Categories: Issue 69 January/February 2013

Bijan Khaleghi, Washington State DOT

The photograph shows a model in four stages: the first stage is the bent cap by itself, the second stage is the bent cap supporting one span’s beam, the third stage is the bent cap supporting two span’s beams, and the fourth stage is concrete on top of the bent cap joining the spans together in an integral joint.
The photograph shows a model in four stages: the first stage is the bent cap by itself, the second stage is the bent cap supporting one span’s beam, the third stage is the bent cap supporting two span’s beams, and the fourth stage is concrete on top of the bent cap joining the spans together in an integral joint.

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 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 first in a two-part series on the bridge project, and it covers the research behind the project.

Precast connections are typically made at the beam-column and column-foundation interfaces to facilitate fabrication and transportation. However, for structures in seismic regions, those interfaces represent the locations of high moments and large inelastic cyclic strain reversals. Provisions must be made for bridges in seismic regions to transfer greater forces through connections and to ensure ductile behavior in both longitudinal and transverse directions. It was envisioned that a fully precast bridge system could be favorably used in accelerated bridge construction (ABC) projects in seismic regions(1).

Monolithic action between the superstructure and substructure components is the key to seismic resistant precast concrete bridge systems. Lack of monolithic action causes the column tops to behave as pin connections resulting in substantial force demands on the foundations of multi-column bents. While the transverse stability of multi-column bents is ensured by frame action in transverse direction, stability in the longitudinal direction requires the column bases to be fixed to the foundation. Developing a moment-resisting connection between the superstructure and substructure makes it possible to develop plastic hinging at the column bases. Integral bent caps introduce moment continuity at the connection between the superstructure and substructure forcing columns into double-curvature bending, which tends to substantially reduce their moment demands at the foundation, affecting the sizes and overall cost of the adjoining foundations. The University of Washington(2) (UW) in two research project with WSDOT demonstrated the satisfactory performance of the column-to-cap connection. The first project performed pullout tests(2) of large size bars placed into corrugated galvanized standard post-tensioning ducts with cementitious grout, and the second project performed the testing of precast column to cap connection made with grouted ducts. The UW pullout tests demonstrated that the development lengths of bars grouted into ducts are significantly reduced compare to those development lengths required by the AASHTO LRFD Specifications(3) Article 5.11.2.1. The development length equation developed for the WSDOT Bridge Design Manual (BDM),(4) shown as Equation (1), is based on the UW experimental tests, and shares the same dependencies on steel strength, bar diameter, and concrete or grout strength with the AASHTO LRFD development length equations.

The second term in Equation (1) represents the effect of the pullout cone, and the difference between duct and bar diameters. AASHTO LRFD requires that the development length be increased by a factor of 1.25 for seismic applications (3) WSDOT BDM(4)recommends using a higher factor of 1.5 for field applications of grouted duct system. Table 1 shows the minimum embedment length of grouted bar-duct sleeves. The minimum development lengths are based on A706 Grade 60 deformed reinforcing bars with expected tensile strength (fue) of 95 ksi, and compressive strength (f’g) of 6.0 ksi.

Table 1. Proposed Grouted Bar-Duct Development Length.

As part of the HFL project, the UW has performed several tests of precast column-to-footing connections. 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 shape of the lower column segment extending into the footing is changed to octagonal to provide more uniform interface surface. The precast column extends just below the footing, to assure that the force transfer at the bottom of the column bars can take place satisfactorily. The column-to-cap beam connection is made with a small number of large bars column grouted into ducts in the cap beam. Their small number, and the correspondingly large ducts sizes that are possible, lead to a connection that can be assembled easily on site.

References

1. Khaleghi, B. WSDOT Plan for Accelerated Bridge Construction. Journal of Transportation Research Board No 2200, Bridge Engineering 2010, Volume 1, pp 3-11.

2. 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

3. AASHTO LRFD Bridge Design Specifications, 5th Edition, 2010.

4. Bridge Design Manual, Publication No. M23-50, Washington State Department of Transportation, Bridge and Structures Office, Olympia, Washington, 2010.

Further Information

For further information, readers are encouraged to contact the author at 360 705-7181 or [email protected], and view project information at the Federal Highway Administration’s Highways for Life website: http://www.fhwa.dot.gov/hfl/partnerships/bergerabam/index.cfm.

Download Issue