Monthly Archives: December 2008

Green Bridges

The concept of sustainability in bridge design is becoming a major issue. Architects (and others) have a certification program for “Leadership in Energy and Environmental Design” or Leed for short.

The purpose is to make the best structures, with environmental friendly products, that leave the smallest footprint possible. Bridge design doesn’t have a certification system yet but it will. (Email me and we can start one!)

Here is a short article in Aspire magazine about Green bridges.

County Accelerated Bridge Construction

I was asked to design a county bridge in my state. Counties typically use high abutment bridges to minimize the overall length of the bridge. if the bridge is under 40′ then they use non-prestressed precast deck panels.

High Abutment Bridge

High Abutment Bridge

The biggest benefit is the cost. A typical 40′ long by 28′ wide bridge costs just over $100,000. The bridge I designed has 2:1 berm slopes and is the DOT favorite spill-through abutment bridge. The advantages of the spill-through bridge are working in the dry, wider stream opening and because it is the most commonly built bridge, it has access to a lot of standard parts.

Spill-through Abutment Bridge

The biggest drawback is that it costs more and counties are strapped for cash. My ABC design should be built in 5 days but the cost will be closer to $200,000. Yikes!

Pier Redundancy

A typical ABC pier can be built with two precast columns tied to concrete drilled shafts (or forget the precast columns and use the drilled shafts as the columns). Then tie the precast pier cap to the columns.

Of course this type of pier is only useful on land. The pier columns are typically not suitable for river environments because of ice and water loads. Also this type of frame pier suffers from non-redundancy (is that a word?). Meaning if one of the drilled shafts settles or gets hit by a vehicle the whole pier could fall down.

For this reason most DOTs like the idea of pile foundations.

Because the chances of multiple piles failing is rare.

Wisconsin Precast Abutment

Some of the work the University of Wisconsin-Madison is doing. Includes a short video of a precast abutment being installed. The abutment is similar to some of the piers shown earlier, precast panels slid down onto steel piles.

You have to place the piles very accurately and lock the system together. A big problem is when you place this in a river channel. You have to excavate fairly deep and hold the water back.

Massachusetts – $3 Billion Accelerated Bridge Bond

I guess this is only news to me. It looks like Governor Patrick signed a $3 billion dollar bond (last August) ” to repair and replace approximately 250-300 bridges in Massachusetts.”

I thought this was interesting because a) it is a ton of money and b) I saw a job posted on the web for a structural Accelerated Bridge program engineer. ABC must be making strides if they are hiring engineers expressly for the ABC program. Maybe this isn’t a fad?

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….

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.

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

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.

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.

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....