THE CONCRETE BRIDGE MAGAZINE

WINTER 2018

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.

Issue link: http://www.aspiremagazinebyengineers.com/i/922349

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The bridge was designed according to AASHTO LRFD specifications, AASHTO's Guide Specifications for LRFD Seismic Bridge Design, and ODOT's Bridge Design and Drafting Manual. The total post-tensioning required was four hundred 0.6-in.-diameter strands tensioned to 75% of ultimate strength for a total force of 17,577 kip. Each web contained four ducts. The number of strands per web varied across the cross section in relation to the lengths of the individual webs as they spanned the curved alignment, with the maximum number of strands being 108 and 92 for webs on the outside and inside of the curve, respectively. Previous experience shows this also helps counteract some twisting of the box due to dead load t o r s i o n . T h e p o s t - t e n s i o n i n g a n d grouting occurred without issue, except that a limited amount of concrete needed to be removed at one of the abutments to allow the expected shortening to take place without engaging the pile cap. ODOT required a two-tiered approach to seismic design: the bridge is to behave elastically during a 500-year event and not collapse under a 1000-year event. The peak spectral accelerations at the site are 0.58g and 0.72g for the 500- and 1000-year events, respectively, where g is the acceleration of gravity. Most of the design was controlled by the 500-year event criteria. A Seismic Design Category (SDC) of D was determined for this site. Because the bridge needed to get back down to grade, the column heights at bents 2 and 3 were much shorter than those at bent 4, which is located in the excavation for the bypass. The AASHTO guide specifications require individual bent stiffnesses to be relatively similar throughout the length of a bridge that is being designed for SDC D. Using two smaller-diameter columns at bents 2 and 3 provided most of the needed flexibility, with the remaining required flexibility achieved by placing the pile caps an additional 6 to 8 ft deeper to increase the column lengths. Lengthening the columns had the additional benefit of reducing the overstrength plastic forces transmitted into the crossbeams and pile caps, which were required to be designed as capacity-protected members, meaning they are to behave elastically while the columns are allowed to develop plastic hinges during a seismic event. One of the design challenges on this bridge was detailing the reinforcement i n t h e c ro s s b e a m s . T h e A A S H T O guide specifications have prescriptive requirements regarding the quantity of reinforcement encasing the intersections of the columns and crossbeams, which can result in significant congestion and interference between the seismic, column, crossbeam, and box-girder reinforcement plus the post-tensioning ducts. Using a three-cell box allowed placement of the column reinforcement between the webs, and extending the ends of the crossbeams 1 ft past the outside face of the box minimized the conflicts as much as possible. Inspection access hatches were provided in every cell at both ends of each span. Access holes were not placed through the crossbeams, to eliminate that additional design and detailing complication. The Wynooski Road Bridge over the Highway 99 Bypass is a great example of how a post-tensioned CIP box girder can be applied to fit a geographically and foundationally challenging site with multiple restrictive design criteria, including horizontally and vertically c u r v e d a l i g n m e n t s , h i g h - s e i s m i c demands, and prescriptive detailing requirements. The flexibility of this structure type allowed for a cost- effective and attractive structure to advance the development of the long- awaited Newberg-Dundee Bypass. ____________ Eric E. Bonn is a senior project engineer with OBEC Consulting Engineers in Salem, Ore. Placing concrete for bent 2 footing. Note the scale of the large footing supporting both columns. Photo: OBEC Consulting Engineers. Placing concrete in web of the box girder. Photo: OBEC Consulting Engineers. Placing concrete in the bottom slab of a box girder near a pier. Post-tensioning ducts can be seen in the webs. Photo: OBEC Consulting Engineers. Top slab formwork being constructed. Detailing the reinforcement to avoid interferences and congestion was challenging. Photo: OBEC Consulting Engineers. 16 | ASPIRE Winter 2018

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