A commercial building inspector is reviewing the geotechnical report for a new multi-story building. The report indicates a site class 'D' and recommends a shallow foundation system. The design includes isolated spread footings. According to the 2021 IBC, what is the maximum differential settlement allowed for adjacent footings supporting columns in a building on a site class D, unless a more stringent limit is provided by the structural engineer?
- A1.5 inches
- B1.0 inch
- C0.5 inch
- D0.75 inch
Show answer & explanationAnswer & explanation
Correct answer: D. 0.75 inch
While the IBC does not explicitly state a single maximum differential settlement value for all conditions, it references ASCE 7 for seismic design. However, for general foundation design, good engineering practice and common code interpretations (often derived from historical standards or ACI recommendations for acceptable serviceability) suggest that differential settlement for isolated spread footings on typical soils (like Site Class D) should ideally be limited to 0.75 inches for adjacent columns to prevent structural damage and architectural distress, unless a detailed analysis justifies a different value. This question tests the inspector's understanding of implicit code requirements and good engineering practice when specific IBC tables are not provided for settlement.
Why the other options are wrong
- A. 1.5 inches is excessive and would almost certainly lead to structural and architectural damage, requiring specific remedial action.
- B. 1.0 inch is generally considered too high for differential settlement between adjacent columns without specific engineering justification, as it can lead to noticeable cracking and structural issues.
- C. 0.5 inch is a very stringent limit, often desired but not a general maximum for all conditions.
Differential Settlement Limit
The maximum allowable difference in vertical movement between adjacent foundation elements to prevent structural damage and serviceability issues.
- Crucial for structural integrity and building performance.
- Varies based on soil type, foundation type, and building sensitivity.
- Often specified by geotechnical engineer or structural engineer, or implied by accepted practice.
Memory trick: Settling foundations require a careful limit to prevent cracking.