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 6
4 CHAPTER 2 FINDINGS This chapter summarizes the results of the surveys, the the bar or strand in the diaphragm area. Seventeen respon- literature search, the analytical work, and the experimental dents (50%) indicated that they provided transverse rein- program. forcing through the beam into the diaphragm. SUMMARY OF THE SURVEYS Type of Girder Used Task 1 required the authors to assess the current state of the The positive moment connection is used most frequently practice for continuous-for-live-load bridges. As part of this with I girders and bulb-T girders, although a number of respon- task, two surveys were conducted of state DOTs, contractors, dents also used it for box girders and shapes specific to a fabricators, and designers. The first survey assessed the gen- given state. eral use of continuous-for-live-load bridges and the use of various connection details. The second survey addressed con- structability issues. Embedment The purpose of the first survey was to determine typical parameters for the connections between precast girders that are Seven respondents indicated that they did not embed the made continuous for live load. This survey was sent to all girder ends in the diaphragm. The remainder embedded the 50 state DOTs and to selected designers and fabricators. girder ends in the diaphragm anywhere from 2 to 12 in. There Answering the survey were 39 state DOTs, 4 designers, and was no clear preference for embedment depth: 10 respon- 8 fabricators for a total of 51 responses. Of the 51 respondents, dents indicated embeddings less than 2 in., 4 respondents 35 indicated that they used, designed, or fabricated continuous- embedded 2 to 4 in., 3 embedded 4 to 6 in., 6 embedded 6 to for-live-load precast/prestressed bridges. The following 8 in., 6 embedded 8 to 10 in., 11 embedded 10 to 12 in., and assessments apply only to those respondents who indicated 12 embedded the girder ends more than 12 in. (responses total that they used continuous-for-live-load bridges. more than 35 as multiple responses were allowed). In all but one case, negative moment continuity was estab- lished by using a reinforced deck over the diaphragm. One respondent used a mechanical splice between the top flanges Concrete Strength of the girders. All those who designed for continuous live load used a positive moment connection. Girder concrete compressive strength ranged from 3,500 It was clear from the outset of the project that the positive to 7,000 psi at release and 4,000 to 9,000 psi at 28 days for moment connection was of greater concern than was the neg- normal mixes. Normal strength deck/diaphragm concrete was ative moment connection, so most of the questions focused 3,000 to 5,000 psi at 28 days. Some respondents (19) indi- on the positive moment connection. The significant findings cated that high-performance mixes were occasionally used. were as follows. For girders, these mixes ranged from 4,000 to 9,000 psi at release and 6,500 to 10,000 psi at 28 days, while the deck/ diaphragm concrete ranged from 3,000 to 7,000 psi. Type of Connection When asked to identify the type of positive moment con- Support Conditions nection used, 17 respondents used a bent-bar connection, 4 used straight bars, 24 used bent strands, 3 used a welded-bar More than 80% of the respondents indicated that the bear- detail, and 3 used mechanical strand connectors (the responses ings were placed under the girder ends. Approximately 10% add up to more than 35 as some respondents used more than used no bearings, leaving the girder and diaphragm to sit one type of connection). It appears the bent-bar and bent-strand directly on the bearing surface. The remaining 10% either connections are the favored connections, with approximately placed the bearings under the diaphragm or under both the equal usage. Of the respondents, 75% (26 total) overlap (mesh) girder and diaphragm.