Trusses and Arches Analyzed and Discussed by Graphical Methods Volume 2 (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: Art. 18. A Triangular Roof Truss. In the wrought iron truss in Fig. 28 the upper chord is divided into eight equal panels, verticals are drawn through the apexes, and the diagonals slope upward toward the center. Let the span be 80 feet, the rise of peak 15 feet, the lower ? ?' Fig. 29. chord horizontal, and the distance between adjacent trusses 18 feet, center to center. The main rafter is found to be 42.72 feet, and each panel I0.68 feet long. The weight of the truss is 9720 pounds, and of the roof covering 18 455 pounds, making the total dead load 28175 pounds. The dead apex load is one.eighth of this amount, or 3 522 pounds, and the half apex loads at the supports are each 1 761 pounds. Since the inclination of each panel of the upper chord is the same the snow apex loads are equal, each one being 1 0 X 18 X 15 = 2 700 pounds. The ratio between any snow apex load 2 700 and a dead apex load being = 0.7666, the stresses caused 3 j by the snow load will bear the same ratio to those due to dead load, and hence only the dead load stress diagram is required. The construction is begun by laying off the load line aa' equal to 28 175 pounds by scale and bisecting it at n. The half apex loads ab and ab' arc next laid off equal to 1 761 pounds and bb' divided into seven equal parts. The reactions are a'n and na, closing the polygon of external forces. The polygons representing the forces acting at each joint are successively formed, as explained in Art. 17, by beginning at the left support, passing to joints alternately on the upper and lower chords until the peak is reached, then going to the right support and passing from joint to joint until the peak is reached again. The last line to be drawn is I'm, which must be parallel to L'M and pass throug...