Structural dynamics / tutorial
Sizing an isolation system
From a target period to a moat width, and the factor of two in between.
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 notesScheme B is taken forward. The bearings are sized and the period is confirmed; now the building has to be made buildable, and that means establishing how much room it needs to move.
The analysis reports one number. The moat is built to another, and the gap between them is where isolation schemes succeed or fail commercially. Work from the first to the second, and keep track of the direction each factor pushes.
- Total mass above the bearings 3 600 tonnes
- Isolator stiffness 13.879 MN/m
- Analysis gives a mean directional isolator displacement of 140 mm over seven records
- Torsional amplification 1.15 at the worst corner; lower-bound bearing factor 1.20; residual offset allowance 20 mm
Practice
Confirm the isolated period, in seconds.
Practice
What is the bidirectional resultant of two equal 140 mm directional peaks, in millimetres?
Practice
Apply the torsional amplification and the lower-bound bearing factor, then add the residual allowance. What displacement results, in millimetres?
Practice
By what factor does the required moat exceed the displacement the analysis reported?
Both bearing property bounds have to be run, and each governs a different quantity: lower bound governs displacement and the moat, upper bound governs force and the superstructure. Running only one is the commonest error in isolation design, and which one you run decides which way you are wrong.