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Another pier post sorry…

Okay I have beaten the pier thingy to death but the point is your typical pier is constructed in one piece. (It may be several actually concrete pours but it is considered an cast-in- place integral pier with no joints.)

Imagine the pier sits on a rock river bed. No joints, no way for the water to get to the steel and the system works as a whole.

When you try for a ABC pier the first thing you are talking about (typically) is segmental construction. Meaning that the pier comes in pieces and you tie it together in the field.

So you have to fit the pier together and add compression, in the form of post tensioning, to make the system act together.

The joints are problematic. How do you make sure the water stays out and does not get to the post tensioning strands?

Realizing, if the strands go, the pier goes….

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FHWA ABC webpage

So here is the FHWA’s webpage on Accelerated Bridge Construction. It is not a very impressive site. For one thing it shows a lot of outdated material and to me it doesn’t really seem serious about investing in ABC designs.

What do I mean by not serious, well, look at the page. Do you see any mention of new projects, help for State DOTs to develop new ABC directions or exciting news about design help. Nope, it plugs a conference from last March on the front page. When I visited the site today, it showed its last page update was seven months ago!

If the FHWA is really interested in ABC designs, why isn’t the page more vibrant? I doubt any business could afford to go seven months without adding something to their site.

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3D Bridge Visualization

One of the things I think can make a better project is the use of 3D visualizations. For example, most engineers can take a 2D drawing and construct a bridge. But 3D representations of the design can really help engineers and their clients understand what the project will look like before it is built.

ABC bridges probably need more visualizations than standard bridges because a small mistake in the design can become a very large mistake in the field.

To that end I have started a little visualization company called 3D Bridge Design. Bridge Concepts draw by an engineer who actually knows how to design bridges.

ABC Bridge
ABC Bridge
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Precast Pier part 2

This is a precast pier cap we used for a county bridge. The pipe piles were concrete filled and the cap was tied into the system by filling the holes. The holes in in the cap were made using corrugated steel pipes. The only problem was supporting the whole thing while you waited for the concrete to cure. We did design it so you could place beams the next day.

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Precast Piers

ABC substructures are one of the more challenging areas of bridge design. Joints in substructures can be problematic (think water getting in and corroding the steel holding the thing together).

A major issue is how to tie piers to piles. Here is a link to the North Carolina’s Beaufort and Morehead Railroad Trestle Bridge, which used precast pier caps tied to preplaced pipe piles.  (say that 3 times)

Beaufort and Morehead Railroad Trestle Bridge
Beaufort and Morehead Railroad Trestle Bridge

This type of system used the speed of precast caps and then the conventional method of using concrete to tie the pier together. We used a similar system in our last two ABC projects.

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Why ABC? Speed of course

Okay why the push to ABC bridges? Speed of course. The most common argument for ABC is the issue of building a bridge quickly which would  minimize the impact to drivers and local businesses.

This is a very strong argument. I have worked on traditional bridges that took up to a year to build. This can have a big impact on traffic patterns but typically its the businesses that are affected the most. Taking away access to businesses or reducing entrances can really disrupt the flow of traffic to a company. Drivers often avoid congested areas which means they avoid the businesses.

The irony is that disruption to drivers and businesses are considered in design but are not readily used in benefit/cost ratios for projects.  So…ABC projects often look more expensive than traditional projects because we can’t quantify the costs to drivers and local businesses.

I wonder what the one car knows that the others don't..
I wonder what the one car knows....
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Rapid Replacement

Obviously ABC would be great for rapid replacement projects, but you would probably need to have the bridge already sitting on the shelf. This means of course standardized design which saves money but stifles creativity.

Scour poblems...
Scour poblems...
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Since this a new blog and Google has not placed it in it’s search engine yet, I can pretty much talk about anything because no one is reading it anyway….

Why Accelerated Bridge Construction?

Despite all the hype it is difficult to change the direction of bridge construction to ABC. Critics of ABC will quote the higher up front costs, but this will be dealt with as standards and mass production come to ABC designs.  The real problem with ABC is that typical cast-in-place techniques usually provide a better final product than ABC.

When you build a bridge the old fashioned way, and I am speaking primarily of concrete bridges, you get a bridge that increases in strength due to integral construction. Meaning, the bridge is “locked” together by the cast-in-place concrete.

Accelerated Bridge Construction will always be appealing for its speed and eventually it will have the longevity of its cast-in-place cousins. As I learned in my steel design class, its all in the connections!