ARTERY Suspension bridge's corroded wires (below)
compose a helical shape that cannot be spliced together
A complex load transfer
operation involving installation of supplemental cables on
a 72-year-old two-lane suspension bridge may be the first
of its kind in the U.S. Consulting engineers for Maines
Dept. of Transportation had discovered extensive degradation
on a main cable that forced bridge load limits and a fast-tracked
The Waldo-Hancock Bridge carries
U.S. Route 1 over the Penobscot River between Prospect and
Verona Island. It has an 800-ft-long center span and 350-ft-long
side spans. The main cables rest on saddles in two truss towers
and are anchored at each end in concrete blocks under approach
ramps. The 10-in.-dia helical cables are composed of 37 strands;
each strand contains 37 galvanized wires.
"The bridge was built during
the Depression for $850,000 and they got the most bridge for
the money," says Jamey A. Barbas, senior project manager
for MDOT consultant Parsons Corp.s New York City office.
"Helical strands are stronger and can stretch longer
spans than parallel wires. [But] you cant splice helical
wires together for a fix."
A $5.3-million rehabilitation program
revealed in July that several hundred wires had broken in
center span sections of the southern main cable, forcing a
12-ton load limit (ENR 7/28 p. 9). The bridge had been rated
for a 50-ton limit. A 1.8 safety factor was verified after
tests showed extensive corrosion, wire brittleness and low
capacity. The rating is well below a comfort zone of 2.5,
Piasecki Steel Construction Corp.,
Castleton, N.Y., had rehabilitated the north cable last year.
For the south cable, MDOT in August hired Pittsfield, Maine-based
Cianbro Corp. under a $4-million emergency contract to be
completed by Nov. 1. "Although [Piasecki] wanted to do
it, this problem required an accelerated schedule that interfered
with other commitments," says Philip D. Roberts, MDOT
Williamsport Wirerope Works Inc,
Williamsport, Pa., rushed production of 16 reels of 2-in.-dia
galvanized helical 91-wire strands. "We had to design
a strand that could be made as quickly as possible, so we
used one wire size instead of three or four," says Thomas
E. Secules, Williamsport structural products project manager.
"We completed the work in about nine weeks, or twice
as fast as normal." Each reel contained over 1,800 ft
and the job cost more than $400,000.
Cianbro completed two new concrete
anchorages with up to 30-ft-long anchor rods. Crews installed
continuous runs of strands on new saddles bolted and welded
on new base plates atop cable bents and main towers. Workers
placed two groups of four strands 12 ft above each main cable.
Each strand weighs 4 tons. A rope pull was walked across,
connected to a 7/8 -in. pull cable, then winched back across
and connected to the strand, which was fed through a tensioner
holding back about 15,000 lb to smooth the pull.
"We hooked and rehooked one
strand per day on average," says Archie J. Wheaton, Cianbro
project superintendent. "The strands were connected to
anchor rods; then we set the sag." The new auxiliary
cables are connected to existing double suspender cables by
11/8 -in. steel rods, then tensioned with 30-ton jacks, bringing
the new cables about 3 ft from the main cables.
CRAWL A rope pull is walked acorss the bridge,
connected to a pull cable and winched back across to install
the supplementary cables.
"We used hydraulic winches
to pull the cables and hydraulic jacks to tension them,"
says Alan D. Fisher, Cianbro chief structural engineer. "There
was no heavy equipment, only 40 workers." Work finished
Oct. 26. The new cables now support 50% of the dead load.
The bridge now carries 40-ton loads with a 3.2 safety factor.
Project officials know of at least one suspension bridge,
in Portugal, that was strengthened by supplemental cables,
but know of no others in the U.S.
"This is a temporary fix to
maintain existing traffic patterns," says Roberts. MDOT
plans a replacement to be completed in 2005. Tallahassee-based
Figg Engineering Group will design the new bridge, estimated
to cost more than $50 million. The historic bridges
future is in doubt and demolition is expected.
(Photos courtesy of Cianbro)