Remove and Replace a Bridge
Class 12 - Session 2

We have a video now showing the underpinning operation, which in this case, is the installation of needle beams to pick up the load - the concentrated load of the column.

This view shows the jacking arrangement used to transfer loads into the needle beams. It is essential to preload and pre deflect the needle beams. You do not want the load of the bridge coming onto the needle beam and then settling. This is a very important principle to preload all of the temporary structure first, before the load is transferred. When the load is transferred, there should be zero movement.

That will create a small space on top of this stack of shim plates This is a good time to remind you that even in support of excavation, when you have a deep excavation with significant loads, you preload all of these struts for the same reason. You don't want any movement of that sidewall, the support of excavation wall. You preload the strut, which, in effect, also preloads the whaler and takes out any sloppiness in the connections. You do all of that first to avoid any movement of the adjoining ground. Also, the jacks in this case were installed at the ends of the beams, so they're actually going to lift up the ends of the beams. That will create a small space on top of this stack of shim plates. You need to have at your disposal some very thin shimming material to fill that space. Then, you can let the jacks relax and the load will come down on those shim plates. All of that has to be prepared in advance.

Here you can see the temporary tower that's been erected to carry the needle beams. The two iron workers are actually standing on the needle beams. They're using a clamp on the existing bridge to help them set the needle beams to precisely in place.

It's really impossible, because of the low headroom, to get a crane to handle the beams, so they're taking advantage of the existing structure. They'll use chain fall or some other device to help them set the beams precisely. You can roughly set them with this machine. You can call that a low overhead crane, but it actually looks much more like a forklift. In fact, it's supporting the needle beams from below. It's not really over the load hanging from a hook. There's just no room to do that. This is actually working from underneath the needle beams. That's a man lift going up in the background- a very quick way to put a man in place.

Now we're a little more advanced. The needle beams are actually in place at this time. That's the column that's going to be to moved just to the left of the ladder. The whole idea is to transfer the load onto the needle beams. When you've done that, you can then remove the column. The process being used here requires that the columns be removed to create a space for the construction of the new abutment. This is somewhat of an unusual approach.

On the far side of the roadway, you can see similar arrangements at temporary towers and needle beams. There are two additional existing columns which need to be removed. Here, you can see two iron workers at work - very, very cramped, crowded space, and they are making final adjustments to the beams and attaching them by feel drilling some holes and holding them together. You need a very stable structure before you transfer the load onto it. You can see the existing column surrounded by the needle beams on either side and, on this end of the structure, you can get a look at the hydraulic jacks that have already been put in place.

This is a good view of the hydraulic jack, which is that yellow cylinder on the right and a stack of steel plates, which is carrying the needle beams. As you introduce the load into the jack, the end of the needle beams will move upward again, by some measurable amounts, to overcome the deflection of the beam and whatever compression takes place and then the temporary tower.

In this view, the load has been transferred, and the existing column is being removed. There's a very distinctive sound of the labor is breaking the concrete. The whole world calls that a jackhammer. The whole world is wrong. That is actually a pavement breaker. Now you've learned something that you won't learn any place else. They are two different tools, and they look alike, and they sound alike. The pavement breaker breaks concrete. It's easy to remember that tool. The jackhammer is actually a rotating tool, and it drills holes. If you were drilling holes and rock and you were going to introduce blasting, you would use a jackhammer to drill the hole, and then you'd fill hole with dynamite, and you can blast the rock. The tool being used here is a pavement breaker.

In this view the column is gone. It was not all taken down with pavement breakers. That would take forever, but the pavement breakers needed to be used to carefully remove the top of the column, which is still joined to the existing bridge. Once it was free from the existing bridge, then much more robust type of breaking tools would be used to speed up the removal.

On the far side of the bridge, there are two existing columns have been removed. Now there is space on both sides for the construction of the new abutment. I took a shot here of the underside of the existing bridge, where the old column stood to show you this arrangement. There are four anchor bolts. which joined the column to the bridge. You can see those, and there's a kind of a cylindrical can at the end of each anchor bolt. That's a detail that's used which will allow you to make some small adjustments to the location of the anchor bolt. When you try to fit a steel bearing plate over it, you might find that it's off by a fraction of an inch. This gives you some small leeway to adjust the location of the anchor bolt and allow you to fit it up to the bearing plate. This is a very nice detail.