Earthquakes, Codes, the R-Factor, and Structural Systems

This page explains what earthquakes are, which standards apply, what the R-factor means, and how different structural systems define R. Images illustrate each system clearly.

What is an Earthquake?

An earthquake is the sudden release of energy in the Earth’s crust, producing seismic waves that shake the ground. Structures don’t fail because the earth “opens” but because the shaking creates inertia forces proportional to their mass and acceleration.

Structural demand depends on: event magnitude, distance to epicenter, soil conditions, and the building’s own natural period and ductility. The design goal is to allow controlled damage and energy dissipation while preventing collapse.

Seismic Design Standards & Codes

Different regions apply different seismic design codes, but the philosophy is the same: permit damage, prevent collapse.

  • ASCE 7 / IBC (USA) – Defines seismic loads, spectra, site coefficients, and R-values.
  • Eurocode 8 (EN 1998) – European seismic design; ductility classes (DCL, DCM, DCH).
  • NBCC (Canada), NZS 1170.5 (NZ), IS 1893 (India), Japan B.C., Iran 2800.
Always follow the code required by your local authority. The concepts are universal, but detailing and factors are code-specific.

What is the R-Factor?

The Response Modification Factor (R) reduces elastic seismic forces to design-level forces. It represents a system’s ductility, redundancy, and energy-dissipation capacity.

  • High R ≈ 7–8: very ductile (special moment frames, dual systems).
  • Moderate R ≈ 5–6: stiff but ductile (reinforced concrete shear walls).
  • Low R ≈ 3–4: limited ductility (ordinary frames, masonry walls).

Design base shear formula (ASCE 7 style):

V = (SDS × W) / (R / I)

Structural Systems & Typical R

Moment Frame (R≈8)

Special Moment Frame R ≈ 8

Ductile hinges at beam-column joints. Requires strong-column/weak-beam detailing and confinement reinforcement.

Braced Frame (R≈6–8)

Steel Braced Frame R ≈ 6–8

Diagonal corner-to-corner braces give stiffness. Ductility comes from yielding braces (CBF) or links (EBF).

RC Shear Wall (R≈5–6)

RC Shear Wall R ≈ 5–6

Concrete walls carry shear and overturning, limiting drift. Needs boundary elements and strong diaphragm anchors.

Dual System (R≈7–8)

Dual System R ≈ 7–8

Combines frame ductility with wall/bracing stiffness. Both must share seismic load per code.

Masonry Shear Wall (R≈3–4)

Masonry Shear Wall R ≈ 3–4

Provides stiffness but low ductility. Needs reinforcement and strong diaphragm ties.

Solid Diaphragm

Solid Diaphragm Critical

Floors/roofs act as rigid plates, transferring seismic forces to frames or walls. Prevents torsional irregularities.

Role of Solid Diaphragms

A solid diaphragm ensures each floor moves together, reducing torsion and sending forces to vertical systems. Without it, even strong walls or frames cannot perform properly.

  • Provide chords at slab edges and collectors (drag struts).
  • Avoid re-entrant corners or openings without reinforcement.
  • Check diaphragm shear and drift compatibility.