2- Show your detailed calculations for the following: A- Using Figure 16.12, select the minimum-size crane required to unload pipe weighing 175,000 lbs. per joint and lower it into a trench when the distance from the centerline of the crane to the trench is 45 ft. (8 points) B- A crane lifts a load with slings, as shown in Figure 16.30. What is the angle between the load and each sling leg for which the horizontal reaction is half the load? (8 points) Figure 16.12 shows typical crane-lifting capacities for four specific crawler cranes of varying sizes. The lifting capacities of units made by different manufacturers will vary from the information in the figure. Individual manufacturers and suppliers will furnish machine-specific information in literature describing their machine Capacity, lb (x1000) 1600 1400 1200 1000 800 600 400 200 0 MLC650 53 al (650) Click to load interactive graph 50 16000 $3 (400t) MLC300 S3 (300t) 100 14000 52 (2001) 150 Radius,ft 200 250 300 Figure 16 12 Safe lifting capacities for four crawler cranes Chart courtesy of Manitowoc Cranes To calculate the stress developed by the load on a rigging arrangement, it is necessary to have all forces in equilibrium. If a 10-ton load is supported by a set of slings in such a manner that the individual sling legs make a 10- degree angle with the load (Figure 16.30), the sling is stressed 28.8 tons, and there is a 28.4-ton horizontal reaction. Changing the sling angle to 45 degrees will reduce the stress in the sling to 7.1 tons and the horizontal reaction to 5.0 tons. 10 tons Figure 16.30. Stresses induced in a set of slings The general formulas are given by the following equations: [16.2] [16.3] where W = weight of the load T= tension in the cable N= stress in the sling Nx cos a = horizontal reaction N = a = angle between the load and each sling leg T(-W) T 2 sin a Nx cos a T 2 tan a When the rigging arrangement creates small sling angles, the resulting sling stress produced will be considerably greater than the load (Figure 16.31). The solution in such situations is to use a supplemental compression member-a spreader bar. The use of a spreader bar will allow for greater sling angles and reduce the induced sling stresses.