Click for next page ( 10

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 9
7 was concluded that the construction sequence greatly affects were PCI BT-54 or BT-63 girders. A limited number of other the development of positive moments. It was recommended AASHTO girders were also used. The girders were composite that if the diaphragm is cast first (without the slab), the slab with a 6.5-in. composite deck, and the decks were formed using should be cast within 230 days to prevent cracking. It was steel stay-in-place forms or removable plywood formwork. also recommended that if the diaphragm is cast first, negative The ends of the prestressed girders were embedded 3 in. moment connections be supplied between the beams to pre- into continuity diaphragms. Mild reinforcement was extended vent cracking and spalling at the joint between the diaphragm from the ends of the girders into the continuity diaphragms to and deck. The study further recommends that when the dia- provide a positive moment connection. The positive moment phragm and deck are cast together, an unbonded joint be used connection reinforcement was hooked in the continuity dia- between the diaphragm and beam to allow the beam to rotate phragm and was embedded into the end of the girder. The under the deck weight. total thickness of the continuity diaphragm was 16 in.; this is Tadros et al. (18) and Ma et al. (19) also explored the use significantly greater than the typical continuity diaphragm of negative moment connections between the tops of the pre- used in Alabama at that time, which was 8 or 10 in. thick. The cast beams. Providing this connection has two advantages. bridge was opened to traffic in 1991. This connection allows the beams to take some of the nega- tive moment rather than relying on the deck to take all of the Details of the Observed Cracks negative moment. The other advantage occurs when the diaphragm is cast before the slab. When the slab weight is Limited cracking in girders as described below was observed added, the end of the beam will attempt to rotate and this during the first inspections in Spring 1992. Cracks had opened will put tension into the negative moment connection. This significantly by the next inspection in March 1994, which tension is balanced by compressing the bottom of the prompted the thorough inspection of all girders on the bridge. diaphragm, and this precompression will mitigate the tensile Three types of cracks were observed: force caused by time-dependent positive moments. Two negative moment connections were tested. The first 1. Spalling in the face of the continuity diaphragm was used threaded rods embedded in the top of an NU 1100 girder. caused as the embedded girders pulled out of the The rods were passed through a connection plate and held with diaphragm. nuts. The second connection used strand that projected from 2. Vertical or inclined cracks in the girders near continu- the top flange of the beams. The strands of adjacent beams ity diaphragms at interior supports were observed. In were connected by a strand connector. Both connection types some cases, the girder cracking initiated at or near the were tested and found to perform well. ends of the embedment of the positive moment rein- forcement, which were all terminated at one location. The positive moment reinforcement terminated within GIRDER CRACKING IN ALABAMA the debonded length of some of the strands. 3. Vertical cracks in the end faces of the continuity dia- A number of bridges constructed in Alabama with pre- phragms were also observed. stressed concrete girders made continuous have experienced significant cracking at interior supports. This experience has All three types of cracking can be attributed to the effects of been widely discussed and was even reported in the national positive moment at the interior support. In no cases were all media (20). As a result, the authors were asked to investigate girders in the bridge cross section cracked. Girders exhibiting and discuss this experience. cracking tended to be grouped in the cross section. Cracked The focus of the Alabama experience of bridges with pre- girders were reported in 2-, 3-, and 4-span units. Cracking stressed concrete girders made continuous is the I-565 viaduct beyond the end region was not reported for any girders where in Huntsville, where cracking was observed in the continuity cracking of any type was reported near an interior support. diaphragms and in girders near the continuity diaphragms. A Cracks large enough to be repaired were injected with summary of information on the viaduct and the cracking epoxy in September 1994. In some cases, new cracks reap- observed in the prestressed girders and continuity diaphragms peared after repair although it was reported that repair spec- follows. The data were gathered from several sources, includ- ifications were not closely followed. In 1995, inspections of ing a report on the cracked girder investigations prepared by the approximately 2,000 girders in the project indicated that the Alabama DOT for these bridges (21). 74 girders were cracked and the continuity diaphragm was cracked from girder pull-out at the ends of 200 to 250 girders. Details of the Viaduct Girder Characteristics and Testing The viaduct is composed of dual 12,200-ft-long bridges with a combination of steel and prestressed concrete spans. The Detailed monitoring and load tests were conducted on two majority of the prestressed concrete girders in the bridge continuous units where a number of girders were cracked.