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Arthur Ravenel Jr Bridge
The Arthur Ravenel, Jr Bridge, with a main span of 471m and a total length of 4558m, is the longest
cable-stayed bridge in North America. It is situated across the Cooper River joining Charleston, South
Carolina to Mount Pleasant. It opened on 16th July 2005 after weeklong celebrations marked the
significance of this historic event. The whole project which includes the roadway, towers, rock islands,
approaches and interchanges took four years to design and build and cost $531 million. It is owned by
the South Carolina Department of Transportation and named in honour of former State Senator Arthur
Ravenel, Jr who helped secure funding for the project by establishing the State Infrastructure Bank.
The bridge was built by Palmetto Bridge Constructors (PBC), a joint venture between Tidewater Skanska
and Flatiron Constructors. It was built as a design-build project, meaning that a single contract was
signed to design and build the bridge. This allows construction to start before the design is finalized.
Parsons Brinckerhoff was hired to complete the design. It has a design life of 100 years (The New Cooper
River Bridge, 2005) and is built to withstand earthquakes, hurricane force winds and ship collisions.
The bridge was constructed in five separate simultaneous projects, the two interchanges, the two
approach spans and the main cable-stayed span. The biggest engineering challenge was the main cable-
stayed span. Cable-stayed bridges are one of the modern techniques of bridge construction. The deck
hangs from cables which are anchored deep into the main towers. They are known for their sleek lines
and exquisite beauty but are notoriously hard to build.
The shape of the main towers was decided upon after many public hearings concluded the local public
approval for the diamond-shape from other options. The two towers reach a height of 175m and are
spaced 305m from each other. The towers foundations are made using 3.05m diameter steel cylinders
drilled 30m into the silt and dense clay below the river bed. Each tower has 11 of these cylinders
beneath them. Shafts are dug inside these cylinders up to a depth of 70m. Mammoth cages of steel
rebar are placed inside the shafts and encased with nearly 150,000 tons of low permeability concrete
(Extreme Engineering: Cooper River Bridge, 2004). These deep footings give the towers the solid
foundations to resist major quakes like the one that struck Charleston in 1886 (Carl W. Stover, 1993). In
order to protect the tower from ship collisions, rock island were constructed around the tower base.
These islands cause the ships to run aground even before it reaches the towers. Each island measures
20000m2 at its base and at the surface covers an area of 4000m2. They are constructed uses three
different sizes of Canadian granite. The bases of the islands are built with small quarry stone. Some of it
is dumped into the river to form perimeter of the islands while the rest is precisely deposited around the
drilled shafts. On top of the quarry stone, 70-140 kg pieces of armor stones are placed. These are
topped off with 1000-1800kg pieces of large armor stones placed around the tower footings. In total, 17
shiploads equaling to 700,000 tons of granite was used to make the island (Extreme Engineering: Cooper
River Bridge, 2004). The towers themselves are made up of cast in place reinforced concrete. B&B
Welding Company created the rebar template cages that were used in the towers. The rebar sections
slope 19˚ outwards from the base up to the road deck. From there on, the rebar sections slopes inwards
until they form the diamond shape. Horizontal beams just below the road deck provide more stability to
the towers and support to the spans. The rebar was encased in in-situ concrete. Due to tight safety
considerations and schedule, the formwork for the towers, supplied by PERI, was designed to be self
climbing. This freed the cranes from raising the formwork to hauling material.
The bridge deck is built in sections made up of I beams composite with precast reinforced concrete
slabs. In total 64 road deck sections were used to make the main span. The road deck is built outwards
from the two main towers. As a time saving measure, construction of the road deck began even before
the towers reached their full height. I-beams measuring 2m deep make up the two edge girders that run
along the sides of the bridge. They also make up the floor beams that connect the two edge girders at
4.8m intervals (Abrahams, 2005). The beams are connected to each other using bolts that go through
bolt holes at each end of the beam. At one end a beam slots into the splice plates of a previously
installed beam while at the other end another beam slits into the splice plates of the current beam. The
bolts are then driven through the aligned holes and fastened. Each beam has 247 bolt holes on its web
and flange at each end. In total, 494 bolt holes connect a beam to another making a full length beam
throughout the span. Similar bolting mechanisms are used to join the floor beams that connect the two
edge girders. The edge girders are made up of Grade 70 steel since they carry heavier loads while the
rest of the floor beams use Grade 50 steel. Precast concrete deck slabs, measuring 25cm thick are then
placed in between the floor beams. The gaps between the slabs are then filled with in-situ concrete to
complete the section. But before the concrete deck panels are laid the cables need to be tensioned and
strung to the girders. These cables are anchored deep into the towers. The cables are made by
Freyssinet from high strength tensile steel. A total of 128 cables are used to suspend the 471m road
deck. Each cable is made up of strands lined parallel alongside each other. Each strand is composed of
seven wires, six wires twisted around one center wire. This is a patented strand called “Monostrand”
and is proven to have excellent durability, strength and anti-ageing properties. The strands are
individually sheathed in polyethylene protection and the cable is enclosed in a white polyethylene pipe
to provide protection against harsh weather conditions. On top of the concrete slab, a 5cm thick
wearing surface made of latex modified concrete is laid for improved driving surface.
The bridge carries eight lanes of traffic, 4 in each direction, with each lane being 3.7m wide unlike the
old bridges which have lanes only 3m wide. This allows for wider driving space, allowing bigger trucks to
safely maneuver through. The main span of the bridge allows a clearance of 57m and expands the
navigation channel to 305m. This allows bigger shipping vessels to safely navigate in and out of
Charleston Harbour, a major boost to the state economy. A bike and pedestrian walkway was added to
the bridge after the overall public insistence. The walkway is 3.7 m in width and runs throughout the
length of the bridge. It is cantilevered out of the south edge girder and faces the ocean.
The two old truss bridges that served the area were severely deteriorated due to poor maintenance and
increasing traffic loads. One of them, the Grace Bridge rated a shocking 4 out of 100 for safety in 1995.
They were regarded as accidents waiting to happen and a replacement was long overdue. The Arthur
Ravenel, Jr Bridge became this long overdue replacement.
While both cable stayed and suspension bridges use cables as the main load bearers of the bridge deck,
they are built with different principles. In a suspension bridge the deck is supported from vertical
suspenders that hang from the main cables. The main cables pass over the towers and are anchored at
the two ends of the bridge. Good soil conditions are required to build strong anchorages. In a cable
stayed bridge the cables are anchored to the main towers, thus they are a better choice for areas with
poor soil conditions since no external anchorages are required. However, cable stayed bridge have their
own Achilles Heel.