Seismic Design Basics — ASCE 7-22

1. Site Classification — ASCE 7-22 Chapter 11 / Table 20.3-1

The site class is determined from measured or estimated soil properties in the upper 100 ft (30 m) of the site profile. It directly amplifies or de-amplifies the ground shaking at the site.

2. Spectral Acceleration Parameters — ASCE 7-22 §11.4

Design spectral accelerations are derived from the 2018 USGS probabilistic hazard maps (ASCE 7-22 Figures 22-1 through 22-6) and amplified for site effects.

Step 1 — Map Values

Step 2 — Adjust for Site Class

Step 3 — Design Spectral Acceleration

Typical SDS Values by US Region

3. Seismic Design Category — ASCE 7-22 §11.6

The Seismic Design Category (SDC) determines which seismic force-resisting systems are permitted, what analysis procedures are required, and what detailing provisions of AISC 341 / ACI 318 Chapter 18 apply.

Also check using SD1 — the more restrictive of the two tables governs. SDC E/F apply near major active faults (§11.6) when S1 ≥ 0.75g for Risk Cat. I–III (E) or I–II (F).

3.5. Structural Irregularities — ASCE 7-22 §12.3

Structural irregularities trigger additional analysis requirements, restrict permitted SFRS types, and may prohibit the ELF procedure. ASCE 7-22 §12.3 identifies horizontal (plan) and vertical irregularities separately.

Horizontal (Plan) Irregularities — Table 12.3-1

Types 1a and 1b are assessed at each floor with rigid diaphragms. Drift ratio is computed including accidental torsion (§12.8.4.2).

Vertical Irregularities — Table 12.3-2

4. Seismic Force-Resisting Systems & Response Modification Factors

The R factor is the response modification coefficient — it reflects the ductility, overstrength, and redundancy of the structural system, reducing the design force relative to elastic demand. The Cd factor amplifies elastic displacements to estimate inelastic drift.

NL = no limit. Source: ASCE 7-22 Table 12.2-1. Ωo = overstrength factor. Height limits shown are for SDC D–F; some systems have higher limits in SDC B–C.

5. Equivalent Lateral Force Procedure — ASCE 7-22 §12.8

The ELF procedure is permitted for most structures in SDC B–D, and in SDC E–F for structures meeting regularity and height criteria. The base shear V is determined as follows:

Fundamental Period T — §12.8.2

Key Terms

• W: Seismic weight — total dead load plus applicable portions of live load per §12.7.2
• Ie: Importance factor (1.0 for Risk Cat. I–II, 1.25 for RC III, 1.50 for RC IV)
• TL: Long-period transition period (from ASCE 7-22 maps, typically 8–16 s in most US regions)

6. Vertical Force Distribution — ASCE 7-22 §12.8.3

The seismic base shear V is distributed vertically over the height of the structure. The distribution is parabolic for taller or more flexible buildings, linear for short rigid structures.

Example — 5-Story Building, T = 0.8 s, k ≈ 1.15

Drift Check — §12.12

7. Modal Response Spectrum Analysis — ASCE 7-22 §12.9

Modal Response Spectrum Analysis (MRSA) explicitly accounts for the dynamic characteristics of a structure — multiple vibration modes — rather than approximating behavior with a single-mode ELF approach. It is more accurate for irregular or taller structures.

When is MRSA Required?

Per ASCE 7-22 Table 12.6-1, MRSA is required (ELF alone is not permitted) when:

MRSA Procedure Summary

1. Compute natural periods and mode shapes — Use structural analysis software (ETABS, SAP2000, RISA). Enough modes must be included to capture at least 90% of the participating mass in each direction (§12.9.1.1).
2. Determine design spectral accelerations Sa(T) for each mode period T from the design response spectrum (§11.4.6): Sa = SDS for T ≤ T0; Sa = SD1/T for T > Ts; Sa = SD1·TL/T² for T > TL.
3. Compute modal base shears Vm for each mode m: Vm = Csm × Wm, where Csm = Sa(Tm) / (R/Ie) and Wm is the effective modal seismic weight.
4. Combine modal responses — Use CQC (Complete Quadratic Combination) method when modes are closely spaced; SRSS (Square Root of Sum of Squares) when modes are well-separated. CQC is generally required.
5. Apply directional combination — 100% along one axis + 30% perpendicular (or SRSS of two orthogonal MRSA results) per §12.5.
6. Scale to minimum base shear if Vt < V_ELF (see below).

Minimum Base Shear Scaling — §12.9.1.4.1

MRSA vs ELF — When MRSA Adds Value

8. Shear Wall Design Overview — ACI 318-25 Ch.11 & 18

Reinforced concrete shear walls (structural walls) are one of the most efficient lateral force-resisting systems. They provide high stiffness and ductility and are economical from SDC C upward.

When to Use Shear Walls

• SDC A–B: Moment frames typically sufficient. Shear walls may be used for drift control.
• SDC C: Either ordinary RC shear walls (R = 5) or intermediate moment frames economical. Walls often preferred over frames for stiffness.
• SDC D–F: Special RC shear walls (R = 6) with full ACI 318-25 Ch.18 detailing required. The high R value and excellent stiffness make them the dominant system choice.

Minimum Dimensions — ACI 318-25 §18.10.2

Behavior: Aspect Ratio

Special Boundary Elements (SBE) — When Required?

Boundary elements are confined end zones that prevent concrete crushing at the wall edges under large seismic demands. Two approaches determine the need:

Pier/Spandrel vs Whole-Wall Behavior

• Whole wall (solid): Single plastic hinge at base. Stiff and strong; controls drift effectively. Preferred for SDC D–F.
• Coupled walls: Two walls connected by coupling beams. Coupling beams dissipate energy in diagonally reinforced (ACI §18.10.7) plastic hinges; overall system has lower stiffness but higher ductility.
• Perforated (pier/spandrel): Window openings create pier-and-spandrel frame-like behavior. More complex analysis; each pier acts as a squat wall — shear-critical behavior.