TABLE VII. Strength of Iron and Steel Wire Ropes, MANUFACTURED BY THE JOHN A. ROEBLING'S SONS Co., NEW YORK. Ropes, Hawsers, and Cables. Ropes of hemp fibres are laid with three or four strands of twisted fibres, and run up to a circumference of twelve inches. Ilawsers are laid with three strands of rope, or with four rope strands. Cables are laid with three strands of rope only. Tarred ropes, hawsers, etc., have twenty-five per cent less strength than white ropes: this is in consequence of the injury the fibres receive from the high temperature of the tar, — 290°. Tarred hemp and manila ropes are of about equal strength. Manila ropes have from twenty-five to thirty per cent less strength than white ropes. Hawsers and cables, from having a less proportionate number of fibres, and from the increased irregularity of the resistance of the fibres, have less strength than ropes; the difference varying from thirty-five to forty-five per cent, being greatest with the least circumference. Ropes of four strands, up to eight inches, are fully sixteen per cent stronger than those having but three strands. Hawsers and cables of three strands, up to twelve inches, are fully ten per cent stronger than those having four strands. The absorption of tar in weight by the several ropes is as foìlows: White ropes are more durable than tarred. The greater the degree of twisting given to the fibres of a rope, etc., the less its strength, as the exterior alone resists the greater portion of the strain. To compute the Strain that can be borne with Safety by New Ropes, Hawsers, and Cables, deduced from the Experiments of the Russian Government upon the Relative Strength of Different Circumferences of Ropes, Hawsers, etc. The United-States nary test is 4200 pounds for a white rope, of three strands of best Riga hemp, of one and three-fourths inches in circumference (i.e., 17,000 pounds per square inch); but in the following table 14,000 pounds is taken as the unit of strain that can be borne with safety. RULE. Square the circumference of the rope, hawser, etc., and multiply it by the following units for ordinary ropes, etc. DESCRIPTION. TABLE VIII. Showing the Units for computing the Safe Strain that may be borne by Ropes, Hawsers, and Cables. When it is required to ascertain the weight or strain that can be borne by ropes, etc., in general use, the above units should be reduced one-third, in order to meet the reduction of their strength by chafing, and exposure to the weather. TABLE IX. Strength and Weight of Manila Rope. Breaking load. 3 Strands. 3 Strands. 3 Strands. TABLE X. Weight and Proof Strength of Chain. MANUFACTUred by the NEW-JERSEY STEEL AND IRON COMPANY. Strength of Oid Iron.-A square link 12 inches broad, 1 inch thick, and about 12 feet long was taken from the Kieff Bridge, then 4 years old, and tested in comparison with a similar link which had been preserved in the stock-house since the bridge was built. The following is a record of a mean of four longitudinal test pieces, 1 x 1 x 8 inches, taken from each link. CHAPTER X. RESISTANCE TO SHEARING. By shearing is meant the pushing of one part of a piece by the other. Thus in Fig. 1, let abcd be a beam resting upon the supports SS, which are very near together. If a sufficiently heavy a S Fig. 1. load were placed upon the beam, it would cause the beam to break, not by bending, but by pushing the whole central part of the beam through between the ends, as represented in the figure. This mode of fracture is called "shearing." The resistance of a body to shearing is, like its resistance to tension, directly proportional to the area to be sheared. Hence, if we denote the resistance of one square inch of the material to shearing by F, we shall have as the safe resistance to shearing, A piece of timber may be sheared either longitudinally or transversely; and, as the resistance is not the same in both cases, the value of F will be different in the two cases. Hence, in substituting values for F, we must distinguish whether the force tends to shear the piece longitudinally (lengthwise), or transversely (across). Table I. gives the values of F, as determined by experiment, for the most common materials employed in architectural construc tion. |