The Huey P. Long Bridge has been vital to the economy of New Orleans and Louisiana for more than seven decades.

One  of the first Mississippi River spans built in Louisiana, the  cantilevered  steel through truss bridge  has been  carrying both rail and highway traffic across the Mississippi River, just outside of New Orleans, and facilitating economic  growth  for the city since it was completed  in 1935. While  it shares a rich history with New Orleans, its most important role lies ahead.

During  the 1930s, the bridge was built to carry both rail and highway traffic, which was relatively common at the  time. At 23,000 ft between railroad abutments,  the main spans included 9-ft highway travel lanes cantilevered off of the railroad bridge. Today, the bridge remains an important artery for both forms of traffic, and until recently the original 9-ft highway traffic lanes were still in use by motorists. With traffic volumes continuing  to increase at this vital crossing of the Mississippi River, however, additional traffic capacity was needed.

In 1982, a study was conducted  for a new bridge crossing on  a nearby  alignment.  Five alternatives  were studied,  but due to the large amount  of right-of-way  required  on either side of the bridge—coupled  with the inherently  high costs associated with a major  river crossing—a new crossing was not considered  a viable option. Additionally, despite the fact that the public wanted and needed additional highway capac- ity across the river, numerous  public meetings all resulted in the same conclusion: No agreement  could be reached on a location for new river crossing. Consequently, in 1986 the Louisiana  Department of Transportation and Development (LADOTD)  decided  to  investigate  widening  the  existing span in order to provide the needed highway capacity on the existing traffic corridor.

Modjeski  and Masters,  the  structural  firm that  designed the  original  bridge,  was engaged  to design  its update.  The first  major  challenge  was to  address  whether  or  not  soils could  support  the  increased  load. The  bridge’s  foundation is a caisson founded  on deep sand layers located  below the typical compressible clay layers found at the surface. For- tunately, the geological investigation determined that these deep sand layers could support  the additional loads of a wid- ened bridge.

Innovative Support

Once   the   soil  was  deemed   adequate   to  support   the   increased foundation  loads, plans for the Huey P. Long Bridge Widening project could begin. The  final approved design involved expanding lanes from two 9-ft lanes to three 11-ft lanes, with a 2-ft inside shoulder and an 8-ft outside  shoulder.  To achieve this, the design used a unique  steel "W" frame pier cap expansion  supported  on a concrete  encasement  of the lower portion  of the existing pier. This expanded pier cap was needed in order to support the additional truss lines that would be used to support the new lanes. The combined total weight of the steel pier caps used for the widening project is nearly 4,750 tons, just under 1,000 tons per pier.

Construction began in April 2006 and with a final construction cost of approximately $1.2 billion, it has become the largest construction project in Louisiana’s history. The first phase of the seven-year, four-phase project involved widening of the main support piers. Four concrete river piers and one land pier were widened by encasing the lower portion  of the exist- ing piers with concrete. The  encasement  began at the top of the caisson distribution block and extended up approximately 97 ft. This encasement, which widened the piers from 60 ft to 80 ft, supported  a new steel frame that was, in turn, used to support the widening trusses, which enabled the widening of the main river spans. The 53-ft-tall steel frame, which was commonly referred to as the “W” frame due to its appearance, is 152 ft wide at the top, but only 75 ft wide at its bearings on top of the encasement.

The  second phase of the work modified portions  of the exist- ing railroad approaches so that the new, wider highway approaches could pass through them. Modifications to the existing railroad trestles were necessary on both sides of the river. On the west bank, one of the existing steel towers had to be removed to permit new at-grade  roadways to pass traffic to the other  side of the railroad trestle. Two straddle bents, consisting of concrete columns support- ing a steel box girder, were designed to support the existing railroad superstructure. Special bearings were designed to permit  the box girder to behave as a simple beam and at the same time resist “roll- ing over” as a result of railroad longitudinal forces.

As the interruption of railroad traffic had to be kept to a mini- mum, the removal of the existing tower, the erection of the steel box girder and the restoration of rail traffic had to be done within a 24-hour  closure period. Like the west bank, a portion  of the east bank trestle  conflicted  with the new roadways, but on the east bank two trestle towers had to be removed and the existing railroad superstructure replaced with longer girder spans, which were supported  on concrete  straddle bents. Again, the interrup- tion of rail traffic had to be minimized for this work, but as there are two tracks, one could allow rail traffic to continue  while the new longer superstructure was erected on the other.

The third phase was the widening of the main bridge. One of the most difficult challenges of this phase was the requirement to maintain  highway, rail and marine  traffic throughout construc- tion. While short outages could be arranged, longer interruptions were not  possible. The  most difficult issue was highway traffic.

The original bridge supported  two lanes of highway traffic on an18-ft-wide  roadway supported  by a floor beam bracket  cantile- vered from the outside of the existing trusses, while the new wider roadway would be supported  on a floor beam that would span 50 ft between the existing and widening trusses. Swapping one for the other would typically require closing the roadways. The problem was solved by incorporating the original floor beam bracket into the widening floor beam, thus permitting highway traffic to con- tinue to use the original roadway while widening construction was performed. Highway traffic impacts were further minimized when the  contractor, MTI  (Massman,  Traylor, and  IHI),  along  with HNTB (who performed  the construction engineering  for MTI), developed an alternative method  of erection that permitted three

530-ft-long  sections of the widening trusses to be erected at one time as opposed to erecting them one individual member at a time. For this erection, an upstream and downstream section of the wid- ening trusses, braced with a stability frame, were floated on barges to the bridge, and then both widening trusses and stability frames were hoisted into position by the constructor’s  team using strand jacks. This approach minimized both vehicular and marine traffic impacts during construction. (See “Lift, Slide, Attach, Repeat” in the 09/2010 issue for more on this erection scheme.)

Approaches Approaching

The fourth and final phase, estimated for completion  next August, involves construction of new approaches. As of this past spring, a portion  of the  new roadways were opened  to traffic, while construction teams continue  to remove the old lanes and replace them  with the remaining  sections of the final widened roadway. When  completed,  phase four will ultimately complete the replacement  of the original  two 9-ft lanes with three  11-ft lanes, adding the shoulders, demolishing the old approaches and main deck and adding signaled intersections and approach ramps at either side of the bridge to improve flow and connectivity.