Bijan Khaleghi, Washington State DOT
The Washington State Department of Transportation (WSDOT) Highways for Life (HFL) project offers a precast concrete bridge system that is simple, rapid to construct, with excellent seismic performance. The WSDOT HFL(1) project includes precast segmental columns, precast bent cap, and precast superstructure. The project is also known as the US 12 Bridge over I-5, Grand Mound to Maytown Interchange Phase 2 Bridge 12/118 Replacement. This article is the second in a two-part series on the bridge project, and it covers the bridge design and construction.
The precast columns are composed of three segments with the lower column segments erected on concrete leveling pads into a cast-in-place (CIP) concrete spread footing. The column-to-cap beam connection is made with a small number of large bars grouted into ducts in the cap beam. The large size bar-ducts lead to a connection that can be assembled easily on site. The precast bent cap beam is built in two pieces that are integrated with a closure pour near its mid-span. The precast bent cap and precast superstructure are then connected together at the intermediate pier with a cast-in-place concrete diaphragm meeting the requirement of the AASHTO Guide Specifications for LRFD Bridge Seismic Design(2) for integral joint, and WSDOT Bridge Design Manual3 (BDM) requirements. Figure 1 shows schematic of a typical WSDOT prestressed girder bridge with lower bent cap and integral diaphragm.
WSDOT HFL Project
The objective of WSDOT HFL project was to demonstrate the constructability of the fully precast bent system on an actual bridge project of US-12 crossing Interstate 5 (I-5) in Washington State. The demonstration project is a replacement bridge that will be built on an alignment parallel to an existing bridge. It is a two-span bridge, with tall abutments on the end and a center bent that is located in the median strip. The HFL bridge features included: unique connection to footing, precast columns in segments, column segment grouted joints, precast bent cap segments, cast-in-place precast bent cap closure, precast superstructure with CIP closure at intermediate pier, and precast end and intermediate diaphragms.
The top of footing reinforcement are not continuous through the precast column segment as is usually done with the cast-in-place applications. To achieve proper interface shear transfer between the precast column and the cast-in-place concrete footing, the exterior of the column is roughened near the bottom to improve the transfer of shear stress. The construction sequences for placement of precast column segments into the cast-in-place footing as shown in Figure 2 includes: excavate for footing and install forms, place leveling pad and set first segment of column, place footing reinforcing and cast footing concrete, and remove forms and backfill.
The columns used in this project are spliced to permit erection in segments. While the columns of the demonstration project are small enough to be handled in a single pick with a crane, the segmental concept will demonstrate the technology for use on projects where the columns are longer and cannot be lifted with a single pick. The precast first-stage cap beam for the demonstration bridge will be built in two pieces that are integrated with a closure pour near its mid-span. This is required because the bridge is 84 feet wide, including sidewalks. Ideally, the precast first-stage cap would be built as a single piece element to avoid the time required for splicing segments, but pick and shipping weight restrictions led to the two-piece solution. The construction sequences for placement of precast column segments and precast bent as shown in Figure 3 are: 1) place and shim middle column segments, 2) place and shim top column segments, 3) install column bracing, 4) place and shim precast bent cap segments, 5) install grout forms and seal and, and 5) pump grout and close grout tubes.
The superstructure of the bridge consisted of WSDOT W35DG Standard precast pretensioned deck bulb tee girders that span 88 feet. These are supported by the center bent connected to the precast bent cap with a cast-in-place diaphragm and a 5-inch cast-in-place slab over the deck bulb tees. The intermediate and end diaphragm were precast with the girders at the fabrication plant. The construction sequences for placement of precast superstructure are: 1) place precast girders on oak blocks, 2) install girder bracing, 3) complete welded ties between girders, 4) place slab reinforcement and cast concrete, 5) cast pier diaphragm concrete 10 days after slab casting, and 6) cast traffic barrier and sidewalk.
LESSONS LEARNED FROM THE DEMONSTRATION PROJECT
Lessons learned from the construction of first WSDOT HFL project included those for precast bent system, and those for precast segmental columns. It is preferable that the columns be in a single precast piece with the grout connection at the precast bent cap and socket into CIP footing. The HFL project consisted of 96 grouted bar-duct connections and a CIP connection in the precast bent cap. Tolerance of precast pieces and erection tolerances were of extreme importance. Grout form quality and ability to seal with column is the key to successful grouting. The contractor preferred the joints that had the ducts in the lower section. They indicated that all the joints where the ducts were below the joint were grouted without any leaking. Shim locations and grout lifting pressures need to be included in erection plan calculations.
References
1. WSDOT HFL Project. US 12 over I-5, Grand Mound to Maytown Interchange Phase 2 Bridge 12/118 Replacement, WSDOT Olympic Region
2. AASHTO Guide Specifications for LRFD Seismic Bridge Design, 1st Edition 2010
3. Bridge Design Manual, Publication No. M23-50, Washington State Department of Transportation, Bridge and Structures Office, Olympia, Washington, 2010.
4. Khaleghi, B. WSDOT Plan for Accelerated Bridge Construction. Journal of Transportation Research Board No 2200, Bridge Engineering 2010, Volume 1, pp 3-11.
5. Kyle P. Steuck Jason B.K. Pang, Marc O. Eberhard, John F. Stanton, Rapidly Constructible Large-Bar Precast Bridge-Bent Seismic Connection, WA-RD 684.2, October 2008
6. AASHTO LRFD Bridge Design Specifications 5th Edition 2010
Further Information
For further information, readers are encouraged to contact the author at [email protected].