Structural dynamics
Question the model.
Examine the assumptions and results in these illustrative cases.
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
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A six-storey steel frame reports a first period of 3.9 s with a first-mode effective mass of 3%. What is wrong?
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A response-spectrum analysis reports a base shear coefficient V/W of 1.85. What is the most likely cause?
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A ten-storey concrete frame, 34 m tall, reports a first period of 0.41 s. What should be suspected first?
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A building's reported total seismic weight is 3.1 kN/m² of floor area. What does that indicate?
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A modal analysis of a rigid-diaphragm model with 8 levels reports 27 modes before the cumulative mass reaches 90%. What does that suggest?
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A time-history analysis of a building with T₁ = 1.4 s and a highest retained mode at T₈ = 0.09 s was run at Δt = 0.05 s. Is the step adequate?
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A modal report shows a first mode where one floor slab distorts in plane while the frames barely move. What is missing?
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Cumulative effective mass reaches 91% at mode 6. The analysis must supply floor accelerations for equipment. What now?
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A model's mass source is set to '1.35 × dead + 1.5 × imposed'. What is wrong?
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A nonlinear time-history run reports 480 non-converged steps out of 30 000. What should be done?
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Two analyses of the same linear building — one direct integration, one modal superposition — give peak roof displacements of 152 mm and 133 mm. What does that mean?
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An engineer computes inter-storey drift by subtracting the reported peak displacements of two adjacent floors. What is wrong?
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A base-isolated building's modal analysis reports a first-mode effective mass of 68%. What does that indicate?
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A designer applies the added-damping formula for viscous dampers, gets 14%, and runs a response-spectrum analysis at 19% total. What is wrong?
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A report states 'the analysis converged'. What is missing?
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A five-storey building, 17 m tall, reports a first period of 0.52 s, a seismic weight of 11.2 kN/m², first-mode effective mass 79% and V/W = 0.21. What is wrong?
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A time-history output shows the structure responding measurably a full second before the strong motion arrives. What causes that?
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A direct-integration run of a finely meshed model shows high-frequency noise superimposed on the response. What is the standard remedy?
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Peak displacements from seven records differ by a factor of 2.4 between the largest and smallest. What should be concluded?
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A model of a building with a large atrium opening applies a rigid diaphragm at that level. What does that hide?
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Rayleigh damping was anchored at modes 1 and 2 of a twelve-mode analysis. What is the consequence?
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A drift check uses the displacements straight from an analysis run with a behaviour factor of 4. What is missing?
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A model reports a first period of 2.9 s for a four-storey braced steel frame. What is the first thing to look for?
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An analysis of a torsionally coupled building combines two modes 4% apart in period by SRSS. What is wrong?
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A synthetic spectrum-compatible record is used for a time-history analysis. What caution applies?
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A record is scaled up by a factor of 3 to match a higher hazard level. What has changed and what has not?
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A modal report lists effective masses that sum to 78% of the total across ALL modes of the model. What does that tell you?
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An engineer reads a design force off a spectrum at the building's ELASTIC period, for a structure expected to yield substantially. What is the flaw?
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A model gives a base shear of 26 000 kN on a building of seismic weight 41 000 kN, with a reported first period of 0.19 s for eight storeys. Which single cause explains both?
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A footfall analysis reuses a lateral seismic model with mass lumped at floor level. What will it get wrong?
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A modal analysis reports the first three periods as 0.84, 0.83 and 0.61 s for a nominally symmetric building. What should be checked?
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An isolated building's analysis was run only with upper-bound bearing stiffness. What is under-estimated?
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A tuned mass damper was manufactured to the period predicted by the finite element model. What is the risk?
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A response-spectrum analysis of a structure with nonlinear viscous dampers is submitted. What is the objection?
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A report gives peak floor accelerations to four significant figures from a model whose yield strengths are known to ±25%. What is wrong?
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A modal output shows mode 1 and mode 2 with identical shapes and periods differing by 0.001 s. What is the likely cause?
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An analysis reports a mode with a NEGATIVE effective mass. What does that mean?
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A twelve-storey building's model is fixed at ground level, but the building has two basement storeys with perimeter retaining walls. What has been assumed?
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The velocity and displacement histories from a time-history run drift steadily away from zero. What is the cause?
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A model reports 24 modes for an 8-level rigid-diaphragm building and the cumulative effective mass reaches exactly 100% at mode 24. What does that confirm?
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A designer reports that adding viscous dampers reduced the peak column axial force. Is that plausible?
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A retrofit adds bracing to an existing building and the reported base shear increases by 40%. Is the analysis wrong?
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A report states the seismic mass as 4 850 tonnes and the seismic weight as 4 850 kN. What is wrong?
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Every one of the six checks passes on a submitted analysis. What can now be stated?