Exam Project-Planning-Design Topic 2 Question 25 Discussion
Actual exam question for NCARB's Project-Planning-Design exam
Question #: 25
Topic #: 2
Question #: 25
Topic #: 2
For a three-story building, which of the following is considered a vertical irregularity with respect to seismic design?
Suggested Answer: C Vote an answer
Comprehensive and Detailed Explanation From Exact Extract:
In seismic design, vertical irregularities are discontinuities or abrupt changes in the building's mass, stiffness, or geometry that can affect seismic response and increase vulnerability during an earthquake. The NCARB ARE 5.0 Project Planning & Design guidelines describe vertical irregularities as changes occurring along the height of the building.
* Option C describes a mass irregularity where story 2 has twice the effective mass of story 1. According to seismic code provisions (such as those referenced in ASCE 7 and adopted by IBC), a vertical mass irregularity is present if the effective seismic mass in any story is more than 150% (1.5 times) or less than 70% (0.7 times) of the mass of an adjacent story. Here, doubling the mass is a significant vertical irregularity that affects the dynamic behavior and design.
* Option A, the roof mass being half that of the floor below, is a decrease in mass but less than the typical threshold of 30% difference (the ratio is 0.5, which is a 50% difference). This might also be considered, but the mass irregularity is more typically flagged at the 1.5x or 0.7x threshold and tends to be more critical in lower floors, making C the clearer choice.
* Option B describes a reentrant corner, which is a horizontal plan irregularity, not vertical. Reentrant corners affect torsional behavior but are not classified as vertical irregularities.
* Option D refers to stiffness differences between interior shear walls and perimeter columns but, when symmetrically placed, this is not necessarily considered an irregularity. Vertical stiffness irregularities are defined by abrupt stiffness changes in vertical elements, but symmetry mitigates torsional effects.
The presence of vertical mass irregularities significantly influences seismic forces distribution, dynamic response, and the potential for torsional motions. Designers must recognize these irregularities per NCARB guidelines and apply appropriate structural detailing and design modifications to meet life-safety requirements.
References:
ARE 5.0 Project Planning & Design Outline: Environmental Conditions and Context - Seismic Design Considerations NCARB ARE 5.0 Guidelines, Seismic Design and Irregularities ASCE 7-16, Chapter 12 - Seismic Design Criteria The Architect's Handbook of Professional Practice, 15th Edition, Chapter 13: Building Codes, Standards, and Regulations
In seismic design, vertical irregularities are discontinuities or abrupt changes in the building's mass, stiffness, or geometry that can affect seismic response and increase vulnerability during an earthquake. The NCARB ARE 5.0 Project Planning & Design guidelines describe vertical irregularities as changes occurring along the height of the building.
* Option C describes a mass irregularity where story 2 has twice the effective mass of story 1. According to seismic code provisions (such as those referenced in ASCE 7 and adopted by IBC), a vertical mass irregularity is present if the effective seismic mass in any story is more than 150% (1.5 times) or less than 70% (0.7 times) of the mass of an adjacent story. Here, doubling the mass is a significant vertical irregularity that affects the dynamic behavior and design.
* Option A, the roof mass being half that of the floor below, is a decrease in mass but less than the typical threshold of 30% difference (the ratio is 0.5, which is a 50% difference). This might also be considered, but the mass irregularity is more typically flagged at the 1.5x or 0.7x threshold and tends to be more critical in lower floors, making C the clearer choice.
* Option B describes a reentrant corner, which is a horizontal plan irregularity, not vertical. Reentrant corners affect torsional behavior but are not classified as vertical irregularities.
* Option D refers to stiffness differences between interior shear walls and perimeter columns but, when symmetrically placed, this is not necessarily considered an irregularity. Vertical stiffness irregularities are defined by abrupt stiffness changes in vertical elements, but symmetry mitigates torsional effects.
The presence of vertical mass irregularities significantly influences seismic forces distribution, dynamic response, and the potential for torsional motions. Designers must recognize these irregularities per NCARB guidelines and apply appropriate structural detailing and design modifications to meet life-safety requirements.
References:
ARE 5.0 Project Planning & Design Outline: Environmental Conditions and Context - Seismic Design Considerations NCARB ARE 5.0 Guidelines, Seismic Design and Irregularities ASCE 7-16, Chapter 12 - Seismic Design Criteria The Architect's Handbook of Professional Practice, 15th Edition, Chapter 13: Building Codes, Standards, and Regulations
by Benjamin at Aug 30, 2026, 02:55 PM
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