What is the primary purpose of the special seismic load combinations required by CBC in high seismic zones?
Correct Answer
A) To account for earthquake forces combined with other loads
Special seismic load combinations ensure structures can handle the simultaneous effects of seismic forces along with dead loads, live loads, and other forces that may occur during an earthquake.
Why This Is the Correct Answer
The primary purpose is to account for earthquake forces combined with other loads. Special seismic load combinations (per ASCE 7 Section 2.3.6 and 12.4) require designers to consider the simultaneous occurrence of seismic forces with dead load, live load, and other applicable loads. This ensures structures have adequate strength for the most critical load scenario during an earthquake.
Why the Other Options Are Wrong
Option B: To meet federal building requirements
Meeting federal building requirements is not the primary purpose of seismic load combinations. While federal agencies (like FEMA) contribute to seismic guidelines, CBC special seismic load combinations are adopted through the state and local building code process, not as a federal mandate. The purpose is structural safety, not regulatory compliance with federal rules.
Option C: To reduce construction costs
Reducing construction costs is incorrect — seismic load combinations typically increase design demands, which generally adds cost rather than reducing it. This is a distractor that misrepresents the purpose of enhanced seismic requirements.
Option D: To simplify structural calculations
Simplifying structural calculations is the opposite of what special seismic load combinations do. These combinations add complexity by requiring engineers to consider additional load scenarios (Em = Emh ± 0.2SDS × D, etc.), making calculations more involved, not simpler.
Memory Technique
Think of an earthquake as a surprise attack on a building that is already carrying its normal load. The building must handle the surprise (seismic) ON TOP of its everyday loads (dead + live). Special seismic load combinations formalize this 'stacked attack' scenario in design.
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