RC105 MarApr2023 - Magazine - Page 31
ed rocking piers and hinges in the existing
girders, as well as critical tolerances for the
rails and gantries, anchorage of the pier
brackets and runway beam without conflicting with existing rebar, plus concerns about
the bridge’s deficient shear strength. And, of
course, it had to remain open to traffic.
The gantry scheme required review
and approval by the director of the city’s
Integrated Infrastructure Department. The
gantries needed to remove 400 pieces of
concrete of up to 80 tonnes each according
to a prescribed order of removal aimed at
maintaining the bridges structural integrity
through all stages of the process.
“First we cut the concrete deck between
the girders and lifted those pieces away using
the gantry cranes, and then would cut and
lifted the girder pieces out,” recalls Grundke. “The sequence followed to remove the
girder segments, at times leaving unbalanced
girders cantilevered over the river, so we
would secure these with ‘strongback’ steel
pieces anchored to the gantry’s support steel.”
Demolition took about four months for each
phase, much of it in the winter. “In addition
to staying below the 80-tonne combined capacity of the cranes, we also needed to control
the sequence we picked each piece to ensure
that loads transferred to the support system
were as designed,” says Grundke. “Many of
the girder lifts were near the cranes capacity
by design. It was critical that our team treated
every lift as seriously as the first one.”
Phase I demolition was very challenging,
recalls Murray. “As they started removing pieces of deck, we found the deck was
delaminated in places, so we needed a new
plan to get the deck panels off in one piece,”
he says. COWI developed a new plan to
revise the anchoring of the panels.
Importantly, the gantry crane approach
proved sound, and, with experience and
adjustments, the work became more efficient.
Consequently, says Murray, “The second half
was far smoother and easier. We had learned
lessons, so we were prepared for everything,
plus we had half of the bridge rebuilt, which
was much stronger than the original.” To further protect the river below during the actual
concrete cutting, Graham installed a mobile
containment system beneath the bridge to
catch the water/concrete slurry during cutting. The concrete slurry was vacuumed and
filtered and then reused for subsequent cuts.
The gantry cranes remained active during
reconstruction, which included hauling
rebar, formwork, the new steel girders, and
other heavy loads. Graham’s innovative
work sequencing saved time by having the
false deck formwork, and rebar installation
commence immediately upon placement
of each span of new girders, rather than
waiting until all seven spans were fully
installed. That too, notes Jennings, “Required
a lot more detailed calculations of where
one expects the steel to be, elevation-wise,
rather than just measuring the girders once
installed.” But the significant time gained
proved well worth the added effort.
In October 2020 the refurbished Groat Bridge
officially opened on-schedule. Graham’s
runway beam was later sold, with parts of
the truss repurposed for the Kicking Horse
Canyon project at Golden, B.C.
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