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Townsend H.E.,Townsend Corrosion Consultants | Frank K.H.,Hirschfeld Industries | Choi C.,T.Y.Lin International
Bridge Structures | Year: 2015

To determine the hydrogen-embrittlement resistance of anchor rods in the new San Francisco-Oakland Bay Bridge, tensile tests of full-size ASTM A354 Grade BD anchor rods were conducted at very slow load rates. Resistance to embrittlement by hydrogen entering the rods while under load, also referred to as stress corrosion cracking (SCC) or environmental hydrogen embrittlement (EHE), was measured by performing the slow-load tests in 3.5 sodium chloride (NaCl) solution. Resistance to embrittlement by hydrogen entering the steel during fabrication processes such as hot-dip galvanizing, also referred to as internal hydrogen embrittlement (IHE), was measured by performing the slow-load tests in air. Testing was conducted on rods representing various sizes, different manufacturers, rolled and cut threads, different alloys, as well as galvanized and ungalvanized rods. Following slow-load hydrogen embrittlement tests, mechanical and chemical properties of the test rods were fully characterized, and fracture surfaces were examined by scanning electron and optical microscopy to establish modes of failure. The results of this work are discussed in terms of 1) material properties, such as strength level and hardness, and toughness; 2) processing variables, including galvanizing, and threading method; 3) the cause of failure of 32 anchor rods in March, 2013; and 4) establishment of safe loads for rods currently in service on the bridge. © 2015 IOS Press and the authors. All rights reserved.

Stith J.C.,University of Texas at Austin | Helwig T.A.,University of Texas at Austin | Williamson E.B.,University of Texas at Austin | Frank K.H.,Hirschfeld Industries | And 4 more authors.
Journal of Structural Engineering (United States) | Year: 2012

The stability of I-girders during erection can be difficult to assess because of the limited presence of bracing and uncertainty in the support conditions of the girders. The behavior of curved girders during the early stages of construction is complicated because the curved geometry can lead to significant torsion. This paper highlights results from a research study that included both field monitoring and parametric finite-element investigations. Curved I-shaped girders were instrumented and monitored during lifting to provide data to validate finite-element models. Both rotational displacements and stress were measured during the lifting process. In this paper, the writers compare data collected from field tests with results computed from detailed finite-element simulations. A prismatic and a nonprismatic girder (with two different cross sections) were considered in the investigation. The I-girders experienced both rigid body rotation and cross-sectional twist. Additionally, the torsional warping stresses were observed to be of the same order of magnitude as the strong-axis bending stresses. However, it should be noted that the total stresses were well below yielding. The fact that the stresses are low during lifting should not be confused with a noncritical stage in the safety of the girders. Although the applied stresses are low, the stresses necessary to buckle the girder or to cause large deformations are also relatively low because usually no bracing exists and limited restraint is provided to the girders during lifting. The finite-element models were able to capture the measured behavior accurately, providing insight into appropriate assumptions and critical features for modeling curved I-girders during lifting. © 2012 American Society of Civil Engineers.

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.

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.

Battistini A.D.,George Mason University | Wang W.,SBM offshore | Helwig T.A.,University of Texas at Austin | Engelhardt M.D.,University of Texas at Austin | Frank K.H.,Hirschfeld Industries
Journal of Bridge Engineering | Year: 2016

Cross frames are critical structural elements in both straight and horizontally curved steel bridges. In order to properly size the brace for the strength and stiffness demands of the superstructure, an accurate model of the elements comprising the cross frame is required. Conventional details most commonly used for cross frames consist of single-angle members connected to form a truss system linking adjacent girders together. Most analyses of the bridges treat the cross frames as truss elements that primarily resist applied forces through the axial stiffness of the members. This paper documents the results of a research study that included full-scale laboratory tests to measure the stiffness and strength of cross frames utilizing both conventional and new details. The tests showed that analytical solutions, as well as computer models, that are routinely used to model the cross frames in analysis software can overestimate the in-plane stiffness of the brace by more than 100%. The primary reason for the discrepancy in the stiffness models was identified to be connection eccentricities that exist in cross frames comprised of single-angle members welded to a gusset plate through only one leg of the angle. Overestimating the stiffness of the braces during construction can lead to unsafe conditions, as well as errors in the geometry of the constructed bridge, resulting from underpredictions of deformations during concrete placement. Extensive parametric analyses were carried out using validated finite-element models to develop correction factors that can be applied to analytical and computer models to significantly improve the accuracy of the simplified models that are used for the cross-frame systems. The correction factors allow a designer to utilize a computationally efficient model for the cross frame while also including the reduction in stiffness that is caused by connection eccentricities. © 2016 American Society of Civil Engineers.

Frank K.H.,Hirschfeld Industries | Samaras V.,University of Texas at Austin | Helwig T.A.,University of Texas at Austin
Engineering Journal | Year: 2012

An experimental study was undertaken to determine if markings used to identify fabricated steel using modern automatic impact stenciling equipment could be used on structures subjected to fatigue loading. The fatigue specimens with both numeric and alphabetic markings were tested. The results showed that the fatigue life of marked steel exceeded the fatigue design strength for Category A, the highest fatigue design category. Marking pieces with this equipment will not affect the fatigue design strength of the member.

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.

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.

Fasl J.,Wiss, Janney, Elstner Associates, Inc. | Helwig T.,University of Texas at Austin | Wood S.L.,University of Texas at Austin | Frank K.,Hirschfeld Industries
Engineering Journal | Year: 2015

As traffic volumes increase, bridges age, and maintenance budgets are cut, transportation officials often need quantitative data to distinguish between bridges that can be kept safely in service and those that need to be replaced or retrofitted. Strain gages can be utilized to evaluate fatigue damage in steel bridges using the techniques that are discussed in this paper. To evaluate fatigue damage, the cycles induced by vehicular traffic must be quantified using a cycle-counting algorithm, such as a rainflow algorithm. The amount of fatigue damage induced during the monitoring period can then be calculated using the traditional method, the effective stress range, or using a new approach based on the index stress range. One distinct advantage of the proposed method is that the relative amount of fatigue damage accumulated at different locations along the bridge can be easily compared. The advantages and limitations of both methods are demonstrated using measured data from a fracture-critical steel bridge. © 2015, American Institute of Steel Construction Inc. All rights reserved.

Williamson E.B.,University of Texas at Austin | Kim J.,University of Texas at Austin | Frank K.H.,Hirschfeld Industries
Structures Congress 2010 | Year: 2010

In the current AASHTO LRFD Bridge Design Specifications [AASHTO, 2007], a fracture critical member is defined as a "component in tension whose failure is expected to result in the collapse of the bridge or the inability of the bridge to perform its function." All bridges designed with fracture critical members or components are designated as fracture critical bridges. The bridge design process is affected by many factors including economics, aesthetics, and function, and many popular bridge structural systems are classified as fracture critical. In fact, approximately 11% of all steel bridges in the United States have this designation [Connor, et al., 2005]. As long as the risk of a brittle fracture of an integral component of a bridge's main load path is minimized, a fracture critical bridge is not inherently unsafe. For this reason, the design of fracture critical bridges is permitted, but a major requirement in the fracture critical member provisions is that a full inspection of all fracture critical bridges be performed every two years. Fracture critical bridge inspections are labor intensive - and therefore costly - requiring the examination of every weld throughout a structure. The fracture critical provisions in the AASHTO Bridge Design Specifications [AASHTO, 2007] assume that, when a fracture critical member fails, the remaining bridge structure lacks a redundant load path to support its loads. A number of incidents involving the full-depth fracture of in-service two-girder bridges (all designated as fracture critical) provide evidence that, in certain cases, a redundant load path does exist in these structures even though they have not been given credit for such. The Texas Department of Transportation (TxDOT) owns and operates a vast inventory of more than 50,000 bridges throughout the state. Many of these bridges are two-girder bridges and are classified as fracture critical by the AASHTO guidelines [AASHTO, 2007], contributing to large annual inspection costs. In search of guidance in reevaluating the inspection schedule for fracture critical bridges, TxDOT and the Federal Highway Administration (FHWA) cosponsored a large-scale research program at the Ferguson Structural Engineering Laboratory (FSEL) at The University of Texas at Austin. The overall goal of the project was to provide transportation engineers with methods for evaluating the redundancy of fracture critical steel bridges. The research focused specifically on investigating the redundant capacity of fracture critical twin steel box-girder bridges, which are common throughout the state of Texas (Figure 1). Using tools to estimate the load carrying capacities of their structures in the event of a failure of a fracture critical member or component, bridge owners would be able to appropriately tailor their maintenance schedules to their bridge inventory. Supported by significant experimental, computational, and financial resources, the comprehensive research program at FSEL continued for four years and comprised a set of interrelated experimental initiatives. The techniques used to work toward the ultimate goals of the research included simplified structural analyses performed by hand or through the use of spreadsheets, analyses performed through detailed finite element simulations, the testing of laboratory specimens to quantify experimentally the capacity of specific bridge elements, and the full-scale testing of a twin steel box-girder bridge (i.e., a fracture critical bridge) reconstructed at FSEL for use in this project. Detailed information on the testing program can be found in Neuman [Neuman, 2009] and Mouras [Mouras, 2008], and a description of the simplified procedure to predict the capacity of twin steel box-girder bridges can be found in Samaras [Samaras, 2009]. The current paper provides an overview of the detailed finite element simulations performed as part of this project. The simulation models include inelastic material behavior and nonlinear geometry, and they also account for the complex interaction of the shear studs with the concrete deck under progressing levels of damage. In addition, the contribution of the bridge rail, including the presence of expansion joints, has been successfully modeled in the finite element analyses carried out for this study. These simulation models have been validated using small-scale laboratory tests and results obtained from full-scale tests on a twin steel box-girder bridge. A brief description of the test program and the finite element models developed for this research are provided below. © 2010 American Society of Civil Engineers.

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