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5 cross section. Specific guidelines on how to determine the developed in this early code have influenced the development number and spacing of interior diaphragms are not provided. of the current Canadian Standards Association (CSA) design Colorado has standards for curved U girders with a cast- specifications (2000) and the updated version of the OHBDC in-place deck that they have used with good success. These (1998). Although curved box-girders are not specifically bridges often consist of several precast segments spliced addressed in these codes, many of the analysis techniques together to form a span. Colorado projects that they will use identified for box-girders, such as the orthotropic plate anal- this form more often in the future. It has the advantage of ogy and the grillage analogy methods may be applicable to potentially eliminating falsework over traffic curved structures. A similar U section, although straight, was developed by In Europe, most countries and most designers base their NRV for use by a contractor on a bridge originally designed designs on the first principles of structural analysis and de- as a cast-in-place multi-cell box. The original design required sign. Although design codes are used, they are generally brief building the superstructure on falsework set above final and bridge designers rely on traditional text books such as grade, lowering it to its final grade position, and then casting those by Menn (1990), Schlaich and Scheef (1982), or Strasky a monolithic bent cap to connect the bridge to the column. (2001); specific course work published by their professors at This approach overcame falsework clearance problems but their university (Leonhardt, Menn, Strasky, etc.); or personal presented some challenges for the contractor. In the con- experience. tractor redesign, precast prestressed U sections set at final In Switzerland, the structural code is split into separate grade spanned the required falsework opening. Cast-in-place booklets. One each for loadings, concrete and prestressed bottom soffit and stem pours were made on either end on concrete, steel, wood, and so forth. The two most applicable to the U girders. The cast-in-place pours were monolithic with NCHRP Project 12-71 (Loadings, and Concrete and Pre- the columns. A cast-in-place deck was then poured and stressed Concrete) are relatively brief documents compared continuity prestressing used to tie the entire structure together. with the current AASHTO LRFD. In general the Concrete This approach has some advantages and, as in Colorado, and Prestressed Concrete booklet does not or only deals could be used on curved structures. briefly with special structural configurations such as hori- The Oyster Point Off-ramp in California also had a span zontal curvature. Instead, students at the two Universities, in consisting of curved precast girders made continuous with a Zurich and Lausanne, study structural design in a practical cast-in-place multi-cell box section. This span crossed railroad manner, preparing them for the professional situation in tracks where falsework was not allowed. As a cost-savings their own country. The textbook discussed above (Menn, measure, the contractor chose to use curved precast beams 1990) is very similar to what students will encounter when at rather than straight beams with a curved cast-in-place deck. the university. Menn, who was a professor for many years, Girder erection required pick points to be located so that the provides some general guidance on the design of horizontally girders would not "roll" and a temporary tie down system at curved beams and skewed bridges. the ends of the girders during the deck pour. Swiss bridges on a curved alignment with a large radius are Despite these examples, curved box-beams appear to be often designed without considering the curvature, except for relatively rare. the bearing design. Many design firms use methods they have developed over the years involving graphs and influ- ence surfaces. Foreign Practice U.S. engineers with experience designing bridges in France Concrete box-girder bridges are used around the world. have been contacted. It is our current understanding that Many of these are on horizontally curved alignments. A par- the French favor precast segmental construction. Typically, tial survey of recently constructed curved concrete box-girder these structures use external tendons with deviator blocks. bridges outside the United States was conducted by reviewing As in most European countries, their design specifications material published in engineering magazines and trade jour- are less prescriptive than those in the United States and de- nals and from personal experience. Many of these bridges were signers rely on their own experience as well as other published built using segmental construction techniques. Although material to analyze and design these bridges. the survey was not exhaustive, its results are indicative of the Germany has been a leader in the design of concrete box- bridges being built today. girder bridges, and engineers like Fritz Leonhardt have been Canada has been very active in developing design specifi- considered pioneers for this type of construction. Germans cations that address the behavior of concrete box-girder tend to favor cast-in-place construction. They have their own bridges. The Ontario Highway Bridge Design Code (OHBDC) DIN code, but, as is typical of most European practice, they (1983) was an early attempt to codify design and analysis re- rely on the engineer to apply first principles in selecting quirements for these types of bridges. Many of the provisions analysis techniques and design details.