Steel Construction Vol 40 no 6 - International Steel Structures | Page 16

SAISC projects
Sellwood neighborhood to restrict traffic to
two lanes to match the capacity of Tacoma
Street to the east. While the existing bridge
had an overall structure width of 32 ft, the
new structure provides 6-ft, 6-in. shoulders,
designated bike lanes and raised 12-ft
sidewalks on each side of the bridge. The
result is a pedestrian-friendly structure that
has a nominal width of 63 ft. The structure
width increases on the western half of the
bridge to 90 ft, allowing for additional turn
lanes to and from Highway 43. Using 5,000
tons of structural steel, the bridge opened
earlier this year.

Nick Garibbo/Nick’s Photo Design

Steel deck arch structure
The 1,275-ft-long three-span steel deck
arch has a span arrangement of 385 ft-425
ft-465 ft, with two arch ribs per span. The
progression of span lengths generally
follows the rise of the bridge in grade from
west to east.
A reinforced concrete Y-arm extends
from the pier and footing substructure to
meet the steel arch rib at the springing
connection in order to keep the steel ribs
above the 100-year flood stage. These
extensions are up to 36 ft in length at the
river piers and follow the curved geometry
of the arch.

Nick Garibbo/Nick’s Photo Design

The solid-ribbed arches are welded box
sections with a constant web depth of 70
in., a flange width of 54 in. and a smooth
parabolic curve profile (all steel curving
was performed in-house by the project’s
fabricator,Thompson Metal Fab). Each of
the three arch spans has four spandrel
columns, which coincide with the location
of the portal bracing between the two
ribs. Each spandrel column supports a
transverse steel cap beam, with longitudinal
girders spanning between them.

Nick Garibbo/Nick’s Photo Design

TOP: The shallow nature of the bridge’s fixed arches
led to increased bending demands.
MIDDLE: There are three segments per rib span,
with lengths up t o 148 ft.
BOTTOM: The 1,275-ft-long three-span steel deck
arch has a span arrangement of 385-ft- 425 ft-465
ft, with two arch ribs per span.
OPPOSITE PAGE: Steel was transported to the site
on barges and placed with cranes operating from
work bridges and barges.

14 Steel Construction Vol. 40 No. 6 2016

and 70 ground anchors with loads up to
850 kips per anchor, the system is designed
to limit seismic deformation to under 4 in.
during a moment magnitude scale (MMS)
9.0 Cascadia Subduction Zone earthquake.
The landslide mitigation was bid at a
construction cost of $14 million.
Both the original truss bridge and new arch
bridge have only two through-traffic lanes.
This was the recommended configuration
from the environmental impact statement
(EIS) stage, driven by the request from the

Both the girders and cap beams have
an overall steel depth of 60 in. and are
composite with the reinforced concrete
deck. The girder system is 15-span
continuous over the 1,275-ft arch structure,
with five to seven girder lines spaced up
to 14 ft, 6 in. Based on pricing feedback
from the CM/GC, plate transition splices
were eliminated and flange and web plate
thickness were held constant for the entire
girder system. Flange plate width varied
based on structural demand but was held
constant within a spandrel span.
Top and bottom girder flanges are
connected across the cap beams with a
continuity connection plate while the
girder web is connected with traditional