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