Structural dynamics / tutorial
Yield, or isolate
Two ways to survive the same earthquake, and what each does to the building.
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 notesThe same client, the same site, two schemes. Scheme A is a ductile frame that yields — the previous tutorial's frame. Scheme B puts the building on isolators and keeps it elastic. Quantify both, then compare what they demand of the rest of the building.
- Scheme A: the ductile frame, k = 9.5 MN/m, fy = 420 kN, α = 0.03, T = 0.75 s, μ = 3.2
- Scheme B: total mass above the bearings 3 600 tonnes, fixed-base period 0.55 s, target isolated period 3.2 s
Practice
Scheme A: what is the secant stiffness at peak response, in MN/m?
Practice
Scheme A: what effective period does that give, in seconds?
Practice
Scheme B: what total isolator stiffness gives a 3.2 s period, in MN/m?
Practice
Scheme B: what period shift does that achieve, against the 0.55 s fixed-base period?
Scheme A reaches an effective period of 1.30 s by damaging itself, and the damage is the mechanism. Scheme B reaches 3.2 s by design, stays elastic, and puts all the movement in bearings that can be inspected and replaced. Scheme A is cheaper to build; Scheme B is far cheaper to own after an event, which is why hospitals and data centres choose it.