Seismic Principles: Ground Motion, Structural Response, and Code-Based Design
What the exam expects
The Seismic Principles domain tests whether you can connect ground motion to structural behavior and then to code provisions. Candidates must understand fault geometry and seismic hazard (fault type, distance, site class, and how soft soils amplify long-period shaking), the difference between magnitude (energy released) and intensity (local effects), and the response spectrum concept. Know that a response spectrum plots peak response versus period for a given damping and earthquake, that spectral acceleration generally peaks at short periods and decays at long periods, and that site class shifts and amplifies the spectrum. Understand how period, mass, and stiffness relate (T grows with mass and with height, shrinks with stiffness), why resonance matters, and how ductility and overstrength reduce design forces relative to elastic demand.
Code framework
Be able to explain the base shear equation conceptually: design base shear depends on seismic weight, a spectral response coefficient, a response modification factor R that rewards ductile systems, and an importance factor. Know the load combinations with overstrength for certain elements, the concept of the redundancy factor, and the difference between strength-level and allowable-stress checks. Understand drift limits, P-delta effects, torsional irregularity, soft and weak stories, and diaphragm flexibility as conditions that change analysis requirements or force penalties.
Traps
Common traps: confusing R with the overstrength factor; assuming a stiffer building always has lower seismic force (short-period plateau); ignoring that reduced R increases design force but also demands greater detailing; mixing ASD and LRFD combinations; treating magnitude as intensity; and forgetting that site amplification is period-dependent, not uniform.
Worked rule of thumb
If period T falls on the descending branch of the spectrum, a small increase in T lowers spectral acceleration, so adding stiffness can raise force. When T is below the plateau corner, forces are roughly constant; then increasing stiffness does not change force much but does reduce drift. Use this to answer conceptual questions about the trade-off between force and drift.