Todd Helwig

High-Temperature Creep Buckling Phenomenon of Steel Columns Subjected to Fire

This paper presents highlights of on-going research, which aims at developing analytical, computational and experimental predictions of the phenomenon of creep buckling in steel columns subjected to fire. Analytical solutions using the concept of time-dependent tangent modulus are developed to model time-dependent buckling behavior of steel columns at elevated temperatures. Results from computational creep buckling studies using Abaqus are also presented, and compared with analytical predictions. Material creep data on ASTM A992M steel is also presented and compared to existing creep models for structural steel. Both analytical and computational methods utilize material creep models for structural steel developed by Harmathy, by Fields and Fields, and by the authors. Predictions from this study are also compared against those from Eurocode 3 and the AISC Specification. Results of this work show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.

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.

Buckling Behavior of Steel Bridge I-Girders Braced by Permanent Metal Deck Forms

Permanent metal deck forms (PMDFs) are often used in the bridge industry to support wet concrete and other loads during construction. Although metal formwork in the building industry is routinely relied on for stability bracing, the forms are not permitted for bracing in the bridge industry, despite the large in-plane stiffness. The forms in bridge applications are typically supported on cold-formed angles, which allow the contractor to adjust the form elevation to account for changes in flange thickness and differential camber between adjacent girders. Although the support angles are beneficial toward the constructability of the bridge, they lead to eccentric connections that substantially reduce the in-plane stiffness of the PMDF systems, which is one of the reasons the forms are not relied on for bracing in bridge applications. This paper documents the results of an investigation focused on improving the bracing potential of bridge deck forms. Modifications to the connection details were developed to improve the stiffness and strength of the forming system. Research included buckling tests on a 15-m (50-ft) long, twin-girder system with PMDFs for bracing. In addition, twin-girder tests were also used to validate computer models of the bracing systems that were used for parametric finite-element analytical studies. The buckling test results demonstrated that modified connection details make PMDF systems a viable bracing alternative in steel bridges, which can significantly reduce the number of cross-frames or diaphragms required for stability bracing of steel bridge I-girders during construction.

Creep Properties of ASTM A992 Steel at Elevated Temperatures

In moving towards an engineered performance-based approach to structural fire safety, a sound knowledge of the elevated-temperature properties of structural steel is crucial. Of all mechanical properties of structural steel at elevated temperatures, material creep is particularly important. Under fire conditions, behavior of steel members and structures can be highly time-dependent. As a result, understanding the time-dependent mechanical properties of structural steel at high temperatures becomes essential. This paper presents preliminary results of a comprehensive on-going research project aimed at characterizing the material creep behavior of ASTM A992 steel at elevated temperatures. Such creep properties are presented in the form of strain-time curves for materials from the web and the flanges of a W4×13 wide flange section and from the web of a W30×99 section. The test results are then compared against material creep models for structural steel developed by Harmathy, and by Fields and Fields to evaluate the predictions of these models. The preliminary results clearly indicate that material creep is significant within the time, temperature, and stress regimes expected in a builing fire. The results also demonstrate the need for a more reliable creep model for steel for strcutural-fire engineering analysis.

Importance of Time-Dependent Material Behavior in Predicting Strength of Steel Columns Exposed to Fire

One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of the applied load. This paper will propose a preliminary methodology to study the phenomenon of creep buckling in steel columns subjected to fire. Analytical solutions using the concept of time-dependent tangent modulus are developed to model time-dependent buckling behavior of steel columns at elevated temperatures. Results from computational creep buckling studies using ABAQUS® are also presented, and compared with analytical predictions. Both analytical and computational methods utilize material creep models for structural steel developed by Harmathy, and by Fields and Fields. The analytical and computational results clearly indicate that accurate knowledge of material creep is essential in studying creep buckling phenomenon at elevated temperatures, and that neglecting creep effects can lead to potentially unsafe predictions of the strength of steel columns subjected to fire.

Analysis of Creep Buckling of Steel Columns Subjected to Fire

One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of applied load. Although material creep and consequently the phenomenon of creep buckling can significantly impact the safety of steel columns subjected to fire, they have received relatively little research attention, and are not currently explicitly considered in code-based design formula for columns at elevated temperatures, such as those in the Eurocode 3 or in the AISC Specification. This paper will propose a preliminary methodology to study the phenomenon of creep buckling in steel columns subjected to fire. Preliminary analytical solutions are presented, and compared with computational predictions for creep buckling. The analytical and computational results clearly indicate that accurate knowledge of material creep is essential in studying creep buckling phenomenon at elevated temperatures. In addition, the results show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.

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.