Bridge Description Grace Memorial Bridge - Old Cooper River Bridge, Charleston South Carolina
The Grace Memorial Bridge is a 14,313.58-foot structure that crosses Town Creek, the Cooper River, and Drum Island connecting the Cities of Charleston and Mount Pleasant. It is part of U.S. Highway Route 17 that is a south to north highway but the actual orientation of the bridge is east to west. Each channel is spanned by two cantilever truss bridges. The longest section, over the Cooper River, clears a space of 1,050 feet making it the fifth longest cantilever span in the world and the third longest in the United States at the time of its construction. The Town Creek span measures 640 feet and was the twenty-third longest cantilever in the world. At its highest point, the Grace Memorial Bridge is taller than the Brooklyn Bridge.
Often described as a roller coaster, the bridge ascends to cross both channels and then descends to and curves across Drum Island. Island on a curved 2,650-foot steel trestle. It begins at Lee Street on the Charleston side of the river with a 432-foot reinforced concrete roadbed. It then rises at its steepest grade of 6 percent to a 1,628-foot steel trestle and then to the Town Creek cantilever spans. All other grades are 5 percent. The bridge descends to cross Drum Island on a curved 2,650-foot steel trestle. It then rises again over the Cooper River cantilever span. The final descent is over a 240-foot steel trestle ending at the Mount Pleasant side. Originally, the bridge included a long earthen embankment over the swampy lowlands on the Cooper River shoreline, but much of the roadway has been altered and is no longer considered part of the structure.
The Foundation Company was responsible for the substructure across the deep-water sections of the span - the main channels of Town Creek and the Cooper River. This essentially consists of the thirteen concrete piers numbered in order beginning on the Charleston side of the crossing. The construction of these primary piers was described in detail in the February 28, 1929 edition of the Manufacturers Record. The article was written by Alexander Allaire, the southern manager of the Foundation Company and foreman for the project in Charleston (see Appendix B for illustrations and views of construction). Six of the piers were constructed using the pneumatic process while the remaining seven are of open cofferdam construction. The shafts of the piers vary in height, reaching a maximum of 134 feet above mean low water. The pneumatic caisson piers were carried to a maximum depth of 100 feet below mean low water. The central portions of the reinforced concrete shafts above the bases are hollow, thus reducing the great weight of the piers without lessening their ability to support the span. The entire process of constructing the piers required 39,400 yards of concrete, two million feet of lumber, and 470 tons of reinforcing steel. Work began on May 21, 1928 and the last pier was complete on January 21, 1929-an unusually short period of time, which Allaire believed, may have been a record.
Piers 1 and 4 are the anchor piers for the cantilever span across Town Creek. Both were constructed using the open cofferdam, single sheet pile method. Piers 2 and 3 are the main supports for the span and were erected using a pneumatic caisson measuring 28' x 58'. The pneumatic caisson was sunk to a depth of 90 feet below water, at which point the cutting edge was firmly embedded in the marl. The intensity of the load allowed on the marl is approximately seven tons per square foot. For this reason, the excavation was carried 10 feet below the cutting edge, belling out a distance of 4 feet around the perimeter. Each of the four major piers possesses this feature where their sides gradually extend outward as they go down. This enlarged the area bearing on the marl to distribute the load over a large enough area to reduce the unit of bearing to a safe amount so that there would be no settlement of the piers. Although not an uncommon practice, usually this was done after the caisson landed on rock, not marl, which made this excavation unusual and potentially hazardous.
Piers 5, 6, 11, 12, and 13 are all located in the Cooper River and support the spans adjacent to the main cantilever. They were all built using the open cofferdam, single sheet pile method. Piers 7 and 10 are the anchor piers for the cantilever span over the Cooper River and were built by the pneumatic caisson process. Dimensions are 18' x 55' and the cutting edge stopped in the marl at elevation minus 80 feet. It is on Pier 10 where the most deadly accident of the bridge's construction occurred. On December 1, 1928, seven African American workers were digging the caisson when it tilted, causing mud to rush in and smother them to death. Piers 8 and 9 are the channel piers on the Cooper River side. They are the largest pneumatic caissons with dimensions of 30' x 70'. These caissons were sunk to an elevation of minus 66 feet for Pier 8 and minus 76 feet for Pier 9. The piers are built to an elevation of over 131 feet above the water.
Except for Piers 1 and 2, concrete for the shafts was poured by a floating mixer plant that was erected on a large barge. Another barge held the floating compressor for the pneumatic caissons. Nearly twenty other boats along with several tugboats were used to moved equipment and supplies to and from the work site. A houseboat quartered the sandhogs, or workers digging the piers, and the medical facilities.
C.E. Hillyer, the engineering contractor from Jacksonville, Florida, was responsible for constructing the concrete approaches on both sides of the river as well as the concrete pedestals for the trestle across Drum Island. The pedestals are rectangular, tapered, reinforced steel concrete structures measuring either 13' x 13' or 10' x 10' at their tops. On the east approach there are eighteen paired pedestals. These rest on woodpiles driven into the marl. Concrete bases encase these piles to far below the bed of the river protecting them from wood-eating worms. On Drum Island and the Charleston shore, the steel trestle bents rest on reinforced concrete piles driven into the marl. The west approach contains twenty-one-paired pedestals. There are thirty-seven paired pedestals over Drum Island. The Hillyer Company worked on both ends of the bridge simultaneously and completed their portion of the project seventy-five days early.
Two different companies constructed the bridge's steel superstructure. The Virginia Bridge and Iron Company completed the steelwork for the Drum Island trestle and the approaches to Mount Pleasant and Charleston. The steelwork on the trestles consists of the sway braces on top of the concrete pedestals and the steel plate girder spans. These are all constructed of carbon steel. There are two primary types of trestle supports. The larger of the two consists of a four sway braces set on top of four concrete pedestals creating a rectangular box or tower which tapers as it climbs towards the deck of the bridge. The other type of trestle support is a single sway brace set on two concrete pedestals.
The main superstructure was constructed and manufactured by McClintic-Marshall who began work on October 10, 1928. All of the truss material is silicon and carbon steel fastened with carbon steel rivets. The bridge members are lattice-braced. The Town Creek span consists of a through truss made up of two 256-foot anchor arms, two 160-foot cantilever arms and a 320-foot central suspended span. This section of the bridge has a 135-foot clearance from the deck to mean high water.
The Cooper River span is comprised of a main through truss cantilever span of 1,050 feet, two through truss anchor spans of 450 feet each, two Warren deck truss spans connecting with the Drum Island trestle and three Warren deck truss spans connecting with the Mount Pleasant approach. The main span is made up of two 306-foot cantilever arms 130 feet in height and a suspended span of 437 feet. The deck of the bridge is 150 feet above high water while the tallest point of the bridge is 270.2 feet above the water. A total of 11,418 tons of steel was used in the superstructure.
The superstructure was created with three travelers or large derricks mounted on carriages. They moved over the bridge itself as it was constructed reaching far ahead and above to set framing members in place.
C.E. Hillyer paved the 20-foot wide roadbed with reinforced concrete and constructed the approaches to the bridge. Rectangular concrete pillars with slightly arched longitudinal girders support the elevated road on the Charleston side of the bridge. The original roadway on the east or Mount Pleasant side of the bridge consisted simply of a concrete roadbed laid directly on the soil surface with wooden posts marking the edge of the right-of-way. This roadway rose immediately to the trestle over the Cooper River.
Paving the bridge was the last step in the entire construction process and was a difficult task simply because of inaccessibility at certain points. Trucks could not be brought onto parts of the bridge because they could not turn around due to the amount of materials present on the bridge deck. Mixers could not be used where the concrete was being poured, so the contractor resorted to a narrow gauge railroad with gasoline locomotives to carry the material directly to where it was needed. Additional tracks were placed above the previous days' work each morning. The deck required 5,980 cubic yards of concrete. Originally most of the bridge was paved with an 8 1/2 inch thick slab across the 20 foot roadbed. On the cantilever spans, the concrete was 7 inches thick and the curves of the road were slightly wider. The paving was complete forty days ahead of schedule on July 20, 1929.