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:

Contents of this Issue


Page 16 of 59

girder has a minimum depth of 7 ft 3 in., with 9-ft 3-in.-deep parabolic haunches at all interior bents. The span-to-depth ratio is approximately 29, resulting in a very slender appearance. The haunches extend for a distance of one-fifth of the span length on either side of the crossbeams to simplify the generally complicated forming geometry over most of the bridge's length. The depth of the haunches was minimized to provide the necessary vertical clearance beneath the bypass without raising the roadway alignment, which would have required additional bridge lengthening. Wynooski Road Bridge's cross section includes a 34-ft roadway and two 7-ft-wide sidewalks. Including the rails, the total bridge width is approximately 50.4 ft. A three-cell box girder with a web spacing of 12 ft 6 in. was selected to coordinate with the column configurations, provide room for the necessary number of post-tensioning ducts, and limit the top slab overhangs to one-half of the web spacing per Oregon Department of Transportation (ODOT) requirements. The exterior webs are sloped 1:4 (horizontal to vertical). The raised sidewalks are integral to the box girder and act as the top flange in the overhangs. The thickness of the top slab above the cells is a constant 9.75 in. The webs and bottom slab have minimum thicknesses of 12 in. and 6 in., respectively, and thicken at the bents. The crossbeams of the interior bents are integral with the box girder. B e n t s 2 a n d 3 e a c h h a v e t w o 4-ft-diameter columns. Bent 4 is located in the future median of the bypass and, with the alignments of Wynooski Road and the bypass intersecting at a 60-degree angle, a single 6-ft-diameter column was selected, thus allowing the crossbeam to remain perpendicular to the box. The bridge is supported on driven steel-pipe piles. Concrete drilled shafts were considered, but they could not develop the necessary capacity in the site's soft, deep foundation soils. One of the geotechnical borings was advanced to a depth of 300 ft and encountered only clay soils. Therefore, large pile caps at the interior bents w e re n e e d e d t o f i t t h e re q u i re d number of piles and space them as necessary to resist overturning forces. At bents 2 and 3, the pile caps are 22 ft by 44 ft and support both columns; the bent 4 pile cap is 34 ft square. During design, the 8-ft thickness of bent 4 caused concern that excessive h e a t o f h y d r a t i o n m i g h t d e v e l o p during the concrete curing. To address this issue, the bent was divided into two 4-ft-thick layers with a horizontal construction joint. Two different concrete mixtures were used in the superstructure. The bottom slab and webs used 5-ksi concrete. The top slab and sidewalks used 5-ksi high- performance concrete (HPC) to increase wear resistance and reduce permeability. T h e H P C i n c l u d e d 6 6 % p o r t l a n d cement, 30% fly ash, and 4% silica fume for the cementitious material, as well as 5 lb/yd 3 of 1½-in.- to 2-in.-long polypropylene macrofibers. The water- cementitious materials ratio was limited to 0.40. All other bridge elements used 4-ksi concrete. In addition to the HPC, other durability measures included limiting tensile service stresses in the box girder to one-half of the stress allowed by the American Association of State Highway and Transportation Officials' AASHTO LRFD Bridge Design Specifications, increasing the concrete cover over the top mat of top slab reinforcement to 2½ in., and eliminating all joints by using a continuous superstructure and semi-integral abutments. When using HPC in decks, ODOT does not require the use of epoxy-coated reinforcement in most regions of the state. 7 ft 0 in. Sidewalk 34 ft 0 in. Roadway 7 ft 0 in. Sidewalk 7 ft 3 in. min. 9 ft 3 in. max. 9 ft ¾ in. 1 ft 6 in. max. 6 in. min. Typical All Webs 2 ft 6 in. max. 1 ft 0 in. min. OREGON DEPARTMENT OF TRANSPORTATION, OWNER BRIDGE DESCRIPTION: 685-ft curved cast-in-place continuous post-tensioned 7 ft 3 in.- to 9 ft 3 in.-deep concrete box girder with spans of 120, 210, 210, and 145 ft and a deck width of approximately 50.4 ft. The bridge was built with integral piers and semi-integral abutments to eliminate all joints on the structure. BRIDGE CONSTRUCTION COST: $5.3 million (approximately $155/ft 2 ) Typical section of cast-in-place box girder. Figure: OBEC Consulting Engineers. Bent 2 elevation. Figure: OBEC Consulting Engineers. Bent 4 elevation. Figure: OBEC Consulting Engineers. ASPIRE Winter 2018 | 15

Articles in this issue

Archives of this issue