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Matt Chynoweth: My name is Matt Chynoweth, I am the engineer of Bridge Field Services for the Michigan Department of Transportation
and today we're talking about our Zilwaukee Bridge bearing replacement project.
The Zilwaukee Bridge is a very large, very complex precast, segmental concrete bridge.
The bearings are the original bearings from the original construction and it was noted years ago that these bearings need to be replaced,
so that's part of the project that we're doing here.
The bearings transmit the vertical loads from the superstructure down to the substructure,
and they also allow the bridge to move and also it allows the superstructure to rotate due to loads on the structure.
If these forces and rotations weren't accommodated for, all of that would be transmitted directly into the substructure
and would decrease the service life of the bridge.
In terms of how we get the old bearings out, it's a very complex, high-capacity jacking operation.
In some areas, we're lifting up to 17 million pounds at a time, about a half-inch.
Once we unload the superstructure, we bring in a wire saw,
it's got ground diamond dust on it and we saw out the existing bearing.
We pour a grout pad, put the new bearing in, and then release the jacks, which puts the force back into the new bearings.
This particular location is part of our expansion joints
and the expansion joints themselves are what I consider to be probably the most complicated part of the structure.
At the expansion joints, there is a cantilever segment and a supported segment.
The supported segment actually sits on the cantilever segment, so we have to actually have to pry the
supported segment off of the cantilever segments so we can get to the bearing, which is in between the two segments.
So what you're looking at here is our very large strong bag beam assembly
and what this does is it transfers the weight from the upper segment to the beams themselves.
There are jacks underneath the beam. Those jacks, those are 400-ton jacks that apply a force
that allow the upper and lower segments to be pried apart so we can get in and remove the bearings.
So the two very large strong bag beams, they sit directly on the web of the box structure
and there's an overhead beam up top, and an underslung beam down below.
The exact opposite of this beam is right below
and then the whole system is held together with 12 2 1/2-inch post-tension rods
and each one of these rods is stressed to several hundred thousand pounds,
So the total force that is in this structure from side to side is about 3 million pounds squeezing the superstructure together,
holding it together so we can pry apart the top and bottom.
So what I'd like to show you now is... these are the post-tensioning rods that connect the overhead beam to the underslung beam.
And to get them to connect, we have to core holes through the top flange of the segmental box.
The box itself, the box structure, has mild rebar that runs throughout it,
it also has transverse and longitudinal post-tensioning.
We use ground-penetrating radar to actually locate the bar locations and the transverse tendon locations in the vicinity,
and then in the case of these holes, which are very close to it, we chip down to locate the conduit.
This hole is kind of filled with water but in there, we were able to locate the edge of the conduit that shows where the tendons are.
The tendons are marked in red so that when we core through, we don't hit any of the transverse post-tensioning tendons
that are critical to the structural capacity of the box section.
So here we have the custom-made 600-ton jacks that were made specific for the Z-bridge project,
and, depending on the size of the pier and the lifting loads,
we may have eight of these per column. So eight and eight, we may have 16 of these acting together to lift the superstructure,
which, at its heaviest lift at piers 10 through 16, is roughly 16 to 17 million pounds.
So these are 600-ton capacity. The jacks themselves have lock rings that we spin into place.
When the piston has come up, we can spin these into place, lock them into place, and then take the hydraulics off,
so in case there's ever a hydraulic failure, that's a safety feature on these jacks.
The jacks also have swivel heads on them to allow up to a 3% slope on the jacks.
These are large jacking and bearing plates that distribute the jacking force to the columns,
into the superstructure itself. So these are the custom-made jacks that we got for this project.
We got about 85 of them for all the different operations we're doing on the bridge.
So what we have here are the PT bars that come up through the deck that actually hold
the work platforms for the piers up underneath the deck
and it holds them into the right position so that we can get all of our equipment down there,
all of the jacks, all of the bearings - everything that we need to replace the bearing at the pier is lowered from the sides here,
so there are total of... there's four on each side,
so four on this side, four on the opposite side. These things are post-tension and they hold the work platform up underneath the box structure.
So we're on the other side of the bridge deck now, and here are the other four post-tensioning rod locations
that support the work platforms from below. And to get access to the work platforms,
the contractor has devised a scheme where we go up over the barrier.
We use our lanyard, we tie off onto the ladder cage,
and then we crawl down onto the work platforms. So this is our access to and from the work platforms.
The platforms themselves are fitted with two exterior smaller platforms that are on Hillman rollers that can roll back and forth.
The contractor uses these to lower heavy equipment on to the platforms and then slide them into place.
The contractor also uses these to remove the existing bearings and insert the new bearings.
The platforms support a significant amount of weight. All of the equipment and workers doing all the work,
along with the temporary works that we need to temporarily brace the columns. We put compression collars on the columns at the jack locations.
We also put transverse post-tensioning through the box section.
OK, so we were underneath the bridge, now we are inside the bridge at Pier 22-S.
This is inside the superstructure of the bridge.
These are concrete diaphragms that are inside the bridge. This helps distribute all the loads from traffic
and the dead weight of the structure into the bearings and then onto the substructure.
So when we put jacks underneath the superstructure to jack it up, we are changing the load path.
This is an additional concrete diaphragm that we poured at the area of jack placement.
This concrete was poured yesterday; we will do a three-day compressive-strength break test.
If its at 4500 PSI, we will strip the forms.
These are post-tensioning bars that we will stress to a large force to pre-compress the concrete in the diaphragm.
That concrete will then act as the resistance and we will jack underneath
this particular portion of the bridge to raise it.
We're only going to raise the bridge about a half an inch, just enough to the old bearing out and put the new bearing in.
So here we are at the north abutment of southbound. The transverse post-tensioning bars are installed.
The bearing is currently being chipped out and its being supported by the jack.
If you look behind me, you can see all of the pier bearings that we have replaced.
And if you look to my right, you'll see the northbound structure
with all the existing bearings, the original bearings.
And that's the work that's ahead of us for 2014.
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