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235 5.1 Introduction The main objective of NCHRP Project 12-105 was to develop proposed AASHTO displacement-based design and construction specifications for the implementation of ABC column connections in moderate- and high-seismic regions. The project was focused on three types of precast column connection: (1) mechanical bar coupler connections, (2) grouted duct connections, and (3) pocket/socket connections. Five phases encompassing 14 tasks were completed in this project to achieve the aforementioned objective. The methods and steps undertaken to develop design and construction specifications for each type of connection are briefly discussed in this chapter. 5.2 Proposed Standard Testing Methods for Mechanical Bar Splices Current design specifications do not allow the incorporation of mechanical bar splices in the plastic hinge region of columns in high-seismic areas. Standard acceptance criteria exist for couplers with nonseismic application, but these criteria are mostly force based in relation to the strength of spliced bars. The type of demand placed on spliced bars under seismic loads is not addressed in the current design and testing documents for couplers. Acceptance criteria for qualifying mechanical bar splices for seismic zones were developed as part of the current study. The criteria were initially selected from the literature and then further refined. Couplers that met the proposed requirements were categorized as âseismic splices.â Furthermore, a stressâstrain material model was selected from the literature to establish the seismic behavior of the splice. The adopted coupler material model, which was calibrated for the displacement-based design, was validated by using the experimental data obtained in the present project and data found in the literature. New testing methods that are consistent with current ASTM/AASHTO specifications were proposed for mechanical bar splices to systematically evaluate and establish their performance under monotonic, cyclic, and dynamic loads and formulate acceptance criteria for seismic couplers. The proposed testing methods were initially prepared by following current stan- dard test methods, such as ASTM A1034 (ASTM 2015) and California Test 670 (California Department of Transportation 2004), with modifications to include various coupler types and methods for extracting the information needed to represent coupler behavior by using the proposed coupler material model discussed above. Thirty bar couplers were tested by the proposed methods to determine the feasibility of the tests. The methods were then refined to facilitate their implementation. C H A P T E R 5 Proposed Specifications and Examples
236 Proposed AASHTO Seismic Specifications for ABC Column Connections Overall, the proposed acceptance criteria, the adopted coupler model, and the proposed testing methods were found viable for identifying existing or emerging couplers that could be used in the plastic hinge region of bridge columns. Appendix B of this report presents these in the AASHTO/ASTM format and language. It is expected that coupler manufacturers follow the proposed methods for each coupler product, type, and bar size and include the required information in the product datasheet. 5.3 Proposed Specifications for Precast Column Connections 5.3.1 Mechanically Spliced Column Connections As discussed above, new specifications were needed to quantify the effect of couplers on the seismic performance of bridge columns incorporating splice longitudinal bars in the plastic hinge regions. The main issues were that current codes do not allow couplers and do not offer any method for the design of mechanically spliced bridge columns. To develop new specifications, a comprehensive literature review was performed, the data were synthesized, and knowledge gaps were identified. Subsequently, a comprehensive analytical study that used the validated coupler model was carried out to understand the coupler effects on the column structural behavior. On the basis of the findings of the analytical study, three design methods were proposed for mechanically spliced bridge columns: 1. A simple equation that reduces the displacement ductility capacity of the column to account for the relatively high stiffness of couplers in the plastic hinge regions, 2. A modified analytical plastic hinge length that includes an adjustment factor to the analytical plastic hinge length specified in the AASHTO Guide Specifications for LRFD Seismic Bridge Design (AASHTO SGS) (AASHTO 2014) to account for the coupler effect, and 3. A distributed plasticity model with a modified steel fiber constitutive relationship to account for the effect of couplers. All three methods were validated against experimental data obtained in this project and data found in the literature. Overall, it was found that the three methods are viable to successfully quantify the cou- pler effects on the seismic performance of mechanically spliced bridge columns. Appendix C includes design and construction specifications for mechanically spliced bridge columns in the format and language of AASHTO. 5.3.2 Grouted Duct Connections The state-of-the-art literature review on grouted duct connections revealed that nearly none of the previous studies accounted for the fact that there are two bond surfaces in a grouted duct connection involving bar bond strength and duct bond strength. A new equation for embed- ment length including all bar and duct bond properties was needed for successful design of precast columns incorporating grouted ducts at their ends. A database including information from previous experimental studies and the test data on grouted duct connections obtained in the present study was generated. Subsequently, the data were filtered and a statistical analysis was carried out on the refined database. An empirical equation was then developed to determine the embedment length of deformed reinforcing steel bars in grouted duct connections to ensure full development of the bar and the grouted duct.
Proposed Specifications and Examples 237 The proposed equation was compared with those from other studies and design codes, and its advantages were discussed. After the embedment length was established, a detailed finite element analysis was performed to investigate the minimum size of column adjoining members when grouted ducts are used at the column ends. Overall, the grouted duct connection was found to be a viable ABC column connection. Appendix C includes design and construction specifications for grouted duct column connec- tions in the format and language of AASHTO. 5.3.3 Pocket/Socket Connections Previous experimental and analytical studies on pocket/socket connections were reviewed and critical information was summarized. It was found that the embedment length of column (as a whole or the column reinforcement) in pockets/sockets was essential for a successful design. Furthermore, the size of the column adjoining member to accommodate the pocket without premature failure needed to be established. On the basis of the data collected from the literature, design embedment length equations were proposed for pocket connections. Subsequently, a detailed finite element analysis was carried out to determine the minimum size of column adjoining members incorporating pocket connections. Overall, the pocket/socket connection was found to be a viable ABC column connection. Appendix C includes design and construction specifications for pocket/socket connections in the format and language of AASHTO. 5.4 Design Examples Five examples were developed to illustrate the design of the three types of ABC column con- nections according to the proposed specifications (Appendix C). Table 5-1 lists the types of connections and the design methods used for each example. Example 1 was a conventional cast- in-place two-span bridge carrying two lanes of traffic designed on the basis of the AASHTO SGS (AASHTO 2014). This example served as a benchmark. Examples 2 to 5 illustrated the design of various types of ABC column connections consisting of three types of mechanical splices, grouted ducts, and pockets/sockets. Example No. Type of Column Connection Design MethodTop Bottom 1 Standard moment connection Standard moment connection Benchmark conventional bridge designed per AASHTO SGS (2014) 2 Grouted sleeve embedded in cap beam Grouted sleeve embedded in columns Design based on the proposed specs (Appendix C) 3 Swaged couplers embedded in columns Swaged couplers embedded in columns Design based on the proposed specs (Appendix C) 4 Grouted ducts in cap beam Headed bar couplers; two on each longitudinal bar Design based on the proposed specs (Appendix C) 5 Pocket/socket Pocket/socket Design based on the proposed specs (Appendix C) Table 5-1. NCHRP 12-105 design examples.
238 Proposed AASHTO Seismic Specifications for ABC Column Connections 5.5 References AASHTO. (2014). AASHTO Guide Specifications for LRFD Seismic Bridge Design. American Association of State Highway and Transportation Officials, Washington, D.C. ASTM. (2015). ASTM A1034: Standard Test Methods for Testing Mechanical Splices for Steel Reinforcing Bars. ASTM International, West Conshohocken, Pa. California Department of Transportation. (2004). Method of Tests for Mechanical and Welded Reinforcing Steel Splices. California Test 670. Division of Engineering Services, Sacramento.
