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Once all the jacks are in and activated, the
remainder of the concrete is removed so all that remains are four
vertical pieces of rebar.
Before the jacking begins the rebar is cut with
a torch and the hydraulic jacks start elevating the bridge.
The bridge is raised and steel plates called
shims, which range from a 0.25 to 2 inches, are placed in the column
until the maximum height of the jack is reached.
The hydraulic pressure is then released until
the bridge rests on the shims. The 2-inch shims are welded at 6-inch
intervals.
When the desired height is reached mechanical
splices are used to replace the missing rebar.
Concrete is poured back into the columns and
allowed to cure for 14 days.
The road approaches on both sides of the bridge
are built up to match the new height of the bridge.
On site
The Lafayette Bridge has four bents; each bent
has four columns for a total of 16 columns plus two end bents.
Twenty-four jacks were used to elevate the entire structure.
The Battle Creek structure has three bents with
two columns per bent and two end bents.
“The columns are probably the best place for
dealing with all the forces of the bridge,” says Bryant. “Because the
only thing you have there is straight compression and no other forces to
deal with.”
The Battle Creek and Lafayette bridges were
raised because of damage caused when over height loads hit the
structures and chipped away concrete from the bottom portion of the
bridge deck and also severed reinforcing steel on the exterior girders.
This past spring, one lane of Interstate 5 was
closed for a short period after the Battle Creek Bridge was struck by a
semi truck and chunks of concrete fell on several vehicles. No one was
seriously hurt.
“Usually what happened is when a low boy with a
track hoe travels under the bridge, and because of the hydraulic bleed
off, the knuckle raises the track hoe arm enough to strike the bottom of
the bridge,” said Bryant.
The Oregon Department of Transportation placed
load restrictions on the bridge because of damage from heavy loads and
cracks due to age.
The Battle Creek work was part of a bigger
project that cost $4.2 million and consisted of paving this section of
I-5, rebuilding 12 bridge approaches, abandoning the Boone Road Bridge,
replacing old median guardrail with new tall concrete barrier, and
replacing old bridge guardrail with new. In addition, shear cracks, some
up to 0.025 inch, on the Battle Creek Bridge were filled with epoxy.
The Lafayette Bridge project cost $633,000.
Raising the Lafayette Bridge, and the Battle
Creek Bridge with a dead load per bent of approximately 300 tons or 75
tons per column, requires a heavy-duty jacking system. ODOT standards
require the system to be capable of supporting one and a half times the
dead load.
There are carbureted hydraulic systems located
at each one of the bents so all four columns at one bent are under the
same hydraulic system.
During the jacking of the bridge each bent is
raised independently and timed accordingly so all bents are elevated at
about the same time. The workers communicate with radios during this
procedure or by whiteboards as was the case at Battle Creek on I-5
because of the noise.
“So that is why you really only go up a quarter
inch at a time,” said Bryant. “You don’t want one bent going up faster
than the other because that could cause some stresses in the bridge you
don’t want.”
Dan E. Knoll represents Region
2 of the Oregon Department of
Transportation.
Reprinted from Better Roads Magazine
February 2005 |
