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3 CHAPTER 2 State-of-Practice Review Domestic Practice more segmental construction are expected. Curved precast girder systems may also increase, particularly in Colorado To obtain a better understanding of current U.S. practice, where this type of construction has been successful. telephone interviews were conducted with representatives 5. Many states have experienced some problems with the from key state DOTs. The states surveyed included California, performance of curved box-girder bridges, but not many Colorado, Florida, Hawaii, Idaho, Oregon, Nevada, New York, as a percentage of the total. Cracking along the tendon and Tennessee, Texas, Washington and Wisconsin. Other states tendon breakout problems are absent or minimal where were contacted but chose not to participate. sufficient space is provided between the ducts. Torsion Based on these interviews, current U.S. practice can be and flexural shear cracking seem to be rare and not neces- summarized as follows: sarily limited to curved bridges. A few bearing failures have occurred, but have been avoided in states that avoid 1. With the exception of California and Washington, most bearings altogether or use conservative bearing designs. states interviewed have a fairly small inventory of curved In some cases, bearing uplift at the abutments has been concrete box-girder bridges (i.e., <1% each of reinforced observed to occur over time and is thought to result from and prestressed) although many see the number increasing the time-dependant behavior of concrete. Unexpected in the future. vertical or horizontal displacements of the superstructure 2. Cast-in-place construction is most popular in the West. are rare, but California has had some problems on skewed Other states are tending toward segmental construction multi-column bents where movement about the c.g. of the (cantilever and span-by-span using both precast and cast- column group has caused the transverse shear keys to in-place concrete) to avoid conflict with traffic. Colorado engage. Lateral displacement of columns has also been has used precast, curved, spliced "U" girders with a cast- observed. in-place deck. The use of precast box-beams in most 6. Some states have special design rules. A few of these are other states is limited to straight girders. Curvature, when discussed below. present, is provided by a curved deck slab (i.e., variable overhangs). Northern and eastern states, where weather Many states either use AASHTO LRFD (2004) or are adopt- conditions cause more rapid deck deterioration, tend to ing it. avoid prestressed boxes because of the need to provide for Most states have no special rules for when a three- future deck replacement. dimensional (3-D) analysis, such as a grillage or finite element 3. Most curved box-girders are relatively narrow continuous analysis, should be performed and leave it to the discretion of ramp structures. A few are single span, and a significant the designer. Many states use an 800-foot radius as the trigger number of all structures (approx. 20% to 30%) have skewed where designers should consider 3-D analysis. Most states have abutments. A small percentage of structures have skewed access to computer programs that can perform such an analy- multi-column bents. Span lengths are usually less than sis. Almost all states use AASHTO wheel load distribution. 160 feet; however, approximately 20% exceed this limit. California commonly uses the whole-width design approach. 4. The trend for the future appears to be dictated by the re- No state had specific guidelines for varying the prestress quirements of a built-up urban environment. More curved force in the individual webs of curved box-girder bridges, alignments, longer spans, more skewed supports, and although at least two states recognize that stresses can vary
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4 transversely across the section and encourage designers to possible that spacing requirements could be relaxed at these take the initiative to specify varying prestress force. The hor- locations. Duct spacing requirements do not affect tendon izontal curvature of the tendons produces additional tension eccentricities where the ducts lie near the mid height of the on the girder toward the inside of the curve, thus mitigating webs. These are the locations of most breakout failures. These the severity of stress distribution across the section and the are also areas where actual duct curvatures may be amplified need to vary prestress force. due to the horizontal deviation of tendons to accommodate end California recently experienced a tendon breakout failure anchorage systems. Therefore, it should be possible to refine on the 405/55 interchange (Seible, Dameron, and Hansen, guidelines on duct spacing so as to both facilitate prestressing 2003). Before that failure, they had published guidelines for economies and prevent breakout failures. designers related to the design of curved post-tensioned Most states interviewed did not have specific guidelines for bridges (Caltrans, 1996). These guidelines dealt with the need the design of bearings in curved box-girder bridges. Some for special detailing in curved webs, including criteria for when states expressed a preference for certain types of bearings and these details are not needed. This memo indicated that, others try to avoid the use of bearings in curved box-girder because of the 405/55 structure's relatively large radius, bridges. tendon ties were not required in this structure. The problem Design for torsion in most states follows the AASHTO resulted because of a separate Caltrans standard plan, not requirements. Colorado expressed a need for better guide- specifically related to curved bridges, that allowed up to six lines for combining shear and flexural stresses. Colorado also tendons 41/2 inches or less in diameter to be stacked on top of uses precast "U" girders, which are temporarily braced for one another without any space in between. Because the torsion during the placement of the cast-in-place deck. At 405/55 was a long-span structure, several tendons needed to least one state said they ignored torsion design, but this might be stacked, resulting in radial forces being applied over a rel- be because they have only designed large radius bridges. atively wide area of essentially unreinforced cover concrete. One point of interest is the combination of global shear This is thought to be the primary cause of the failure. Caltrans and regional transverse bending stresses in the webs of curved indicated that they currently have no special policy for pre- box-girder bridges. Caltrans, which uses mostly cast-in-place vention of tendon breakout failures except that designers are bridges constructed on falsework, does not combine these to provide tendon ties under certain circumstances. Breakout stresses. The reasoning is that when the bridge is stressed, and failures have not occurred when these ties are present. regional transverse bending stresses are first realized, the Some other states indicated that they used the Caltrans bridge is on falsework and experiences no flexural or torsion tendon tie details to prevent tendon breakout failures. shear stress. By the time falsework has been released, the Several states had requirements for providing space be- prestress force is reduced because of relaxation and is not as tween tendons. Oregon requires that no more than three critical for regional transverse bending. Other states have no 4-inch-diameter or less ducts be stacked without a space of specific guidelines and leave it to the designer to determine 11/2 inches between a subsequent stack of ducts. The num- how these stresses should be combined. ber of stacked ducts is reduced to two for duct diameters Several states have standard details for concrete box-girder exceeding 4 inches. The current AASHTO LRFD specifications bridges. Most of these deal with prestress duct patterns and have duct spacing requirements that are similar to Oregon's. web reinforcing. Some of these were discussed above. Texas also indicated that they have similar duct spacing The requirements for the number and spacing of interior requirements. diaphragms vary among the states. The current AASHTO Colorado requires a duct spacing of 44% of the duct di- Standard Specifications for Highway Bridges (AASHTO, 1996) ameter or 11/2 inches minimum. This applies to all ducts (i.e., has specific requirements for the number and spacing of in- no stacks). This is more conservative than AASHTO and terior diaphragms in concrete box-girder bridges and several most other states, but Colorado reports no breakout failures states use these or similar requirements. Diaphragms are resulting from web curvature. not required in curved bridges with a radius of 800 feet or It appears that duct ties and duct spacing requirements greater. For a radius between 400 and 800 feet the maximum have been successful in preventing tendon breakout failures. diaphragm spacing shall not exceed 80 feet, and when the However, excessive duct spacing requirements can present radius falls below 400 feet the maximum diaphragm spacing problems at midspan and over the bents in continuous con- is 40 feet. crete box-girder superstructures because of the reduction in The AASHTO LRFD Bridge Design Specifications state that prestressing eccentricity and the corresponding increase in diaphragms are required in curved concrete box-girder prestress forces that results. Because the action of the deck bridges with a radius of 800 feet or less, but the code implies and soffit slabs tends to prevent breakout failures at points of that their number and spacing are to be determined by the maximum tendon eccentricity in box-girder structures, it is designer and depends on the radius and the dimensions of the