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28 tendons in the multicell boxes), summarizes occurrences of for both of these bridges local slab action of the concrete similar problems at Las Lomas (California) and Kapiolani cover over the tendons was the primary cause of the failure. (Hawaii), and then documents a step-by-step analysis of the The article discusses the theory of transverse stresses in 405/55 bridge. The steps followed include global, regional, curved box-girder cross sections due to post-tensioning in- and local analysis, as suggested by Podolny. The regional and cluding "distributed radial force arch action." Horizontal local analysis use detailed finite element modeling with crack- curved post-tensioned bridges are subjected to the following ing concrete constitutive models, but various hand calcula- three separate actions: tion methods for evaluating regional and local effects are also described and demonstrated as a check on the detailed 1. The global or overall girder action of the bridge together analysis. The report also provides an in depth summary of with its supporting piers and abutments. Caltrans's Memo to Designers (MTD) 11-31 and points to 2. Regional beam action of each web supported at the top some potential shortcomings (under the right set of unusual and bottom flanges as a beam. circumstances) of the Memo. The report then lists eight con- 3. Local slab action of the concrete cover over the tendons. tributory causes for the damage that occurred, but concludes that the one cause which, if corrected would have prevented The article discusses the current design philosophy of the the damage, was the omission of duct ties from the design. California DOT, the Texas DOT, the AASHTO Guide Speci- Although this report is very structure-specific and contains fications for the Design and Construction of Segmental Concrete detailed project information that should not be directly ref- Bridges, and the AASHTO Load and Resistance Factor Design erenced in a set of design specifications, it provides a useful Bridge Design Specifications for tendon confinement. reference for summarizing the various analyses (both hand A test program was carried out by the authors on webs calculation and finite element) appropriate to this class of without tendon confinement reinforcement. Both closely and problems, especially at the "regional" and "local" level for widely spaced ducts (duct spacing less than or greater than girder cross-section and web evaluation. one duct diameter, respectively) were tested, and different lateral shear failure modes were observed. Recommendations Strasky, J., 2001, Influence of Prestressing in Curved Mem- for lateral shear capacity are proposed that are more conser- bers, Betonve Mosty, Report TK21, Prauge, Czech Republic. vative than the AASHTO LRFD Specifications and yielded a consistently narrow range of factors of safety for the four test The influence of prestressing in curved members is discussed. specimens (1.99 to 2.34). A recommendation for a design To mitigate the effect of radial prestressing forces in the webs of methodology for the flexure of the concrete cover is not pro- box-girders, it is recommended that prestress tendons be sepa- posed because of the lack of understanding of its behavior. rated vertically where they pass near the middle of the web. This The test specimens did not fail in regional transverse bending will spread the radial forces and result in lower stresses tending of the web due to the formation of a second load path after to rip the strand out of the side of the web. Care is also required formation of flexural cracks in the web. The load path is en- for tendons in the soffit of segmentally constructed bridges visioned primarily as a vertical one with the lateral prestress (straight or curved) where tendons are anchored in blisters load carried through flexural bending of the web until crack- along the length of the bridge and deviated across the width of ing. Once cracking occurred, the stiffness was reduced and the soffit, resulting in transverse tensile stresses that can fail the the load was carried primarily through longitudinal arching soffit. Such a failure occurred in a bridge constructed with a until a local lateral shearing failure occurred. A University of gantry in Austria. The vertical curvature of these tendons in a Texas thesis (Van Landuyt, 1991) that discusses this research haunched bridge can also present a problem. in greater detail was also reviewed. Van Landuyt, D., and Breen, J. E. (1997) Tendon Breakout Failures in Bridges, Concrete International, American Con- Time Dependency crete Institute, Farmington Hills, MI, November 1997. The redistribution of stresses due to creep and shrinkage may be important in curved concrete bridges. This issue is The article discusses the pullout failure of horizontal discussed in at least one of the references previously described curved tendons that occurred on several cast-in-place post- (Menn, 1990). A paper further exploring this issue is de- tensioned concrete box-girder bridges. In both of these struc- scribed below. tures, there was a combination of relatively sharp horizontal curvature, thin concrete cover over the tendons, and the Zhang, L., Liu, M., and Huang, L. (1993) Time-Dependent bundling of large-sized tendons close together. It appears that Analysis of Nonprismatic Curved PC Box-Girder Bridges,