For a bolted assembly with six bolts, the stiffness of each bolt is kb = 3 Mlbf/in and the stiffness of the members is km = 15 Mlbf/in per bolt. An external load of 80 kips is applied to the entire joint. Assume the load is equally distributed to all the bolts. It has been determined to use 1/2 in-13 UNC grade 8 bolts with rolled threads. It is desired to find the range of torque that a mechanic could apply to initially preload the bolts without expecting failure once the joint is loaded. Assume a torque coefficient of K = 0.2.

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Determine the maximum bolt preload that can be applied without exceeding the proof strength of the bolts once the load is applied.