Under API 650 requirements, what should the bottom plates of tanks supported by grillage be joined by?

• A. Spot welds
• B. Fillet welds
• C. Butt welds
• D. Screw joints

Answer: (C)

Under API 650 requirements, the bottom plates of tanks supported by grillage should be joined by butt welds. This type of weld is preferred because it provides a continuous and strong connection between the plates, which is essential for maintaining the structural integrity and leak-tightness of the tank. Butt welds also enable better load distribution across the bottom plates, which is especially important in tanks that hold liquids and endure various operational stresses.

The design requires the use of butt welds to ensure that there are no gaps or weaknesses at the joints. This is critical, as any failure in the welds can lead to leaks or the structural failure of the tank. In contrast, techniques like spot welding or fillet welding may not provide the same level of strength or reliability in load-bearing applications. Screw joints, while useful in certain contexts, do not offer the needed durability or resistance to environmental factors, which makes them inappropriate for this application.