Structural Dynamics / module 17
Base isolation
Period shift as a design strategy: what it buys in force, what it costs in displacement, how the isolated first mode should look, and the practical constraints — moat, services, wind restraint, re-centring.
What you will understand
- Derive the isolated period and explain why the superstructure's own stiffness barely enters
- Quantify the force–displacement trade using a response spectrum
- Recognise a healthy isolated first mode from its effective mass and period separation
- Size the isolator displacement and the moat that has to accommodate it
- State the practical constraints that decide whether isolation is viable at all
- Explain when isolation is the wrong answer
Lessons
The period shift
Why putting a soft layer under a building reduces the force it attracts, and exactly what that reduction costs.
Designing an isolated building
The moat, the services, the wind restraint, the re-centring — and the sites and structures where isolation is the wrong answer.
Module summary
- Isolation combines a period shift with device-dependent energy dissipation
- Tiso = 2π√(M/Kiso) — the superstructure's stiffness is nearly irrelevant
- Moving to a longer period moves down the acceleration branch and up the displacement branch
- Force reduction is bought with displacement, and the displacement is large
- For the simple teaching model, concentrated directional participation and period separation support the intended isolation idealisation
- The superstructure moves nearly as a rigid block, which is the design intent
- Isolator damping changes displacement and transmitted forces; evaluate the combined response
- The moat, the services and the wind restraint are what make isolation buildable or not
- Isolation is often attractive for stiff structures; soft sites and flexible buildings require site- and system-specific assessment
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
What total isolator stiffness gives a 1 800 tonne building a period of 3.2 s? Answer in MN/m.
Practice
A fixed-base period of 0.52 s becomes 3.2 s when isolated. What is the period shift ratio?
Practice
An analysis gives a directional isolator displacement of 130 mm. Take the bidirectional resultant of two equal peaks, then apply torsional amplification 1.15 and a lower-bound bearing factor 1.20. What displacement results, in millimetres?
Check yourself
Which proposal needs particular caution because the proposed isolation period could coincide with a site-spectrum peak?