventually are covered with the white corrosion product. However, there is no ferrous corrosion under the white corrosion product.
Stage 3:	signs of ferrous rust are visible on wire surfaces. The zinc coating is almost completely consumed.
Stage 4:	ferrous rust stains displace most of the white corrosion product on wires. Wire surfaces become very rough and pitted.

The Williamsburg Bridge cables were investigated almost 30 years ago (1988) also utilizing a visual-based method. It was the first bridge with parallel wires whose cables conditions were inspected in-depth. The Williamsburg Bridge main cable is composed of parallel bright wires. Therefore the bridge investigation adopted a modified six (6) degrees of corrosion classification system for the metal surface of wires, ranging from Grade 0; no corrosion, almost new condition, to Grade 5; worst corrosion (Steinman 1988).

When published in 2004, Report 534 generalized the definition of Hopwood and Havens, which was originally developed for helical wire strands, to classify visual corrosion on surface of parallel wire cables (NCHRP Report 534 2004).

2.2 Reliability-based evaluation of wire degradation

A typical suspension bridge cable is composed of thousands of wires, and the assessment for cable strength is based on a small sample size. Therefore it is essential to employ reliability-based method, which infers the strength of the cable from a small sample of wires.

With this understanding of the limited sample from a large population of wires, the BTC method utilizes modern assessment techniques that employ reliability criteria (very similar to the Load and Resistance Factor Design “LRFD” criteria), in which the wire mechanical properties obtained from a random sample, including strength and ductility (strain) are known as probabilistic entities, from which a “probability of failure” could be estimated. If an evaluation is conducted using these criteria, it can help establish, with high level of confidence, the tempo of inspection and further evaluations in the future.

The BTC method employs random sampling to eliminate visual bias, and fracture mechanics principles to assess the strength of cracked wires. Designing the sampling plan in each investigated panel, which marks the wires to be sampled from each wedge opening, prior to the field inspection ensures that the sampling is random. The inspectors and the Contractor are strictly instructed to sample only the wires marked on the sampling plan. With the use of a random sample, the analysis evaluates the sampling error in the estimated cable strength. The method forecasts the remaining life of the bridge cable based on strength degradation and rates of growth in broken and cracked wires proportions detected over a time frame, and the rate of change in fracture toughness over same time frame (NYSDOT Report C-07-11 2011) and (FHWA 2012).

The BTC method was first utilized to evaluate the strength and residual life of the main suspension cables at the Bronx-Whitestone Bridge in New York City. The BTC method was afterwards applied at the Mid-Hudson Bridge, in New York, and to check and validate the previously applied NCHRP Report 534 Guidelines, at the Forth Road Bridge, in Scotland.

The following section presents a brief comparative analysis of the BTC method and Report 534 based on the dominant metrics of degradation, which affect the estimated strength and residual life of the cable.

3 METRICS FOR WIRE DEGRADATION

The accuracy and reliability of the estimated cable strength depend on the influential metrics of degradation listed below:

• Hydrogen embrittlement

• Broken wires

• Cracked wire

• Forecast of cable life

3.1 Hydrogen embrittlement

The main inputs for the assessment of remaining strength of bridge cables are the test results of wire specimens and data collected during inspection. Following Report 534 Guidelines, during inspection, a number of wires are sampled, based on the visual assessment of the c...