Torsional Irregularity in Seismic Design: Plan Irregularities and Amplification Factors
Ground shaking pushes a building sideways, but a building doesn't always just slide back and forth in response. If the point where the earthquake force effectively acts, the center of mass, doesn't line up with the point the building's lateral system resists that force from, the center of rigidity, the building twists around a vertical axis on top of translating. That twist is torsion, and it's one of the most common reasons a building performs worse in an earthquake than a simple base shear calculation would suggest.
Why Mass and Rigidity Don't Line Up
In a perfectly symmetric building, with shear walls or braced frames balanced evenly on every side and mass distributed uniformly on each floor, the center of mass and center of rigidity coincide and torsion is minimal. Real buildings rarely cooperate. A shear wall core pushed to one side of the plan to keep an open floor plate on the other, a parking garage with a solid wall along the property line and open framing at the entrance, or a corner unit with more glazing and less shear wall than the rest of the floor, all shift the center of rigidity away from the center of mass. Even a torsionally regular building can develop irregularity at upper floors if the tenant layout, mechanical equipment, or unit mix isn't consistent floor to floor.
The distance between those two centers, multiplied by the seismic base shear, produces a torsional moment on top of the direct translational shear, and every element of the lateral system, not just the ones farthest from the center of rigidity, has to be checked for that added demand layered on top of whatever it's already carrying from lateral load-resisting systems doing their ordinary job.
Elements on the side of the building farthest from the center of rigidity see the largest displacement under torsion, since displacement grows with distance from the twist's center of rotation. Codes call this the flexible side, and it's usually where torsional irregularity checks and amplified forces end up governing the design of individual walls or frames.
How Codes Quantify and Penalize the Irregularity
Model codes like ASCE 7 define torsional irregularity by comparing the maximum story drift at one end of the building to the average drift across both ends, under a load case that already includes an assumed accidental eccentricity, typically five percent of the building's plan dimension, added to whatever calculated eccentricity exists between the two centers. That accidental eccentricity exists because real buildings never behave exactly as modeled; actual mass distribution, material properties, and construction tolerances all introduce some torsion even in a nominally symmetric design.
Once a building crosses the ratio threshold that defines torsional irregularity, and an extreme version of it crosses a stricter threshold for extreme torsional irregularity, the consequences cascade through the design. Amplification factors get applied to accidental torsional moments on the flexible side, certain simplified analysis procedures become unavailable and a more rigorous dynamic analysis is required instead, and in high seismic design categories extreme torsional irregularity can trigger outright height or configuration restrictions. This mirrors how seismic design principles generally treat irregularity: the code doesn't ban irregular buildings, it demands the analysis be rigorous enough to actually capture what the irregularity does to force distribution.
Design Strategies to Reduce Torsional Demand
The most effective fix is architectural, not structural: pushing lateral elements toward the perimeter and balancing their stiffness on opposing sides reduces eccentricity at the source, which is far more efficient than trying to out-design a torsion problem with heavier reinforcement after the fact. Where the architectural program won't allow perfect symmetry, engineers sometimes add supplemental lateral elements specifically on the flexible side to pull the center of rigidity back toward the center of mass, accepting some redundancy loss elsewhere in exchange for a torsionally cleaner building. Three-dimensional dynamic analysis that explicitly models mass and stiffness eccentricity at every floor, rather than a simplified equivalent lateral force procedure, is what ultimately confirms whether a given layout's torsional response stays within acceptable bounds.
FEMA's seismic design resources, including guidance developed under the National Earthquake Hazards Reduction Program, document torsional irregularity provisions and their engineering basis in detail, available through the Federal Emergency Management Agency.