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NCHRP Project Number 12-71 Design Specifications and Commentary for Horizontally Curved Concrete Box Girder Highway Bridges Final Report January 29, 2008 NRV Nutt, Redfield & Valentine A Professional Partnership ________________________________________________________________ in association with David Evans & Associates, Inc. Zocon Consulting Engineers, Inc. NCHRP Project 12-71 Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Highway Bridges Final Report TABLE OF CONTENTS 1. INTRODUCTION 1 2. STATE-OF-PRACTICE REVIEW 3 a. Domestic Practice 3 b. Foreign Practice 6 c. Field Problems 8 3. PUBLISHED LITERATURE REVIEW 11 a. Codes and Design Standards 11 b. Design Philosophy 19 c. Response of Curved Concrete Box Girder Bridges 22 i. Global Analysis 22 ii. Laboratory Experiments 31 d. Design Issues 32 i. Bearings 32 ii. Diaphragms 33 iii. Flexure and Shear 33 iv. Torsion 37 v. Wheel Load Distribution 39 vi. Tendon Breakout and Deviation Saddles 39 vii. Time Dependency 45 viii. Vehicular Impact 45 ix. Seismic Response 46 x. Design Optimization 46 xi. Detailing 47 4. GLOBAL RESPONSE ANALYSIS STUDIES 49 a. Objective 49 b. Model Verification 49 c. Parameter Study 51 d. Special Studies 65 5. LOCAL RESPONSE ANALYSIS STUDIES 71 a. Local Analysis Validation 72 b. Local Analysis of Multi-cell Box Girders 88 c. Local Analysis of Single-cell Box Girders 111 d. Conclusions from Local Analysis 122 Page i 6. CONCLUSIONS 129 7. REFERENCES 137 APPENDICES A. PROPOSED LRFD DESIGN SPECIFICATIONS A-1 B. DESIGN EXAMPLES B-1 a. Example B-1 - Comprehensive Problem (Spine and Grillage) B-5 b. Example B-2 - Tendon Confinement B-121 c. Example B-3 - Tendon Confinement B-131 d. Example B-4 – Global plus Regional Combination (Menn) B-137 e. Example B-5 – Global plus Regional Combination (Poldony) B-147 f. Example B-6 - Deviation Saddle Design B-149 C. GLOBAL ANALYSIS GUIDELINES C-1 a. Three Dimensional Spine Beam Analysis C-1 b. Grillage Analogy Analysis C-4 D. STATE OF PRACTICE SUMMARY FOR UNITED STATES D-1 E. DETAILED GLOBAL ANALYSIS RESULTS E-1 a. Bridge Model Scattergrams – Spine vs. Grillage E-3 b. Bridge Model Ratio Plots – Spine vs. Grillage E27 c. Bridge Model Ratio Plots - Straight to Curve (spine and grillage) E39 d. Effect of Diaphragms – Grillage Analysis E51 e. Effect of Diaphragms – Ratio Plots E55 f. Effect of Hinges – Grillage Analysis E57 g. Effect of Hinges – Ratio Plots E61 h. Effect of Skews E63 F. DETAILED LOCAL RESPONSE ANALYSIS RESULTS F-1 a. Single Cell Box Girder F-1 b. Multi-Cell Box Girder F-63 Page ii NCHRP 12-71 Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Highway Bridges Final Report 1. INTRODUCTION At one time, bridges on curved alignments were rare. However, times have changed and modern highway bridges and traffic separation structures are commonly built on a horizontal curve. This has come about because of higher traffic volumes and speeds, geometric constraints of the urban environment, and improved structural forms that lend themselves to curved construction. The concrete box girder, particularly post-tensioned prestressed concrete that can span large distances, is one such structural form. The cross section of these structures is inherently strong in torsion. This is important, because curvature induces high torsion forces. Also, because concrete can be easily molded into the required shape, it is ideal for curved construction. For these reasons, prestressed concrete box girders have become the structure typeof choice in many jurisdictions. A common application of curved structures is in freeway interchanges where connector ramps or “fly-overs” carry traffic from one freeway to another at relatively high speed. Cross sections of curved box girders may consist of single cell, multi-cell or spread box beams as shown in Figure 1-1. Because only a very few spread box beam bridges use curved beams, only the first two types were considered in this study. Single-cell Box Girder Multi-cell Box Girder Spread Box Beams Figure 1-1 Types of Cross Sections Page 1 It has become common practice to analyze and design these structures as if they were straight. Live load distribution is often addressed using the whole-width design approach. Local problems, such as the lateral forces induced by curved prestressing ducts, are often handled using specific design rules and details that have been developed over the years. For the most part, this design approach has been used successfully, but some recent cases of poor bridge performance have made it clear that this approach has its limitations. Because it is likely that the use of curved structures is going to increase, and that the geometries of some of these structures will continue to push the envelope with respect to the degree of curvature, span lengths and depths, the amount of required prestressing force, etc., it is evident that better guidelines are required for their design. That is the purpose of this project. There has been a significant body of research and development relative to the design and analysis of curved prestressed concrete box girder bridges. Some of this information has found its way into design specifications, while much of it has not. There is a need to collect and analyze this information with the goal of evaluating its merit for nationwide design rules. While much of this work has been conducted domestically, there is also a significant body of work conducted by other countries that also needs to be considered. Although most issues relative to design of curved concrete box girders have been studied to some degree, there is a need to fill in the gaps in our understanding. With modern computer programs and analytical models calibrated to existing physical and experimental results, it is possible to do most of this through analytical studies. It is evident that additional physical testing of existing structures or laboratory experimentation, while important in and of itself, are beyond the scope of this project, and are not necessary to accomplish this project’s intended goals. This report is intended to present the results of a review of the literature and the state-of-practice with respect to curved concrete box girder bridges. In addition, detailed results from both global and local response analysis studies are presented. Final recommendations are presented in the final chapter and are implemented in the form of recommended changes to the AASHTO LRFD Bridge Design Specifications and Commentary and in analysis guidelines for these types of bridges. Example problems are also presented that illustrate the application of these recommendations. Page 2 2. STATE-OF-PRACTICE REVIEW a. Domestic Practice To obtain a better understanding of current US practice, telephone interviews were conducted with representatives from key state Departments of Transportation (DOT’s). The states surveyed included California, Colorado, Florida, Hawaii, Idaho, Oregon, Nevada, New York, Tennessee, Texas, Washington and Wisconsin. Other states were contacted but elected not to participate. Based on our interviews current US 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. Many other states are tending toward segmental construction (cantilever and span-by-span using both pre-cast and cast- in-place concrete) in order to avoid conflict with traffic. Colorado has used pre-cast, curved, spliced “U” girders with a cast-in-place deck. The use of pre-cast box-beams in most otherstates 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 due to 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 but approximately 20% exceed this limit. 4. The trend for the future appears to be dictated by the requirements 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 good success has been experienced with this type of construction 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 seems to be rare and not necessarily 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 are thought to be due to 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 already use AASHTO LRFD (2004) or are in the process of adopting it. Page 3 Most states have no special rules for when a 3 dimensional analysis such as a grillage or finite element analysis should be performed and leave it to the discretion of the designer.

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