Proposed AASHTO Seismic Specifications for ABC Column Connections (2020)

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239 6.1 Summary Accelerated bridge construction (ABC) utilizes rigorous planning, new technologies, and improved methods to expedite construction. Prefabricated columns and their connections to adjoining bridge members (cap beams, footings, pile caps, and pile shafts) are the most critical components of ABC in moderate- and high-seismic regions. Recent studies on the seismic performance of various types of ABC connections have generated a significant amount of information that has demonstrated the feasibility of ABC in these seismic areas. Some gaps remained. To help expand utilization of ABC in moderate- and high-seismic regions, seismic design specifications were needed. The objective of NCHRP Project 12-105 was to develop AASHTO displacement-based design and construction specifications for the implementation of ABC column connections in moderate- and high-seismic regions. The specifications were focused on three types of precast column connections: mechanical bar couplers, grouted ducts, and pocket/socket connections. The objective of the project was accomplished through five phases encompassing 14 tasks. A state-of-the-art literature review that covered recent experimental and analytical studies on the seismic response and constructability of the three selected ABC column connections was performed. As part of this project, a survey of state departments of transportation, federal agencies, and consulting engineers was conducted about their experience, preferences, and past and future utilization of ABC connections. The findings of the literature review and the survey were synthesized to identify the knowledge gaps that needed to be filled before speci- fications could be developed. An experimental program was designed to address some of the gaps identified for the three types of precast column connections. This program included tensile testing of 30 coupler speci- mens, pullout testing of 12 specimens with bars anchored in grouted ducts, and slow-cyclic lateral load testing of a half-scale bridge column incorporating grouted sleeve couplers. No. 10 deformed reinforcing steel bars were utilized in all 43 test specimens to generate data for realistic bar sizes used in the field. Furthermore, an analytical program was completed to fill the remaining knowledge gaps identified for the three precast column connections. A set of new acceptance criteria for mechanical bar splices in the plastic hinge of bridge columns was presented. Then, a material model suitable for all types of tension-compression couplers was adopted from the literature. Finally, testing protocols, including monotonic, cyclic, and dynamic loads for any type of cou- pler, were presented. A comprehensive parametric study was performed to quantify the effect of couplers on the seismic performance of mechanically spliced columns. Simple design equa- tions were proposed, including a modified equation for plastic hinge length, to include the coupler effect in the design of bridge columns. A design equation was developed to determine C H A P T E R 6 Summary and Conclusions

240 Proposed AASHTO Seismic Specifications for ABC Column Connections the embedment length of deformed bars in grouted duct connections. Finite element (FE) parametric analyses were carried out to determine the minimum size of column adjoining members incorporating grouted duct or pocket connections. On the basis of the findings of the experimental and analytical studies, design and construction specifications were developed for mechanical bar splices, mechanically spliced bridge columns, columns incorporating grouted duct connections, and columns with pocket/socket connections. All of the proposed design equations were validated with test data. Finally, five design examples were developed to illustrate the implementation of the proposed specifications. The first was a conventional cast-in-place (CIP) bridge to serve as a benchmark design. The same bridge was redesigned in the other examples, each implementing a different type of ABC column connection. 6.2 Conclusions The study presented in this report consisted of many phases and tasks, all aimed at accom- plishing the primary objective of the project, which was the development of AASHTO speci- fications for three types of precast column connections to facilitate ABC implementation in moderate- and high-seismic regions. The deliverables addressing the primary objective are presented in Appendices B and C of the report. The supporting studies related to the primary objective are presented in the main body of the report and Appendices A and D. This section presents the main conclusions of the project. 6.2.1 Literature Review and Survey of State Departments of Transportation and Other Agencies The main outcomes of the literature review and the survey of state departments of trans- portation and other agencies were identifying knowledge gaps for different types of ABC column connections, developing comprehensive experimental databases, and understanding the familiarity of bridge engineers with ABC column connections. • Mechanical bar splices. It was found that current acceptance criteria and testing meth- ods for mechanical bar splices were not sufficient to comment on the suitability of a coupler type for use in bridge columns. New acceptance criteria and testing standards were needed. • Mechanically spliced bridge columns. It was found that current design specifications prohib- ited the use of couplers in plastic hinge regions mainly because coupler effects on the seismic performance of columns were largely unknown. The available test data showed that couplers tend to reduce the column displacement capacity. However, the bulk of test data were not sufficient to empirically quantify the effect of different coupler types and sizes on column behavior. Furthermore, the effect of bar debonding on the performance of coupler columns was not quantified in previous studies. Reliable and simple design methods for mechanically spliced bridge columns were needed. • Grouted duct connections. Even though there have been several experiments on the per- formance of grouted duct connections, the literature lacked a complete design equation for grouted duct connections incorporating all critical bond parameters. Previous studies did not address the effect of duct bond strength, which could potentially control the required anchor- age length. A comprehensive design equation for the embedment length of bars in normal- strength grout-filled duct connections was needed. • Grouted duct column connections. The available test data confirmed that precast columns incorporating grouted duct connections usually perform the same as conventional CIP

Summary and Conclusions 241 columns in terms of strength and ductility. However, previous studies did not investigate the minimum size of column adjoining members (cap beams and footings) when grouted duct connections were used at the column ends. • Pocket/socket connections. As with grouted duct column connections, previous studies reported promising performance for precast columns that utilize pocket/socket connections. However, a reliable design embedment length as well as the minimum size of column adjoining members needed to be established for this type of connection. • Survey of state departments of transportation and other agencies. The survey showed a clear correlation between the familiarity of bridge engineers with ABC column connections and the field application of these connections. For example, the bridge engineering com- munity was most familiar with mechanical bar splice connections (mainly grouted couplers), and the survey showed that this type of connection has been utilized more than any other type of ABC connection. The same trend was seen in plans for future application of this type of connection. Furthermore, the survey clearly indicated that the availability of design guidelines and examples is an important motivation for field deployment. 6.2.2 Experimental Programs The most important observations and conclusions from the experimental studies performed in the present project were as follows: • Mechanical bar splice testing. Three types of mechanical bar splices were tested. Ten speci- mens per coupler type were tested: three each under monotonic loading and cyclic loading and four under dynamic loading. The testing methods proposed in this study for couplers were used. It was found that consistent and reliable measurement could be obtained with the proposed testing methods. Furthermore, the proposed criteria were found sufficient to comment on the suitability of a coupler type for use in the plastic hinge region of bridge columns. • Mechanically spliced column testing. A half-scale bridge column that used grouted sleeve couplers at the base was tested to failure. The data for a conventional CIP column were used to evaluate the effect of the couplers on the local and global response of the bridge column. From these and other test data, it was found that long and rigid couplers could reduce the displacement ductility capacity by approximately 30%. With respect to the spliced column testing method, the available methods for reinforced concrete columns (e.g., ACI 374.2R-13) were found sufficient to experimentally establish the seismic performance of mechanically spliced bridge columns. • Grouted duct connection testing. Twelve corrugated duct specimens filled with conven- tional grout were monotonically tested to failure under tensile loads. The test results con- firmed that a preliminary proposed design equation for grouted ducts was reliable because the bars were fully developed in all the specimens meeting the proposed anchorage length. The additional data collected in this study were then used to further refine the proposed design equation. 6.2.3 Analytical Programs The most important observations and conclusions from the analytical studies performed in the present project were as follows: • Mechanical bar splices. It was found that the stress–strain model for couplers was relatively simple and accurate in estimating the coupler behavior. Only one parameter is needed in this model to represent the coupler behavior and the coupler rigid length ratio. This parameter needs to be established through testing for each coupler type and bar size.

242 Proposed AASHTO Seismic Specifications for ABC Column Connections • Mechanically spliced bridge columns. The verified coupler material model was used in an extensive parametric study to quantify the effect of couplers on the displacement ductility capacity of bridge columns. It was found that stiff and long couplers installed at the interface of the column and the adjoining member may reduce the calculated column displacement ductility capacity by as much as 40%. The three analytical methods for the design of mechani- cally spliced columns offer designers a menu of methods with different levels of sophistica- tion. The parametric study showed that debonding of longitudinal bars in the vicinity of couplers may improve the displacement capacity of columns with couplers. • Grouted duct connections. All of the previous studies of grouted duct bonds were reviewed in the present study to generate a database. A subset of the data that were complete and applicable was used to develop equations that accounted for the failure of both bars and grouted duct bonds. Comparison of the proposed equation with the previous bond studies showed that the duct diameter is an important parameter in the design of grouted duct connections. • Grouted duct column connections. The effects of the size of the adjoining member connected to a precast column by means of grouted ducts were investigated through detailed FE analyses. The analysis showed that following the current AASHTO SGS (AASHTO 2014) requirements for CIP integral cap-beams is adequate to keep the connection capacity protected. The same requirements apply to connections of a column to an enlarged pile shaft. Therefore, the mini- mum diameter of an enlarged pile shaft or the minimum width of an integral cap beam that uses grout duct connections should exceed the column diameter by at least 24 in. to ensure capacity-protected behavior. • Pocket/socket connections. A reliable FE modeling method was developed for column pocket connections. This FE model was used in parametric studies to quantify the minimum size of cap beams and pile shafts under lateral loading. The analytical results indicated that the requirements for seismic design of the CIP joints could be extended to pocket connections with some modifications. On the basis of the available literature and the results of the analyti- cal studies, design and construction guidelines were proposed for the pocket connections in cap beams, footings, and pile shafts. 6.2.4 Proposed Specifications and Examples The present study showed that it is crucial to establish the behavior of a mechanical bar splice through testing. Only couplers categorized as seismic splices should be used in the plastic hinge region of bridge columns. Couplers not meeting the proposed acceptance criteria should not be used in bridge columns, and the coupler rigid length ratio should not be reported for nonseismic couplers, to avoid confusion. The study also showed that many of the provisions of the AASHTO SGS (AASHTO 2014) are applicable to analysis and design of precast columns that incorporate the three types of ABC connections, but the design also has to incorporate the results of this and previous studies that address the characteristics of a particular type of connection used in the column. Therefore, the ABC column specifications were proposed as an appendix to the AASHTO SGS highlight- ing the specific requirements. The five examples covering all three types of connections showed that the proposed specifications are relatively easy to use and require minimal variations from the AASHTO SGS. Furthermore, the examples showed that the ABC column connections that were the subject of NCHRP Project 12-105 do not alter the overall dimensions of cap beams. 6.3 References AASHTO. (2014). AASHTO Guide Specifications for LRFD Seismic Bridge Design. American Association of State Highway and Transportation Officials, Washington, D.C.