Associated Engineering's fast thinking and novel solution minimizes closure of overpass
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| Fully-restored Moody Street Overpass reopened to traffic |
On June 24, 2009, Associated Engineering was retained by the City
of Port Moody, BC to assess damage to the Moody Street Overpass
after a garbage truck, with its lifting arms elevated, accidentally hit the overpass, seriously damaging the superstructure. The overpass
provides an
important commuter
corridor
connecting the
areas north of the
CP Rail mainline
with the remainder
of the City,
while businesses
to the west use
Murray Street,
below, for access.
Closure of the
overpass effectively
cut off
access to the
west. Therefore
the immediate
priority was to
secure the structure above Murray Street for controlled traffic use
beneath the span. This required removing fractured superstructure
concrete that was potentially hazardous, along with a rapid assessment
of the safety of the damaged superstructure. Closure of the
overpass forced a major detour onto many commuters, so the City
was keen to reopen the structure as soon as possible. Our assessment
was that the required structural repairs would take some time,
but that following the immediate demolition phase, a one-lane detour
plan across the east side of the deck was feasible with suitable traffic
control.
Although serious damage was limited to only one girder, replacing
the girder was a challenge. Dating from the 1970's, the 20 metre
long precast girder was a type no longer produced in BC.
Furthermore, the section was the central portion of three semi-continuous
spans and was acting compositely with the concrete bridge
deck. Replacing the damaged girder would therefore have required
a considerable amount of deck to be removed which would have triggered
an investigation into the safety of the partially-demolished
bridge under traffic.
The truck had removed a large portion of the west fascia girder
above Murray Street, near the north end of the ten-span structure.
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| Accident-damaged fascia girder |
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| Steel girder installation using fork-lift trucks |
Bridge reopened to traffic after only five weeks
When the Associated Engineering team of Shane Cook and David Chen examined the damaged girder,
they determined that the original prestressing strands were mostly undamaged. However, with most of
the concrete section fractured, they suspected that much of the prestressing force had been lost and
would be very difficult to restore. Rapid analysis suggested that if the girder was repaired, it could safely
carry its own weight without the intended prestressing force, but additional capacity for the superimposed
loads would need to be added.
Shane and David devised a novel solution: adding a section of steel plate girder that could be rapidly
procured and inserted alongside the fascia girder, but was slightly shorter in height to facilitate installation
beneath the deck. The steel girder needed to project through the pier diaphragms into the adjacent
spans to replicate the structurally continuous behaviour of the original design. To permit installation, the
girder was designed in two sections with a bolted field-splice. Openings were sawn through the concrete
diaphragms and the girder sections were shoehorned into place using fork-lift trucks, and bolted together.
Bearing pads were placed at the piers caps and grouted.
While the girder was being fabricated, the fascia girder was repaired. Removing the damaged concrete
did not result in strand pull-out, and
so the straight strands were in the
correct position and self-supporting.
Additional reinforcing bars were
selectively added to replace damaged
stirrups and assist with crack
control of the replacement concrete
(which was non-prestressed). Selfcompacting
concrete was used to fill
the custom-built girder formwork.
Following installation, the steel girder
was shimmed against the deck
and preloaded by suspending
weights near mid-span, followed by
grouting of the gap between steel
girder and bridge deck. The preloading
ensured that the steel girder
picked up its assigned portion of
superstructure dead load, thereby
relieving the fascia girder. After the
steel girder was grouted, the pier diaphragms were reinstated. While the fascia girder will continue to
undergo live load service deformations, any contribution to resisting imposed loading is unnecessary.
The critical construction work was carried out by West Shore Constructors, who coordinated a number of
subcontractors and performed repairs. Shane Cook reports, "Thanks to fast and effective work all round,
the Moody Street Overpass was opened to full two-way traffic operation on July 31 at 7 a.m."
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