Adjust building mass, isolator stiffness, damping ratio and soil type to instantly compute natural period, peak Sa, isolator displacement and acceleration reduction vs fixed-base.
The core of the analysis is calculating the isolated structure's natural period of vibration. A longer period is the primary goal of isolation, moving the building's response away from the peak energy of typical earthquake ground motions.
$$T_i = 2\pi \sqrt{\frac{M}{K}}$$Where $T_i$ is the isolated period (s), $M$ is the building mass (ton), and $K$ is the total effective stiffness of the isolation system (kN/m). In the simulator, adjusting M and K changes this period instantly.
Once the period is known, we estimate the maximum displacement of the isolators. This is a critical design check to ensure there is enough clearance (a "moat") around the building. The displacement depends on the earthquake's intensity (spectral acceleration, Sa) and is reduced by damping.
$$D = \frac{S_a \cdot T_i^2}{4\pi^2}\cdot B_f, \quad B_f = \sqrt{\frac{0.05}{0.01 + \zeta}}$$Where $D$ is the isolator displacement (m), $S_a$ is the spectral acceleration (g), and $B_f$ is a damping reduction factor based on the damping ratio $\zeta$. The Soil Type selector in the tool changes the $S_a$ value used in this calculation.
Critical Facilities (Hospitals, Data Centers): Seismic isolation is paramount for buildings that must remain operational immediately after a major quake. For instance, a hospital with isolated foundations can protect sensitive equipment like MRI machines and ensure life-saving functions continue without interruption, even during strong aftershocks.
Heritage and Monument Preservation: Isolating historical buildings or museums is often the only non-invasive way to protect irreplaceable artifacts and structures. A common case is installing isolators in the basement or at the plinth level of a museum to shield fragile statues and paintings from damaging accelerations.
High-Tech Manufacturing: Semiconductor fabrication plants (fabs) require extreme stability. Even minor vibrations can ruin a batch of microchips. Seismic isolation pads under the cleanroom floor protect billions of dollars in equipment and production from seismic disturbances.
Bridges and Infrastructure: While often used for buildings, isolation principles are also applied to bridges. Lead-rubber bearings or friction pendulum bearings are installed between the bridge deck and its piers. This allows the deck to sway safely during an earthquake, protecting the piers from catastrophic shear forces.
There are a few key points you should be especially mindful of when starting to use this tool. First is the idea that "increasing the damping ratio ζ solves everything." While raising ζ does suppress the isolation layer displacement, the acceleration transmitted to the building actually increases slightly. Try fixing K and M in the tool and raising ζ from 0.05 to 0.30. You'll see the displacement decreases, but the response point moves slightly upward, right? This happens because excessive damping not only dissipates seismic energy as "heat" but also exerts a "braking" effect that pulls on the building itself. In practice, note that there is an appropriate range for the damping ratio based on the characteristics of the seismic isolators used (e.g., lead-plug laminated rubber).
Next is how to interpret "Mass M." The tool simply labels it as "building mass," but it's better to think of it as the "effective mass contributing to natural vibration." For instance, if all floors of a building sway identically, it would be the total mass, but real buildings have higher modes. This tool's single-degree-of-freedom model assumes the "first mode," where the entire building behaves like a rigid body. Therefore, inputting the total weight of an actual building directly into M might result in a calculated period longer than reality. Use it first as an estimation tool; the typical workflow is to proceed to more advanced multi-degree-of-freedom analysis for detailed study.
Finally, blind faith in the "design spectrum." This tool's spectrum is merely a standard model. In actual design, you are often required to use a "site-specific spectrum" based on particular faults or past earthquake records at the construction site. Selecting "soft ground" in the tool elevates the long-period side, but real spectra often have more complex shapes. Remember, this tool's purpose is to grasp the correlation between parameters and response, not to determine final design values.
A 3-story hospital (mass=1,200,000 kg) on Site Class D soil with elastomeric isolators: stiffness K=15 MN/m, damping ζ=0.10. Computed natural period Ti=1.78 s, well above the 0.5 s peak region. Spectral acceleration Sa=0.42 g (vs. 0.65 g fixed-base). Isolation displacement δ=224 mm under MCE (Maximum Credible Earthquake, 0.50 g peak ground acceleration). Acceleration reduction: 35% compared to conventional foundation.