Michael J. Chajes

The retrofitting of reinforced concrete column-to-beam connections

Abstract This paper reports the research effort in improving the ductility of concrete column-to-beam connection and the capability of connections containing insufficient development length. CFRP tow sheets were wrapped around the column near the joint region for ductility retrofitting, and were longitudinally bonded to and/or wrapped around the column near the joint with a set of steel angles and rods for development retrofitting. Repeated loading–unloading–reloading were applied on ductility specimens for simulating seismic loads. Development specimens were tested under monotonic loading. A total of 19 concrete column-to-beam connection specimens were tested. Ductility retrofitting has resulted in significant improvement in ductility and 24–35% increase in ultimate loading capacity. The development retrofitting has demonstrated 154–172% increase in ultimate loading capacity.

Analysis of concrete beams reinforced with externally bonded woven composite fabrics

Abstract With the deterioration of this nations infrastructure comes the growing need for effective means of rehabilitating structures. Possibly one of the most challenging tasks is to upgrade the overall capacity of concrete structures. The utilization of composite materials in rehabilitating such structures represents an innovative use of new technology. Laboratory experiments have recently been conducted on a series of reinforced concrete T-beams to study the effectiveness of using externally applied composite fabrics as a method of increasing a beams shear capacity. In the experiments, woven composite fabrics made of aramid, E-glass, and graphite fibers were bonded to the webs of T-beams using a two-component epoxy. The beams were loaded in flexure and tested to failure. All beams failed in shear. The ultimate strengths of the externally reinforced beams were 60–150% higher than the strengths of beams without external reinforcement. This paper presents analyses of the externally reinforced concrete beams using finite element models. The quality of numerical simulations are assessed by comparing them with experimental results. To gain better insight into the behavior of externally reinforced concrete T-beams, several numerical parametric analyses were also performed. The significance of the results of these analyses with respect to potential retrofits of existing concrete beams in the civil infrastructure is addressed.

Bridge-Condition Assessment and Load Rating Using Nondestructive Evaluation Methods

In most cases, bridge-condition assessment is made according to visual inspections, and bridge-load ratings are determined with fairly simple analytical methods and without site-specific, live-load, bridge-response data. As a result, estimates of bridge load-carrying capacity are often quite conservative. The increased weight of today’s trucks compared with design loads that are used for older bridges, combined with the continued aging and deterioration of our nation’s bridges, has resulted in a significant number of them being classified as structurally deficient. Reliable condition assessments are essential to ensure the safety of the traveling public. Furthermore, because load-carrying capacity is often used to prioritize bridges for repair, rehabilitation, and replacement, and because funds for these actions are limited, it is more important than ever that these estimates be as accurate as possible. To achieve this goal, researchers at the University of Delaware have been working with engineers at the Delaware Department of Transportation to develop methods for improving the accuracy of bridge-capacity evaluation through use of nondestructive evaluation techniques. Among the methods currently used are diagnostic load testing and in-service monitoring. These methods are described, and a detailed case study that illustrates the applied methodologies is discussed.

Bridge rating using in-service data in the presence of strength deterioration and correlation in load processes

This paper presents a probability-based methodology for load rating bridges that can accommodate detailed site-specific in-service structural deterioration and response data in a load and resistance factor rating (LRFR) format. The use of site-specific structural response allows the elimination of a substantial portion of modelling uncertainty in live load characterization. Inclusion of structural ageing allows the bridge owner the choice to rate for longer intervals than, say, the usual two-year inspection cycle. This methodology allows the live load-effect sequence on bridges to be statistically stationary with a weakened mixing-type dependence that asymptotically decreases to zero with increasing separation in time, instead of making the common assumption of independent and identically distributed sequences of live loads. In addition, uncertainties in field measurement, modelling uncertainties and Bayesian updating of the empirical distribution function are considered to obtain an extreme value distribution of the time-dependent maximum live load. Gross section loss due to corrosion occurring with a random rate governed by an exponentiated Ornstein-Uhlenbeck type stochastic noise is considered. An illustrative example utilizes in-service peak strain data from ambient traffic collected on a high-volume steel girder bridge. In-service load and ageing resistance factor rating (ISLARFR) equations corresponding to plastic collapse of critical girder cross-section over a range of service lives are developed.

Ultimate Capacity Destructive Testing and Finite-Element Analysis of Steel I-Girder Bridges

Current bridge design and rating techniques are based at the component level and thus cannot predict the ultimate capacity of bridges, which is a function of system-level interactions. While advances in computer technology have made it possible to conduct accurate system-level analyses, which can be used to design more efficient bridges and produce more accurate ratings of existing structures, the knowledge base surrounding system-level bridge behavior is still too small for these methods to be widely considered reliable. Thus, to advance system-level design and rating, a 1/5-scale slab-on-steel girder bridge was tested to ultimate capacity and then analytically modeled. The test demonstrated the significant reserve capacity of the steel girders, and the response of the specimen was governed by the degradation of the reinforced-concrete deck. To accurately capture the response of the specimen in an analytical model, the degradation of the deck and other key features of the specimen were modeled by using a...

Bridge 1-351 over muddy run : Design, testing, and erection of an all-composite bridge

Bridge 1.351 on Business Route 896 in Glasgow, Delaware, was replaced with one of the first state-owned all-composite bridges in the nation. Composites are lightweight construction materials that do not corrode, which results in benefits such as ease of construction and reduced maintenance costs. A summary of the design, large-scale testing, fabrication, erection, and monitoring of this bridge is presented. The bridge was designed to AASHTO load and resistance factor design specifications. A methodology was developed to incorporate the engineering properties of these unique composite materials into the design. The bridge consists of two 13 × 32 ft (3.96 × 9.75 m) sections joined by a unique longitudinal joint. The sections have sandwich construction consisting of a core [28 in. (71.12 cm) deep] and facesheets [0.4 to 0.6 in. (10.16 to 15.24 mm) thick] that provide shear and flexural rigidity, respectively. The composite bridge was fabricated with E-glass preforms and vinyl-ester resin, which offers excellent structural performance and long-term durability. Each of the sections was fabricated to near-net shape in a single step by a vacuum-assisted resin transfer molding process. The overall structural behavior has been accurately predicted with simple design equations based on sandwich theory for anisotropic materials. Large-scale testing of full-sized subcomponents was conducted to prove that the design satisfied deflection, fatigue, and strength limit states. A redundant longitudinal joint was designed that consisted of both an adhesively bonded vertical joint between sections and splice plates. Assembly procedures were developed, and transverse testing of the full-sized joint was conducted. Final bridge sections were proof-tested to the strength limit state. The construction phase included section positioning, joint assembly, and application of a latex-modified concrete wear surface. The bridge was reopened to traffic on November 20, 1998. Results from the long-term monitoring effort will be documented.

Using diagnostic load tests for accurate load rating of typical bridges

Historically, bridge load ratings based solely on theoretical calculations tend to be overly conservative, due to the many assumptions made in the modelling of a bridge. A controlled load test can be used to determine a more accurate load rating of the bridge. An overview of typical controlled load tests is presented in this paper. Specific attention is given to determining the bridges actual live load distribution, assessing support fixity, unintended composite action, and contributions from non-structural components. The manner in which these factors are computed and applied directly to standard beam models currently used to load rate bridges is also discussed.

Behavior of open steel grid decks for bridges

Abstract The life cycle of grid decks has come full circle from their introduction in the 1920s and 1930s, through their maturity in 1950s and 1960s, to their reintroduction in the 1980s. Many of these decks have been performing satisfactorily for 50 or more years of service. Open grid decks offer a lightweight deck alternative to reinforced concrete decks. Despite the good performance history of grid decks, some bridge owners are hesitant to utilize them, even in situations where weight savings is at a high premium. With a better understanding of grid deck behavior, the manufacturing process can be optimized, and design methods improved. Hence, poor details that may lead to fatigue problems can be avoided and design efficiency can be achieved. This paper presents results of research conducted with the goal of providing a better understanding of open steel grid deck behavior through experimental testing and numerical and analytical analyses. Four full-scale open grid decks were tested to experimentally quantify their structural behavior. Three-dimensional finite element models were developed for the grid decks and calibrated using the experimental results. Classic orthotropic thin plate theory and the theory for beams on elastic foundation were applied to the open decks and compared with the finite element (FE) results. Finally, parametric studies were conducted and used to quantify the effect of variations in the significant design parameters. The results of the parametric studies can be applied to optimize future grid deck designs.

Detection and characterization of corrosion of bridge cables by time domain reflectometry

In this paper, we develop and demonstrate a nondestructive evaluation technique for corrosion detection of embedded or encased steel cables. This technique utilizes time domain reflectometry (TDR), which has been traditionally used to detect electrical discontinuities in transmission lines. By applying a sensor wire along with the bridge cable, we can model the cable as an asymmetric, twin-conductor transmission line. Physical defects of the bridge cable will change the electromagnetic properties of the line and can be detected by TDR. Furthermore, different types of defects can be modeled analytically, and identified using TDR. TDR measurement results from several fabricated bridge cable sections with built-in defects are reported.

Steel Girder Fracture on Delaware’s I-95 Bridge over the Brandywine River

A significant crack was recently discovered on an I-95 bridge over the Brandywine River in Delaware. The steel girder bridge carries six lanes of traffic just north of downtown Wilmington. The crack was located on the fascia girder at midspan of the bridge’s main span. The entire bottom flange was found to be fractured, with the crack extending upwards to within 0.3 meters of the upper flange. This paper will review the circumstances leading up to the crack, discuss the cause of the crack, review the repair strategy, and summarize the results of load tests performed prior to and during the repair.