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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 paper deals first with the historical facts leading to the publication in 1965 of the BS Code of Practice for the Structural Use of Precast Concrete, CP 116. The author then sketches briefly the considerations which led to the decision that a code, separated from the ‘ in-situ ’ Codes CP 114 and CP 115 was desirable.
W. Hunter Rose