High-cycle fatigue tests of pretensioned concrete beams

Pretensioned concrete beams are widely used as bridge girders for simply supported bridges. Understanding the fatigue behavior of such beams is very important for design and construction to prevent fatigue failure. The fatigue behavior of pretensioned concrete beams is mainly influenced by the fatigue of the prestressing strands. The evaluation of previous test results from the literature indicated a reduced fatigue life in the long-life region compared with current design methods and specifications. Therefore, nine additional high-cycle fatigue tests were conducted on pretensioned concrete beams with strand stress ranges of about 100 MPa (14.5 ksi). The test results confirmed that current design methods and specifications overestimate the fatigue life of embedded strands in pretensioned concrete beams.

Impact of pile-to-cap fixity on the design and behavior of sensitive structures

This paper presents the results of analytical studies on the connection between piles and pile caps or footings. Two nonlinear finite element analysis software packages were used to investigate the behavior of the connection itself and the impact of connection assumptions on the overall behavior of different sensitive structures such as simple spans with uneven span lengths, segmental box girders with fixed pier tables, and straddle bents with temperature loading. Results show that the behavior of the connection is affected by variables such as pile size, pile embedment length, pile cap concrete strength, interface reinforcement, and distance between the edge of the pile and the edge of the pile cap. The study also demonstrated that significant moment can develop even with shallow pile embedment lengths. The assumed level of fixity between the pile and pile cap was found to significantly influence the behavior of some of the bridges investigated in this study.

Structural grouting of load-bearing precast concrete elements: Issues and solutions

The purpose of this paper is to raise construction industry awareness regarding structural grouting of precast concrete projects in the United States. Incidents related to structural grouting have occurred with both precast concrete systems and other building systems. Designers, architects, owners, insurance carriers, and the construction industry in general are becoming increasingly aware of deficient grouting installation methods and verification procedures. Concerns include the timeliness of grout installation during the erection process, the lack of grout on some components of finished projects, and the poor quality of installed grout. These concerns pertain to horizontal connection joints of critical load-bearing elements between foundations and precast concrete columns and walls, stacked precast concrete columns, and stacked precast concrete walls. Current building codes and standards provide no requirements and limited guidance for the installation or special inspection of these critical horizontal joints. In an effort to address this gap in building codes and standards, as it specifically relates to precast concrete structural products, some precast concrete producers and erectors are implementing several strategies and new procedures, as noted and further developed in this paper.

Experimental investigation of 0.6 in. diameter strand lifting loops

Very little experimental data have been published relating to the pullout capacity of prestressing strand lifting loops. To address this gap in knowledge, 13 pullout tests were conducted on strand lifting loops with 0.6 in. (15.24 mm) diameter, 270 ksi (1860 MPa) strand. Straight and bent orientations were tested for single loops at different embedment depths. Loops were embedded in 12 in. (304.8 mm) wide and 44 in. (1117.6 mm) deep concrete blocks and subjected to monotonic, static loading until failure. Marginal bond quality of the strand (18.2 kip [81 kN]), Mohs hardness (3.6), and concrete strength (3000 psi [20.7 MPa]) resulted in an average bond stress value of 400 psi (2758 kPa) at failure. Most tests exhibited pullout failure modes and adequate ductility. Three loops tested at 32 in. (812.8 mm) embedment with 6 in. (152.4 mm), 90-degree bends experienced brittle side-face blowout failures. These failures were due to inclination of the lifting, which led to a reduced edge distance. A safe uniform bond stress of 199 psi (1372 kPa) is recommended for 0.6 in. diameter strand.

Saint-Venant torsion constant of modern precast concrete bridge girders

Many bridge owners have developed new precast, prestressed concrete bridge girder sections that are optimized for high-performance concrete and pretensioning strands with diameters greater than 0.5 in. (12.7 mm). Girder sections have been developed for increased span capacities, while others fill a need in shorter span ranges. Accurate geometric properties are essential for design. Common properties, including cross-sectional area, location of centroid, and major axis moment of inertia, are generally easy to compute and are readily available in standard design references. Computation of the torsion constant is a different matter. This paper presents the methods and results of a study to determine the torsion constant for many of the modern precast, prestressed concrete bridge girders used in the United States and compares the results with values from the approximate methods of the AASHTO LRFD specifications.