Answer:
a) To determine the desired preload, we need to find 75% of the proof load for each bolt.
Proof load for a 12 in-13 UNC grade 8 bolt can be found using reference tables or engineering specifications. Let's assume the proof load is 130 kips.
Desired preload = 75% of the proof load
Desired preload = 0.75 * 130 kips
Desired preload = 97.5 kips
b) The yielding factor of safety can be calculated by dividing the proof load by the desired preload.
Yielding factor of safety = Proof load / Desired preload
Yielding factor of safety = 130 kips / 97.5 kips
Yielding factor of safety = 1.33