Earthquake Engineering

Earthquake engineering keeps structures standing when the ground shakes. Learn seismic waves and magnitude, natural period and resonance, response spectra, equivalent static base shear (IS 1893), ductility and response reduction, capacity design, soft storeys and base isolation.

Earthquakes shake the ground back and forth; the building's own mass resists that motion and generates inertia forces. Buildings whose natural period matches the dominant ground period suffer most.

Seismic design doesn't try to make buildings stay elastic in a big earthquake — that would be uneconomical. Instead it makes them ductile so they can bend, crack and yield in controlled places without collapsing.

An analogy

Shake a tray with tall and short bottles: some rock wildly depending on their height. Buildings, too, respond according to their natural period.

Earthquakes and ground motion

Fault rupture releases energy as P, S and surface waves. Magnitude (Mw) measures energy released; intensity (MMI) measures local shaking effects. Soft soils amplify shaking and can liquefy.

Period, resonance and response spectra

Short, stiff buildings have short periods (0.1–0.5 s); tall flexible ones long periods (1–5 s). A response spectrum plots peak response vs period for a given ground motion; codes provide smoothed design spectra for soil types, which peak for short-period buildings.

Equivalent static method

For regular buildings, base shear V_B = A_h W is distributed up the height (roughly as W h²) and applied as lateral loads. R (3–5 for ordinary/special frames) reflects ductility and overstrength; I raises design forces for hospitals and schools.

Ductile design and advanced methods

Capacity design: strong columns/weak beams, shear capacity exceeding flexural capacity, confined concrete with closely spaced stirrups and 135° hooks. Avoid soft/weak storeys, torsional irregularity and short columns. Response spectrum and time-history analysis, pushover, base isolation and dampers are used for important or irregular structures.

Formulas

• Natural period (SDOF): T = 2π √(m/k)
• Design horizontal coefficient (IS 1893): A_h = (Z/2) · (I/R) · (S_a/g)
• Base shear: V_B = A_h · W
• Approx. period (RC moment frame): T_a = 0.075 h^0.75
• Vertical distribution: Q_i = V_B · W_i h_i² / Σ W_j h_j²

Worked example: Base shear (short-period building)

Given: Zone IV (Z = 0.24), I = 1, R = 5; S_a/g = 2.5, W = 20 000 kN

Find: A_h and base shear.

• A_h = 0.12 × 0.2 × 2.5 = 0.06
• V_B = 0.06 × 20 000 = 1200 kN

Answer: 1200 kN (6 % of weight)

Worked example: Using the approximate period

Given: RC frame 30 m tall, medium soil

Find: T, S_a/g, V_B.

• T = 0.075 × 30^0.75 = 0.96 s
• S_a/g = 1.36/0.96 = 1.41
• A_h = 0.12 × 0.2 × 1.41 = 0.034 → V_B = 679 kN

Answer: ≈ 680 kN — less than the plateau value

Common mistakes

• Wrong: Open ground floors (parking) with infilled floors above. — Right: Soft storeys collapse first — add walls or design ground columns for amplified forces.
• Wrong: Stirrups with 90° hooks and wide spacing. — Right: Use 135° hooks and close spacing for confinement.

Applications

• Buildings: Ductile RC and steel frames, shear walls, seismic detailing (IS 13920).
• Critical facilities: Hospitals, schools and power plants with higher importance factors and isolation.
• Retrofit: Jacketing, FRP wrapping, added walls and dampers for vulnerable buildings.

Key takeaways

• Inertia forces depend on mass, stiffness and period.
• A_h = (Z/2)(I/R)(S_a/g); V_B = A_h W.
• Distribute forces ∝ W h².
• Ductile detailing and regular configuration save lives.

Going further

• Pushover analysis: Nonlinear static analysis traces capacity until collapse mechanism to verify performance levels.
• Time-history analysis: Recorded or synthetic ground motions applied to nonlinear models for critical structures.
• Liquefaction: Saturated loose sands lose strength in shaking — ground improvement or deep foundations needed.