Engineering Wonders of the World
Volume I

442 ENGINEERING WONDERS OF THE WORLD. of two planks 9" x %" placed one on either side of the two verticals, and having wooden blocks at intervals bolted between them of slightly less thickness than the verticals. This method, as will bo seen by reference to page 436, opposes to wind pressure from any direction a surface supported by the convex side of an arch, thus putting a compressive strain on the timber—a desideratum always to be aimed at in timber con- struction. The base and crown are formed of 12" square timbers tenoned and bolted in the form of a square and rigidly fixed, the verticals being again tenoned into these frames. At heights of 75, 150, and 215 feet respectively, steel stranded wire stays are attached and carried down at an angle of about 30° from the vertical to a suitable anchorage, the attachment to the towers being such as to maintain the compres- sion strain on the timber. All those carried to anchors on the outside of the square formed by the four towers have an ultimate tensile strength of 18 tons, those inside the square an ultimate strength of 40 tons. Funnel stays only could be used inside the square, and of these the longest had to be carried up INSERTING DIAGONAL TIE-BOLTS. at a very acute angle to the perpen- dicular ; hence the need for extra strength. The work of erection was carried out in two distinct stages. Each side of a tower in turn was first laid out and bolted together Erecting1 the Towers. on the ground ; and when complete, all parts were clearly marked for position, unbolted and stacked in the order in which they would be required. All four sides having been treated in this manner, the base frame was lowered on to the foundations already pre- pared, carefully levelled, and set in position relatively to the other towers. Then the first section of the vertical members was fixed in position together with the first set of braces, and concrete was added round the base to maintain the position permanently. After the concrete had set, the second section of verticals and the braces connecting them were placed in position ; then the next section ; and so on, until the first staying point was reached. A timber frame having been fitted and the stays attached, the work proceeded to the next point of stay attachment, ånd thence, to the top, where the crown was placed in position. Nine men worked aloft on the actual building of each tower, and twenty on the ground, the latter collecting the sections and braces,