Audubon Takes Flight

As summer approaches, the John James Audubon Bridge Project continues with significant progress in all phases of construction. The two towers currently reach 400 feet above the mighty Mississippi River with the construction of the lower cross beams well underway. Since the last blog, these towers have doubled in height allowing a distant view all the way to downtown Baton Rouge on a clear day! They still have approximately 120 feet to climb making them comparable to the Big Cajun II smokestacks.

The towers are not the only element of the main span beginning to form. Recently, the first sections of bridge deck have been placed providing the first perspective of where vehicles will drive. In addition, the first stay cables, the defining feature of a cable-stayed bridge, have been installed. The shape of a premier, world-class bridge now is coming into focus.

The approach roadways and other bridges on the Project also are moving forward. Two layers of asphalt already have been placed from U.S. 61 to the low- and high-level approaches. Paving has begun on the west approach roadways with significant development on the new Delta Road configuration.

The entire project will continue to experience leaps and bounds throughout the summer. Check back periodically in the photo gallery to see pictures of the construction. Also, don’t forget to check out the three web cams to track the Audubon Bridge’s main-span development.

Construction on the John James Audubon Bridge project is progressing around the clock. The past year saw many new developments on all elements of the historic structure. Construction on of the enormous and complex footings completed allowing for the towers to rise from the water. Also, a majority of the other smaller bridges and approaches are completed. The past year was not without its challenges. The spring of 2009 produced another bout of high water. This year, as opposed to previous years, the water did not force the job to completely shut down all operations. Many important operations had to be suspended, but workers were able to make some progress. The end of year, though, produced some of the wettest months for the season on record.

With the dawn of a new year, 2010 promises to be an exciting and pivotal time for the Bridge. Currently, the towers rise approximately 200 feet above the Mississippi River reaching for their final height of 520 feet. Completion for both “H-form” towers is scheduled for end of summer. Workers will also be hanging some of the deck steel that will support the bridge deck. Installation of some of the cable also will accompany this work. By the end of 2010, the formation of the cable-stayed main span will begin to come into focus. These operations will occur on the west and east banks simultaneously approaching each other in the center. This coming year will not be without its challenges, but everyone working on the Audubon project is committed to working hard to deliver a quality bridge for Louisiana.

Wind, lightning, rain, traffic, and the environment are all things that engineers have to take into account when designing things such as bridges.  Forces also play a big role in the layout of the bridge. 

Two forces that are present in all bridges are compression and tension.  The engineer designing the bridge needs to take these forces into consideration so that the bridge does not buckle or snap.  Compression is a force that shortens the object that it is acting on.  Tension is the opposite because it lengthens the object that it is acting on.

A spring can be used to demonstrate compression and tension.  When we press down, or push the ends of the spring together, we compress it.  This force shortens the spring.  When we pull the spring apart we create tension.  This force lengthens the spring.

The best way to handle these forces is to either disperse or convert them.  When you disperse the force you are spreading it out over a larger area, so that no one spot has to carry the intensely sustained force.  When you convert the force you are moving it from an area of weakness to an area of strength.

There are dozens of other forces that also must be taken into consideration when designing a bridge.  These forces are usually specific to a particular location or bridge design.

The method of construction of a cable stayed bridge is faster and simpler than that of a suspension bridge.  The advantage of the cables being connected directly to the bridge deck and not draped like a suspension bridge allows building a lighter deck which saves money.  The draped cables of a suspended bridge are enormous and incredibly heavy and require equipment such as helicopters or cableways to install.  There is no need for this kind of equipment when draping the cables of a cable stayed bridge from one tower to the other because the cables are smaller and easier to install.  The process involves laying the cables down on the deck and raising them to the towers to make the connection.  At the beginning of the project you have a small bridge deck and short cables to install.  Towards the end of the project you have longer cables to install but already have the longer deck constructed cantilevering from the towers.

The cable stayed bridge does not need big anchorages, because the bridge deck is attached to a multitude of cables that are directly connected to the towers.  The more cables that are used means smaller forces to be carried and therefore smaller anchorages are sufficient.  A suspended bridge is connected by two main cables, with other cables transferring the load from the deck to the main cables.  Therefore they carry an incredible tension force that must be resisted by big anchorages placed at the two extremities on land.  

In conclusion, from 750 ft up to 3000 ft a cable stayed bridge represents the best technical and economical solution.  The project is scheduled to be complete in 2010.    

The John James Audubon Bridge will be the longest cable-stayed bridge in North America.  Have you ever wondered what the differences are between a cable-stayed bridge and a suspension bridge?  This is a question that is asked frequently.

A cable-stayed bridge consists of one or more towers with cables supporting the bridge deck.  The cable-stay design is the optimum bridge for a span length between that of cantilever bridges (a bridge that has structures that project horizontally into space, supported on only one end) and suspension bridges (a type of bridge where the main load-bearing elements are hung from suspension cables).  Within this range of span lengths a suspension bridge would require more cable, while a cantilever bridge would require more material, thus increasing the cost of the project.

A cable-stayed bridge may look similar to a suspension bridge, but in fact is very different in principle and in the method of construction.  The cables form the primary load-bearing structure for the bridge deck in a suspension bridge design.  In a cable-stayed bridge design, the towers form the primary load-bearing structure.

The John James Audubon cable stayed bridge will incorporate many advanced features that represent the state of the art in cable stayed bridge construction.  These features include the galvanized strands for the cables, the high density polyethylene stay pipes with an external helix for wind stability, the anchorage system of the cables inside the towers with steel anchorage boxes, the single inspection traveler running along the bottom of the whole deck, the provision of elevators inside the towers, the simplicity of design of the towers and of the deck, and the efficient semi-fan arrangement of the cables.  All of these features are combined with the most advanced design provisions for withstanding extreme events like vessel collision, hurricane force winds, sudden loss of one cable or the scouring action of the river.

The Audubon Bridge project, as with all TIMED projects, was strategically chosen to be built in order to stimulate economic growth across Louisiana.  By stimulating economic development this will help to strengthen the communities of New Roads and St. Francisville, as well as other communities in this area. 

From a policy perspective, economic development can be defined as efforts that seek to improve the economic well-being and quality of life for a community by creating and/or retaining jobs and supporting or growing incomes and the tax base.  It typically refers to improvements in a variety of indicators such as literacy rates, life expectancy, and poverty rates.

Construction of the work trestles (a structure consisting of spans supported upon frame bents) at the Mississippi River crossing is at a standstill due to the high water, but work continues on the 12 miles of two-lane roadway approaches and 8 conventional bridges that will connect LA 1 east of Hospital Road at New Roads to US 61 south of LA 966 at St. Francisville. 

Work is being done in the Prefabrication Yard where testing is being performed on the jacking system used for lowering and raising pre-cast formwork panels.  Piles (a shaft-like linear member which carries loads through weak layers of soil to those which are capable of supporting such loads) are currently being driven for many of the conventional bridges.  Grading and drainage work continues on the west side.

Work on the trestles will resume when the water level is safe to do so and is being monitored daily for changes in the water level.  Once these work trestles are complete this will allow crews to continue work on the main bridge structure despite river conditions.

Box culverts on the west side.  On the right side, surcharge soil has been placed on the embankment for compaction.

A Pier of the Main Span is underwater.  The river has flooded the side on the East bank.

The foundation shafts and the access ramp to the trestle are underwater.  The trestle is just above water.

A pier of the Main Span.  Only the top of the shafts are visible.  The access trestle on the left is at elevation +54 ft and is a few feet above water.

View of the west access to the Main Span.  The west storage area is visible at the top of the photo, beyond the levee and LA 981.  The protruding pipe piles are the top of the templates needed for the installation of the shafts.  The top of the steel casing of the shafts is barely visible in the river.

Photos provided by Tommy Comeaux with The Pointe Coupee Banner