Watch a fixed-base building and a base-isolated building shake under the same ground motion side by side, and see in real time how lengthening the period lowers the spectral acceleration.
Parameters
Fixed-base period
s
Building period before isolation.
Isolated period
s
Target isolated period.
Equivalent damping
%
Equivalent damping including isolators.
Input PGA
g
Peak ground acceleration.
Effective mass
t
Participating building mass.
Results
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Acceleration reduction
—
Isolation displacement
—
Base shear reduction
—
Period shift
Earthquake response (fixed-base vs isolated)
Fixed base (large sway) Isolated (small superstructure sway) Isolator deformation
$T$ is the natural period, $M$ the effective mass, $K$ the lateral stiffness, and $\xi$ the damping ratio. The response spectrum $S_a$ has a short-period plateau and decays roughly as $1/T$ beyond the corner period $T_c$. Lengthening $T$ to 2-3 s with base isolation lowers $S_a$, reducing the base shear $V$ and superstructure acceleration. The trade-off is that the isolator displacement $D$ grows in proportion to $T^2$. The damping correction used here is $\eta=\sqrt{0.10/(0.05+\xi)}$.
What is base isolation period shift
Base isolation inserts horizontally flexible isolators (laminated rubber bearings, sliding bearings, and so on) between the building and its foundation, deliberately lengthening the natural period of the whole structure. A fixed-base building usually has a period of about 0.3-0.8 s, which tends to overlap the plateau where the seismic response spectrum is largest, so the ground acceleration is amplified and transmitted to the superstructure.
Lengthening the period to 2-3 s moves the structure into the descending branch of the spectrum (which decays roughly as 1/T). There, the spectral acceleration $S_a$ is much smaller, and the base shear $V=M\,S_a$ drops by the same ratio. In other words, isolation does not "stop" the shaking; it shifts the period out of the energy-rich band of the earthquake.
This simulator shakes a fixed-base building and an isolated building side by side under the same ground motion, visualizing in real time the difference in superstructure acceleration, the deformation of the isolation layer, and the movement of the period points on the response spectrum.
How to read it
In the animation, the left fixed-base building sways strongly, while the right isolated building barely tilts at the top; instead the isolation layer at its base (the yellow shear deformation) moves a lot. This visualizes the relationship "lower acceleration at the price of larger displacement."
In the response-spectrum view, the fixed-base period point (red) sits near the plateau and the isolated period point (blue) moves to the low-acceleration region at the lower right. The view shows where a longer period lowers acceleration.
The bar view directly compares the fixed-base and isolated spectral accelerations and reads off the base-shear reduction.
Learn base isolation period shift by dialogue
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Looking at the animation, only the left building sways a lot, and the top of the right isolated building barely moves. Is the earthquake weaker for it?
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No, the ground input is the same for both. What differs is the building's period. The isolated building has a soft isolation layer at its base that stretches the period to 2-3 s. Remember the response spectrum? At short periods you sit on the high-acceleration plateau, but a longer period moves you into the low descending region. So the acceleration transmitted to the upper part, the shaking, becomes smaller.
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So it shifts the period rather than stopping the shaking. But the base of the right building (the yellow part) moves a lot. Is that okay?
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Good catch, that is the trade-off of isolation. Spectral displacement is $D=S_a(T/2\pi)^2$, so it scales with the square of the period; even though acceleration drops, the isolator displacement tends to grow. A 3 s isolation system can move tens of centimeters, for example. That is why in practice we always check the clearance to moat walls, the expansion joints, and whether the piping can follow.
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Does raising the damping reduce the displacement? The numbers change when I increase the equivalent damping slider.
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Yes, more damping lowers both $S_a$ and $D$. We get damping from lead-rubber bearings or sliding bearings. But pushing damping too high can slightly increase the acceleration transmitted upward, so the optimum depends on whether you want to limit displacement or acceleration. Design means going back and forth between the bar view and the isolation-displacement number to find the right spot.
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Then is a longer period always better? What if I set the isolated period to about 6 s?
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That is the pitfall. Stretching the period too far makes the displacement explode, and you risk resonance with long-period ground motion (the component that dominates in distant great earthquakes or sedimentary basins). It is especially dangerous on soft soil. So in practice we usually keep it around 2-3.5 s. Final decisions still require standards, measured data, detailed time-history analysis, and the device limits from the manufacturer.
Real-world applications
Used in hospitals, data centers, and semiconductor fabs where function must continue after an earthquake, to limit the floor response acceleration.
Explaining the initial benefit of seismic retrofit (adding isolation to an existing building) and comparing isolated-period and damping candidates.
Estimating the order of isolator displacement and clearance planning before moving to detailed time-history analysis.
Common misconceptions and notes
"Isolation = removing the earthquake" is wrong. In exchange for shifting the period and lowering acceleration, the isolation layer experiences large displacement. Insufficient clearance leads to pounding against moat walls.
On soft soil or where long-period ground motion dominates, excessive period lengthening (over 4 s) can amplify the response instead. Avoid resonance with the site fundamental period.
This tool is an equivalent-linear estimate. Confirm the nonlinear hysteresis, hardening, velocity dependence, and displacement limits of the isolators with detailed analysis and vendor conditions.
FAQ
Start with Acceleration reduction and Isolation displacement. Then use Isolation response spectrum to confirm the assumed state and Isolation displacement to read distribution or bias. The spectrum view shows where longer period lowers acceleration.
Move Fixed-base period alone, then move Isolated period by a comparable amount and compare the change in Acceleration reduction. Base shear reduction shows combinations where margin or performance changes quickly.
A response spectrum has a near-constant acceleration plateau at short periods and decays roughly as 1/T beyond the corner period Tc (about 0.5-0.8 s). Fixed-base buildings (0.3-0.8 s) tend to sit on the plateau; shifting the period to 2-3 s with base isolation moves the structure into the descending branch where the spectral acceleration Sa is much lower. This lowers the base shear V = M*Sa and the superstructure acceleration.
The isolator displacement increases. Spectral displacement is D = Sa*(T/2pi)^2, which grows with the square of the period T. Even though acceleration drops, D itself tends to grow, so clearance to moat walls, expansion joints, and the ability of piping to follow the motion become the key design points. This matters most where long-period ground motion dominates.
This tool is an equivalent-linear estimate (equivalent period and equivalent damping) using a response spectrum. Real design checks the nonlinear hysteresis of the isolators, displacement limits, hardening, wind response, vertical motion, torsion, and soil-structure interaction with time-history analysis. Final decisions still require standards, measured data, detailed analysis, and vendor limits.
How to Use
Enter the fixed-base period (typically 0.3–2.0 s for buildings) in the fixedT field
Input the isolated period (usually 2–4 s for base isolation systems) in the isoT field
Set damping ratio as percentage (5–20% for elastomeric bearings, 10–30% for friction pendulum)
Enter building mass in tonnes and peak ground acceleration (PGA) in g units
Read off the acceleration reduction, isolation displacement, base shear reduction, and period shift ratio
Worked Example
A 500-tonne hospital building on fixed base has period T₁=0.8 s. Installing elastomeric bearings shifts isolated period to T₂=3.2 s with 18% damping. Under PGA=0.4 g: period shift ratio = 3.2/0.8 = 4.0; the fixed-base structure sits on the spectrum plateau (Sa ≈ 2.5×PGA = 1.0 g) while the isolated structure drops to Sa ≈ 0.12 g, an acceleration reduction of about 88%; isolation displacement ≈ Sa·(T/2π)² ≈ 0.32 m. The displacement grows as the price of acceleration reduction. Treat this as an order-of-magnitude estimate and confirm with nonlinear time-history analysis in real design.
Practical Notes
Higher damping (15%+) reduces isolation displacement but can increase transmitted acceleration—verify seat width and clearances
Isolated period should avoid resonance with site soil fundamental period (typically 0.5–1.5 s) to prevent amplification
Isolation displacement grows with PGA and period; allow 0.8–1.2 m clearance for moat walls in critical facilities
Friction pendulum systems tolerate higher damping; elastomeric bearings degrade if stiffness increases beyond 20% over bearing lifetime