Huey P. Long Bridge (Jefferson Parish) - Structure

Structure

The widest clean span is 790 feet (240 m) long and sits 135 feet (41 m) above the water. There are three navigation channels below the bridge, the widest being 750 feet (230 m). The distinctive rail structure is 22,996 feet (7,009 m) long and extends as a rail viaduct well into the city. It has sometimes been described as the longest rail bridge in the US, but the nearby Norfolk Southern Lake Pontchartrain Bridge, at 5.8 miles (9.3 km), is considerably longer. The highway structure is 8,076 feet (2,462 m) long with extremely steep grades on both sides. Each roadway deck is a precarious 18 feet (5.5 m) wide, with two nine-foot lanes, but because of the railroad component, is unusually flat for a bridge of this height. Normally, bridges this high have a hump to accommodate the height but this bridge is flat to accommodate rail traffic.

The bridge is a favorite railfan location. It is owned by the New Orleans Public Belt Railroad, which is owned by the City of New Orleans and managed by the Public Belt Railroad Commission. The bridge is hated by many drivers in the New Orleans area due to the narrow 9-foot (2.7 m) wide lanes without shoulders. Also, as the East Bank approach meets the superstructure of the bridge, the two vehicular roadways "jog" or shift inwards towards the bridge centerline about 1+1⁄₂ feet (0.46 m) since the through-truss portion of the superstructure is 3 feet (0.91 m) wider than the deck truss portion of the east approach.

The foundation of the bridge is also unique. The land in and around New Orleans was formed by silt deposits brought down the Mississippi River. The clay topsoil (notorious for its role in the Hurricane Katrina levee failures) is compressible and unsuitable for foundation loads. However, bedrock is around 1,000 feet (300 m) below the surface, making it too deep for normal bridge foundation construction. So, the main piers are seated on a layer of fine sand 160 to 170 feet (52 m) below Mean Gulf Level and rely on their massive weight and girth to hold them in place.

The bridge dates from an era when the construction of large works presented significant engineering challenges and the needs of rail and auto travel were more matched than they are today. Large bridges mixing rail tracks and highways were common, as typified by the MacArthur Bridge and McKinley Bridge in St. Louis, Missouri and the Harahan Bridge in Memphis, Tennessee. A second Huey P. Long Bridge, which is very similar to the design of this bridge before its renovation, was built further upstream in 1940 in Baton Rouge, Louisiana and was mainly the last of its kind. While both of the Long bridges still carry both types of traffic, most of the others have been converted either to entirely rail use (Harahan since 1949, MacArthur since 1981) or entirely auto use (McKinley since 1978), sometimes with bicycle and/or pedestrian use added (McKinley in 2007, Harahan by mid-2014), and new large bridges are always devoted exclusively to meeting increasing vehicular traffic needs. Current rail demands are well met by existing bridges that are a testament to the care and craftsmanship of early 20th century bridge builders.