Seismic Design and Earthquake Engineering
Core concepts
Understand seismic hazard (ground motion, site effects, response spectra), structural dynamics (period, damping, mode shapes), and ductile design philosophy (capacity design, strong column–weak beam). Know base shear calculation, load distribution over height, drift limits, and detailing requirements for ductile behavior. Be familiar with irregularity classifications (plan and vertical) and when they trigger special analysis.
Common traps
- Period estimation: approximate formulas vs. analytical; using the wrong period shifts the spectral acceleration.
- Irregularities: torsional, soft-story, and vertical geometric irregularities each have distinct triggers and consequences.
- Redundancy and overstrength factors: mixing them up changes the base shear.
- Drift: computed at the design level, but amplified for inelastic effects.
Worked rule of thumb
Base shear is proportional to the spectral acceleration at the fundamental period, scaled by the seismic weight and divided by the response modification factor R. A stiffer, shorter building attracts more force but drifts less; a taller, flexible building attracts less force but drifts more. The design must satisfy both strength and drift - a common exam trap is a design that passes strength but fails drift.
Capacity design means detailing so that a chosen ductile mechanism (beam hinging) forms before brittle mechanisms (column or joint failure).