THE CONCRETE BRIDGE MAGAZINE

WINTER 2012

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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each span. In span 6, due to traffic clearance limitations, the superstructure depth is reduced to 6.5 ft, decreasing to 4.75 ft at the face of the exterior webs. The columns are rectangular, 8 by 10 ft with the longer faces curved in a 13-ft radius. The viaduct is joined to a 320-ft radius, reinforced concrete connector bridge that leads to Pacific Coast Highway 1. The connector bridge has five spans ranging from 100 to 108 ft in length, with a constant superstructure depth of 6 ft. The connector varies from 33 to 40 ft in width. Foundation Type Geological conditions at the site vary drastically along the bridge alignment. The soil strata contain varying depths of sandy/silt layers, along with stiff clay layers underlain by bedrock. The depth to bedrock varies dramatically along the longitudinal alignment of the bridge The high liquefaction potentials at Bents 3 and 4 dictated the use of pile shafts for the bridge foundations. Historically, these foundation types have performed well in seismic events under similar soil conditions and are superior to spread footings and pile caps as they reduce the possibility of lateral spreading. Cast-in-drilled-hole shafts with rock sockets were used at all bents. To mitigate the possibility of caving during construction, 12-ft-diameter, permanent steel casings were installed into bedrock at Bents 2, 3, and 4. Additionally, 11.5-ft-diameter rock sockets were installed into bedrock at these bents to a depth of 30 to 40 ft below the permanent steel casing tip elevation. Seismic Design Considerations T h e S a n A n d r e a s F a u l t l i e s approximately 6 miles southwest of the project site and has a maximum m o m e n t m a g n i t u d e o f 7 . 9 . T h e Presidio Viaduct is classified as a post- earthquake "Recovery Route" and as such, seismic design of the viaduct considered two levels of earthquakes, Functional Evaluation Earthquakes (FEE) and Safety Evaluation Earthquakes (SEE). A FEE has a smaller magnitude and a probabilistic hazard for such an event with a mean return period of 108 years (i.e., 50% probability of exceedance in 75 years). A SEE has a greater magnitude with an acceleration response spectrum derived from the envelope of the median deterministic Maximum Credible Earthquake for the region, with a probabilistic hazard for such an event with a mean return period of 1000 years (i.e., 7.5% probability of exceedance in 75 years). Stiffness balancing The drastic variation of the soil profile along the bridge alignment resulted in very stiff columns at Bents 5 and 6 compared to Bents 3 and 4. The related CAST-IN-PLACE, POST-TENSIONED, CONCRETE BOX GIRDER BRIDGE WITH CAST-IN-PLACE RECTANGULAR COLUMNS AND CAST-IN-DRILLED-HOLE SHAFTS / CALIFORNIA DEPARTMENT OF TRANSPORTATION, OWNER bRIDge DeSCRIpTIoN: A 1340-ft-long, cast-in-place concrete box girder bridge with variable width and bifurcation. The span lengths are 188, 275, 275, 275, 184, and 143 ft. The section depth is constant at 12.75 ft, except the last span that has a 6.5 ft depth. Cross section has 14-ft overhangs supported by steel fins spaced evenly at about 15 ft centers. The substructure is 8 by 10 ft single or double column bents supported on 12-ft-diameter, cast-in-drilled-hole shafts. bRIDge CoNSTRuCTIoN CoST: $38.5 million AwARDS: 2011 ASCE Geotechnical Design Bridge Project of the year Award, San Francisco, Calif.; 2011 Roads and Bridges , Top 10 Bridges in the U.S. A E S T H E T I C S C O M M E N T A R Y by Frederick Gottemoeller In too many viaducts, the design focus is restricted to the bridge itself. The need to knit back together the spaces under the bridge and relate the bridge to the uses around it is often forgotten. The visual quality and sometimes even the security of the space underneath are ignored. The Presidio Viaduct makes none of those mistakes. A major goal of the project is to recreate and restore, in so far as it can reasonably be done, the topography and landscape of the Presidio before the Golden Gate Bridge was built, and to make the visible elements of the Golden Gate approach structures as unobtrusive as possible. The aesthetics of this viaduct are really not about the bridge itself; but about what goes on under and around it. The long spans minimize the number of piers, making it easy to see through the bridge from all angles. The bridge presents little obstacle to the flow of space through it. The piers themselves are simple shapes with no visible pier caps or articulation. The common geometrical shapes tend to fade from our notice. The curved underside of the post-tensioned concrete box girder is shaped to blend in with the steel braces for the overhangs, visually unifying the parts into one continuous element. The box presents a smooth and featureless underside, with no details that would draw our eye or create visual contrasts. The concrete soffit reflects light into the space under the bridge, keeping the underside spaces bright and supporting the planting. The regularly spaced steel overhang braces establish a rhythm that relates well to the features of nearby buildings, allowing viewers to measure the size of the bridge in comparison to its surroundings. Plus, they create an opportunity to visually tie the viaduct to the Golden Gate by the use of color. Future users of the Presidio will find it a pleasing structure to be around, one that is an asset to the Golden Gate National Recreational Area. ASPIRE , Winter 2012 | 31 Book_Win12.indb 31 12/29/11 11:12 AM

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