CA Seismic common mistakes
CA Seismic · Understand the traps and how to avoid them.
Working the structural analysis problem with unfactored service loads and then checking the result against a strength-level limit or vice versa, submitting an answer that is off by the load factor.
Candidates carry ASD habits from gravity-design practice into seismic problems, where the base shear and member demands are almost always strength-level, and the exam deliberately offers the service-level result as a distractor.
What to do instead
Before any calculation, write the code basis in the margin (ASCE 7 strength combinations, load factor 1.0E for seismic) and label every intermediate number as service or strength. If two answer choices differ by roughly 1.2 or 1.6, that is a signal you mixed levels.
Computing the seismic base shear with the approximate period Ta when the exact period T was given, or forgetting to cap T at CuTa, and then picking the resulting force without checking the Cs minimum.
Candidates memorize V = CsW and skip the two governing limits (upper period bound and the Cs,min floor of 0.044SDS I), so the arithmetic looks right but the controlling value is wrong.
What to do instead
Build a fixed four-line checklist for every base-shear problem: compute Ta, compute CuTa, compare with given T, then compute Cs and compare against Cs,min and the S1-based cap. Write the governing Cs with a circle before multiplying by W.
Assigning the wrong response modification coefficient R, overstrength factor, or deflection amplification factor Cd to the lateral system actually described, or mixing values from the vertical and horizontal combinations.
The tables are long and similar, and candidates pattern-match on a keyword such as 'special' or 'ordinary' without confirming the full system description, material, and height limits.
What to do instead
Practice reading the system description aloud and matching all three descriptors (material, frame type, detailing level) before touching the table. Memorize R and Cd for the six or seven systems most likely to appear rather than skimming the whole table.
Computing the story drift as the raw difference in deflections between adjacent levels and comparing it directly to the drift limit, omitting the Cd amplification and the importance factor.
The drift limit in the code applies to amplified drift, and the deflection output from an analysis is elastic; candidates forget the Cd/I conversion under time pressure.
What to do instead
Write the drift equation in full (Cd times the elastic difference divided by I) at the top of the scratch page before starting, and always state whether the computed drift is elastic or amplified in the answer line.
Applying the redundancy factor rho = 1.3 by default to every structure instead of testing the two conditions that permit rho = 1.0.
Candidates remember that rho exists and that 1.3 is conservative, so they skip the condition check, which changes base shear and member demands by 30 percent.
What to do instead
For each problem, explicitly test both exceptions (the story-drift-ratio condition and the removal-of-a-member condition) and record the outcome. If neither is satisfied, use 1.3 and say so.
Designing a diaphragm or collector using only the inertial force from the mass tributary to that element, ignoring the transfer forces produced by discontinuous lateral systems or by the difference in stiffness between adjacent vertical elements.
Candidates treat diaphragm design as a simple tributary-area problem because that is how gravity members are designed, but seismic diaphragm demands are frequently governed by transfer and collector forces.
What to do instead
Before sizing a diaphragm or collector, sketch the load path from the mass to the vertical elements and identify any offsets, openings, or discontinuities. If the path is not straight, add the transfer force explicitly to the inertial force.
Treating an irregularity as a single checkbox and applying only one consequence, when the configuration triggers several independent requirements such as a 25 percent force increase, an extreme-irregularity review, and a specific analysis procedure.
The irregularity tables and their consequences live in different sections, and candidates stop reading once they find the irregularity itself.
What to do instead
Keep a one-page matrix of each irregularity against its consequences (analysis procedure allowed, force increase, design penalty) and rehearse it until it can be reproduced from memory. On the exam, list every triggered consequence before computing.
Assuming a rigid diaphragm and distributing story shear to the lateral elements in proportion to stiffness, when the problem describes a flexible diaphragm or a torsionally irregular plan that requires an accidental eccentricity.
Rigid-diaphragm distribution is the default taught in most coursework, and candidates do not check the diaphragm classification or add the 5 percent accidental torsion before distributing forces.
What to do instead
First classify the diaphragm as rigid, semi-rigid, or flexible using the span-to-depth criterion, then choose the distribution method. Always add the accidental eccentricity unless the problem explicitly states it is not required.
Answering a detailing or proportioning question from general good practice, such as 'provide adequate confinement', instead of the specific numeric requirement for the system, such as a minimum hoop spacing or a strong-column/weak-beam ratio.
Candidates run out of time and switch to qualitative reasoning, but seismic detailing questions are almost always testing a specific dimensional or ratio limit.
What to do instead
Maintain a memorized short list of the numeric limits that recur (confinement spacing, splice locations, strong-column weak-beam ratio, boundary element requirements) and drill them as flashcards so the number, not the concept, is recalled.
Spending disproportionate time on one long analysis problem and then rushing the remaining questions, leaving conceptual questions unanswered or answered without reading all four choices.
Seismic exams are time-compressed and the analysis problems are front-loaded with setup work, so a single stuck calculation consumes the budget for several quick items.
What to do instead
Set a hard per-question time cap during practice, mark and skip any problem that exceeds it, and return only after all quick conceptual items are answered. Simulate full-length timed sets so the pacing decision becomes automatic.