Innovative Prestressed Concrete Bridges Mark Caltrans Centennial I

Innovative Prestressed Concrete Bridges Mark Caltrans Centennial In this centennial anniversary of Caltrans, the authors trace the evolution of bridges in the State of California and describe an important research program and new hit/atives that will insure a bright future for California’s bridges. n 1895, the State of California first FIRST PRESTRESSED .4 established a State Bureau of High CONCRETE BRIDGE I ways Commission. The California I Department of Transportation (Cal The first prestressed concrete Eric Thorkildsen, RE. trans) is currently celebrating this cen bridge built in California can be de Senior Bridge Engineer tennial by reflecting back on its his fined in modern terminology as a pre Imbsen & Associates, Inc. tory and achievements and also cast segmental bridge. Constructed in Sacramento, California looking towards the future. 1951 (near Los Angeles), the Arroyo Formerly, Senior Bridge Engineer A major technological breakthrough Seco pedestrian bridge California Department of Transportation joined two Sacramento, California in the evolution and development of 55 ft (16.8 m) precast segments to highways in California was the intro gether at midspan (see Fig. 1). A tem duction of prestressed concrete in porary falsework bent provided sup structures more than 44 years ago. port until the post-tensioning was What started out as an experiment in completed. 1951 has exploded into the construc This lightly loaded simple span tion of thousands of prestressed con bridge over a flood channel was a con crete bridges on the California high servative first application of pre way system. stressed concrete technology (see Fig. Currently, 85 percent of all bridges 2). The bridge was designed and constructed in California use pre checked by two legends in the field of stressed concrete. While the majority prestressed concrete, Caltrans engineer of recent bridges constructed in Jim Jurkovich and University of Cali California have cast-in-place post- fornia at Berkeley professor T. Y. Lin. tensioned box girder superstructures, Dr. Lin turned the bridge into an on- Jay Holombo, RE. recent developments in precast, pre site laboratory by placing strain Graduate Research Assistant stressed concrete technology has set gauges at key locations on the struc Division of Structural Engineering the stage for a return to the early days ture to check the predicted stresses University of California at San Diego when precast concrete was the domi and deformations. The “Proof Test” La Jolla, California nant structure type. was successful, and confidence in de 34 PCI JOURNAL U) C’) quickly solved this problem. California contractors invested heavily in the falsework needed for cast-in-place construction. Once this initial investment was made, the cost of cast-in-place construction dropped to a level at or below precast girders. The seismic resistance and aesthetics of the box girder gained favor among designers. The cast-in-place post- tensioned concrete box girder became California’s favorite bridge. PRECAST GIRDERS SOLVE DIFFICULT PROBLEMS While cast-in-place construction was feasible for new highway align ment in uncongested areas, an increas ing number of bridges required inno vative solutions using precast concrete. A safety program, begun in the 1960s to eliminate bents directly adjacent to roadways, provided con straints that only a precast, prestressed concrete option could accommodate. Many four-span overcrossings were replaced with two-span structures, eliminating the end bents nearest to the abutment. The existing roadway pro files could not be altered and vertical clearance to the existing freeway was at a minimum. This eliminated the pos sibility of falsework placement and de construction. manded a replacement two-span struc ture depth comparable to the existing four-span type. Half-span segments sign gained rapidly. Within two years, structures not affected by such situa were precast and erected at night on a prestressed concrete bridge was con tions was to use the conventionally re temporary supports while the freeway structed in Fresno to carry heavy truck inforced cast-in-place T-girder struc was closed. They were immediately traffic over a congested highway. ture type for spans less than 100 ft post-tensioned together, the support re Many California bridge engineers (30.5 m) and the conventionally rein moved, and the freeway was clear and soon considered prestressing ‘just an forced box girder for spans greater open to traffic by the next morning. other familiar construction method.” than 100 ft (30.5 m). By 1956, the The solution was very successful. controlling span length decreased to Further growth within the populated CAST-IN-PLACE 80 ft (24.3 m) as the box girder’s pop regions of California, such as San PRESTRESSED BOX ularity grew. Contractors preferred the Francisco, brought about new difficul flat platform falsework surface for the ties for cast-in-place construction. In GIRDER box girder and the cheap material used terstate 280 in the China Basin area of Prestressed concrete bridges in the to form the interior girders reduced San Francisco, constructed in the early 1950s were precast and used overall costs (see Fig. 3). 1970s, used hundreds of precast gird mainly in special situations, such as to Because concrete forming costs were ers for its elevated viaduct (see Fig. 4). work around site constraints on false- much less for the box, the only item Many more bridges needed to be work placement, to speed up construc restricting all around use of this widened with no additional vertical tion, and to meet roadway vertical superstructure system was the in clearance for falsework placement, so clearance restrictions that required creased amount of reinforcement re precast pretensioned concrete girders more slender superstructures. The eco quired. The reduction of steel atthbuted were often the choice. Precast girders nomic rule of thumb in those days for to the new prestressing technology were erected over environmentally 36 PCI JOURNAL ____ sensitive bodies of water and became the preferred structural type to span over or carry rail traffic. Although it remained the minority bridge system during this time period, precast con crete bridges filled an important gap. PRESENT NEED FOR PRECAST BRIDGES Special situations that historically required precast construction are now very common in California. Growing congestion, environmental concerns, and speed of construction are just a few of the contributing factors. Cal- trans is currently working in close co operation with the precast concrete in dustry to develop technologically advanced precast bridges that feature many of the desirable characteristics found in cast-in-place post-tensioned concrete box girder bridges. These characteristics include continuous post-tensioning, seismic resistance, aesthetic appeal, and a low depth-to- span ratio. 4. Route 280 Viaduct in San Francisco, built in the early 1970s, used Although the California I-girder Fig. large quantities of precast prestressed girders. would still be the workhorse for spans less than 125 ft (38 m), a new girder was needed for longer spans that could Horizontal Actuators tot. 2 be spliced together to take advantage reaction frame offered of the low depth-to-span ratios 32 by continuous post-tensioning. The bulb tee structure shape had been rou tinely used in other states for typical bridge spans ranging from 150 to 180 ft (45.7 to 54.8 m), but lacked the hold—down aesthetic appeal of the cast-in-place delta girder, box girder. The precast Vertical Actuators tot. 4 ‘U historically used in aesthetically sensi tive locations, was considered uneco - girders nomical due to its heavy weight. A precast delta girder without a precast type of bent cap deck slab, currently used in Canada, post—tensioning Oregon, Washington, and Texas, was metric (SI) conversIon factors: itt = O.305m tin = 25.4mm then considered. This type of structural in Califor shape, called the “Bathtub” Fig. 5. UCSD model test setup. nia, was chosen to be the next genera tion precast concrete girder shape. The bathtub girder was not the concrete girder types, the bulb tee and manageable repairs and presumably rigid definitive precast solution for all prob bathtub, should be developed. allows continued traffic flow. A the su lems. This girder still required tempo type of connection at column bents until the initial perstructure interface allows the col rary falsework SEISMIC CONCERNS post-tensioning was complete, was umn to better resist lateral seismic girder heavier and more costly than the bulb Current Caltrans seismic design pol forces. Past designs for precast these areas. tee, and would be difficult to continu icy requires superstructures to elasti bridges were suspect in cast- ously mount over a support bent. Cal- cally resist plastic hinging demands The design usually consisted of a with trans decided that two distinct precast from the column. This leads to more in-place inverted T-bent cap 37 November-December 1995 notched precast girders seated on the designed to increase the width of the sary dead load and seismic shears into cap. The deck was continuous over the superstructure effective in resisting the the girders. bent, but there was no effort to obtain plastic overstrength moment of the Testing of both units will consist of the kind of positive moment connec column, reduce reinforcement conges incremental horizontal fully reversed tion needed to resist column hinging tion in the joint region, and increase displacement cycles until target ductil demands. the cap beam torsional capacity. ities have been reached. If a successful The lack of a rigid connection at the Design and construction of the first response is observed, the majority of column superstructure connection be model to be tested has begun and is the damage will occur in the column comes shown in Fig. This is a 4/io-scale, evident when considering a 5. or with only minor distress in the super typical bridge with multiple support 40 percent full scale, model of a typi structure elements and cap beam ele columns per bent.

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