4.8 Computer Analysis of Trusses 155 Using the RISA-2D computer program, analyze the determinate truss in Figure 4.21 and compare the magnitude of the bar forces and joint dis- placements, assuming (1) joints are rigid and (2) joints are pinned. Joints are denoted by numbers in a circle; members, by numbers in a rectangular box. A preliminary analysis of the truss was used to establish initial values of Each member's cross-sectional properties (Table 4.1). For the case of pinned joints, the member data are similar, but the word pinned appears in the columns titled End Releases. (Note, RISA-2D uses internal force sign convention opposite to that assumed in this book, i.e., tension is negative and compression is positive.) To facilitate the connection of the members to the gusset plates, the truss members are often fabricated from pairs of double angles oriented back to back. The cross-sectional properties of these structural shapes, tabu- lated in the AISC Manual of Steel Construction, are used in this example. CONCLUSIONS: The results of the computer analysis shown in Tables 4.2 and 4.3 indicate that the magnitude of the axial forces in the truss bars, as well as the joint displacements, are approximately the same for both pinned and rigid joints. The axial forces are slightly smaller in 200 kips 5 8' 2 4 40 kips 200 kips- (3 6' 3 60.kips 10' 10' Figure 4.21: Cantilever truss. 6' TABLE 4.1 Member Data for Case of Rigid Joints EXAMPLE 4.9 Member Label | Joint J Joint Area (in.) Moment of Inertia (in.) Elastic Modulus (ksi) End Releases I-End J-End Length (ft) 1 1 2 5.72/ 14.7 29,000 8 2 2 3 11.5 77 29,000 20.396 3 3 4 11.5 77 29,000 11.662 4 4 1 15.4 75.6 29,000 11.662 5 2 4 5.72 14.7 29,000 10.198 [continues on next page]

Fig: 1