Eric B. Williamson

Blast-resistant Highway Bridges: Design and Detailing Guidelines

This paper presents preliminary results and observations from blast tests on concrete bridge columns conducted during a U.S. national study to develop design and detail guidelines for blast-resistant highway bridges.

Response of Reinforced Concrete Bridge Columns Subjected to Blast Loads

The vast majority of past research on blast-resistant structural design focuses on buildings, with limited attention directed specifically towards bridges. Although many of the same principles apply, bridges pose unique challenges that are not often encountered when designing buildings for blast resistance. Specifically, establishing standoff with bridges is difficult because they are intended to provide open access to the traveling public, and structural components are directly loaded rather than having loads transferred to them through a facade system. Thus, relative to buildings, bridge components may be exposed to large blast threats that can be in close proximity to the potential target. To address these unique challenges, experimental and computational research was carried out, through support from the National Cooperative Highway Research Program (NCHRP), to understand the behavior of blast-loaded concrete bridge members. Although spalling of concrete cover off the back of reinforced concrete walls...

DoD research and Criteria for the Design of Buildings to Resist Progressive Collapse

The collapse of conventional/nonhardened structures was a concern of the U.S. Department of Defense (DoD) for years before the collapse of the World Trade Center (WTC) towers during the terrorist attacks on September 11, 2011 (9-11), owing to the bombings of the Murrah Federal Building in Oklahoma City, the U.S. embassies in Africa, and the U.S. Marine barracks in Lebanon. Since 9-11, motivated by the lack of any meaningful U.S. progressive collapse design requirements, DoD has worked with the civilian community on a number of significant efforts to improve the design of buildings to resist disproportionate collapse. The DoD efforts have included laboratory and field experiments, numerical simulations, and development of design requirements. Synergy and coordination with the civilian community resulted in combined programs with the General Services Administration, guidance and feedback provided by the ASCE Structural Engineering Institute (SEI) Committee on Disproportionate Collapse Standards and Guidance...

Performance of Bridge Columns Subjected to Blast Loads. I: Experimental Program

Statistical data from past terrorist attacks show that transportation infrastructure has been widely targeted, and a significant percentage of the attacks against transportation structures have been directed towards bridges. Recent threats to bridges in the United States validate this concern and have attracted the attention of the bridge engineering community. To address these concerns, the National Cooperative Highway Research Program sponsored a research project to investigate the response of critical bridge components subjected to blast loads. This paper includes a description of an experimental research program on ten different half-scale column designs in which the design parameters that have the greatest impact on the performance of blast-loaded bridge columns were evaluated. Interpretation of the test results and guidelines for the blast-resistant design of reinforced concrete bridge columns are provided in the companion paper written by the writers.

Performance of Bridge Columns Subjected to Blast Loads. II: Results and Recommendations

Guidelines for the design of critical bridge components subjected to blast loads are currently not available to the general bridge engineering community. Historically, however, transportation assets have proven to be attractive targets for terrorists because of their open access, utilization by large numbers of people, symbolic importance, and significance to commerce, in addition to a host of other reasons. To improve the current state of practice, the National Cooperative Highway Research Program sponsored a research project to investigate the response of reinforced concrete bridge columns subjected to blast loads. Part I of this manuscript details the unique experimental program carried out to assess the effects of different design parameters on overall performance under these types of loads. In the current paper, results from the test program are analyzed to identify the design parameters that most significantly influence the performance of blast-loaded reinforced concrete bridge columns. Using the scaled standoff distance as the primary variable to assess threat severity, three separate blast design categories are recommended.

Development of computational software for analysis of curved girders under construction loads

The analysis of horizontally curved, trapezoidal steel girders presents a variety of computational challenges. During the erection and construction stages before a concrete deck is available to form a closed section, these girders are weak in torsion and susceptible to warping. Considering the design of an entire bridge system, current design approaches favor the use of a grid analysis methodology. While the use of a grid analysis procedure offers the advantage of computational efficiency, it is unable to capture girder stresses and brace member forces with sufficient accuracy, particularly during the critical erection and construction stages. In this paper, we present an alternative analysis approach based on the finite element method. The developed software has been designed to be computationally efficient and easy to use for bridge designers.

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.

Numerical Simulation of Damage Localization in Polyester Mooring Ropes

This paper presents the derivation of a mechanical model to estimate the effects of damage on the response of ropes. Damage can be represented through a degradation of the properties of individual rope elements, and it can also include the complete rupture of one or more elements. The general assumptions made to estimate the length over which damage propagates along the length of a rope and how this length is considered in modeling damaged rope behavior are explained. Consistent with tests on damaged polyester (PET) mooring ropes, numerical simulations demonstrate the existence of strain localization around the failure region and, due to degradation of rope element properties, damage localization as well. This damage localization causes the premature failure of rope elements, reducing the maximum load capacity and maximum failure strain that a damaged rope is capable of resisting relative to that of an intact rope. The proposed model suggests that some of the variables that affect damaged rope behavior are the degree of damage present at a given cross-section, the location of broken rope elements, and the length over which damage propagates along the rope length. Experimental data are used to validate the model.

Evaluation of Conventional Construction Techniques for Enhancing the Blast Resistance of Steel Stud Walls

AbstractCold-formed steel stud walls are an attractive alternative to wooden stud walls because of their ductility, sustainability, and resistance to insects, mold, and rot. To date, engineers have faced challenges using steel stud walls to mitigate blast loads because of strict response limits set forth by existing standards that address blast-resistant design. Because of the limited past research on the response of steel stud walls subject to blasts, uncertainties exist regarding how these systems transition from elastic to inelastic response, particularly when lateral torsional buckling or local buckling of a section occurs. Existing standards used to design structures to mitigate blast loads penalize the use of steel stud walls by incorporating large safety factors that make the design of these systems uneconomical, which prevents engineers from taking full advantage of their benefits. The goal of the research described in this paper is to evaluate the suitability of conventional construction techniqu...