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Seismic Load Calculator

Work out seismic load instantly with clear inputs, formula shown and shareable results.

About Seismic Load Calculator

The equivalent static method expresses seismic demand as a fraction of the building's own weight: Ah = Z I (Sa/g) / (2 R). The zone factor sets regional hazard, importance reflects consequence of failure, the spectral acceleration ratio captures the building period and R credits the ductility of the structural system. Base shear is that coefficient times seismic weight.

Formulas

Design coefficient

Ah = Z x I x (Sa/g) / (2 R), with Sa/g taken as 2.5 in the plateau region

Base shear

Vb = Ah x seismic weight

Worked example

With the default values below, Seismic Load Calculator returns Design base shear of 576.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Seismic weight of buildingminimum 1 kN
12,000 kN
Zone factor Zaccepts 0.01–0.5
0.16
Importance factor Iaccepts 0.5–2
1.2
Response reduction factor Raccepts 1–10
5

Results

Design base shear
576.0 kN
Design horizontal coefficient Ah
0.04800
Base shear as share of weight
4.80% %
Base shear in tonnes force
58.7 t

Step by step

  1. 1Values used
  2. 2Design coefficient
  3. 3Base shear
  4. 4Design base shear= 576.0 kN
  5. 5Design horizontal coefficient Ah= 0.04800
  6. 6Base shear as share of weight= 4.80 %
  7. 7Base shear in tonnes force= 58.7 t

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Seismic design must be performed by a qualified structural engineer to the seismic code in force, including period calculation, storey distribution and drift checks.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Seismic design must be performed by a qualified structural engineer to the seismic code in force, including period calculation, storey distribution and drift checks.

Frequently Asked Questions

How is the Seismic Load figure worked out?

Ah = Z x I x (Sa/g) / (2 R), with Sa/g taken as 2.5 in the plateau region. The equivalent static method expresses seismic demand as a fraction of the building's own weight: Ah = Z I (Sa/g) / (2 R).

Why divide by R?

R credits inelastic energy dissipation. A ductile moment frame with R = 5 is designed for a fifth of the elastic force because it is detailed to yield without collapsing.

What values do I need to enter?

This calculator takes 4 inputs: Seismic weight of building, Zone factor Z, Importance factor I, Response reduction factor R. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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Seismic Load Calculator – Calculate Design base shear