Structural Dynamics / Base isolation
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.
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.
- Isolation specialist review needed for bearing qualification, upper/lower property bounds, bidirectional histories, torsion, stability/uplift, recentering, moat and services. Example factors and record statistics are not code verification.
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
The period shift is arithmetic. Everything difficult about base isolation is on the other side of it: making a building that can actually move a hundred millimetres in any direction, several times, and then be used the next day.
This lesson is the practical half. It is also where isolation projects are most often abandoned — not because the dynamics did not work, but because the moat could not be built, or the site was wrong to begin with.
By the end of this lesson you should be able to
- Size a moat from the analysis displacement and state why the design value is larger
- List what has to cross the isolation plane and how each is detailed
- Explain the wind-restraint problem and how it is resolved
- Identify sites and structures for which isolation is unsuitable
What you should already know
- The period shift and the force–displacement trade (Lesson 17.1)
- Spectra and their long-period behaviour (Module 14)
- Selecting records and reporting an envelope (Module 15)
Movement clearance
Provide clearance wherever the moving isolated structure could contact fixed construction. Determine the required movement at each location from the applicable hazard, response procedure and physical arrangement. Moat walls, covers, entrances and attached services must accommodate that movement.
Include the effects required for the project: simultaneous horizontal response, torsion, bearing-property bounds, record variability and any residual offset. The resultant of separate directional maxima is an upper geometric bound on simultaneous movement, not necessarily the actual peak.
Evaluate upper and lower bearing properties. Softer bearings often increase displacement and stiffer bearings often increase force, but spectral shape and nonlinear response can change which case governs. Do not substitute a generic multiplier for those analyses.
Impact against surrounding construction can introduce high accelerations and must be prevented or explicitly assessed.
Details crossing the isolation plane
The superstructure and its supports move relative to one another. Water, drainage, gas, fire, electrical and data services need appropriate joints or loops. Resolve stairs, ramps, lift pits, guides and structural attachments at the plane. Check movement, force, durability and inspection access; an unintended rigid connection can alter the isolation system.
Service loads and recentering
Wind and other service actions may require additional resistance or restraint. Lead cores, friction systems and specifically designed restraints offer different force–displacement behaviour. Assess service movement, comfort, restraint release and the associated load path for the selected system.
After shaking, residual offset depends on bearing restoring force, friction and the history of motion. Elastomeric stiffness and curved sliding surfaces can provide recentering, but the final offset is not automatically zero.
Feasibility
Soft deposits and near-fault pulses can produce large long-period demands. Flexible buildings may offer less useful period separation and may introduce stability or uplift difficulties. These are reasons for site- and system-specific assessment, not categorical bans. Stiff low-rise buildings on suitable sites are often good candidates, particularly where reducing equipment acceleration is important. Space for movement and maintenance remains a practical constraint.
Worked example
From analysis displacement to moat width
Given
- Analysis gives a mean isolator displacement of 105 mm per direction, from a set of seven records
- Plan eccentricity between the centre of mass and the centre of isolator stiffness gives a torsional amplification of 1.15 at the worst corner
- Bounding bearing properties: the low-stiffness bound increases displacement by 20%
- Residual offset from a previous event, allowed for: 15 mm
Find
The moat width to be provided.
Assumptions
- Bidirectional resultant taken as the SRSS of two equal directional peaks
- Effects combined multiplicatively where they act on the same quantity
Predict first
A hospital is to be built on a deep soft-soil site where the site spectrum peaks at about 3 s. Isolation to a 3 s period is proposed. What is the concern?
Practice
A directional isolator displacement of 105 mm is combined bidirectionally by SRSS with an equal peak in the orthogonal direction. What is the resultant, in millimetres?
Practice
Starting from a 148.5 mm resultant, apply a torsional amplification of 1.15 and a lower-bound bearing factor of 1.20. What displacement results, in millimetres?
Check yourself
Why do isolation systems need a wind-restraint mechanism?
Check yourself
Which building is the best candidate for base isolation?
Check yourself
Why should both displacement and transmitted force be checked when isolator damping changes?
Summary
- Determine clearance from the full movement envelope, including applicable directional, torsional, property and offset effects
- Run the required bearing-property bounds and identify the governing displacement and force for the actual system
- Everything crossing the isolation plane must take the full design displacement
- One rigid connection across the plane can defeat the whole system
- Wind restraint resolves the conflict between service-load stiffness and severe-load softness
- Check restoring action and residual offset for the selected elastomeric or sliding system
- Isolation suits stiff low-rise structures on firm ground where the contents matter most