Construction of Large Prestressed Concrete Girders

The main function of the girders (Bruce Nuclear Power Station) located at the top of the reactor tower is to support the boilers, steam drums, piping and pumps, and at the same time the girders need to be crack free to serve as radiation shields. Each girder had a clear span of 92 ft, was fixed at both ends and post-tensioned. They were 42 ft deep, 6 ft wide, cast continuously from bottom to top, and contained about 1,000 cu yd of concrete. In a semimass structural concrete of this dimension, the cement content is generally high, resulting in high internal temperature. With time, cooling takes place, and if the girder is restrained at both ends, stresses will develop that could cause cracking. This paper examines the way in which these stresses due to thermal cooling have been minimized in practice.