Structural Dynamics / Dynamic modelling and software-result checking
Checking a dynamic result
Six checks in order, what each one catches, and how to write them down so someone else can repeat them.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Checks needed for project use
- A dynamics reviewer must confirm excitation models, damping, modal assumptions and any comfort or serviceability interpretation. Small displacement alone is not an acceptance criterion.
- Independent validation of the associated interactive implementation, input domain and project applicability remains required; this review checks lesson context and selected calculations, not design certification.
- Independent model audit should compare with a physical mass take-off and justified stiffness/period estimates. Generic V/W,period,weight and modal-mass bands are not acceptance criteria.
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.
Read the lesson register and outstanding review notesWhy this matters
Analysis software does not report uncertainty. It reports a period to four decimal places whether the model is right or not, and it plots a mode shape just as neatly for a mechanism as for a building.
Every fault in this lesson has occurred on real projects, and every one of them produces output that looks entirely normal. What catches them is not experience or intuition — it is a short sequence of checks, run in a fixed order, every time.
This is the last lesson of the course, and it is the one that decides whether everything before it was useful.
By the end of this lesson you should be able to
- Run six checks in order on any dynamic analysis output
- Identify the specific fault each check catches
- Recognise the signature of a missing constraint or an accidental mechanism
- Record checks so that another engineer can repeat them
What you should already know
- Modelling decisions (Lesson 20.1)
- Effective modal mass (Module 11)
- Time step and convergence (Module 8)
- Base shear coefficient (Module 15)
1. Compare periods with a justified estimate
Use mechanics or an empirical relation appropriate to the structural system. Rules such as0.1N or0.075H^0.75 are preliminary indicators, not universal bounds. Investigate substantial discrepancies by checking stiffness, mass, constraints, connectivity and the mode shape. A discrepancy does not uniquely identify its cause.
2. Reconcile physical mass
Compare total mass with an independent take-off, including the loads required by the governing seismic mass definition. Weight per floor area can help spot anomalies, but8–15kN/m² is only an illustrative building range. Lightweight, heavy industrial and other structures may legitimately fall outside it. Generalised modal mass depends on mode-shape normalisation and is not a substitute for total physical mass.
3. Inspect modes and directional participation
Plot important mode shapes and inspect participation in each direction. Low directional effective mass can belong to a local mode, torsion or motion mainly in another direction; it does not by itself prove a mechanism. High participation can be physically reasonable. Investigate near-zero frequencies against the intended restraints and rigid-body freedoms.
4. Check the base-shear coefficient
Compute V/W and compare it with the actual spectrum, damping, modal properties and design procedure. Independently verify units and scale factors. Values outside0.02–0.5 are not automatically errors: high elastic spectral accelerations and small hazards both occur. Entering m/s² values as multiples of g overstates the input by9.81; the reverse understates it.
5. Check modal completeness
Use the current governing mass-participation and residual-response requirements, then demonstrate convergence of the quantities needed. A90%example is not a universal criterion and does not establish convergence of base shear, local forces or floor accelerations on its own.
6. Check integration and convergence
Resolve the relevant frequency range and the input history. A step such as Tmin/20 is a starting point, not proof. Refine the step, retained modes and mesh as appropriate, and report changes in each required response.
Record the evidence
For each check, record the modelled result, the independently justified expectation, the investigation and the conclusion. Separate a plausibility screen from verification of the inputs, equations and response. Passing a set of screens does not establish that the model is correct.
Worked example
Auditing an analysis output, in order
Given
- A ten-storey concrete frame, 34 m tall, plan 30 m × 20 m
- Software reports: T₁ = 0.42 s; total seismic weight 41 000 kN; first-mode effective mass 78%
- Base shear 21 500 kN; cumulative effective mass over 12 modes: 96%
- The analysis is a response-spectrum analysis for a design earthquake
Find
Whether the output can be accepted, and if not, what is wrong.
Assumptions
- Ordinary regular concrete frame on firm ground
Predict first
A modal analysis reports a first mode with a period of 4.8 s and an effective mass of 3%. The building is a six-storey steel frame. What is the most likely explanation?
Practice
A ten-storey building 34 m tall reports a first period of 0.42 s. What does the rule T ≈ 0.075H^0.75 predict, in seconds?
Practice
An analysis reports a base shear of 21 500 kN on a building of seismic weight 41 000 kN. What is the base shear coefficient?
Practice
For two SDOF models with the same mass, periods are0.42s and1.0s. What is the ratio of their stiffnesses?
Practice
A building of plan area 600 m² and ten floors has a reported total seismic weight of 41 000 kN. What is the seismic weight per unit floor area, in kN/m²?
Check yourself
A single-mode check with a specified spectral ordinate of0.16g reports V/W=1.6. What should be investigated first?
Check yourself
Cumulative effective mass reaches 92% at mode 8. The analysis must produce floor accelerations for equipment qualification. What should be done?
Check yourself
Which of these checks is independent of the stiffness model, and therefore useful when the period check has already failed?
Worked example
Three checks on a reported frequency
Given
- A model reports a fundamental frequency of 4.8 Hz for a 12 m span composite floor
Find
Whether to believe it
Worked example
A frequency that came out too high
Given
- A concrete floor model reports 9.5 Hz
- The 18/√δ estimate gives 6.0 Hz
- The two disagree by 58 %
Find
Where to look
Summary
- Check the period against a hand estimate first; if it fails, stop
- Reconcile total physical mass with an independent take-off; weight-per-area ranges are only screening aids
- Low directional effective mass warrants inspection for local, torsional or orthogonal modes and possible mechanisms
- Review V/W against a project-specific expectation and independently confirm spectrum units
- Check the governing modal-mass requirement and independently demonstrate convergence of base shear and local responses
- Convergence is demonstrated by rerunning and reporting the change, not asserted
- Record what was checked, what was found, what was expected and on what basis