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6 Based on the published literature (Branco and Martins, 1984; from the stresses developed from horizontal alignment of the Danesi and Edwards, 1982 & 1983; Evans and Al-Rifaie, 1975; bridge, depending on the direction of the tendon flare. Goodall, 1971; Grant, 1993; Lim, Kilford, and Moffatt, Several members of the project team were involved in in- 1971; Maisal and Roll, 1974; Perry, Waldron, and Pinkney, vestigating the 405/55 HOV Connector Overcrossing (OC) 1985; Pinkney, Perry, and Waldron, 1985; Rahai, 1996; failure. Although the curvature of this structure was less than Rasmussn and Baker, 1998; Trikha and Edwards, 1972) the many, several other geometric characteristics of this structure British have been quite active in researching the behavior of led to the failure. First, because prestress radial forces per duct curved concrete box-girder bridges. The British use their own were under the limit beyond which Caltrans Memo 11-31 re- code (BS5400) for bridge design. quired tie reinforcement, none was placed. Also, because the The new "Eurocode" is intended to supersede the codes of structure had fairly long spans, the structure depth was rela- the major European countries. The Eurocode has been de- tively large as were the prestressing forces in each web. The veloped over a number of years and is in use. However, this resulting large number of ducts required (five per stem) for code has appendices that direct the designer to special provi- the increased prestress force were placed one on top of the sions by individual countries (e.g., the DIN code for Germany other without any space between. The combination of pro- and BS5400 for England) and, for the most part, practice still portionately larger radial prestress forces applied to a deeper follows the traditional codes of the countries involved. web exacerbated regional concrete stresses. When these In Asia, the British BS5400 (India, Malaysia and Hong Kong) stresses were combined with the local stresses generated in the and AASHTO (Thailand, Taiwan, Korea and Philippines) concrete cover over the stacked ducts, concrete cracking and codes are widely used. Japan, which has its own code, fre- spalling occurred. This particular design pushed the limits quently builds curved concrete box-girder bridges. for Caltrans design requirements to prevent a breakout fail- The structural code in Brazil is quite brief and all encom- ure, and it is generally agreed that had the Caltrans lateral tie passing. It is much more concise than the current U.S. design detail been used, the failure could have been prevented. codes. Curved beams are not covered directly, although there Abutment bearing failure progressing with time was experi- is a section on torsion, but only with general instructions enced on the I-5 NB to Hwy 217 NB ramp in Oregon. The sin- found in most textbooks. Bridges that have alignments with gle cell box-girder is supported on two large bearings at the slight curvature are generally designed as straight bridges south abutment. Over time, the entire load at these bearings without consideration for the curve, except that bridge bear- has shifted to only one of the bearings while the other has ex- ings are designed for eccentric loads taking into account the perienced uplift. These problems are thought to result from the curve of the superstructures. time-dependent behavior of concrete. This theory is corrobo- As of today, Brazil, Argentina, Chile, and Mexico, and rated somewhat by the results observed in the time-dependent some Latin American countries use computers and similar analyses of similar structures, although it is thought that cur- programs to those of the United States. rently commercially available software will tend to overpredict The AASHTO design specifications (not necessarily LRFD) the problem because torsion creep is not considered. are widely used by many other countries around the world. Another recent bearing failure occurred on the bridge at Wildcat Road in Shasta County, California. This single-span curved prestressed concrete box-girder bridge was under Field Problems construction. When the falsework was being removed, the Several failures of stem concrete due to the radial forces de- bearings at the abutments began to fail. The outside elas- veloped by curved prestress tendons have occurred over the tomeric bearing was overloaded and was destroyed and the years. These include the Las Lomas Bridge in 1978, the bearing at the inside of the curve began to lift off. This prob- Kapiolani Interchange in 1981, and the 405/55 HOV Con- lem was corrected at the abutments by retrofitting the bridge nector OC in 2002. Repair costs for some of these structures with prestress bar tie-downs and eliminating the bearings. were significant (the Kapiolani Interchange was $4,000,000). This essentially converted the seat abutments to end di- Prestress breakout failures have been linked to a combina- aphragm abutments. Fortunately, the relatively short bridge tion of the regional action of the web acting as a beam be- length and the fact that most of the prestress shortening had tween the top and bottom deck and the local slab action of the already taken place made this possible. concrete cover over the prestressing tendons. Global actions, Stirrups in the outside web were also inadequate to resist although theoretically a factor, have been found not to be the combined effects of torsion and flexural shear in this important in these failures. structure. The web was retrofitted with external prestressing Many such failures have occurred even in straight bridges tendons that will correct the problem. This repair was deemed where local curvatures of prestressing ducts occur near the pre- to be preferable to adding extra mild reinforcement within a stress anchorages. These stresses can either add to or subtract web overlay.
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7 Figure 2-1. Uplift at edge of bearing. Figure 2-2. Construction of widened column. A recent problem with two box-girder bridges in Coahuila, movement was severe enough to remove the superelevation Mexico, that is apparently due to the curvature of the structures placed in the bridge at the time of construction. Significant has developed. These bridges, which are relatively new, are cracking of the superstructure was also observed. The bridge cast-in-place, post-tensioned, continuous concrete box-girder owner is attempting to correct the problem by increasing bridges supported on single bearings at each non-integral sin- the size of the piers in the transverse direction as shown in Fig- gle column. These relatively narrow multi-span ramp structures ure 2-2 and jacking and shimming the superstructure back to are experiencing ongoing deflections and lateral movement. It its original as-built position. The wider piers will allow bearings is not clear what is causing this behavior, but it is fairly certain to be placed eccentric to the centerline of the superstructure and that curvature is a factor. The bearings have experienced uplift hopefully stabilize the situation. A lightweight overlay is also from rotation of the superstructure as shown in Figure 2-1. The being considered to completely restore the superelevation.