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Title:Illinois integral abutment bridges: behavior under extreme thermal loading and design recommendations
Author(s):Holloway, Kurt
Advisor(s):LaFave, James M.
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):integral abutment bridge
thermal behavior
pile foundations
parametric study
three-dimensional (3D) model
skewed abutments
Abstract:The Illinois Department of Transportation (IDOT) has increasingly constructed integral abutment bridges (IABs) over the past few decades, similar to many other states. Throughout the United States, IABs are preferred over conventional bridge construction because they are generally cheaper and easier to construct; have lower maintenance costs due to reduced damage to bearings, expansion joints, and other conventional appurtenances; and will correspondingly enjoy longer service life-spans. However, limitations on overall length and skew of IABs, as well as typical design details, vary considerably from state to state. Because the length and skew limitations currently employed by IDOT have not necessarily been based on rigorous engineering analyses, IDOT contracted our team of researchers from the University of Illinois at Urbana-Champaign for Illinois Center for Transportation (ICT) project ICT-R27-55. In partial fulfillment of the ICT project scope, the project team has conducted an extensive 3-D parametric study, targeting typical IDOT IAB construction details, to potentially expand the use of IABs in Illinois. In conjunction with the parametric study, we performed an extensive literature review and corresponded with researchers and DOT officials from other states in the region for insight into other states’ design and construction practices for IABs. Results of these efforts comprise the material of this thesis, which will also be published as part of ICT Research Report ICT-R27-55, Thermal Behavior of IDOT Integral Abutment Bridges and Proposed Design Modifications. The 3-D parametric study focuses on identifying key interactions between the bridge superstructure, the abutment-foundation system, and the soil under thermally induced movements. This study builds on a previous effort at the University of Illinois that identified some of the key parameters influencing the overall behavior of IABs. By fully modeling a large assortment of IABs in three dimensions with varying lengths, intermediate spans, skews, pile types, and loading conditions, we obtained a more comprehensive picture of parametric influences on IAB behavior, particularly for skewed bridges. The current 3-D numerical analyses have yielded numerous findings about the behavior of IABs subjected to extreme thermal loading. Some notable findings from these analyses are: • IABs with extreme skew of the abutments (e.g., 60°) show an additional increase in pile stresses compared to similar bridges with 40° skews, but this trend is generally not excessive. Because with proper design, even IAB configurations with extreme skew can be successful, we recommend that IDOT not place any strict limitation on IAB skew and instead provide limitations that address the combined effects of length and skew. • The current orientation scheme that IDOT uses for H-piles in IABs is unfavorable in skewed bridges because it permits excessive weak-axis bending. Revising the standard H-pile orientation to align all H-pile webs parallel to the longitudinal axis of the bridge, regardless of skew, would substantially reduce weak-axis bending. As a result, lighter H-pile sections and/or greater IAB lengths may be possible in skewed bridges. • The authors recommend use of compacted granular backfill behind the abutments. Except in bridges with extreme skews (beyond 45°), backfill pressures are beneficial to piles resisting thermal expansion. Furthermore, friction between the abutment and backfill plays an important role in resisting transverse abutment movements that are detrimental to pile performance in skewed bridges. Therefore, we recommend that details of drainage and other non-structural components at the abutment-backfill interface be evaluated to allow significant friction between the abutment and backfill to develop. • Live loads affect thermally-induced pile stresses by altering the amount of abutment rotation. While pile head displacement remains virtually unchanged, the net change in abutment rotation substantially modifies the pile head fixity conditions. This increases pile head loads under thermal expansion and relieves them during bridge contraction. Accordingly, thermal expansion combined with live loading is typically the critical load combination in IAB piles. Analyses also revealed that only a portion of the full HL-93 live load needs to be present to cause the majority of the change in abutment rotation. This indicates that the critical load combination likely will be reached in an extreme thermal event. • Longer intermediate spans tend to increase pile stresses, but this effect varies with individual bridge configuration. Changes in intermediate span length are typically accompanied by changes to other parameters such as girder cross-section and abutment height. Thus, these combined variables affect the stiffness and geometry of the IAB, and, by extension, the pile response. • Some of our analyses corroborate other recent studies indicating that time-dependent behaviors, such as concrete shrinkage, may significantly influence maximum pile stresses for some IABs. Also, shrinkage may induce additional axial loads in the superstructure. Specifically, IABs using PPCI girders may exhibit unusually high contraction displacements over the life of the bridge due to superstructure shrinkage. Therefore, we suggest that IDOT consider incorporating shrinkage and other significant time-dependent behaviors such as concrete creep, pre-stress relaxation in concrete girders, and yearly increases in backfill pressure due to cyclic compaction in any subsequent IAB research. • Research and design practices in other states are increasingly demonstrating the effectiveness of using the plastic capacity of the foundation pile steel in IABs. Our parametric study reveals that in skewed bridges, there is a potential for substantial re-distribution of loads over the pile group after the critical pile has started to yield. We recommend that IDOT re-evaluate the potential benefits and risks of permitting limited plastic deformation in pile steel in its IAB design practice. Also, IDOT should incorporate some work assessing pile plasticity as a mechanism to further extend allowable IAB lengths and skews in Illinois as part of any additional IAB research programs. As a final result of these analyses, a more rigorously developed set of recommendations for maximum IAB lengths and skews in Illinois has been proposed. These recommendations apply to a wide set of commonly-used H-piles and concrete-filled shell piles. When the full 3-D behavioral complexities were considered, numerous acceptable IAB length and skew combinations that did not induce stresses in the foundation piles above the steel yield stress were identified. However, to potentially expand the use of IABs to accommodate most of the applications desired by IDOT, further exploration of mechanisms that reduce pile moments may be necessary. Additionally, the proposed permissible lengths and skews are based on the behavior of the piles under thermal loadings; accordingly, more evaluation of the performance of IAB abutments and superstructures is warranted to ensure that the detailing of the entire bridge system is adequate for the induced thermal loads.
Issue Date:2012-05-22
Rights Information:Copyright 2012 Kurt Holloway.
Date Available in IDEALS:2012-05-22
Date Deposited:2012-05

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