Commercial Building Inspector B2Footings and FoundationsHard
A commercial building inspector is reviewing the structural plans for a new building in a seismic design category 'D'. The foundation system includes continuous concrete footings. The plans show the longitudinal reinforcement in the footings. According to the 2021 IBC, what is the minimum area of longitudinal reinforcement required for continuous concrete footings in Seismic Design Categories C, D, E, and F?
- A0.0010 times the gross concrete area
- B0.0025 times the gross concrete area
- C0.0018 times the gross concrete area
- D0.0020 times the gross concrete area
Show answer & explanationAnswer & explanation
Correct answer: D. 0.0020 times the gross concrete area
IBC Section 1809.4.2 (and by reference, ACI 318 Section 10.5.4) specifies requirements for longitudinal reinforcement in continuous footings in seismic design categories C, D, E, and F. The minimum area of longitudinal reinforcement shall not be less than 0.002 times the gross cross-sectional area of the footing.
Why the other options are wrong
- A. 0.0010 is too low for the minimum longitudinal reinforcement ratio in seismic categories C-F.
- B. 0.0025 is higher than the minimum required and would be acceptable, but it's not the specified minimum.
- C. 0.0018 is the minimum shrinkage and temperature reinforcement ratio for slabs, but not for continuous footings in high seismic zones.
Footing Longitudinal Reinforcement (Seismic)
The minimum amount of continuous steel reinforcement required along the length of footings in high seismic areas to resist bending and shear forces during an earthquake.
- Crucial for ductility and integrity in seismic events.
- Expressed as a ratio of steel area to concrete area.
- Specific requirements vary by Seismic Design Category.
Memory trick: Two thousandths of concrete area, for seismic strength, banishing all fear.