Author: Rose, W Hunter
First published: N/A
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Rose, W Hunter
This paper reviews the present knowledge on the prestressing of steel girders. In particuIar, the factors affecting the design of girders prestressed by means of high-tensile steel tendons anchored to the bottom flange are recorded, together with the design principles adopted. The question of an acceptable load factor is discussed, and the paper concludes by indicating the type of structure in which prestressing may result in worthwhile economy.
Introducing the paper, Mr. Mears said that when, in 1962, the Independent Television Authority announced their plans to build three new high transmitting masts it was apparent from the specification that there was a need to depart from the conventional open-latticed structure. The problem of servicing the extensive aerial systems suggested that serious consideration should be given to an all-weather structure and attention was therefore directed towards the design of a cylindrical plate mast. The television aerials, by reason of their sensitive performance, could be mounted only on
relatively slender latticed columns above the main support cylinder. Fibreglass shrouds completely encircling these sections of mast would afford the necessary weather protection. A further feature was the provision of an electrically powered passenger lift operating from ground level up to the lowest aerial aperture.
The building of underwater tunnels by sinking preconstructed sections into a trench dates back to the end of the 19th century, not too many years after the first shield tunnels were Completed. The method has come into its own with the era of highway tunnels beginning in the 1920s. A description is given of the design and construction methods of several different types of submerged tube tunnels. One, used largely in the eastern part of the United States, takes a circular welded steel shell as the basic element of construction. This is lined with reinforced concrete and protected with a concrete envelope. Individual sections are about 300 ft long. It is particularly well adapted to a two-lane tunnel. In Europe a rectangular, reinforced concrete section has been preferred for multiple-lane tunnels. Different methods of placing and connecting the sections are outlined. The use of a movable offshore platform for sinking sections of a 12-ft-diameter sewer tunnel in open ocean waters to a depth of 200 ft is described. The last examplc is a two-track, 3 1/2-mile rapid-transit tunnel to be constructed in the earthquake region of San Francisco, California.
John O. Bickel