Karl H. Frank

Simplified Method for Evaluating the Redundancy of Twin Steel Box-Girder Bridges

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.

Comparisons of the Computed and Measured Behavior of Curved Steel I-Girders during Lifting

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.

Development of interim recommendations for improved welded moment connections in response to the northridge earthquake

Publisher Summary A short-term research and testing program is conducted in response to damage observed at a large number of welded steel moment connections following the 1994 Northridge Earthquake. The test program investigated several changes to joint welding and design procedures intended to improve earthquake response of steel moment frame connections. Sixteen very large scale beam-to-column connections are tested under cyclic load. The most successful connections tested in this program are those in which the beam flanges are reinforced with cover plates or vertical ribs. The test results showed that reinforcing the connection to reduce stress at the beam flange groove welds, combined with reasonable care in welding can significantly enhance joint performance. The results of this test program suggest that improved welding workmanship, by itself, may not be adequate to assure satisfactory performance of the conventional welded flange-bolted web connection detail under inelastic cyclic loading. The results also indicate that a large improvement in cyclic loading performance is possible at steel moment flame joints by the use of a reinforced connection combined with careful attention to welding.

Experimental Investigation of Bending Fatigue Response of Grouted Stay Cables

An experimental investigation was conducted to determine the susceptibility of grouted stay cables to bending fatigue damage. The results from twelve, bending fatigue tests are reported in this paper. Fretting of adjacent wires within a single strand was the dominant cause of bending fatigue damage in the grouted stay-cable specimens. This damage tended to be concentrated at the ends of the specimens and at locations where concentrated loads were applied to the stays. The laboratory tests indicated that the risk of bending fatigue damage was low at the tension rings, along the free length of the stays, and in the vicinity of unintentionally crossed strands.

Use of Strain Data to Estimate Remaining Fatigue Life of a Fracture-Critical Bridge

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–Miners 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 AASHTOs 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.

Long-term gage reliability for structural health monitoring of steel bridges

Real-time monitoring of fracture critical steel bridges can potentially enhance inspection practices by tracking the behavior of the bridge. Significant advances have occurred in recent years on the development of robust hardware for field monitoring applications. These systems can monitor, process, and store data from a variety of sensors (e.g. strain gages, crack propagation gages etc.) to track changes in the behavior of the bridge. Thus, for a long-term monitoring system to be successful, the reliability of gages that are to be monitored for several years is very important. This paper focuses on the results of a research study focused on developing a wireless monitoring system with a useful life of more than 10 years. An important aspect of the study is to identify strain gages and installation procedures that result in long lives as well as characterizing the effect of temperature fluctuations and other environmental factors on the sensor drift and noise. In long-term monitoring applications, slight sensor drift and noise can build up over time to produce misleading results. Thus, a wide variety of gages that can be used to monitor bridges have been tested for over a year through environmental tests. The environmental tests were developed to determine the durability of the gages and their protective coatings (e.g. zinc-based spray, wax and silicon, etc.) against humidity, sun exposure and other environmental effects that are expected in long-term bridge monitoring applications. Moreover, fatigue tests were performed to determine the fatigue category of the weldable gages and to reveal any debonding issues of the bondable gages. This paper focuses on the results of laboratory tests on gage durability that were conducted as part of a research project sponsored by the National Institute of Standards and Technology (NIST).

Development of a wireless monitoring system for fracture-critical bridges

This paper provides a summary of ongoing research sponsored by the National Institute of Standards and Technology (NIST) that seeks to improve inspection practices for steel bridges by providing the technology and methodology for real-time monitoring. In order to reduce the time and cost of installing a monitoring system, the research team elected to use wireless communications within the sensor network. The investigation considered both IEEE 802.11 and IEEE 802.15.4 communications protocols and identified the latter as more practical for bridge monitoring applications. Studies were conducted to investigate possible improvements in the network performance using high-gain antennas. Results from experiments conducted outside and on bridges with different antennas are presented in this paper. Although some benefits were observed using high-gain antennas, the inconsistent performance and higher cost relative to the current stock, omni-directional antennas does not justify their use.

A Method to Calculate Rotational Deformations of Curved Plate Girders during Lifting

The design of curved I-girders is complicated due to the many performance stages that must be evaluated, including erection, construction, and in-service. Evaluation of girder stability during early stages of construction is particularly complicated due to the limited presence of bracing. In addition to stability issues during girder lifting, guidelines for the necessity of shore towers during erection are also not available. The Texas Department of Transportation has funded a research project to investigate girder stability during early stages of construction. Results from this research project will be used to develop guidelines for the evaluation of girder stability during early construction stages as well as proportion the girder cross sections for safe and economical construction. The research includes: (1) field monitoring, (2) a parametric study using finite element analysis, and (3) the development of a user-friendly finite element analytical software that will serve as a tool for designers. Field studies have been successfully conducted on a bridge that is a part of the SH 130 Turnpike Project near Austin, TX. Data was collected for the validation of a 3-D finite element model. Recommendations were made to provide guidance for safely lifting horizontally curved steel I-girders. This paper will provide a summary of results from the field studies that have been used to validate finite element models of the curved girders during girder lifting. A method for calculating the rotational deformation of curved I-girders during lifting and results from the parametric investigations that are currently underway will also be discussed.

Cyclic fatigue life of cables

Abstract Methods for calculating the fatigue life of cables with an infinite number of parallel wires are presented. The methods account for the effects of cable legnth and the increase in stress range that occurs as cable wires break. Comparisons are made with fatigue lives of cables with finite numbers of wires calculated by Monte Carlo simulation. A previously published model is extended and applied to the case where the fatigue lives of short pieces (test specimens) of the cable wire have a log-normal distribution. Also, an approximate solution for the life of long cables is presented.

Stiffness Behavior of Cross Frames in Steel Bridge Systems

AbstractCross 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 fo...