Carin L Roberts-Wollmann

Stresses in External Tendons at Ultimate

Post-tensioned segmental concrete bridges are often used for long, multispan viaducts or medium-span valley and river crossings. This article presents the research that led to the development of the equation for predicting stresses in unbonded tendons at ultimate; this equation is currently used in the AASHTO LRFD Specifications and AASHTO Guide Specifications for the Design and Construction of Segmental Concrete Bridges. The research, performed by the late Robert J. G. MacGregor, involved the construction and testing of a 1/4 scale model of a three-span, continuous, precast segmental-concrete box girder bridge, erected using span-by-span techniques and post-tensioned with external tendons. The authors discuss the results of the tests to ultimate that were dominated by flexural behavior. Predicted tendon stress increases are then compared to a large data base from other tests of beams and slabs with unbonded tendons. The authors conclude that the equation proposed by MacGregor predicts the tendon stress increases in unbonded tendons conservatively and relatively well, compared to expressions that ignore tendon length.

High-Performance/High-Strength Lightweight Concrete for Bridge Girders and Decks

This report presents proposed changes to the AASHTO LRFD bridge design and construction specifications to address the use of lightweight concrete in bridge girders and decks. These modified specifications will provide highway agencies with the information necessary to develop comparable designs of lightweight and normal weight concrete bridge elements for use in evaluating alternatives and selecting the alternative that will yield economic benefits. The material contained in the report should be of immediate interest to state bridge engineers and others involved in the design and construction of concrete bridges.

Experiments on a Hybrid-Composite Beam for Bridge Applications

This paper details a study of the structural behavior of hybrid-composite beams (HCBs), which consist of a fiber-reinforced polymer (FRP) shell with a tied concrete arch. The HCB offers advantages in life-cycle costs through reduced transportation weight and increased corrosion resistance. Through a better understanding of system behavior, the proportion of load in each component can be determined, and each component can be designed for the appropriate forces. A long-term outcome of this research will be a general, structural analytical framework, which can be used by transportation departments to design HCBs as rapidly constructible bridge components. This study focused on the identification of the load paths and load sharing between the arch and FRP shell in an HCB and on the test of an HCB with a composite bridge deck. Tests were performed through the application of point loads on simple span beams (before the bridge deck was placed) and with a three-beam, skewed composite bridge system, which resulted in strain data for the arch and FRP shell. The test results showed that strain behavior was linear elastic at service loads, and the FRP shell had a linear strain profile. Curvature from strain data was used to find internal bending forces, and the proportion of load within the arch was found. A stress integration method was used to confirm the internal force contributions. The arch carried about 80% of the total load for the noncomposite case without a bridge deck. When composite with a bridge deck, the arch made a minimal contribution to the HCB stiffness and strength, because most of the arch was below the neutral axis and cracked under the maximum live load expected for the bridge. For this composite case, the FRP shell and prestressing strands resisted about 80% of the applied load, while the bridge deck carried the remaining 20% to the end diaphragms and bearings.

Shear strength of a lightweight self-consolidating concrete bridge girder

Lightweight self-consolidating concrete (LWSCC) is advantageous in the bridge industry because members made with this material have a significantly lower self-weight, and in its fresh state, LWSCC has a low viscosity which eliminates the need for vibration during fabrication. A composite section was fabricated with a single precast bulb-tee LWSCC beam and a lightweight concrete cast-in-place deck. A simply supported test configuration was constructed with two point loads to quantify the web-shear strength of the girder. The experimental shear strength is compared to four analytical models from different AASHTO specifications. Based on the results of this limited study, the theoretical predictions for the web-shear strength of this girder were all conservative when compared to the experimentally measured failure strength. With these results in mind, further research is recommended on the use of LWSCC girders in the bridge industry to better understand the material properties, structural properties, and cos...

Field Verification of Simplified Analysis Procedures for Segmental Concrete Bridges

Load tests on segmental bridges are uncommon in the literature given their relatively short history and comparatively smaller presence in the national bridge inventory. This paper presents results from two segmental concrete bridge field tests and compares them with common simplified longitudinal and transverse analysis procedures. These single-cell structures, built with balanced cantilever construction, represent two significantly different segmental concrete bridges. Designers frequently use a beamline model for longitudinal analysis. When compared with the load test results, this simple method produces conservative predictions of longitudinal behavior within 20%, which is also reflected in the literature. Conversely, little information exists in the literature on transverse bending analysis. When analyzing the localized transverse bending from concentrated wheel loads, designers commonly use an equivalent frame model. Most frequently, designers use influence surfaces to estimate the scaled loads to apply to these two-dimensional frame models. This simplified approach is shown to be conservative overall but cannot always predict bending sense and frequently overpredicts demand in excess of 100%.

Inspecting the lightweight precast concrete panels in the Woodrow Wilson bridge deck of 1982

This paper presents the results of a detailed inspection of the deck panels of the Woodrow Wilson Bridge installed in 1982. The original cast-in-place concrete deck, constructed in 1962, was replaced with full-depth lightweight precast concrete deck panels that enabled rapid construction with minimal traffic disruption. The inspection of the Woodrow Wilson deck provides valuable information about the performance of the precast concrete panels, joints, and connections after 20 years of very harsh traffic loads and environmental stressors. The deck panels performed well overall, with the only serious problems at expansion and contraction joints. All of these joints exhibited cracking and rusting. The most prevalent type of cracking appeared to be due to restrained shrinkage between the new polymer concrete, the older precast panels, and the rigid steel joints. This location is more vulnerable to cracking and leaking because there is no prestress across the joint. The multilayered corrosion protection methods used for the transverse and longitudinal post-tensioning tendons were very successful.

Effect of Vertical Casting Position on Transfer and Development Length

This investigation includes the effect of vertical casting position on transfer and development lengths of prestressing strand. Current code provisions account for the top-bar effect in the calculation of development length for standard reinforcing bars, but fail to recognize the phenomenon when calculating transfer and development lengths of prestressing strand. Historically, this phenomenon has been dependent on the amount of concrete cast below a bar or strand. Recent research shows the amount of concrete cast above a strand to be more influential. This paper presents the results of an experimental investigation on the influence of vertical casting position of prestressing strand on transfer and development lengths. The study includes data from 20 T-beam test specimens and four sets of top-strand blocks. The results from 119 transfer zones and 39 flexural tests were evaluated and compared to vertical casting position along with current code provisions.

Pedestrian Bridge Collapse and Failure Analysis in Giles County, Virginia

A pedestrian suspension bridge over Walker Creek near Route 749 in Giles County, Virginia, partially collapsed in October of 2008, causing 10 people to fall into the creek below. The failure occurred when one of the anchors holding a suspension cable fractured. This paper presents the investigation of the failure, including a structural analysis of the bridge to determine cable forces, material property testing of the fractured anchor, laboratory testing of a different anchor from the bridge, and a simple strength evaluation of the anchor under combined axial force and moment. Based on the results of the study, it was concluded that the anchor should not have failed carrying the loads on the bridge on the day of failure. The anchor failed in brittle fracture because of a local defect in the material at the perimeter of a corrosion pit on the surface of the anchor hook. The structural analysis and laboratory testing indicated that the material around the defect was at yield before fracture initiation. The material flaws significantly influenced the bridge failure after yielding and inelastic deformation of the anchor material at the inside of the hook radius occurred.