SAISC projects clip angles. The cap beam has an internal diaphragm at the girder line, and the entire connection is bolted. The CM / GC requested slotted holes at specific girder locations to increase tolerances for fit-up during erection.
The transverse cap beams are built-up boxshape members composed of two welded I-girders with top and bottom cover plates. The entire assembly is bolted to eliminate the possibility of crack propagation across the entire section and is designed for the loss of either I-shape or cover plate.
The spandrel columns are welded box sections with dimensions of 42 in. × 36 in. and plate thicknesses varying between 1.25 in. and 2 in. The connection of the spandrel columns to the arch rib is a bolted endplated moment connection.
Establishing the articulation of the spandrel columns was an important aspect of the design. Design iterations evaluated various configurations of“ pinned,”“ fixed,” and“ free” boundary conditions at the 12 column locations, with the primary challenge being to balance structure stiffness and load path during seismic and thermal response.
The final articulation uses unidirectional bearings at the top of spandrel columns in the flanking spans 3 and 5 and fixed endplate moment connections for the columns in the center span 4( the middle arch span). These fixed columns function similarly to a closed arch crown, while the deck structure is free to move at the ends.
Engineering Development Like many replacement projects, local site conditions and the associated built environment imposed a number of engineering challenges. The structural system of the new Sellwood Bridge had to meet the following constraints:
• Provide a horizontal and vertical navigational opening that meets or exceeds that of the existing bridge
• Provide a span layout that, when combined with the existing bridge, would allow continued navigation throughout construction
• Limit the amount of structure constructed in the waterway to comply with no-net-river-level-rise criteria.
• Provide a similar roadway profile as the existing bridge in order to limit project extents and facilitate construction staging
Meeting the profile grade requirement resulted in limited rise in the west arch. The three arches have a rise-to-span ratio that varies from 1:7.7( 0.13) to 1:6.4( 0.16). The shallow nature of the fixed arches led to increased bending demands compared to the more efficient arching action that could be attained with more ideal geometry.
In order to limit the effects of flexural demands on the size of the arch section, the springing connections were left in a pinned condition during construction from initial rib placement through concrete deck placement. The two-hinged arch freely rotated during construction loading, resulting in“ simple span” bending, with zero negative moment at the springing support and increased positive moment at the crown. After deck placement the springing connection was fixed, shifting the flexural response toward“ fixed-fixed” beam action for subsequent loading.
The springing connection consists of ten 4-in.-diameter ASTM A354 Gr. BC highstrength steel rods that are embedded up to 15 ft into the concrete substructure. In the temporary hinged condition, the rods are not tightened to the end of the arch ribs. A high-strength( 15-ksi) UHMW pin plate was placed at the springing connection to transfer axial thrust while allowing rotation, and was coupled with an external frame support for vertical loads. Upon completion of staged construction, the fixed connection was completed by grouting the pin plate gap and prestressing the anchor rods for service level moments.
Thompson Metal Fab proposed pieceby-piece stick erection, with arch ribs placed on shoring towers instead of a float-in system originally considered for arch erection. Each rib span contained two bolted field splices to match the optimum weights chosen by the CM / GC for fabrication and erection, resulting in three segments per span with lengths up to 148 ft and weights up to 146 tons each. Steel was transported to the site on barges and placed with cranes operating from work bridges and barges.
When the bridge opened to traffic, the crossing immediately jumped to a sufficiency rating of 100.
Project Team
Owner: Multnomah County, Ore. General Contractor: Slayden / Sundt Joint Venture Slayden Construction Group, Stayton, Ore. Sundt Construction, Tempe, Ariz. Structural Engineer: T. Y. Lin International, Beaverton, Ore. Architect: Safdie Rabines Architects, San Diego Steel Team: Fabricator: Thompson Metal Fab, Vancouver, Wash. Detailer: Candraft Detailing, Inc., New Westminster, B. C., Canada
Steel Construction Vol. 40 No. 6 2016 15