Integrated numerical and field approaches to improve bridge designs

"Bridge design codes provide a set of technical requirements for safe and acceptable design. However, several design requirements need evaluation and field verification to asses their accuracy. Those associated with the full-depth shear key joints in multi-beam decks, load distribution in highly skew continuous bridges, and permit load bridge rating are reviewed in the context. Multi-beam decks can be made by precast units that are post-tensioned together with shear keys and transverse post-tensioning. The performance of the longitudinal joints with regards to load transfer and water tightness is one of the design features of interest. To transfer load between the adjacent units, the joints are filled with grout material. The transverse post-tensioning stress is employed to prevent excessive opening of these joints. Live load tests on multi-beam bridge were performed to obtain actual load distribution data. The study presents the test results and an associated finite element analyses. A simplified method based upon grillage analogy is developed to quantify required transverse post-tensioning stresses in the longitudinal joints. The efficiency of the joints was evaluated by comparing with a hypothetical monolithic slab structure and by the distribution of loads in the transverse direction. The post-tensioning stresses as calculated according to the method were compared with the requirements of the AASHTO LRFD provisions. Estimation of the span and support moment is required to develop a realistic design for skew continuous bridges. Distribution factors recommended by the code provide convenience and simplistic methods to calculate the moments on bridge girders. Refined formulas for estimating the load distribution factors are provided in the current AASHTO LRFD Bridge Design Specifications. A considerable amount of literature is available on field verification studies of right angle steel girder bridges; however studies on highly skew bridges are relatively limited. Previous studies on highly skew bridges have verified the distribution characteristics either analytically or experimentally using prototype bridges. The current study presents a live load field test carried out on a 60-degree skew three-span continuous steel girder bridge. The results are used to derive quantitative information regarding the actual transverse load distribution characteristics of the bridge to asses the accuracy of the current methods. To account for the application of overloads, the LRFR permit load rating was modified by introducing the fatigue damage of overloads. The study used a modification factor referred to as the fatigue index to point to the damage causing effect of overloads. A method is developed to calculate the modification factor. The method considers the fatigue damage that may be caused by overloads as well as the stresses induced by normal traffic. The approach assumes that fatigue index for a specific overload is proportional to the total number of stress cycles required to increase the crack size to the crack threshold at stress induced by normal traffic. The number represents the overload stress cycles that would produce a crack size at which normal traffic stress cycle is able to contribute to the crack growth. The fatigue index as determined by the method reflects the severity of an overload and its potential for causing fatigue damage."--p. iii-iv.

Language

  • English

Media Info

  • Features: Bibliography;
  • Pagination: xvi, 137 leaves

Subject/Index Terms

Filing Info

  • Accession Number: 01094533
  • Record Type: Publication
  • Source Agency: Northwestern University, Evanston
  • Report/Paper Numbers: 31-84578 UMI
  • Files: TLIB
  • Created Date: Apr 24 2008 8:22PM