Nabil F. Grace Ph.D, PE
From the surface, the Bridge Street Bridge Project in Southfield, Michigan does not appear to be much different than any other bridge installation. As the sole means of access to the Bridge Street Industrial Subdivision, the project consists of two parallel bridges with one serving incoming traffic and the other, outgoing. Each is constructed of reinforced concrete with similar traffic barrier and sidewalk configurations. This similarity of appearance belies the radical difference in the actual structures with regard to their reinforcement systems and materials.
Structure A, which services traffic entering the subdivision, was constructed of standard AASHTO precast concrete beams, reinforced with conventional steel elements. Structure B, carrying outgoing traffic, was constructed of precast concrete double-tee beams reinforced not with steel, but with elements manufactured of carbon fiber reinforced polymer (CFPR). The use of CFRP offers several distinct advantages over reinforcing steel in concrete construction, on of which is the virtual elimination of deterioration due to corrosion, a common problem among conventionally reinforced bridges.
The quantity and extent to which CFRP was used in place of steel in Structure B has set a new precedent in advanced composite technology in a civil engineering application. To assure the success of a project with no precursor, special programs for the production, testing, erection, instrumentation, and monitoring of these CFRP-reinforced beams were implemented. The design, management, and construction of this $8.5 million structural system project form the basis of our ACEC 2003 Engineering Excellence Awards joint entry.
The Bridge Street Bridge Project made extensive use of a cutting edge material with the input and support of a team consisting of members from around the world. For example, quality CFRP materials were purchased from manufacturers in Japan and Canada. Funding was provided by the Federal Highway Administration through the cooperation of the Michigan Department of Transportation. In addition, four (4) separate contracts were established: one for the fabrication of the precast beams, one for the Test Beam program, and separate agreements for the site construction and structural instrumentation and monitoring. Joining the City of Southfield, HRC, and LTU as part of the Project Team were:
· The Federal Highway Administration (FHWA) - Funding Procurement
· The Michigan Department of Transportation (MDOT) - Design Guidance & Support
· Tokyo Rope Mfg., Ltd., Mitsubishi Chemical Corp. & Autocon Composites, Inc. - CFRP Material Manufacturers
· Angelo Iafrate Construction Co. - General Contractor
· Hollowcore, Inc./Prestressed Systems, Inc. - Precast Contractor
· Construction Technology Laboratories - Testing, Instrumentation & Monitoring Contractor
The innovative design approach for Structure B, which was based on research conducted by LTU and the University of Windsor, Ontario, called for the use of CFRP in nearly all aspects of the structural reinforcement. Inside each double-tee beam are sixty (60) strands of pretensioned CFRP cable as well as a variety of non-prestressed CFRP reinforcement manufactured in bent shapes and straight rods. Steel reinforcing bars used for shear reinforcement, and all miscellaneous embedded materials used in Structure B, were made of stainless steel.
Standard techniques for prestressing of conventional steel strands could not be used during the CFRP prestressing operations. A simplified method for the prestressing of the CFRP tendons was first developed in Japan using conventional steel strands and steel transition couplers. Due to severe geometric constraints associated with the placement of thirty (30) tendons in each stem and beam production economies, a new and innovative prestressing method had to be developed. This new method utilized rotational restraining transition couplers, both conventional steel strand and CFRP strand, and separate anchor devices for each of the sixty (60) tendons that were prestressed in each beam.
Structure B features a unique method of external post-tensioning using a series of four (4) - 1½ in. (40mm) diameter cables made of CFRP on the underside of each double-tee beam. An innovative bearing support technique called a tendon alignment bearing shoe was designed for these cables. Various engineering constraints and constructability issues were successfully resolved through this original design.
FUTURE VALUE TO THE ENGINEERING PROFESSION
The Bridge Street Bridge Project has met the immediate need to provide continuous access to the Bridge Street Industrial Park. At the same time, the dual bridge concept has provided an opportunity for an on-going study and comparison between the conventional steel and innovative CFRP-reinforced structural systems. A complex system of instrumentation and monitoring devices was embedded and exterior-mounted to the structural elements of both bridges during construction. Through this instrumentation, the behavior of the structural system will be continuously monitored over the next five (5) years. The data collected will be made available to researchers, federal and state transportation authorities, and the National Science Foundation. It is hoped that the successful construction of this innovative structural system and the study of its behavior, will result in future investigation and usage of advanced CFRP composites in many more applications in and beyond civil engineering.
It is anticipated that an increased life span and significantly reduced maintenance costs over that of a conventionally reinforced bridge will be the primary benefits of choosing to reinforce Structure B with CFRP. When maintenance or repair does become necessary in the future, the dual bridge concept will allow the City to completely close one bridge down for service while traffic is temporarily redirected to the second bridge. This will provide uninterrupted access to the Bridge Street Industrial Subdivision as was done during construction.