The two C12 x 30s shown in Fig. 4.2 have been selected to support a dead tensile working load of 120 k and a 240-k live tensile working load. The member is 30 ft long. consists of A36 steel, and has one line of three 7/8-in bolts in each channel flange 3 in on center. Using the AISC Specification, determine whether the member is satisfactory and design the necessary tie plates. Assume centers of bolt holes are 1.75 in from the backs of the channels. Solution. Using Cl12 × 30s (A, 8.81 in? cach, t, 0.501 in, I, 162 in each, 1, = 5.12 in" each, y axis 0.674 in from back of C,r, = 0.762 in) 5.326 in 0.674 in cg of C -x 1200 in 2-C12 x 30s (A = 881 in' each)n -1 in sị in- in 12 in (a) Tie plateo Length of tie plate Width of tie plate FIGURE 4.2 Built-up section for Example 4-3. (b) O o o mles
Design Against Fluctuating Loads
Machine elements are subjected to varieties of loads, some components are subjected to static loads, while some machine components are subjected to fluctuating loads, whose load magnitude tends to fluctuate. The components of a machine, when rotating at a high speed, are subjected to a high degree of load, which fluctuates from a high value to a low value. For the machine elements under the action of static loads, static failure theories are applied to know the safe and hazardous working conditions and regions. However, most of the machine elements are subjected to variable or fluctuating stresses, due to the nature of load that fluctuates from high magnitude to low magnitude. Also, the nature of the loads is repetitive. For instance, shafts, bearings, cams and followers, and so on.
Design Against Fluctuating Load
Stress is defined as force per unit area. When there is localization of huge stresses in mechanical components, due to irregularities present in components and sudden changes in cross-section is known as stress concentration. For example, groves, keyways, screw threads, oil holes, splines etc. are irregularities.
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