The Seohae Grand Bridge is the longest bridge in South Korea.
It is a cable-stayed bridge crossing the Asan Bay 65 km south of Seoul. The bridge is 9.4 km long with a 870 m cable-stayed portion. The main span is 470 m long and is 62 m above the water.
The bridge accomodates six traffic lanes between P'yongtaek to the north and Dangjin to the south. It was designed by T.Y. Lin International, of San Francisco. The construction of bridge took seven years to complete. It was opened to traffic in November 2000.
The Seohae Grand Bridge is particularly remarkable for two reasons: the inclusion of piston-like lock-up devices and the design of the cable-staying pylons.
The lock-up devices (LUDs) are fluid-filled chambered cylinders joined to the bridge girders. When the bridge changes shape, fluid flows through holes in the chamber division to compensate for the change. The size and shape of the holes and the viscosity of the fluid can be adjusted to make the piston react differently to different loads. These LUDs are "tuned" to allow slow movements such as those due to temperature change, shrinkage of building materials, or creep. Quicker and more abrupt forces, like those resulting from seismic activity or the high winds common to Asan Bay, "freeze" the LUDs and prevent movement.
The pylons of the bridge consist of two 180 m tall hollow concrete towers. Each tower is "H" shaped, a variation from most cable-stayed bridges which use "A" or inverted "Y" shapes. The cables supporting the bridge deck are spread out through the vertical portion of the pylon. There are three horizontal crossbeams, one near the bridge deck and two roughly one-third of the way from the top of the pylons. This double crossbeam style was chosen by the owner (the Korean Highway Corporation) for aesthetic reasons.
I recommend going to the ASCE web site below for excellent pictures of the bridge during construction and after completion.
Sources: ASCE 2002 calendar, http://www.pubs.asce.org/ceonline/ceonline01/0102feat.html