2l6 TOWERS AND TANKS FOR WATER-WORKS. 



of 5 for tower members, the safe unit stress is 6,444 Ibs. per square 

 inch, and the metal required for the column section would be 



= 49.4 square inches, and which may be secured by a 

 0444 



pair of laced channels or other suitable combination of shapes. 

 Determining to use a Z-bar column, according to Carnegie's 

 handbook, a standard f-inch column has 26.3 square inches, weighs 

 84.1 Ibs. per foot of length, and its minimum radius of gyration 

 is given as 2.58, and its safe bearing for lengths of 18 feet and 

 under as 157.5 tons. Since this is the nearest section to safely 

 support the given load, it will be selected. With a vertical height 

 of 70 feet, the inclination of the tower will increase the length 

 of the column beyond 70 feet, but the extra length may be neglected 

 in determining the approximate weights. Similarly the load of 

 each pannel exerts a horizontal stress which should be combined 

 with that found graphically for the compression stress due to 

 the action of the wind, but these loads being small, will also be 

 omitted. The exact lengths of the horizontal and diagonal 

 members of the tower truss are hardly ever calculated except 

 in shop details, but are usually scaled from the diagram as being 

 closely approximate. In the case under consideration, it will 

 be seen from the diagram that the set of diagonal rods at the top 

 of the tower are subjected to the greatest stress, shown as 9640 

 Ibs. tension. With an allowable unit strength of 12,000 per 



square inch of rod metal, the required area is =0.8 square 



inch, or about i inch round rod, which may be adopted for all 

 of the diagonals in this case. The first horizontal member from 

 the top being in compression 7060 Ibs. and its length being 23 

 feet, its section must be determined. Commonly two channels 

 or four angles, riveted back to back, are used for such members. 

 Selecting a pair of 4-inch channels, weighing 5.25 Ibs. per lineal 

 foot, from Carnegie's handbook, their least radius of gyration 

 for neutral axis perpendicular to their web at centre is given 



