CFS Joint Design & Load Calculations 3 — Questions and Answers
Question 1: The 'separation load' (Ps) in a preloaded bolted joint is the external tensile load at which:
- The bolt yields
- The joint faces begin to separate (clamp load = 0) (Correct answer)
- The nut strips
- The bolt fractures
Correct answer: The joint faces begin to separate (clamp load = 0)
Separation occurs when the external load reduces the compressive stress between joint members to zero, eliminating clamping force.
Question 2: Using the Shigley joint diagram approach, what percentage of an external tensile load is carried by the bolt in a well-designed preloaded joint?
- 100%
- 50%
- A small fraction determined by Kb/(Kb+Km) (Correct answer)
- A fraction determined by Km/(Kb+Km)
Correct answer: A small fraction determined by Kb/(Kb+Km)
The bolt takes only the fraction Kb/(Kb+Km) of the external load because the compliant clamped material absorbs most of it.
Question 3: Which thread form has a 60° flank angle and is the most common in US industrial fastener applications?
- Acme thread
- Buttress thread
- Unified National (UN/UNC/UNF) (Correct answer)
- Square thread
Correct answer: Unified National (UN/UNC/UNF)
The Unified National thread with its 60° flank angle is the dominant fastener thread form in the United States for clamping applications.
Question 4: In a bolted joint subjected to fatigue loading, which design change most effectively reduces the alternating stress in the bolt?
- Using a larger diameter bolt
- Increasing bolt stiffness
- Decreasing bolt stiffness (longer grip, reduced shank diameter) (Correct answer)
- Reducing preload
Correct answer: Decreasing bolt stiffness (longer grip, reduced shank diameter)
A more compliant bolt takes a smaller fraction of the alternating external load, reducing the amplitude of cyclic stress and improving fatigue life.
Question 5: What does the 'k factor' (nut factor) represent in the torque-tension equation T = k × F × d?
- The bolt's elastic modulus ratio
- An empirical coefficient combining all frictional and geometric effects (Correct answer)
- The thread helix angle only
- The ratio of clamp load to proof load
Correct answer: An empirical coefficient combining all frictional and geometric effects
The nut factor k ≈ 0.1–0.2 for typical lubricated bolts is an empirical constant that lumps thread friction, under-head friction, and geometry into a single multiplier.
Question 6: A fastener in a structural steel connection is said to be in 'double shear' when:
- Two bolts share a single shear plane
- The fastener passes through three members creating two shear planes (Correct answer)
- The bolt is loaded in both tension and shear simultaneously
- The bolt is tightened to twice the specified torque
Correct answer: The fastener passes through three members creating two shear planes
Double shear occurs when a fastener has two shear planes (e.g., a clevis pin), effectively doubling its shear capacity compared to single shear.
Question 7: What is the primary purpose of a prevailing-torque nut (e.g., nylon insert or all-metal distorted thread) in joint design?
- Increase clamp load above proof load
- Resist loosening due to vibration or transverse movement (Correct answer)
- Reduce installation torque
- Increase joint stiffness
Correct answer: Resist loosening due to vibration or transverse movement
Prevailing-torque nuts create a resisting torque that must be overcome during both installation and removal, preventing self-loosening under dynamic loads.
CFS Joint Design & Load Calculations 3
The 'separation load' (Ps) in a preloaded bolted joint is the external tensile load at which: