Fatigue Evaluation of Steel Bridges (2012)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

5 Optimal performance of a bridge under normal service loads is essential for full and effective use by the motoring public. Problems that occur as a result of excessive deflec- tions, deteriorating deck conditions, or fatigue cracking of steel girders or beams under normal operating service loads can cause delays and inconvenience for the public as these problems are being corrected. In extreme cases, inadequate serviceability performance may require that portions of a bridge be closed as it is being repaired, or the bridge may need to be replaced altogether. Moreover, repair operations also pose a safety hazard for both the motoring public and the construction personnel. Clearly, the need exists to develop modern and effective methods to assess the serviceability of a bridge structure so that the optimal performance can be achieved. This research project has a focus on Section 7 “Fatigue Evaluation of Steel Bridges” in AASHTO’s The Manual for Bridge Evaluation (MBE) Second Edition issued in 2011. The MBE combines the Manual for Condition Evaluation of Bridges, Second Edition (2000) and its 2001 and 2003 Interim Revisions with the Guide Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges, First Edition and its 2005 Interim Revisions. The objective of this research is to develop a revised and updated Section 7 for the MBE, to meet the needs of the user. A view exists among some fatigue evaluation engineers that the MBE is overly conservative because some bridges with satisfactory service history are accordingly determined to have negative remaining fatigue lives. A number of fac- tors may have contributed to this conservatism: overesti- mated load distribution factors, unintended composite action ignored, the S-N curve’s lower bound being used, etc. On the other hand, not all cases of fatigue evaluation are believed to be overly conservative. For example, truss or two-girder bridges carrying more than one lane of traffic may have un- conservative fatigue life estimate because of the single lane loading prescribed in the MBE. When multiple lanes are carried by the two trusses or girders, the fatigue life may be significantly overestimated because possible simultaneous loads on other lanes are ignored. On the other hand, conven- tional analysis methods generally overestimate the live load stress ranges in truss bridges because unintended composite actions are often ignored. In general, a larger amount of uncertainty is involved in fatigue evaluations compared with bridge strength evalua- tions or load ratings. Furthermore, the demand for a realis- tic fatigue evaluation is much higher than that for a fatigue design because an over-conservative evaluation result could cost considerably more than an over-conservative design. An un-conservative result is, of course, not desired either. Besides the uncertainty factors mentioned herein, there are also other sources of uncertainty in the fatigue evaluation process. They include the scatter nature of the S-N curves, variable truck loads (including significant site-to-site variation), approximations in structural analysis or load effect estimation, etc. The inherent uncertainties, however, can be reduced using more refined analyses or field mea- surements to better define the stress range at the details in question. The research program of this project aims toward the revision of Section 7 of the MBE to advance the state of the art and the practice (AASHTO 2011). This research includes analytical and experimental studies which will lead to the effective development of an improved Section 7 of the MBE. The primary objective of the research is to revise and update Section 7, “Fatigue Evaluation of Steel Bridges” of the Manual for Bridge Evaluation. Items specifically identified as in need of improvement include the following: 1. Improved methods utilizing a reliability-based approach to assess the fatigue behavior and aid bridge owners in making appropriate operational decisions. C h a p t e r 1 Background

6welds. The objective of the tests is to classify the tack weld detail into the appropriate fatigue category based on the observed fatigue life of the tack welds. Experimental tests to evaluate distortion-induced fatigue cracks examine the effectiveness and fatigue behavior of different types of retrofit options. The retrofits used to address the distortion- induced fatigue include only stiffening retrofits: particularly WT, double-angle retrofits, and single-angle retrofits. New methodologies and provisions to enhance the fatigue evalu- ation procedure of bridge details are researched in order to improve Section 7 of the MBE. Existing fatigue provisions are also examined for improvement. 2. Guidance on the evaluation of retrofit and repair details used to assess fatigue cracks. 3. Guidance for the evaluation of distortion-induced fatigue cracks. 4. Guidance for evaluation of tack weld induced fatigue cracks. 5. Adjustment of truck loading factors to account for mul- tiple lane loading. Experimental tests performed to evaluate tack weld induced fatigue cracks examine the effect of different stress ranges and weld parameters on the fatigue life of the tack