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94 CHAPTER 4 Conclusions This chapter summarizes all major observations and con- smaller area of vertical stirrups within the joint and smaller clusions from the precast bent cap connection research con- area of bent cap longitudinal reinforcement--the Grouted ducted under NCHRP Project 12-74, including results from Duct (GD) specimen satisfied the performance goal of the the seven bent cap to column connection tests, one girder design, achieving an extensive drift without appreciable to bent cap connection test, design specifications including strength degradation and exhibiting extensive plastic hing- design methodologies, design flow charts and design exam- ing of the column, limited joint distress, and essentially ples, construction specifications, example connection details, elastic behavior of the bent cap (2, 1). and implementation plan. Emulative performance is concluded for the GD specimen based on the close match between its overall behavior and that of the CIP control specimen, including lateral force- 4.1 Test Specimens displacement response; plastic hinging; joint shear stiffness; Based on the observed specimen response and data analy- level of joint distress; pattern of joint cracking; strain pat- sis, the following conclusions can be drawn. terns of bent cap and joint reinforcement; integral behavior between the bedding layer, column, ducts, and bent cap; and minor bar slip. 4.1.1 Cast-in-Place (CIP) Control Specimen GD response indicates that design specifications for a full Despite the less conservative design basis used from the 2006 ductility grouted duct connection should address vertical Recommended LRFD Guidelines for the Seismic Design of joint stirrups inside and outside the joint, horizontal cross Highway Bridges (2006 LRFD RSGS) compared to the 2009 ties inside the joint, transverse joint shear reinforcement, AASHTO Guide Specifications for LRFD Seismic Bridge and additional longitudinal bent cap reinforcement. Design (2009 LRFD SGS), including a smaller area of verti- Construction specifications should address fabrication and cal stirrups within the joint and smaller area of bent cap assembly processes as well as grout used for the connection. longitudinal reinforcement, the CIP specimen satisfied the performance goal of the design--achieving an extensive 4.1.3 Cap Pocket Full Ductility drift without appreciable strength degradation and exhibit- (CPFD) Specimen ing extensive plastic hinging of the column, limited joint dis- tress, and essentially elastic behavior of the bent cap (2, 1). Despite the less conservative design basis used from the 2006 The CIP specimen provided an appropriate benchmark LRFD RSGS compared to the 2009 LRFD SGS--including a (control) for comparison with the precast grouted duct and smaller area of vertical stirrups within the joint and smaller cap pocket specimens. In addition, test results can be reliably area of bent cap longitudinal reinforcement--the Cap Pocket used as a supporting basis for developing design and con- Full Ductility (CPFD) specimen satisfied the performance struction specifications for seismic precast bent cap systems. goal of the design, achieving an extensive drift without appreciable strength degradation and exhibiting extensive plastic hinging of the column, limited joint distress, and 4.1.2 Grouted Duct (GD) Specimen essentially elastic behavior of the bent cap (2, 1). Despite the less conservative design used from the 2006 Emulative performance is concluded for the CPFD speci- LRFD RSGS compared to the 2009 LRFD SGS--including a men based on the close match between its overall behavior

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95 and that of the CIP control specimen, including lateral ior between the bedding layer, column, pipe, and bent cap force-displacement response; plastic hinging; joint shear support this conclusion. stiffness; strain patterns of bent cap longitudinal reinforce- Despite the extensive plastic hinging, the development of ment; integral behavior between the bedding layer, col- significant joint shear damage (but not failure) observed for umn, pipe, and bent cap; and minor bar slip. the CPLD specimen does not match the expressed intent of CPFD response indicates that design specifications for a full Article 4.7.1 of the 2009 LRFD SGS for limited ductility ductility cap pocket connection should address vertical joint structures, including the requirement that "Inelastic action stirrups inside and outside the joint, pipe thickness based on is intended to be restricted to flexural plastic hinges in the providing the same circumferential hoop force in the joint column (1)." as that required by transverse reinforcement provisions of CPLD response indicates that design specifications for a Article 8.13.3 of the 2009 LRFD SGS, supplementary hoop limited ductility cap pocket connection should incorporate at ends of the pipe, and additional longitudinal bent cap minimum reinforcement requirements to help produce reinforcement (1). emulative behavior characterized by flexural plastic hinging Construction specifications should address fabrication and with limited effects of joint shear cracking: (1) minimum assembly processes as well as concrete within the cap pocket. area of vertical joint stirrups and (2) pipe thickness based on providing the same circumferential hoop force in the joint as that required by minimum transverse reinforcement pro- 4.1.4 Cap Pocket Limited Ductility visions of Article 8.13.3 of the 2009 LRFD SGS (1). In addi- (CPLD) Specimen tion, where the principal tensile stress, pt, is greater than or Despite elimination of the joint reinforcement used in the equal to 3.5 fc psi (or 0.11 fc ksi), additional joint re- full ductility specimens (vertical stirrups within the joint, inforcement should be required. joint-related stirrups and horizontal cross ties external to CPLD response also has important implications for CIP the joint, and hoops at the ends of the pipe) and reduction design. The following provisions are recommended for of bent cap flexural reinforcement and bent cap transverse inclusion in the LRFD SGS for CIP structures in SDC B (lim- reinforcement, the CPLD specimen satisfied the main per- ited ductility) to help produce emulative behavior charac- formance goal of the Seismic Design Category (SDC) B terized by flexural plastic hinging with limited effects of joint design. The CPLD specimen exhibited ductile plastic hing- shear cracking: (1) minimum area of vertical joint stirrups ing and reached an extensive drift of 5.1% (8 nominal), and (2) minimum joint transverse reinforcement based on well beyond a displacement ductility of 2.0 (2), with only Article 8.13.3 of 2009 LRFD SGS (1). This reinforcement can minor (12%) load degradation at maximum drift. This is be determined prescriptively, avoiding extensive seismic attributed to the effectiveness of the corrugated steel pipe analysis, and can result in constructible details. Similar pro- within the joint. visions can also be adopted for SDC A. Extensive joint shear cracking softened the CPLD joint, contributed significantly to column drift, and delayed (but 4.1.5 All Emulative Precast Specimens did not prevent) flexural plastic hinging. This response is (GD, CPFD, CPLD) attributed to the absence of vertical joint stirrups, which permitted unrestrained development, growth, and widen- Additional analysis is required to develop a new model that ing of joint shear cracks. This response was in contrast to fully characterizes grouted duct and cap pocket joint behavior the full ductility CIP and CPFD specimens. However, it can including joint forces, pipe effects, crack patterns, pipe effects, be reasonably deduced that similar, or more severe, joint and differences in strain distributions between the specimens behavior would likely develop for a similarly detailed CIP and the CIP control specimen. limited ductility connection because an SDC B CIP joint would incorporate less extensive and less effective trans- 4.1.6 Conventional Hybrid Specimen verse reinforcement (based on the limited provisions of current AASHTO LRFD Bridge Design Specifications) than The design methodology used for the conventional hybrid that provided by the steel pipe. specimen resulted in a system that satisfied performance Based on the foregoing conclusions, emulative behavior objectives up to the design level drift. The ultimate lateral (relative to a limited ductility CIP connection) can be con- deformation capacity was in excess of a 6% drift ratio, with cluded for the CPLD specimen. Similarities in performance significant reductions in damage and residual offset as com- between the limited ductility and full ductility speci- pared to CIP and emulative systems. mens including plastic hinging; lateral force-displacement Lateral force-displacement predictions based on the pro- response; equivalent viscous damping; and integral behav- cedures presented by Tobolski (5) and in the attachments

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96 match well with the recorded system response up to the pre- 6% drift ratio, with appreciable reduction in damage and dicted failure point. The predicted ultimate displacement residual displacements as compared to the CIP specimen. capacity was conservative in comparison to the actual Lateral force-displacement predictions based on the pro- observed lateral capacity. Predictions indicated that the fail- cedures presented by Tobolski (5) and in the attachments ure of the system would be attributable to the crushing of match well with the recorded system response up to approx- the confined concrete core whereas the observed failure imately a 2% drift ratio. After cycles at a 2% drift ratio, dam- mode was fracture of column reinforcement. age was observed in the grout bedding layer, ultimately Use of current joint force transfer models as presented in leading to a continual reduction in the lateral capacity until the 2009 LRFD SGS are reasonable and conservative for the ultimate fracture of a column reinforcing bar. The reduction design of joints in hybrid bridge column systems with in capacity is attributable to the progressive damage to the the consideration of column post-tensioning forces (1). bedding layer, which resulted in a reduction in the effective Larger-than-expected column post-tensioning forces were column diameter at the base. obtained due to a smaller-than-anticipated anchor set loss Use of current joint force transfer models as presented in in the tendons. Based on observed damage and residual off- the 2009 LRFD SGS are reasonable and conservative for sets, it is recommended that the ratio of the neutral axis the design of joints in hybrid bridge column systems with the depth to column diameter be limited to 0.25 to minimize the consideration of column post-tensioning forces (1). level of compressive straining in the column and enhance Results indicated that the use of fiber-reinforced grout in the the self-centering capacity of the system. bedding layer may enhance overall system performance by maintaining the integrity of the bedding layer and thereby maintaining the column compression toe. 4.1.7 Concrete Filled Pipe Hybrid Specimen The design methodology used for the concrete filled pipe 4.1.9 All Hybrid Specimens hybrid specimen resulted in a system that satisfied perfor- The use of fiber-reinforced grout in the bedding layer of mance objectives up to approximately the design level drift. hybrid specimens is expected to enhance overall perfor- The ultimate deformation capacity was approximately equal mance by maintaining the integrity of the compression toe dur- to a 6% drift ratio, with appreciable reduction in damage and ing cyclic loading. This is expected to minimize the observed residual displacements as compared to the CIP specimen. reductions in lateral capacity during larger deformation cycles Lateral force-displacement predictions based on the pro- and enhance the self-centering performance of the systems. cedures presented by Tobolski (5) and in the attachments match well with the recorded system response up to approx- imately a 2% drift ratio. After cycles at a 2% drift ratio, 4.1.10 Integral Specimen damage was observed in the grout bedding layer, ultimately During cycling at essentially elastic service demands, the leading to a continual reduction in the lateral capacity until superstructure responded without any observed reduction ultimate fracture of a column reinforcing bar. The reduction in stiffness or slip between the girder and reaction block. in capacity is attributable to the progressive damage to the Under essentially elastic seismic demands, there was simi- bedding layer, which resulted in a reduction in the effective larly no observed reduction in stiffness or slip indicating that column diameter at the base. the system is capable of satisfying operational and service Use of current joint force transfer models as presented in level demands in accordance with LRFD code provisions. the 2009 LRFD SGS are reasonable and conservative for The superstructure connection studied is capable of under- the design of joints in hybrid bridge column systems with the going rotation demands in excess of 0.01 radians in a safe consideration of column post-tensioning forces (1). and reliable manner. Under both positive and negative flex- Results indicated that the use of fiber-reinforced grout in the ural loading, the flexural capacity was maintained with sig- bedding layer may enhance overall system performance by nificant energy dissipation under negative loading due to maintaining the integrity of the bedding layer and thereby yielding of deck flexural reinforcement. Under positive flex- maintaining the column compression toe. ural loading, deformations were concentrated at the joint with pronounced joint opening. At cycles to approximately 0.006 radians, a horizontal crack 4.1.8 Dual Steel Shell Hybrid Specimen was observed between the deck and the top of the girder. The design methodology used for the dual steel shell hybrid This crack is attributable to the inadequate anchorage of the specimen resulted in a system that satisfied performance girder shear reinforcement provided by traditional 90-deg objectives up to approximately the design level drift. The hooks. It is recommended that headed reinforcement or ultimate deformation capacity was approximately equal to a similarly well-anchored reinforcement is used to minimize