Structural Dynamics / Structural control
Passive, semi-active and active control
What supplies the force, what each needs to work, and why fail-safe behaviour decides which is buildable.
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.
- Control specialist review needed for modal normalisation, TMD stroke/forces, detuning, pedestrian feedback, robust stability and safe failure states. No regulatory life-safety permission has been verified.
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
Passive devices respond without a powered command. Semi-active devices adjust properties using control power; active actuators command force using an external supply. Assess capacities, stroke, attachments, maintenance and credible failure states for each. Passivity alone does not establish safety.
By the end of this lesson you should be able to
- Classify a control device by what supplies its force
- State what a semi-active system can and cannot do
- List what an active system needs to operate
- State the fail-safe requirement and explain why it governs
What you should already know
- Tuned mass dampers (Lesson 19.1)
- Viscous dampers (Module 18)
- Base isolation (Module 17)
Categories
Passive devices respond mechanically without a powered command. Semi-active devices adjust properties such as damping; active actuators command forces using external energy.
A dissipative semi-active damper’s control law should preserve non-negative dissipation. Variable-stiffness systems need a separate energy and stability assessment: stiffness changes exchange stored energy, so the label semi-active does not prove passivity.
Stability and adaptation
Changing device settings can help across operating conditions. Assess delays, saturation, sensor faults and model uncertainty. A passive or dissipative device can still exceed capacity or leave the structure inadequately controlled.
Active control requires sensors, a controller, actuators, power and maintenance. Assess closed-loop stability and robustness against plausible structural states and delays, using appropriate control-system measures.
Failure states
Identify power loss, sensor and controller faults, leakage, seizure, actuator runaway and other relevant failures. Define the resulting mechanical states and demonstrate the required structural performance. A device that becomes passive on power loss is not automatically safe or adequate.
Provide mechanical limits, fault detection, inspection and access as appropriate. Address common-cause faults as well as independent failures. Reliance on control for safety depends on the applicable approval route and reliability case; this course establishes no blanket legal permission or prohibition.
Selection
Compare feasible devices against response targets, uncertainty, reaction points, maintenance and failure consequences. A passive option may simplify the system; more complex control needs a demonstrated benefit and a proportionate reliability case.
Worked example
A hypothetical resonant response target for a footbridge
Given
- A 92 m steel footbridge; first lateral mode at 0.94 Hz, inherent damping 0.6%
- For this teaching comparison, a fixed-amplitude harmonic lateral force acts at exactly 0.94 Hz, the mode’s undamped natural frequency
- Assumed uncontrolled steady-state acceleration amplitude under that force: 2.1 m/s²
- Comfort criterion: 0.35 m/s²
Find
A control strategy, and the damping needed.
Assumptions
- Single linear mode with unchanged mass, stiffness and harmonic force amplitude; forcing equals the undamped natural frequency
- Equivalent SDOF damping comparison, not a coupled TMD design or a pedestrian synchronisation model
Predict first
An active control system's model has the building's first frequency 8% too high. What is the risk?
Practice
Under unchanged harmonic force exactly at the undamped natural frequency, a linear SDOF bridge-mode model has 0.6% damping. What total damping percentage gives a sixfold reduction in acceleration amplitude?
Check yourself
What distinguishes a semi-active control device from an active one?
Check yourself
Why does a structural control proposal need an explicit reliability and failure-state assessment?
Check yourself
A tuned mass damper's own damping ζ₂ is halved from its optimum. What happens to the response curve?
Summary
- Passive takes its force from the motion; semi-active modulates it; active supplies it from outside
- A dissipative semi-active device must maintain passivity and a verified safe failure state; these are design requirements
- Active control needs sensors, an algorithm, actuators, power and decades of maintenance
- A model-frequency error can put the commanded force in phase with velocity and add energy
- Demonstrate performance in credible operating, degraded and failed states
- Compare devices on response, reliability, maintenance and failure consequences