Nodinc

Node number increment of automatically generated loads (default = 1)

a Indicates that real value (F or E format) should be entered; otherwise input the integer value (I format).

a Indicates that real value (F or E format) should be entered; otherwise input the integer value (I format).

The design examples are limited to two-dimensional steel frames, so that the spatial behavior is not considered. Lateral torsional buckling is assumed to be prevented by adequate lateral braces. Compact W sections are assumed so that sections can develop their full plastic moment capacity without buckling locally. All loads are statically applied.

5.6.1 Roof Truss

Figure 5.38 shows a hinged-jointed roof truss subject to gravity loads of 201 kip at the joints. A36 steel pipe is used. All member sizes are assumed identical.

5.6.1.1 Design by Advanced Analysis

Step 1: Load condition and preliminary member sizing. The critical factored load condition is shown in Figure 5.38. The member forces of the truss maybe obtained (Figure 5.39) using equilibrium conditions. The maximum compressive force is 67.1 kip. The effective length is the same as the actual length (22.4 ft) since K is 1.0. The preliminary member size of steel pipe is 6 in. diameter with 0.28 in. thickness (fPn = 81 kip), obtained using the column design table in the LRFD Specification.

Step 2: Structural modeling. Each member is modeled with one truss element without geometric imperfection since the program computes the axial strength of the truss member with the LRFD column strength equations, which indirectly account for geometric imperfections. An incremental load of 0.51 kip is determined by dividing the factored load of 201 kip by a scaling factor of 40 as shown in Figure 5.40.

Step 3: Analysis. Referring to the input instructions described in Section 5.5.4, the input data may be easily generated, as listed in Table 5.8. Note that the total number of supports (NBOUND) in the hinged-jointed truss must be equal to the total number of nodal points, since the nodes of a truss element are restrained against rotation. Programs DATAGEN, INPUT, and PAAP are executed in sequence by entering the batch file command RUN on the screen.

Step 4: Check of load-carrying capacity. Truss elements 10 and 13 fail at load step 48, with loads at nodes 6, 7, and 8 being 241 kip. Since this truss is statically determinant, failure of one member leads to failure of the whole system. Load step 49 shows a sharp increase in displacement and indicates a system failure. The member force of element 10 is 80.41 kip (Fx = 72.0 kip, Fy = 35.7 kip). Since the load-carrying capacity of 241 kip at nodes 6, 7, and 8 is greater than the applied load of 20 kip, the member size is adequate.

Step 5: Check of serviceability. Referring to P.OUT2, the deflection at node 3 corresponding to load step 1 is equal to 0.02 in. This deflection may be considered elastic since the behavior of the beam

FIGURE 5.38 Configuration and load condition of the hinged-jointed roof truss.

20 ft

FIGURE 5.38 Configuration and load condition of the hinged-jointed roof truss.

Pu = 20 kip
0 0

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