Advanced Seismic Hazard Calculator

Seismic Hazard Input Form

This page uses a single-column page flow, while the calculator fields switch to 3 columns on large screens, 2 on medium screens, and 1 on mobile.

Reference PGA (g)

Mapped or regional peak ground acceleration on rock.

Short-Period Spectral Acceleration, Ss (g)

Mapped spectral acceleration at short periods.

1-Second Spectral Acceleration, S1 (g)

Mapped spectral acceleration at 1 second.

Site Class

Used to estimate Fa, Fv, and Fpga amplification factors.

Importance Factor, Ie

Raises design demand for more critical structures.

Damping Ratio (%)

Used with damping correction factor eta.

Exposure Years

Typical values include 50, 75, or 100 years.

Exceedance Probability (%)

Probability of exceedance over the chosen exposure years.

Structural Period, T (s)

Fundamental period used to read spectral acceleration.

Response Modification Factor, R

Used to reduce elastic demand for design-level force estimation.

Effective Seismic Weight (kN)

Approximate seismic weight used for base shear estimation.

Long-Period Transition, TL (s)

Controls the long-period descending branch of the spectrum.

Example Data Table

Case	Reference PGA (g)	Ss (g)	S1 (g)	Site Class	Ie	Damping (%)	Pe in 50 Years (%)	R	Weight (kN)	Design PGA (g)	SDS (g)	SD1 (g)	Base Shear (kN)
Office Tower	0.25	1.00	0.40	D	1.25	5	10	5.0	2500	0.333	0.917	0.533	416.67
Hospital Block	0.30	1.20	0.55	E	1.50	5	10	4.5	4200	0.360+	Higher	Higher	Elevated
Warehouse	0.12	0.45	0.18	C	1.00	5	10	3.5	1800	Moderate	Moderate	Moderate	Lower

The first row is aligned with the default example inputs used in this calculator. Other rows illustrate how softer soil and higher importance raise design demand.

Formula Used

Annual Exceedance Rate, λ = -ln(1 - Pe) / Y

Where Pe is exceedance probability as a decimal and Y is exposure years.

Return Period, Tr = 1 / λ

This converts the annual exceedance rate into an equivalent return period.

Damping Factor, η = max[0.55, √(10 / (5 + ξ))]

Here ξ is damping ratio in percent. Higher damping usually reduces spectral demand.

Risk Factor = clamp[√(Tr / 475), 0.60, 1.80]

This normalizes the computed return period against a 475-year reference and prevents unrealistic extremes.

PGA_M = Reference PGA × Fpga

Fpga is the site amplification factor for peak ground acceleration.

SMS = Fa × Ss and SM1 = Fv × S1

Fa and Fv are site coefficients selected from simplified code-style tables.

Design PGA = (2/3) × PGA_M × η × Ie × Risk Factor

This produces a design-level peak acceleration after damping, importance, and risk adjustments.

SDS = (2/3) × SMS × η × Ie × Risk Factor

SD1 = (2/3) × SM1 × η × Ie × Risk Factor

SDS and SD1 define the code-style design spectrum shape.

T0 = 0.2 × SD1 / SDS and Ts = SD1 / SDS

These characteristic periods define the rising branch and constant-acceleration plateau.

Sa(T) =
• PGA_design + (SDS - PGA_design) × T / T0, for 0 ≤ T ≤ T0
• SDS, for T0 < T ≤ Ts
• SD1 / T, for Ts < T ≤ TL
• SD1 × TL / T², for T > TL

This piecewise relationship is used to build the Plotly response spectrum.

Cs = max[ Sa(T) / (R / Ie), 0.044 × SDS × Ie, 0.01 ]

Base Shear, V = Cs × W

The calculator uses a simplified design-force estimate with effective seismic weight W.

How to Use This Calculator

Enter the mapped hazard values: Reference PGA, Ss, and S1.
Select the site class that best represents local soil stiffness.
Choose the importance factor for the structure’s occupancy category.
Input damping ratio, exposure years, and probability of exceedance.
Provide structural period, response modification factor, and seismic weight.
Set TL if long-period behavior matters for your structure type.
Click Calculate Seismic Hazard to generate the results block above the form.
Review Design PGA, SDS, SD1, hazard level, response coefficient, and base shear.
Use the Plotly chart to inspect the full response spectrum shape.
Export the final results using the CSV or PDF buttons.

Frequently Asked Questions

1) What does this seismic hazard calculator estimate?

It estimates site-adjusted hazard measures such as design PGA, SDS, SD1, annual exceedance rate, return period, spectral acceleration at the structural period, and an approximate base shear.

2) Why do I need both Ss and S1?

Ss controls short-period demand, while S1 controls longer-period demand. Using both allows the calculator to build a more realistic response spectrum for different structural periods.

3) What does site class change in the results?

Site class changes the amplification coefficients Fa, Fv, and Fpga. Softer soils often increase seismic demand, especially in short-period and 1-second spectral ranges.

4) How is the return period calculated?

The calculator converts exceedance probability over exposure years into an annual exceedance rate, then inverts that rate to estimate an equivalent return period.

5) Does damping really affect seismic demand?

Yes. Higher damping usually lowers spectral response. This tool applies a damping correction factor so you can compare conventional 5% damping against systems with greater energy dissipation.

6) Is the base shear output code-ready for final design?

It is best used for screening and early-stage engineering studies. Final design may require jurisdiction-specific load combinations, irregularity checks, drift limits, and detailed code provisions.

7) What is TL in the spectrum model?

TL is the long-period transition where the descending spectral branch changes from a 1/T form to a 1/T² form. It matters most for taller or more flexible structures.

8) When should I use a site-specific seismic study instead?

Use a site-specific study for critical facilities, unusual geology, soft or liquefiable soils, near-fault effects, major projects, or whenever regulations require detailed geotechnical and seismic hazard assessment.