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Refining the fatigue assessment procedure of existing steel bridges
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.ORCID iD: 0000-0002-2833-4585
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis treats the fatigue assessment process of steel bridges. The purpose is to identify areas with potential of enhancement with an overall aim of attaining a longer service life. The aging bridge stock in Sweden and in many other developed countries is an impending economical burden. Many bridges have reached their expected service life and increased axle loads, speeds, and traffic intensity further accelerate the deterioration. An immediate replacement of all our bridges approaching their expected service life will not be possible. For economical and environmental reasons, effort should be put on extending the theoretically safe service life as far as possible.

 Fatigue is one of the major reasons limiting the service life of steel bridges .A specific example is the Söderström Bridge in Stockholm, Sweden. Fatigue cracks have been found in the webs of the main beams and theoretical assessments have shown an exhausted service life. As a mean to reduce the uncertainties in the theoretical assessments a monitoring campaign was started in 2008 and continued in periods until 2011. The first continuous period of 43 days of measurements in 2008 forms the experimental foundation for this thesis.

A fatigue life prediction involves (i) an estimation of the load effect, (ii) an estimation of the resistance, and (iii) the selection of a prediction model. This thesis treats in some sense all three of them. Considering part (i), the load effect, a theoretical study on the influence of dynamics has been performed. The quasi static approach suggested in the standards does not necessarily reflect the true dynamic behavior of the structure. Performing a dynamic moving load analysis gives for all cases studied a more favorable fatigue life. A further enhancement of the assessment is to perform in situ measurements. The uncertainties related to dynamics, loads, and structural behavior are thereby captured in the response. Routines for processing the measured response and performing life predictions are treated, moreover the quality of the measured response.

The resistance, part (ii), in form of the fatigue endurance has a funda- mental influence on the fatigue life. As an attempt to reach a more adequate fatigue endurance a refined assessment of a typical joint is performed using linear elastic fracture mechanics (LEFM). A descriptive detail category is suggested which renders a somewhat more favorable resistance.

Part (iii), the prediction model, is treated considering the safety format. A reliability-based model is suggested which enables a consideration of the uncertainties in each stochastic variable. Thereby, all aforementioned parts and uncertainties can be combined within the same prediction. The reliability-based model is used for a code calibration of partial safety factors to be used in semi-probabilistic assessments according to the standards.

The implementation of the suggested procedures is shown with a numerical example. The outcome should not be generalized but the example clearly shows an increase in fatigue life with the proposed enhancements of the assessment procedure.

 

 

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013.
Series
Trita-BKN. Bulletin, ISSN 1103-4270 ; 118
Keyword [sv]
Fatigue, Steel bridges, Assessment, Reliability, Monitoring, Utmattning, stålbroar, bärighetsberäkning, tillförlitlighet, mätningar, Söderström, getingmidjan
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:kth:diva-127707OAI: oai:DiVA.org:kth-127707DiVA: diva2:645375
Public defence
2013-10-11, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130917

Available from: 2013-09-17 Created: 2013-09-04 Last updated: 2013-09-17Bibliographically approved
List of papers
1. Dynamics of thick bridge beams and its influence on fatigue life predictions
Open this publication in new window or tab >>Dynamics of thick bridge beams and its influence on fatigue life predictions
2013 (English)In: Journal of constructional steel research, ISSN 0143-974X, E-ISSN 1873-5983, Vol. 89, 262-271 p.Article in journal (Refereed) Published
Abstract [en]

An analytical model for moving load analysis is derived based on the Timoshenko beam theory. The model enables rapid and reliable analyses of the influence of dynamics on the fatigue life prediction of thick beams. This type of beams is typical for open deck railway bridges. The number of modes required in a modal superpositioning is studied as well as the influence of damping and support stiffness. A total of 22 bridge beams from open deck bridges presented in the literature is included in the study. The results clearly show a fundamental difference in the behaviour of one span beams and continuous beams. Especially in the case of assessment of existing bridges, the quasi static model of the Eurocode renders an excessive safety margin. For the majority of continuous beams studied, the dynamic response is lower than the static response resulting in no dynamic amplification. Such an outcome can have a substantial influence on the remaining fatigue life prediction due to the exponential relation between the stress range and the fatigue life.

Keyword
Dynamics, Fatigue, Timoshenko beam theory, Railway bridge, Moving loads
National Category
Infrastructure Engineering
Identifiers
urn:nbn:se:kth:diva-126020 (URN)10.1016/j.jcsr.2013.07.012 (DOI)000326206800023 ()2-s2.0-84881268458 (Scopus ID)
Note

QC 20130820

Available from: 2013-08-18 Created: 2013-08-18 Last updated: 2017-12-06Bibliographically approved
2. Monitoring and enhanced fatigue evaluation of a steel railway bridge
Open this publication in new window or tab >>Monitoring and enhanced fatigue evaluation of a steel railway bridge
2010 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 32, no 3, 854-863 p.Article in journal (Refereed) Published
Abstract [en]

During routine inspections of the Soderstrom Bridge in central Stockholm, one of Sweden's most important railway bridges, cracks were found in the web of the main steel beams. The finding initiated theoretical Studies which showed that the cracks developed mainly due to poorly designed connections of the cross beams and out-of-plane bending of the web. The Studies also showed an alarming result regarding the remaining fatigue life of the stringers and the cross beams. However, no cracks or other damage have been found on these components during the inspections. To explore the differences between the theoretical indications and the inspected reality, an extensive monitoring program has been performed. This article describes the monitoring program and the analysis methods used. Some results regarding the remaining fatigue life based on measured and theoretical values are presented. (C) 2009 Elsevier Ltd. All rights reserved.

Keyword
Railway bridge, Monitoring, Bridge assessment, Fatigue, Dynamics
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-28452 (URN)10.1016/j.engstruct.2009.12.011 (DOI)000275488000021 ()2-s2.0-75749154751 (Scopus ID)
Note
QC 20110117Available from: 2011-01-17 Created: 2011-01-14 Last updated: 2017-12-11Bibliographically approved
3. Quality Assurance of Measured Response Intended for Fatigue Life Prediction
Open this publication in new window or tab >>Quality Assurance of Measured Response Intended for Fatigue Life Prediction
2012 (English)In: Journal of Bridge Engineering, ISSN 1084-0702, E-ISSN 1943-5592, Vol. 17, no 4, 711-719 p.Article in journal (Refereed) Published
Abstract [en]

By in situ measurements, the real distribution of stress ranges caused by service loads can be recorded. Inherent effects in the measured response as disturbance can, however, create puzzles in the interpretation of the result. In large monitoring campaigns, it is not possible to examine the result from every gauge over the whole measured period by visual control. In this paper, an approach for a quality assurance of the measured response is presented based on established statistical methods. The stress range spectra, the product of the monitoring program intended for fatigue assessment, are analyzed. The aim of the analysis is to find deviant spectra and identify corrupt gauges. An additional aspect is the length of the monitoring period, that is the required duration for obtaining a stable result. A case study of a monitored Swedish steel railway bridge is incorporated in the paper to exemplify the approaches. Some statistical distributions for the monitored stress ranges are also presented and incorporated in a fatigue assessment.

Keyword
Monitoring, Analysis of Variance, Fatigue, Stress range, Railway bridge, Bridge assessment
National Category
Civil Engineering
Research subject
Järnvägsgruppen - Infrastruktur
Identifiers
urn:nbn:se:kth:diva-70218 (URN)10.1061/(ASCE)BE.1943-5592.0000291 (DOI)000312660700017 ()2-s2.0-84878366155 (Scopus ID)
Note

QC 20120208

Available from: 2012-01-30 Created: 2012-01-30 Last updated: 2017-12-08Bibliographically approved
4. Refined fatigue assessment of joints with welded in-plane attachments by LEFM
Open this publication in new window or tab >>Refined fatigue assessment of joints with welded in-plane attachments by LEFM
2013 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 56, 25-32 p.Article in journal (Refereed) Published
Abstract [en]

The fatigue life of joints with welded in-plane gusset plates is studied. The purpose is to suggest a descriptive detail category to be used in assessment of existing structures. Linear elastic fracture mechanics is used together with fictitious initial crack depths to predict the fatigue life. By calibrating the fictitious crack depths to real fatigue tests and simulating new test results, fatigue strengths dependent on the dimensions of the joint have been derived. The results show that a decreasing length and thickness of the gusset give decreasing stress intensity and higher fatigue strength. A numerical example is presented showing the merit of the proposed strengths in fatigue life.

Keyword
Fatigue, Gusset plate, Detail category, Steel bridge, Stress intensity factor, XFEM
National Category
Infrastructure Engineering
Identifiers
urn:nbn:se:kth:diva-126021 (URN)10.1016/j.ijfatigue.2013.07.013 (DOI)000325120900004 ()2-s2.0-84883104595 (Scopus ID)
Note

QC 20130828

Available from: 2013-08-18 Created: 2013-08-18 Last updated: 2017-12-06Bibliographically approved
5. Reliability-based calibration of fatigue safety factors for existing steel bridges
Open this publication in new window or tab >>Reliability-based calibration of fatigue safety factors for existing steel bridges
2015 (English)In: Journal of Bridge Engineering, ISSN 1084-0702, E-ISSN 1943-5592, Vol. 20, no 10, 04014107Article in journal (Refereed) Published
Abstract [en]

This paper investigates the fatigue assessment of existing steel bridges. Measurements can be used to reduce uncertainties in the structural behavior and the effects of action. However, the treatment of the measured response is not clearly defined in today's codes of practice. A reliability model is suggested that treats the uncertainties for bilinear fatigue endurance and the uncertainties in the measured response. A parametric study on the modeling of the load effect is performed and shows a significant influence on the reliability. The reliability model is used for calibration of the partial safety factors in a deterministic design equation. The suggested partial safety factors enable practical use of measured stresses in the fatigue assessment of existing steel bridges. A numerical example shows the merit of the proposed model and partial safety factors as an increase in fatigue life.

Keyword
Fatigue, Steel Bridges, Monitoring, Measurements, Reliability, Code calibration
National Category
Infrastructure Engineering
Identifiers
urn:nbn:se:kth:diva-126022 (URN)10.1061/(ASCE)BE.1943-5592.0000716 (DOI)000361557800004 ()2-s2.0-84941670668 (Scopus ID)
Funder
Swedish Transport Administration
Note

QC 20151028

Available from: 2013-08-18 Created: 2013-08-18 Last updated: 2017-12-06Bibliographically approved

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