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Suggested Citation:"Chapter 2 - State-of-Practice Review." National Academies of Sciences, Engineering, and Medicine. 2008. Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges. Washington, DC: The National Academies Press. doi: 10.17226/14186.
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Suggested Citation:"Chapter 2 - State-of-Practice Review." National Academies of Sciences, Engineering, and Medicine. 2008. Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges. Washington, DC: The National Academies Press. doi: 10.17226/14186.
×
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Suggested Citation:"Chapter 2 - State-of-Practice Review." National Academies of Sciences, Engineering, and Medicine. 2008. Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges. Washington, DC: The National Academies Press. doi: 10.17226/14186.
×
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Suggested Citation:"Chapter 2 - State-of-Practice Review." National Academies of Sciences, Engineering, and Medicine. 2008. Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges. Washington, DC: The National Academies Press. doi: 10.17226/14186.
×
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Suggested Citation:"Chapter 2 - State-of-Practice Review." National Academies of Sciences, Engineering, and Medicine. 2008. Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges. Washington, DC: The National Academies Press. doi: 10.17226/14186.
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3Domestic Practice To obtain a better understanding of current U.S. practice, telephone interviews were conducted with representatives from key state DOTs. The states surveyed included California, Colorado, Florida, Hawaii, Idaho, Oregon, Nevada, New York, Tennessee, Texas, Washington and Wisconsin. Other states were contacted but chose not to participate. Based on these interviews, current U.S. practice can be summarized as follows: 1. With the exception of California and Washington, most states interviewed have a fairly small inventory of curved concrete box-girder bridges (i.e., <1% each of reinforced and prestressed) although many see the number increasing in the future. 2. Cast-in-place construction is most popular in the West. Other states are tending toward segmental construction (cantilever and span-by-span using both precast and cast- in-place concrete) to avoid conflict with traffic. Colorado has used precast, curved, spliced “U” girders with a cast- in-place deck. The use of precast box-beams in most other states is limited to straight girders. Curvature, when present, is provided by a curved deck slab (i.e., variable overhangs). Northern and eastern states, where weather conditions cause more rapid deck deterioration, tend to avoid prestressed boxes because of the need to provide for future deck replacement. 3. Most curved box-girders are relatively narrow continuous ramp structures. A few are single span, and a significant number of all structures (approx. 20% to 30%) have skewed abutments. A small percentage of structures have skewed multi-column bents. Span lengths are usually less than 160 feet; however, approximately 20% exceed this limit. 4. The trend for the future appears to be dictated by the re- quirements of a built-up urban environment. More curved alignments, longer spans, more skewed supports, and more segmental construction are expected. Curved precast girder systems may also increase, particularly in Colorado where this type of construction has been successful. 5. Many states have experienced some problems with the performance of curved box-girder bridges, but not many as a percentage of the total. Cracking along the tendon and tendon breakout problems are absent or minimal where sufficient space is provided between the ducts. Torsion and flexural shear cracking seem to be rare and not neces- sarily limited to curved bridges. A few bearing failures have occurred, but have been avoided in states that avoid bearings altogether or use conservative bearing designs. In some cases, bearing uplift at the abutments has been observed to occur over time and is thought to result from the time-dependant behavior of concrete. Unexpected vertical or horizontal displacements of the superstructure are rare, but California has had some problems on skewed multi-column bents where movement about the c.g. of the column group has caused the transverse shear keys to engage. Lateral displacement of columns has also been observed. 6. Some states have special design rules. A few of these are discussed below. Many states either use AASHTO LRFD (2004) or are adopt- ing it. Most states have no special rules for when a three- dimensional (3-D) analysis, such as a grillage or finite element analysis, should be performed and leave it to the discretion of the designer. Many states use an 800-foot radius as the trigger where designers should consider 3-D analysis. Most states have access to computer programs that can perform such an analy- sis. Almost all states use AASHTO wheel load distribution. California commonly uses the whole-width design approach. No state had specific guidelines for varying the prestress force in the individual webs of curved box-girder bridges, although at least two states recognize that stresses can vary C H A P T E R 2 State-of-Practice Review

transversely across the section and encourage designers to take the initiative to specify varying prestress force. The hor- izontal curvature of the tendons produces additional tension on the girder toward the inside of the curve, thus mitigating the severity of stress distribution across the section and the need to vary prestress force. California recently experienced a tendon breakout failure on the 405/55 interchange (Seible, Dameron, and Hansen, 2003). Before that failure, they had published guidelines for designers related to the design of curved post-tensioned bridges (Caltrans, 1996). These guidelines dealt with the need for special detailing in curved webs, including criteria for when these details are not needed. This memo indicated that, because of the 405/55 structure’s relatively large radius, tendon ties were not required in this structure. The problem resulted because of a separate Caltrans standard plan, not specifically related to curved bridges, that allowed up to six tendons 41⁄2 inches or less in diameter to be stacked on top of one another without any space in between. Because the 405/55 was a long-span structure, several tendons needed to be stacked, resulting in radial forces being applied over a rel- atively wide area of essentially unreinforced cover concrete. This is thought to be the primary cause of the failure. Caltrans indicated that they currently have no special policy for pre- vention of tendon breakout failures except that designers are to provide tendon ties under certain circumstances. Breakout failures have not occurred when these ties are present. Some other states indicated that they used the Caltrans tendon tie details to prevent tendon breakout failures. Several states had requirements for providing space be- tween tendons. Oregon requires that no more than three 4-inch-diameter or less ducts be stacked without a space of 11⁄2 inches between a subsequent stack of ducts. The num- ber of stacked ducts is reduced to two for duct diameters exceeding 4 inches. The current AASHTO LRFD specifications have duct spacing requirements that are similar to Oregon’s. Texas also indicated that they have similar duct spacing requirements. Colorado requires a duct spacing of 44% of the duct di- ameter or 11⁄2 inches minimum. This applies to all ducts (i.e., no stacks). This is more conservative than AASHTO and most other states, but Colorado reports no breakout failures resulting from web curvature. It appears that duct ties and duct spacing requirements have been successful in preventing tendon breakout failures. However, excessive duct spacing requirements can present problems at midspan and over the bents in continuous con- crete box-girder superstructures because of the reduction in prestressing eccentricity and the corresponding increase in prestress forces that results. Because the action of the deck and soffit slabs tends to prevent breakout failures at points of maximum tendon eccentricity in box-girder structures, it is possible that spacing requirements could be relaxed at these locations. Duct spacing requirements do not affect tendon eccentricities where the ducts lie near the mid height of the webs. These are the locations of most breakout failures. These are also areas where actual duct curvatures may be amplified due to the horizontal deviation of tendons to accommodate end anchorage systems. Therefore, it should be possible to refine guidelines on duct spacing so as to both facilitate prestressing economies and prevent breakout failures. Most states interviewed did not have specific guidelines for the design of bearings in curved box-girder bridges. Some states expressed a preference for certain types of bearings and others try to avoid the use of bearings in curved box-girder bridges. Design for torsion in most states follows the AASHTO requirements. Colorado expressed a need for better guide- lines for combining shear and flexural stresses. Colorado also uses precast “U” girders, which are temporarily braced for torsion during the placement of the cast-in-place deck. At least one state said they ignored torsion design, but this might be because they have only designed large radius bridges. One point of interest is the combination of global shear and regional transverse bending stresses in the webs of curved box-girder bridges. Caltrans, which uses mostly cast-in-place bridges constructed on falsework, does not combine these stresses. The reasoning is that when the bridge is stressed, and regional transverse bending stresses are first realized, the bridge is on falsework and experiences no flexural or torsion shear stress. By the time falsework has been released, the prestress force is reduced because of relaxation and is not as critical for regional transverse bending. Other states have no specific guidelines and leave it to the designer to determine how these stresses should be combined. Several states have standard details for concrete box-girder bridges. Most of these deal with prestress duct patterns and web reinforcing. Some of these were discussed above. The requirements for the number and spacing of interior diaphragms vary among the states. The current AASHTO Standard Specifications for Highway Bridges (AASHTO, 1996) has specific requirements for the number and spacing of in- terior diaphragms in concrete box-girder bridges and several states use these or similar requirements. Diaphragms are not required in curved bridges with a radius of 800 feet or greater. For a radius between 400 and 800 feet the maximum diaphragm spacing shall not exceed 80 feet, and when the radius falls below 400 feet the maximum diaphragm spacing is 40 feet. The AASHTO LRFD Bridge Design Specifications state that diaphragms are required in curved concrete box-girder bridges with a radius of 800 feet or less, but the code implies that their number and spacing are to be determined by the designer and depends on the radius and the dimensions of the 4

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

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

A recent problem with two box-girder bridges in Coahuila, Mexico, that is apparently due to the curvature of the structures has developed. These bridges, which are relatively new, are cast-in-place, post-tensioned, continuous concrete box-girder bridges supported on single bearings at each non-integral sin- gle column. These relatively narrow multi-span ramp structures are experiencing ongoing deflections and lateral movement. It is not clear what is causing this behavior, but it is fairly certain that curvature is a factor. The bearings have experienced uplift from rotation of the superstructure as shown in Figure 2-1. The movement was severe enough to remove the superelevation placed in the bridge at the time of construction. Significant cracking of the superstructure was also observed. The bridge owner is attempting to correct the problem by increasing the size of the piers in the transverse direction as shown in Fig- ure 2-2 and jacking and shimming the superstructure back to its original as-built position. The wider piers will allow bearings to be placed eccentric to the centerline of the superstructure and hopefully stabilize the situation. A lightweight overlay is also being considered to completely restore the superelevation. 7 Figure 2-1. Uplift at edge of bearing. Figure 2-2. Construction of widened column.

Next: Chapter 3 - Published Literature Review »
Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Report 620: Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges explores proposed specifications and examples for the design of horizontally curved concrete box-girder highway bridges.

Potential LRFD specifications and design examples illustrating the application of the design methods and specifications are available online as appendixes to NCHRP Report 620.

Appendix A - Proposed LRFD Specifications and Commentary

Appendix B - Example Problems

Appendix C - Global Analysis Guidelines

Appendix D - State of Practice Summary for the United States

Appendix E - Detailed Global Analysis Results

Appendix F - Detailed Local Analysis Results

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