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Architecture Engineering QUESTION #2357
Question 1
In the design of a waffle slab, the minimum rib width and depth requirements per ACI 318 specify that the rib width shall not be less than:
  • 100 mm with rib depth not more than 3.5 times rib width
  • 75 mm with rib depth not more than 3.5 times rib width✔️
  • 100 mm with rib depth not more than 4 times rib width
  • 75 mm with rib depth not more than 4.5 times rib width
Correct Answer Explanation
Per ACI 318 Section 9.8.1, for joist (ribbed) and waffle slab systems, ribs shall not be less than 100 mm wide, and the overall depth of ribs shall not exceed 3.5 times the minimum rib width. Therefore rib width ≥ 100 mm and depth ≤ 3.5 × 100 mm = 350 mm. The clear spacing between ribs shall not exceed 750 mm. These dimensional limits ensure proper concrete placement, adequate cover, and structural integrity of the one-way or two-way ribbed slab system.