Start with the concrete, chosen as the primary material since it is less vulnerable to saltwater than steel, said Dave Lambert, the department’s director of bridge engineering and infrastructure management. If one were to pile it all up on a football field, the amount of concrete used in the bridge would rise 10 stories high.
The bridge was built because its predecessor, built in 1933, was starting to deteriorate and was sometimes unable to handle traffic on its narrow lanes with no shoulders. The old bridge, now torn down, also was so low that waves would wash over it during heavy storms, rendering it inadequate as an evacuation route. The roadway surface on the new bridge is substantially higher — at its lowest point, more than a foot above the water level in a 100-year flood. But a Rutgers University professor says that still may not be enough in a few decades, given rising sea levels.
The new bridge is supported by a series of stout "piers" — what most people would probably call columns — some of them standing in water and others on small islands in the bay. These piers are supported underneath by concrete piles that were driven as deep as 100 feet into the ground, in some cases by blasting high-pressure water down into hollow channels, or jet ports, located inside the pile.
The spans — the sections of roadway that stretch from pier to pier — are supported by massive concrete girders. A typical one measures 160 feet long, about half the length of a football field, and weighs 220,000 pounds. The girders, or beams, are so big that they had to be brought by barge, 10 at a time, up the coast from the manufacturer in Virginia. There are more than 600 beams in all.
They are made of prestressed concrete, meaning they contain sturdy strands of steel that are pulled taut before the concrete is poured around them. After the concrete hardens, the manufacturer releases the tension on the steel strands, allowing them to contract. This causes a compressive force to be transferred to the surrounding concrete, enabling it to support greater loads.
It’s a bit like how you can pick up a row of books by squeezing hard on each end, said William Nickas, managing director for transportation systems at the Precast/Prestressed Concrete Institute, an industry group in Chicago.
This technique makes the new bridge to Ocean City the technological descendant of a historic structure in Philadelphia. The Walnut Lane Memorial Bridge in Fairmount Park, built in 1950, was the first major structure in North America to use prestressed concrete.
In a behind-the-scenes tour of the Ocean City bridge this month, Lambert, the state transportation engineer, showed off the beams and other structural elements to members of the media and the local chapter of the American Society of Highway Engineers. Construction workers were still putting finishing touches on the structure, which at its highest point allows 55 feet of clearance for sailboats.
That’s a big improvement over the old causeway, which seemed almost to skim the surface of the water and required two drawbridges to accommodate boats.
"On a bad day, like the Fourth of July, that bridge would go up 20 to 25 times," Lambert recalled.
And in hot weather the steel would expand, sometimes so much that the bridge could not close properly, and firefighters had to be summoned to hose down the broiling metal, he said.
At its lowest point, the new road deck is about 10.8 feet above mean sea level, said Tim Greeley, a transportation department spokesman. The water level in a 100-year flood, meaning a flood that has a 1 percent chance of occurring in a given year, would be 9.6 feet.
Ken Miller, a professor of earth and planetary sciences at Rutgers, said the low point of the bridge will not allow much breathing room by 2050, given that the sea level is steadily rising. On the Jersey Shore, the sea level is rising for two reasons: The land is sinking by a millimeter or two every year, and warmer temperatures are causing the water level to rise.
Some of that is due to melting glaciers and ice sheets, while some is due to the fact that water expands in warmer temperatures. There is a wide range in the sea-level projections made by climate scientists, but Miller said a middle-of-the-road estimate is that the waters will be 1 foot higher by 2050.
"They’re not planning for risk," Miller said, when told of the bridge dimensions.
Not so, countered Joe Dee, another transportation department spokesman.
"We’re confident that this was designed and engineered appropriately," Dee said. He added that when an especially severe storm is forecast, people in Ocean City and other Shore communities would be evacuated beforehand, as happened last year in advance of Hurricane Irene.
A more immediate concern for most people is likely to be traffic, and on that score the new bridge is an unquestioned improvement. Aside from not having drawbridges, it also features 12-foot-wide lanes, up from 10 feet on the old bridge, and it has 8-foot shoulders to accommodate broken-down cars and emergency responders. The project also includes road improvements on either end.
In addition, the bridge boasts fishing piers, a sidewalk for pedestrians and bicyclists, and a still-to-be-finished visitor center. All in all, a concrete example of modern engineering.
Contact Tom Avril at 215-854-2430 or email@example.com.