Research leads to new possibilities for
Accelerated Bridge Construction in Idaho

Authors: Arya Ebrahimpour, Ph.D., P.E. (Idaho State University) and Leonard Ruminski, P.E. (ITD Bridge Section)

ITD continues to find more effective processes, reduce construction timeframes, improve performance and minimize impacts to the traveling public through Accelerated Bridge Construction (ABC) methods. Grouted couplers may be the latest example, thanks to work recently completed by Idaho State University under a contract initiated by the ITD Research Program.

The general concept of ABC is to prefabricate most bridge components and quickly assemble and connect them in the field, which significantly reduces on-site field work compared to conventional techniques, and increases safety for the public and for workers by reducing exposure to construction activities. Prefabricated bridge components are used and connections made using couplers and grout/concrete. But before using these methods in areas with higher seismic potential, ITD Bridge staff needed information about how grouted couplers would perform under these conditions. The study found that grouted couplers can safely be used under seismic conditions common in Idaho.

ABC methods are relatively new in the U.S. and Idaho, but have grown recently in popularity amid the push to reduce congestion, traffic interruptions and inconvenience to the motoring public..

One of the challenges facing bridge designers implementing ABC, however, is providing adequate connections between prefabricated elements and connections of precast columns with foundations and pier caps. These connections are required by current design codes to provide a ductile behavior under high seismic events and truck-collision impact loads. The ductile column connections can dissipate large amounts of energy while preserving the column's load-carrying capacity and the overall integrity of the structure.

ITD's Bridge Section decided to tackle this challenge head-on by evaluating the seismic performance of columns with grouted couplers in ABC applications. The research evaluated the suitability of grouted couplers in Idaho's low-to-moderate seismic conditions and to provide bridge designers with design implementation guidelines in ABC.

The attributes of ABC dovetail nicely with ITD's mission statement of providing safety, mobility and economic opportunity. For these reasons, ITD's Bridge Section is increasingly committed in implementing ABC techniques whenever feasible. Several bridge projects in Idaho were already delivered in recent years using these innovative methods.

Despite higher initial construction cost, ABC methods may result in other savings by minimizing traffic interruptions and delays and by improving long-term performance. Prefabricated bridge elements are usually manufactured in certified plants under controlled factory conditions following strict industry standards, offering superior quality and durability.

Current design codes don't adequately address the precast column connections, but there are several experimental methods in existence. The one usually preferred by fabricators due to the ease of handling and installation incorporates grouted bar couplers installed at each joint of precast column. In principal, grouted couplers are classified as mechanical bar splices and consist of steel sleeves with two reinforcing bars inserted into each end. High-strength grout is then pumped within the sleeve, providing bonding connection between bars. Although mechanical bar splices are not new to concrete construction, their use in bridge column connections is currently not recommended by AASHTO design code, due to lack of adequate research supporting their required ductile behavior in high-seismic areas.

To achieve the objective of the project, several tasks were undertaken. First, the research team performed an extensive literature review of seismic requirements of the column-to-footing and column-to-bent cap connections in ABC applications. Two recent experimental projects were identified as very relevant; these are the work by the University of Nevada, Reno (UNR) and the University of Utah. Both projects concluded that grouted couplers are a viable option in ABC applications in seismic zones. Next, the ISU team developed computer models of the cast-in-place (CIP) column and the precast column with grouted couplers and no pedestal (GCNP) that were used in the UNR project. The computer models predicted the force-displacement behavior of the UNR laboratory columns very well. Using the experimentally-verified computer models, the team performed seismic analyses of three Idaho bridges with (a) CIP columns, and (b) precast columns with grouted couplers at the bottom and top of the columns. The three bridges were selected by the ITD team. The bridges were analyzed for seismic accelerations corresponding to the most seismically active location in Idaho (i.e., Montpelier) and a less desirable soil condition of Site Class D. All three bridges, having either CIP columns or precast columns with grouted couplers, performed well. The maximum column drift was 1.6 percent (the column drift is defined as the ratio of top of the column horizontal displacement to the length of the column). In addition, the stresses in the column longitudinal reinforcing bars and the grouted couplers were well within acceptable range.

Although all three Idaho bridges performed well, it was still unclear what upper limit of drift would be appropriate for columns with grouted couplers. To obtain this limit, the research team performed analyses of single columns from two of the bridges under large drifts. It was concluded that grouted couplers may be used to connect precast columns to footings or cap beams for columns with less than 4 percent drift. The team also provided a list of acceptable grouted couplers and typical connection detail drawings.

The conclusions of this research, including recommendations, will be soon included in ITD's Bridge Design LRFD Manual. This will provide all Idaho bridge designers with formalized acceptance of this new innovative approach to ABC.

The research project started in January 2015 and ended in October 2016 with a cost of approximately $41,000.

Contributing members of the ISU research team include: Arya Ebrahimpour, Ph. D., P.E. (Principal Investigator), Andrew Sorensen, Ph.D. (Co-Principal Investigator), Barbara Earles (Graduate Student), and Supreme Maskey (Graduate Student).

Contributing members of the ITD team include: Matt Farrar, P.E., ME-2 (State Bridge Engineer), Dan Gorley, P.E., ME-1 (Project Manager), Leonard Ruminski, P.E., TE-2 (TAC), Ned Parrish (Research Program Manager)

 

Published 03-03-17