The new bridge over the Willamette River at Salem, designed and built
under the supervision of the State Highway Department, is one of the
largest strictly highway bridges erected in the United States in 1918.
This structure has a total length of 2,220 feet, and consists of 352
feet of reinforced concrete approach on the Marion County end, six steel
spans over the river aggregating 885 lineal feet, and 780 feet of high
class pile trestle approach on the Polk County end.
To provide for the river navigation, the U. S. Engineers required a
minimum horizontal clearance normal to the channel of 120 feet and a
vertical clearance of at least sixty-six feet above low water, in case a
high level type of bridge were adopted. After careful study it was
decided to construct a high level bridge of the deck type, and to carry
the pony channel span on cantilevers projecting from the adjoining
spans, giving an arch effect over the channel. By this type of
construction, a clear distance of about 145 feet between channel piers
was obtained.
To minimize the cost of fabrication and erection of steel, four spans
were made practically the same. The east span was necessarily shorter
than the typical ones, since otherwise it would place the channel
opening too far across the river. This span is not so deep as the
adjoining one, and the difference in height is made up by means of a
rocker under the small span, which, of course, also serves the purpose
of movable shoes. The channel span rests on cast steel rockers on one
end, which are carried by a shelf on the cantilever panel. The opposite
end of the span was pin connected.
Previous to the design of the bridge, wash borings were made, which
indicated in a general way that the bed of the stream was composed of a
few feet of gravel, underlaid with sand for a considerable depth. It was
accordingly decided to carry the foundations down below the probable
point of future scour, and carry the loads entirely upon piling. The two
channel piers were sunk about thirty-five feet below low water.
The west approach is carried on fir piling treated in an open vat with
carbolineum to a temperature of about 220 degrees F.
Lumber being comparatively cheap, the entire roadway deck of the bridge
and wooden approach is composed of fir, three by seven inches and
twenty-six feet long on edge, spiked together, thus projecting a foot
outside the curb on either side, the roadway being twenty-four feet
between curbs. An asphaltic wearing surface three and one-half inches
thick effectively waterproofs the wood, and the small interstices
between the pieces will allow sufficient circulation of air to preclude
any possibility of attack of dry rot. The ends of the floor members were
painted with hot carbolineum, as were also the wooden members contiguous
to openings in the floor occasioned by expansion joints between spans.
The two five-foot concrete sidewalks are carried on brackets beyond the
trusses. The curbs serve as reinforced concrete beams to carry half the
sidewalk load to the trusses, there being small concrete struts
extending up from the top of the top chords at panel and midpanel
points. Between the bottom of the curb and the top of the wooden floor
is a three-quarter-inch cushion of "Sarco." Since the bridge is on a
grade, it was necessary to precast this in sheets the width of the curb.
When the material hardened, it was rolled up in convenient lengths and
placed. It will thus be possible in the future to renew the wooden floor
with comparatively little difficulty, although it is believed that the
wood floor base, protected as it is now, should last at least fifty
years.
On account of the rather extreme length for pony trusses of the channel
span, 123 feet between end pins, it was decided to tie the trusses
together, and this was accomplished by means of two arched lattice
struts placed each twenty feet and six inches from the center of the
span.
A final coat of battleship gray paint was applied to the steel work, and
has evoked considerable favorable comment, being an innovation from the
black paint commonly used in this part of the country.
The bridge was built by the Coast Bridge Company and Robert Wakefield of
Portland. The cost of the structure complete was $250,000.00, of which
Marion County paid approximately $200,000.00 and Polk County $50,000.00.
The engineering cost for the work is remarkably low. It includes
surveys, borings, inspection of cement and steel fabrications, resident
engineer's salary, cost of design, blue prints and supplies and
stenographic work, and amounts to $4,600.00, or about one and
eight-tenths per cent of the total cost.