If joint efficiency is reduced from 1.0 to 0.85, what happens to the required thickness?

• A. Increases
• B. Decreases
• C. Remains unchanged
• D. Becomes infinite

Answer: (A)

The main idea is that wall thickness depends on how strong the joint is considered in the design. Joint efficiency is a factor that reduces the effective strength of the welded joints compared to the base metal. In the standard thickness calculation for a pressure vessel, the required thickness is inversely proportional to the product of allowable stress and joint efficiency. So if the joint efficiency drops, the allowable strength goes down and the wall must be thicker to carry the same internal pressure.

If the joint efficiency goes from 1.0 to 0.85, the thickness required increases. In a simplified view, t is proportional to 1/(S × E); keeping all else the same, lowering E from 1.0 to 0.85 makes t grow by about 1/0.85 ≈ 1.18, i.e., roughly 18% thicker. So the correct outcome is that the required thickness increases.