SEISMIC

Damping

Energy dissipation in structure. RCC: 5% (design). Steel: 2-3%. Higher damping → smaller seismic response.

Also calleddamping ratioviscous damping
Related on InfraLens
CODES
IS 1893
Definition

Damping is the dissipation of vibrational energy in a structure. Per IS 1893:2016, design damping for typical structures: RCC = 5%, Steel = 2-3%, Masonry = 5-10%. Higher damping reduces dynamic response; lower damping causes structures to vibrate longer with larger amplitudes. Critical damping is the value that prevents oscillation; structures are designed for sub-critical damping (less than critical) — typically 2-15% in real-world buildings.

Damping mechanisms: (1) Material damping — internal friction in concrete and steel; small (1-3% typical). (2) Aerodynamic damping — air resistance to building motion. (3) Soil damping — soil radiates energy from foundation movement. (4) Friction damping — connections, joints, partition walls. (5) Tuned mass dampers (TMDs) — explicit damping devices for tall buildings (Burj Khalifa, etc.). For RCC framed buildings, total damping: material 2-3% + soil 1-2% + friction 1-2% = total ~5%.

Applications in seismic design: (a) Lower damping = larger seismic response; conservatively 5% used in IS 1893 design spectrum. (b) For specialty structures with explicit damping devices: higher effective damping reduces design force. (c) Wind serviceability — damping affects building motion under wind; lower damping (≤ 1%) in tall buildings causes occupant discomfort. (d) Dynamic analysis — damping is essential parameter in time-history analysis.

Where used
  • Seismic design (IS 1893 — 5% damping default)
  • Wind serviceability analysis for tall buildings
  • Dynamic analysis (response spectrum, time history)
  • Specialty buildings with tuned mass dampers
  • Mechanical equipment foundations
Acceptance / threshold
Per IS 1893:2016: damping per structural type (RCC 5%, Steel 2-3%, Masonry 5-10%); design spectrum based on 5% damping; specialty structures with explicit damping devices may use higher value with verification.
Frequently asked
What is damping in structures?
Damping is the dissipation of vibrational energy in a structure. Per IS 1893:2016: RCC 5%, Steel 2-3%, Masonry 5-10%. Mechanisms: material damping, aerodynamic, soil, friction, tuned mass dampers (TMDs). Affects seismic and wind response — higher damping reduces dynamic motion.
What is damping ratio?
Damping ratio is the actual damping divided by critical damping. Critical damping = value that prevents oscillation. For typical buildings: 2-15% of critical (sub-critical). Per IS 1893: design uses 5% damping for RCC, 2-3% for Steel, 5-10% for Masonry. Specialty structures with explicit damping (TMDs, viscous dampers) can have effective damping 10-15%.
How does damping affect seismic design?
Lower damping = larger seismic response (longer vibration with larger amplitudes). IS 1893 design spectrum is calibrated for 5% damping; if actual damping is higher (e.g., with TMDs), design forces can be reduced proportionally. For wind serviceability of tall buildings: very low damping (≤ 1%) causes occupant discomfort from building sway; specialty damping systems address this.
Related seismic terms
Earthquake / Seismic Zones (II-V)
Zone II (least)-V (most) per IS 1893
Ductile Detailing (Seismic)
Special seismic detailing per IS 13920
Base Shear
Horizontal seismic force at the base of a structure. Vb = Ah × W, where Ah = (Z/2)(I/R)(Sa/g). IS 1893 Cl. 7.5.
Response Spectrum
Plot of peak structural response vs. natural period for an earthquake. IS 1893 Fig. 2 gives design spectra for 5% damping.
Importance Factor (I)
Multiplier on seismic force based on building criticality. Hospitals: 1.5. Schools: 1.5. Residential: 1.0.
Response Reduction Factor (R)
Reduces elastic seismic forces to design forces accounting for ductility. SMRF: 5.0. OMRF: 3.0. Shear wall: 4.0.
Special Moment Resisting Frame (SMRF)
Frame designed and detailed for high ductility per IS 13920. R = 5.0. Required in Zone IV/V for important buildings.
Soft Storey / Stilt Floor
Storey with stiffness <70% of the storey above — typical of stilt parking. IS 1893 Cl. 7.10 mandates 2.5× design force.