Structural Dynamics / module 20
Dynamic modelling and software-result checking
The faults that analysis software reports without a warning — missing mass, wrong units, accidental mechanisms, absent diaphragm constraints, too few modes, an unresolved time step — and the checks that catch each one.
What you will understand
- Build a dynamic model whose assumptions match the question being asked
- Check a reported period against a hand estimate before looking at anything else
- Verify the seismic mass against the building's weight, independently of the model
- Detect missing diaphragm constraints, accidental mechanisms and unit errors from the output
- Confirm mode count, time step and convergence from the numbers the software reports
- Write a check that another engineer can repeat
Lessons
Building a model that answers the question
Mass, stiffness, damping and constraints — the four modelling decisions that decide whether the analysis is about your building.
Checking a dynamic result
Six checks in order, what each one catches, and how to write them down so someone else can repeat them.
Module summary
- Software reports a wrong answer in exactly the same format as a right one
- Compare reported periods with justified estimates and inspect their mode shapes
- The second is the seismic mass against the building's weight, computed independently
- Compare V/W with the actual hazard spectrum, damping and design procedure; a generic range cannot diagnose an error
- Low directional effective mass requires inspection of local, torsional and orthogonal modes and possible mechanisms
- Missing diaphragm constraints show up as a very low first frequency and a floor that distorts
- Check the governing modal-completeness rule and convergence of each required response
- A converged answer is one that has been shown not to change; convergence is demonstrated, not assumed
- Every check must be recorded so someone else can repeat it
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.
Read the lesson register and outstanding review notesCheck your understanding
Practice
A twelve-storey building is 41 m tall. What first period does T ≈ 0.075H^0.75 predict, in seconds?
Practice
An analysis reports a base shear of 3 900 kN on a building of seismic weight 52 000 kN. What is the base shear coefficient?
Practice
A building of plan area 480 m² and twelve floors reports a total seismic weight of 52 000 kN. What is the seismic weight per unit floor area, in kN/m²?
Check yourself
A modal analysis of a six-storey frame reports a first mode at 3.9 s carrying 4% of the mass. What does that indicate?