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Sutton J.P.,Modjeski and Masters Inc. | Mouras J.M.,Magnusson Klemencic Associates | Samaras V.A.,Kellogg Brown and Root | Williamson E.B.,University of Texas at Austin | Frank K.H.,Hirschfeld Industries
Journal of Bridge Engineering | Year: 2014

AASHTO defines fracture-critical members (FCMs) as components in tension whose failure is expected to lead to bridge collapse. Accordingly, the bottom flanges of a twin steel box-girder bridge are considered to be fracture-critical members in the positive-bending-moment region. In the event of a fracture propagating through the entire depth of a box girder, the shear studs connecting the fractured girder to the bridge deck play a crucial role in the performance of the bridge. To characterize the response of these connections, a series of laboratory tests were performed to determine the capacity and behavior associated with different stud layouts. Based on the test results, modifications to the current American Concrete Institute (ACI) equations to predict the tensile strength of shear stud connections are proposed. © 2014 American Society of Civil Engineers. Source


Battistini A.D.,Washington State University | Donahue S.M.,University of Texas at Austin | Wang W.H.,SBM offshore | Helwig T.A.,University of Texas at Austin | And 2 more authors.
Structural Stability Research Council Annual Stability Conference 2014, SSRC 2014 | Year: 2014

The stability of steel bridges is improved by using cross frames, which provide lateral and torsional restraint along the girder length. In order to be considered an effective brace, the cross frame must satisfy both strength and stiffness requirements. It is imperative the stiffness of the cross frame be accurately calculated to ensure the associated cross frame forces and behavior are realistic representations of the brace. Cross frames can utilize a variety of layouts: the X-Type and K-Type cross frames are commonly used in current practice for steel I-girder bridges, while the single diagonal Z-Type cross frame is being researched at the University of Texas at Austin as part of a TxDOT sponsored research project. During the design of a steel bridge, the engineer must select a cross frame that provides adequate stiffness to allow for buckling between the brace points while also adhering to maximum fatigue stress limits. If the fatigue stresses are exceeded, the engineer sometimes chooses to use cross frame members with a larger area. The increased stiffness of the members with more area causes the cross frame to attract more forces under fatigue loading, which can lead to exceeding the maximum allowable fatigue stresses. The circular nature of this problem can lead engineers to require larger, heavier cross frames, to use more braces along the girder length, and in some cases, to add an extra girder line. Previous research at the University of Texas at Austin has shown the stiffness of cross frames with eccentrically connected members, like the angles typically used in X-Type and K-Type cross frames, is significantly less than predicted by analytical equations and truss-type computer models. A case study using a validated finite element model was performed to examine the interaction between brace stiffness and fatigue-induced forces. Recommendations for cross frame stiffness from large-scale experimental testing will be discussed, as well as suggestions for modifying the stiffness in commercial design software. Source


Battistini A.D.,George Mason University | Donahue S.M.,University of Texas at Austin | Helwig T.A.,University of Texas at Austin | Engelhardt M.D.,University of Texas at Austin | Frank K.H.,Hirschfeld Industries
Structural Stability Research Council Annual Stability Conference 2015, SSRC 2015 | Year: 2015

Cross frames are used in steel bridges to improve the stability of the girder by providing lateral and torsional restraint at discrete points along the girder length. To establish the strength requirements for the cross frame members, large displacement analyses on imperfect systems can be performed. The selection of the imperfection magnitude and shape can significantly impact the forces developed in the braces. To maximize the cross frame forces, previous research on simply supported spans suggested applying the critical imperfection at the brace nearest to the location of the maximum moment, with zero twist at the adjacent brace points. The recommended imperfection shape was a pure twist, where the bottom flange remains perfectly straight while the top flange displaces laterally. However, in two-span continuous girders, the location of maximum moment typically occurs at the center support, a location which is not likely to have the critical imperfection. In addition, in the negative moment region, the compression flange will correspond to the bottom flange instead of the top flange, potentially changing the critical shape. In order to provide guidance on maximizing cross frame forces in two-span continuous steel I-girder bridge systems, various imperfection locations and magnitudes will be studied using a three-dimensional finite element analysis program. Preliminary results of cross frame forces for both straight and skewed bridge layouts are provided. Copyright © 2015 by the Structural Stability Research Council. Source


Samaras V.A.,University of Texas at Austin | Sutton J.P.,Modjeski and Masters Inc. | Williamson E.B.,University of Texas at Austin | Frank K.H.,Hirschfeld Industries
Journal of Bridge Engineering | Year: 2012

A fracture-critical bridge (FCB) is a structure that is expected to collapse after the failure of an essential tension component. In the positive bending moment region, the bottom flanges of a twin steel box-girder bridge are considered to be fracture-critical elements. Bridges with fracture-critical elements are required to undergo stringent hands-on inspections at least every two years. These inspections, which often require lane closures, are labor intensive and costly. There have been multiple cases of FCBs that have experienced a failure in one of their fracture-critical elements without collapsing, which suggests that current provisions may not accurately account for the inherent redundancy that exists in various FCB structural systems. To improve the understanding of how a twin steel box-girder bridge behaves after suffering a full-depth fracture in one of its girders, simplified analytical methods have been developed and are presented in this paper. The proposed methodology has been validated against data from full-scale tests and provides a convenient means for predicting response. © 2012 American Society of Civil Engineers. Source


Fasl J.,University of Texas at Austin | Helwig T.,University of Texas at Austin | Wood S.,University of Texas at Austin | Frank K.,Hirschfeld Industries
Transportation Research Record | Year: 2012

A fracture-critical steel I-girder bridge was instrumented with strain gauges to estimate the remaining design fatigue life. The two girders on the bridge had extensive fatigue cracking. Continuous, dynamic strain data were collected for nearly 2 months to determine an effective stress range and cycle count according to Palmgren-Miner's rule. A simplified rainflow counting algorithm was developed and used to calculate the amplitude of each fatigue cycle. The effective stress range and cycle count were combined with AASHTO's S (stress range)-N (number of cycles to failure) curves to estimate the remaining design fatigue life of certain bridge details. The data revealed that the estimated design fatigue life was exceeded in the east girder (right lane), whereas some life remained in the west girder (left lane). The distribution of observed cracks in the girders was closely correlated with the calculated fatigue life. A method is presented in this paper to index the effective stress range so that strain measurements can be compared over extended periods. Source

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