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MISSOURI DIVISION |
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PROCEEDINGS OF THE SEPTEMBER 2000 POST EARTHQUAKE HIGHWAY RESPONSE AND RECOVERY SEMINAR HELD IN ST. LOUIS MISSOURI
BRIDGE REPAIR & TRAFFIC IMPACT BY HOWARD HWANG
DAVID JOHNSON: Thank you, Glenn. The final speaker will be Howard Hwang. He's with the University of Memphis. If you've ever been to a seminar, even this one, you've heard his name come up numerous times as far as dealing with earthquakes. I'll turn it over to Howard.
Good afternoon. I'm going to talk a while today about the expert opinion on the bridge repair strategy and traffic survey. How they're going to repair it and how to repair it and what the impacts on the traffic are.
Most of the bridges in the metro area now are not really designed for earthquakes. I think we are convinced if an earthquake occurs in this area, the bridges are going to be damaged. If a bridge is damaged, of course it's going to interrupt traffic and may even hurt the economy. Fortunately, we have not had an earthquake for a long, long time.
I carried out a study on the Memphis area. If a 7.1 earthquake occurred in the New Madrid area, half of the bridges will sustain extensive damage and that's going to interrupt traffic flow. Some people maybe know FEMA spent one hundred million dollars to try to put in a bridge through Memphis to Arkansas across the Mississippi River. We are doing it right now. That bridge was down to one-lane traffic. It takes three hours to cross the bridge from Memphis to Arkansas.
The only problem you notice in this area is that when we talk to the people who cross the bridge, then advise we have no idea. We never had experience; we have no idea. Back in 1999, we had a workshop in Memphis. All state, DOT engineers came to Memphis to talk about how to minimize bridge damage. But the New Madrid earthquake and everybody talk about different ideas. There is no consensus for that.
That is why we're sponsored by the MidAmerican Earthquake Center. We tried to do a bit more controlled opinion surveys on how to repair a bridge damaged by the earthquake and how much it cost and so forth.
The bridge we used, we called 60311 bridge. It's actually a continuous concrete deck and
supported by concrete columns. It is a typical bridge in this area. This bridge carries the state highway and expressway. We can see many like it on I-40 or I-55. There are many state bridge overpasses on the expressway.
Basically what we are doing is selecting a type of bridge and describing the damage to the bridge. We write a nice letter and sign your name on any conference attended so we can get. Then we send it out to 50 people in the state DOT or Federal Highway people and even consulting engineers.
We ask, given this kind of bridge, how much will it cost for repairs, including debris removal and the new cost of the bridge. Besides that, we ask, given this kind of damage, how are you going to repair it? How much is the cost to repair it and how long will it take to repair it? What kind of traffic to we expect on it?
We sent it to about 50 people and we got a response. Responses came from six of the DOT, including Illinois, Missouri, Arkansas, Tennessee, even Georgia in the south. We only got two responses from the consultant engineers: one from the St. Louis area and one in the Memphis area. So we got responses.
We collected data on the first one and we made a summary. We call this 1, 2, 3, and 4. We summarize it and we sent it to them. Besides that, we also sent it back if their response was not clear. We ask specific questions from DOT. We ask them to follow up and to respond. If you want more defined, change his mind and he can do it. Then he sent back the results. From that, in the beginning, we had four people including Stu Werner who is from California. We summarized the results.
First, is called damage slight or minor damage. Abutment 1 or 5, minor crack. In the bridge Bent 2 or 4, the column is perfect, there's no damage. But repairing and maybe has some movement or yielding.
Bent 3, the column in the base, because the most typical here all in the base of the column is spalling, so most of the damage is in the base of the column and began to show some minor crack.
So given these conditions, we ask how would you repair it. Even though we don't have every response, we have different approaches. Some people say if it's just a minor crack only use epoxy injects to fix the crack. So that is the cost of repairing some damage. Then they're going to realign and reset it.
Basically, Approach A, just use epoxy and the cost -- the repair cost, this is a ratio, the repair cost, replacement cost. As I say, we ask them what do you expect to repair this bridge. If you use epoxy, there is less than 1 percent of the replacement cost.
For the second approach, the abutment and the bearing, the same, use the epoxy to fix the crack in the abutment or in the columns. In fact, this second approach is different is that they're going to say, okay, the column is cracked. We're going to get a steel jacket or fiber wrap of the whole column.
If you use the second approach, the cost is about 2.4 percent of the replacement. If you use steel jacket to the column in the center, remember the whole thing is not damaged, only the center is damaged. And that costs about 10 percent of the replacement cost.
For all this minor cracking, all of the responses say that there is not an affect on traffic. That is slightly damaged conditions. The traffic is not interrupted.
The second is what we call moderate damage. The same just minor crack in the column, and Bent 2 and 4 beginning to show some crack. In the center, the concrete begins to the spall. Given this condition, some of the people still say, well, even the column concrete begins to spall in the Bent 3, but the column still intact, they feel still use epoxy to fix it. So this is inspection of damage and causing realignment.
Basically, use steel, use epoxy because the damaged area is larger and we figure about 2 percent of the replacement costs.
B and C are the same. They still want to use post-tension strength for Approach B, but others use epoxy. They want to strengthen the column with post-tension strand hoops. Steel would be the same. Use the steel jacket to reinforce it. And because the damage is more, the percentage of the repair cost is higher.
Now, this begins to affect the traffic. We expect to close the bridge for one day to inspect it and then reopen it to the traffic. So one day is closed and the other one is 100 percent, the percentage before earthquake.
So this is for the highway and the bridge. But for the expressway -- expressway is underneath the bridge, they don't need to close it, but they want to reduce the speed. This basically reduces the speed from the original speed of 70. Usually, if you see the construction on the highway right now, they reduce it to like 45. About 60 or 55 percent is due to reduce the speed passing through the expressway. So they don't close the expressway that's underneath the bridge. But if you use Approach A and B, that takes about two weeks to fix it. If you use steel jacket, it takes about one month to fix it. So this other 14 days, they can reach to the 100 percent, and this other one reach 100 percent.
Not the abutment only showing the crack -- major cracking and some sagging of the superstructure. Bent 2 and 3, the concrete beginning to show the spalling and the cost of damage. Bent 3, the column, the core begins to show disintegrated.
So on this, the bridge abutment damage, crack and some sagging and all the columns show some damage. Basically, one approach because you need to inspect the damage, shore-up the bridges. Under this condition, you need to shore up a bridge. And basically they are going to check out the superstructure and fix the alignment and so forth.
But for the Bent 2 and 4, repair it and put a steel jacket. And for Bent 3, take it out and put in a new one. So replacement means take out the old one and put in new one.
Under this approach, I think is about 35 percent of the replacement cost. How they affect the traffic, think about the bridge closed down for about six days. The red line is for the roadway and the bridge. So the shoring up of the bridge, then you can allow the traffic to go through, but reduce the speed. So this is the first six days, shutting down the bridge, shore it up. The other six days, basically come from reducing the speed, but open for traffic with the reduced speed. It takes 90 days to fix it. So this is for the roadway and the bridge.
For the expressway, it's the same; shut down the expressway for six days to allow for the bridge to be shored up, but then they're going to open up one lane. Originally, two lanes with 70 mile speed limit. But another six days, they're going to open up one lane with reduced speed to 45. That's how you get 70 percent of the capacity, compared to before the earthquake.
You take about 60 days to do replacement. Some of like a Bent 2 and 4, you can open up the both lanes with the reduced speed, and the other 90 days to completely fix the bridge.
I would make one comment that this repair duration under this condition actually is a little bit faster. Not always does the emergency management consider it. It is maybe California people seem to take a long time, but for the people they think about take much longer than this.
Also this kind of repair, we are assuming all the material and labor is there. So we cannot come to you and you don't have labor and don't have material. This assumption, for this study, is that assuming you have labor and material for fixing the structure. If it's not there, you need to add additional time for that.
This is a little bit high. It's called concrete damage, basically more severe and extensive damage. Abutment is the same, showing a large crack and superstructure and maybe abutment and soil have sunk a bit. And Bent 2 and 4 begin to have spalling concrete and the bars have damage. And this is the superstructure. It has some sagging. For Bent 3, the pile, and pile cap show some of the damage.
Most of the construction in this area there are no positive connections for the pile cap to the pile. So in a severe earthquake, there is damage. These are the columns that are basically disintegrated and so forth.
This is damage before collapse basically. One approach is just to demolish and build a new one; that's Approach B. So you take down the bridge and build a new one and that cost would be 100 percent of the cost.
Some people say repair it and basically you jack up and shore up the bridge and, of course, fix the crack in the abutment, but repair with a new one. You keep the superstructure in position and fix the abutment and replace the 3 Column Bent. So the cost for that is 60 percent of the replacement cost. The cost is quite high comparatively.
This is the week before that. We did not expect that. My personal feeling, if you show me less column damage, I say, okay, take it off and build a new one.
Let's talk about Approach B first. That's take off the bridge. You take about two weeks to demolish it and get rid of the debris. Then you have lost that bridge so for about half a year you have no bridge. The traffic cannot cross that bridge. For the expressway, you have another two weeks.
Then you go to this, which is the same. You open traffic for one day with a reduced speed, then you take about another three months, then you open up the two lanes, then eventually in 180 days, the expressway will be fully function.
If you use Approach A, it takes about ten days to shore it up. After the shoring up, it's about the same. You get the traffic on the bridge and the state highway for the reduced speed we open up the other end. That can take about four months to fix it. And for the expressway, about the same way, takes about ten days. Then you eventually open one lane with reduced speed, then later open two lanes with reduced speed, and so forth. It will take about 120 days to fix it.
So Approach A, this is really crucial in a rural area. Most people in this area, the state highway is far apart. That will probably take a long time. That's Approach A, which probably has some variance under conditions.
The result we got from this survey, as I just described. We described 4 Bents damaged and asked people are they going to fix it, how long it's going to take, how long closed, and so forth. As I mentioned, this is sponsored by the American Earthquake Center. And so far, this is the result we got on this bridge.
We present this to you and hopefully you feel this is useful. Maybe some organization may or even people will continue. We hope more people want to stay on Federal Highway or even consult to participate.
One that we show here, I cannot say I take the average or this makes populations because some areas are too small and we do different approaches. We really cannot do any kind of just by four people sit down together and look at the average result and say, okay, this is what everybody thinks about it. So the percentage we put here is really just by what a number of people think about it.
So we hope you people learn more about this. Thank you.
DAVID JOHNSON: I've got a question. On that last example, when you talk about building a brand new structure, did you look at possibly building that in stages where you could open part of it earlier? I assume the 180 days is the complete bridge. Could you build it in stages and open half of it to traffic?
HOWARD HWANG: No. We did not; however, we did consider that somehow the emergency could cross instead of normal maybe take more than a year. But we did not consider about the bridge and open part of it.
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