A Text Book on Roofs and Bridges Volume 1 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1888 edition. Excerpt: ...can also be used, the live load being placed for each member in the position to give the largest positive or negative shear (Art. 32). Thus, for the vertical Dd we cut it by a section, place the live load on the right, and take the center of moments at the intersection of CD and cd, which is 105 feet to the left of A. The reaction for this loading is y.c R = 6.2$ +- (1 + 2) = 10 tons, and the moment equation for Dd is--10 X 105 + 2.5 (120 + 135 + 150)--5 X 150 = 0, from which.S =--0.25, which is the minimum stress. The greatest tension in Dd will occur when the truss is fully loaded and is g tons. For the maximum stress in the diagonal Cd the live load is placed on the right, the center of moments is on the lower chord produced at 105 feet to the left of A, the lever arm of Cd is 92.27 feet, and the stress is found from the equation--13.75 X 105 + 2.5 (120 + 135) + 5 XQ2.27 = whence S = + 8.7. For live load on the left the stress in Cd is O and the counter Dc comes into action. To find the stress for Dc, we have--17.5 X 105 + 10 (120 + 135)--5 X 88.41 = 0, from which 5--+ 8.0 tons. In the same manner all the other stresses are found and marked on the diagram. The method of resolution of forces can also be used to find the stresses in the diagonals; the load being put on the truss in the proper position and the two adjacent chord stresses being found by moments, the difference of these is the horizontal component of the stress for the given diagonal. The bowstring truss is sometimes built without counter-ties, in which case the main ties take compression as well as tension, like the Warren truss. Prob. 63. Compute the maximum and minimum stresses for the members bc, Cc, Cb, and Bc in Fig. 48. Art. 39. The Parabolic Bowstring Truss. The apex...