The equation for Kb shows the value of 'At' as 115. This is the value of 'At' for an M14x2 bolt from Shigley's Table 8-1. The value for an M14x1.5 bolt is 125. Final answer for Kb is 926.4.
I got this problem in my PE: MDM exam this week *Givens:* Bolt (E = 10 x 10^6 PSI) Shank: A(Unthreaded) = 0.565 in^2 & L(Unthreaded) = 1.2 in Thread: A(t) = 0.29 in^2 & L(t) = 0.8 in *Answer:* K = E x A / L Kb = (Ku x Kt) / (Ku + Kt) = 2.05 x 10^6 [lbf/in] *Values changed from that in the exam for copy rights only ;)