Cumulative tutorial 16
Sizing an isolation system
From a target period to a moat width, and the factor of two in between.
Scheme 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.
What you are given
- 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
Tutorial 16
Work the problem
4 questions
Question 1
Confirm the isolated period, in seconds.
Question 2
What is the bidirectional resultant of two equal 140 mm directional peaks, in millimetres?
Question 3
Apply the torsional amplification and the lower-bound bearing factor, then add the residual allowance. What displacement results, in millimetres?
Question 4
By what factor does the required moat exceed the displacement the analysis reported?
What this establishes
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.