Understanding Structural Irregularities in ASCE 7: A Guide to Remedial Actions

In modern structural design, achieving both safety and efficiency requires more than just calculating forces and designing members—it demands an awareness of how a building behaves under seismic loads, especially when its geometry or system layout deviates from the norm. These deviations, known as structural irregularities, can significantly influence a building’s performance during an earthquake.

The American Society of Civil Engineers (ASCE 7) outlines clear definitions and classifications for these irregularities, along with required actions when they are present. Whether it’s a torsional response from an L-shaped floor plan, a weak story from open ground levels, or a misalignment in vertical elements, each irregularity introduces vulnerabilities that must be addressed through enhanced analysis and design.

This article summarizes the different types of plan and vertical irregularities defined in ASCE 7-16 and 7-22, and provides a practical reference table highlighting their implications and the required remedial steps. If you’re a practicing engineer, structural modeler, or designer aiming for seismic resilience and code compliance, this guide is for you.

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ASCE 7 Irregularity Types & Required Remedial Actions

Irregularity Type	Description	Section	Remedial Instructions / Required Actions
1. Torsional Irregularity	Extreme difference in lateral displacements across plan (diaphragm rotation)	12.3.3.1	- Design for torsional amplification (Amp ≥ 1.0)   - Center of mass shift  - Limit in drift per diaphragm corner  - May need modal analysis with 3D modeling
2. Re-entrant Corners	L-shaped or U-shaped plans with stress concentrations	12.3.3.2	- Use 3D modeling or ensure transfer of seismic forces across re-entrant corner  - Consider additional detailing at notch area
3. Diaphragm Discontinuity	Sudden diaphragm stiffness/strength change (e.g., openings, cut-outs)	12.3.3.3	- Explicit modeling of diaphragm  - Consider collector/strut detailing  - Avoid using flexible diaphragm assumption
4. Out-of-plane offsets	Vertical elements (walls/frames) misaligned between floors	12.3.3.4	- Ensure vertical force transfer path continuity  - Additional collector and transfer elements
5. Non-parallel system	Lateral systems not aligned with grid axes	12.3.3.5	- Orthogonal loading required (12.5)  - 100% + 30% rule for bidirectional loading
6. Vertical Irregularity: Stiffness	Weak story: stiffness < 70% of above story	Table 12.3-2	- Must use nonlinear analysis for design  - Design ductile detailing (per 12.3.3.4 & 12.10)
7. Vertical Irregularity: Strength	Story strength < 80% of above	Table 12.3-2	- Check for soft/weak story failure  - Increase lateral resistance (e.g., brace, wall)
8. Vertical Geometric Irregularity	Large changes in story geometry (setbacks, transfer stories)	Table 12.3-2	- Full 3D dynamic analysis recommended  - Check force transfer through setback floor
9. In-plane Discontinuity in Vertical Elements	Discontinuity in lateral-force-resisting elements (e.g., piloti, open ground floor)	Table 12.3-2	- No simplified analysis allowed  - Require nonlinear procedure or detailed modeling
10. Mass Irregularity	Story mass > 150% of adjacent stories	Table 12.3-2	- Explicit mass modeling in modal analysis  - Adjust base shear and period accordingly
11. Torsional Strength Irregularity	Lateral system strength uneven in plan (new in ASCE 7-22)	ASCE 7-22 §12.3.3.6	- Requires balanced strength distribution  - Special checks for torsional yielding

✅ General ASCE 7 Remedial Requirements Summary

• Modal Response Spectrum Analysis (MRSA) required for major irregularities (especially torsional or vertical).

• Nonlinear Static or Dynamic Analysis for soft story, discontinuities, or setbacks.

• Detailed Collector/Chord Design in diaphragms with re-entrant corners or openings.

• Torsional Amplification Factor (Ax) must be ≥ 1.0 when irregularities exceed thresholds.

• Orthogonal Load Combination when irregular systems or non-parallel frames exist.

• No simplified Equivalent Lateral Force (ELF) method for certain irregularities (must use dynamic methods).