Click for next page ( 2


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
INTEGRAL STEEL BOX-BEAM PIER CAPS SUMMARY Conventional I-girder bridge superstructures usually are supported on bearings placed on the bridge substructure. The bearings are designed to support the vertical reaction of the bridge girders and may also be designed to restrain the horizontal movements of the bridge. The bearings are usually detailed to allow the superstructure to rotate at pier locations. The superstructures and substructures of conventional bridges essentially are designed as separate systems. Unlike conventional bridges, an integral connection between the superstructure and substructure provides some degree of continuity between the two systems, thus enhanc- ing the seismic performance of the structure. In addition, the elevation of the bottom of the integral pier cap may be the same as that of the bottom of girders. This reduces the need to elevate the bridge approaches to provide adequate clearance under the bridge while orienting the pier caps in a direction perpendicular to the girders. This orienta- tion is preferred as it eliminates the problems associated with sharp skew angles. This report presents the details and results of the work on the use of integral con- nections for steel I-girder bridge superstructures connected integrally to concrete sub- structures. This work was conducted under NCHRP Project 12-54. A questionnaire on past use and performance of integral pier caps was used to study the state of the practice of integral connections. The questionnaire was sent to all AASHTO members, domestic researchers, and domestic and international bridge designers. In addition to the questionnaire, an extensive literature search was performed to identify and review relevant past research. Analyzing the response to the questionnaire and the results of the literature search allowed the research team to identify the issues related to the design and construction of the integral connections, to identify the connection systems used in the past and the rea- sons for selecting these systems, and to develop several new systems that were poten- tial candidates for this study. In total, 14 connection configurations were examined. Selection criteria were developed to determine the most viable systems. The selec- tion criteria were based on the expected economy, constructability, and expected per- formance of each system. The system selected for final study consists of a steel box- beam pier cap connected integrally with a steel I-girder superstructure and a single- column reinforced concrete pier. The integral connection between the column and the

OCR for page 1
2 pier cap was accomplished by extending the column longitudinal reinforcement through holes in the bottom flange of the pier cap into the pier cap compartment directly above the column. This compartment is bounded by the four sides of the box-beam pier cap and two interior diaphragms of the box-beam. The compartment was filled with con- crete which transfers the load from the pier cap to the column reinforcement. Analytical and experimental studies were conducted to validate the selected con- nection system. A bridge with two equal spans, each 30.5 m (100 ft) long, was selected as the basis for these studies. Throughout this report, this bridge is referred to as "the prototype bridge." The experimental studies were accomplished by testing two, one-third-scale models of the pier region of the prototype bridge. The two test specimens were similar except that the depth of the girders, and consequently the depth of the pier cap, was deeper in the first specimen than in the second one. The deeper girders of the first specimen allowed the full development of the column longitudinal bars within the depth of the pier cap. The shallower depth of the girders in the second specimen was not sufficient for the full development of the column longitudinal reinforcement within the depth of the pier cap and, thus, mechanical connections were used to provide the required additional anchorage for the column reinforcement. The mechanical connections were provided by threading the ends of the column longitudinal reinforcement and installing nuts embedded in the concrete in the integral connection region. The analytical studies were conducted on finite element models of both the proto- type bridge and the test specimens. The analytical studies of the prototype bridge were used to validate the applicability of some of the existing design provisions, which orig- inally were developed for girders supported on conventional bearings, to girders of structures with integral connections. The analytical studies on the test specimen computer model were used to determine the anticipated forces acting on different components of the laboratory specimens and to validate the modeling technique by comparing the analytical results with the labo- ratory results. Observation from testing of the first test specimen under cyclic loading revealed that the specimen behaved as expected under lateral loading (i.e., a plastic hinge was formed in the column adjacent to the cap beam). The superstructure behaved elastically through- out the entire test, also in accordance with the intent of the design. As expected, flexural cracking of the column occurred at loads below the predicted yield of the column lon- gitudinal reinforcement. Defining the displacement ductility, D, as the ratio between the maximum displacement during a load cycle during the test divided by the dis- placement required to cause the yield of the column longitudinal reinforcement, con- crete spalling at the column-to-cap beam connection began to occur at displacement ductility = 1.5. At ductility = 4.0, several column longitudinal bars were visible, with a few showing indications of buckling. At ductility = 6.0, the three extreme col- umn longitudinal bars on each side of the column fractured. The buckling is believed to have been caused by loss of confinement because of interaction effects between the steel cap beam and concrete column. The second test specimen also displayed satisfactory seismic performance, exhibit- ing the formation of a plastic hinge in the column adjacent to the cap beam and show- ing elastic behavior of the superstructure. Early stages of the test revealed similar behavior to the first test specimen. Increased cracking was observed in the slab as could be expected for the more flexible superstructure in the second specimen. The primary difference in results in the second test specimen was the failure mechanism of the lon- gitudinal bars, which appeared to lose anchorage in the connection region and fractured the mechanical connections.

OCR for page 1
3 The test specimens were also tested under low-level loads to simulate service loads. The results from this testing were compared with the results of the analytical model of the pier region to validate the analytical modeling techniques used. The results of the analytical and experimental studies were used to finalize the details of the integral connections and to develop design methodologies, specifications, com- mentary, and a detailed design example based on the proposed specifications.