**What is Closely-Spaced Modes:** The natural modes of oscillation of a structure, whose natural frequencies differ from each other by 10 percent or less of the lower frequency.

**What is damping?** The effect of internal friction, inelasticity of materials, slipping, sliding, etc. in reducing the amplitude of oscillation, expressed as a fraction of critical damping.

**Design Acceleration Spectrum:** An average smoothed graph of maximum acceleration as a function of natural frequency or natural period of oscillation for a specified damping ratio for the expected earthquake excitations at the base of a single-degree-of-freedom system.

**What is ductility? **The capacity of a structure (or its members) to undergo large inelastic deformations without significant loss of strength or stiffness.

**Earthquake Zone Factor (Z): **The value of peak ground acceleration considered by this standard for the design of structures located in each Earthquake Zone.

**Elastic Force Reduction Factor (R): **The factor by which the base shear induced in a structure, if it were to remain elastic, is reduced to obtain the design base shear.

It depends on the following factors:

- the perceived earthquake damage performance of the structure,
- characterized by ductile or brittle deformations,
- redundancy in the structure, or
- overstrength inherent in the design process.

**Floor Response Spectrum:** The response of a floor (expressed as responses of a spectrum of single degree of freedom systems, for a chosen structural damping value) is presented as a spectrum when the structure on which it is rested is subjected to earthquake ground shaking at its base.

**Importance Factor (I):** Factor used to estimate design earthquake force depending on the functional use of the structure, characterized by

- consequences of its failure,
- post-earthquake functional needs,
- historical value, or
- economic importance.

**What is Limit State:** A state at which one condition about a structure has reached a limiting or critical value. A limit state is said to have been attained when the loads or actions imposed on a structure are such that their effects (in terms of stresses or stress-resultant(s) in the members of the structure, or of strains, deflection, or rotations of the structure or a member) just reach a specified limiting value.

**What is the Limit State of Strength:** It is said to have been attained by a structure when, under the action of the designated loads or load actions imposed on it, the stresses or strains in its members just reach a specified limiting value

**What is Limit State of Serviceability?** It is said to have been attained by a structure when, under the action of the designated loads or load actions imposed on it, the deflection or rotations in its members just reach a specified limiting value.

**What is Liquefaction:** A state primarily in saturated cohesionless soils wherein the effective shear strength is reduced to a negligible value for all engineering purposes when the pore pressure approaches the total confining pressure during earthquake shaking. In this condition, the soil tends to behave like a fluid mass.