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# Calculate Seismic Loads (NBCC 2025)
> Design base shear and story forces to NBCC 2025 Cl. 4.1.8 from the six-period hazard values, site class and seismic force-resisting system.
Determine the design base shear using the Equivalent Static Force Procedure of NBCC 2025 (Art. 4.1.8). Builds the design spectrum from the six-period hazard values and the X450 site parameters, applies the higher mode factor, checks base shear against the code limits, screens Table 4.1.8.6 irregularities, and distributes forces story by story.
Base shear and story forces come from the six-period NBCC 2025 hazard values, site class and your seismic force-resisting system, with the design spectrum built for you. Story forces export straight to diaphragm design. Design-level and elastic results are reported side by side.
## Method & scope
| Property | Detail |
| ------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| Design standards | NBCC 2025, Cl. 4.1.8 |
| Regions | Canada |
| Who it's for | Canadian structural engineers designing to NBCC 2025 who need equivalent-static-force seismic base shear and story force distribution before designing lateral-system elements. Available now to Core Calcs users (Beta). |
| What it helps with | Replaces roughly 2-4 hours per building of manually sourcing the six-period NBCC 2025 hazard values, constructing the design spectral acceleration, estimating the fundamental period, and hand-computing base shear, vertical force distribution, and the irregularity and system-restriction checks. |
| What it checks | Site hazard and site coefficient, Importance factor and seismic category, Fundamental period, Design spectral acceleration, Higher mode factor, Design base shear, Irregularity and system restrictions, Force distribution and elastic analysis |
#### What it calculates
Canadian structural engineers moving a building onto the NBCC 2025 seismic provisions, where the six-period hazard values, the X450 site parameters and the new site coefficient all have to be assembled before any lateral element can be sized. Story forces export straight into diaphragm design.
Replaces roughly 2-4 hours per building of manually sourcing the six-period NBCC 2025 hazard values, constructing the design spectral acceleration, estimating the fundamental period, and hand-computing base shear, vertical force distribution, and the irregularity and system-restriction checks.
#### Calculation method
This calculator determines the design base shear of a building using the Equivalent Static Force Procedure of the National Building Code of Canada 2025 (Article 4.1.8), then distributes that shear over the height of the structure. The workflow runs from the site seismic hazard, through the design spectrum and system parameters, to the base shear, its code limits and the story force distribution.
##### Site hazard and site coefficient
The seismic hazard is defined by the design spectral response accelerations Sa at 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 seconds, together with the peak ground acceleration and peak ground velocity. NBCC 2025 adds the X450 spectral accelerations at 0.2 and 2.0 seconds. The site is classified by site class or by the average shear wave velocity over the top 30 m, and the site coefficient Fs follows from the site condition. These values can flow from the project defaults or be overridden per calculation.
##### Importance factor and seismic category
The building importance category sets the earthquake importance factor IE. The seismic category is then assigned from the importance-weighted hazard: a building reaches SC4 where IE·Sa(0.2) exceeds 0.75 or IE·Sa(1.0) exceeds 0.3, with lower categories following the corresponding thresholds. The seismic category governs which irregularity and system restrictions apply.
##### Fundamental period
The fundamental lateral period Ta comes from the code empirical expression for the selected system type, or from a custom value. The empirical period is capped by a maximum period Tmax that depends on the system category, for example 1.5 × 0.085 × hn^0.75 for a steel moment frame. Single-story buildings with a steel deck or wood roof diaphragm on a moment or braced frame use the diaphragm-specific expression that includes the diaphragm span. Separate periods can be carried for the X and Y directions.
##### Design spectral acceleration
The design spectral acceleration S(Ta) is interpolated from the six-period spectrum at the fundamental period, with the site coefficient applied. Below 0.2 seconds the 0.2 second value governs, and the spectrum is built across the full period range so the value at Ta is available for the base shear equation.
##### Higher mode factor
The higher mode factor Mv depends on the spectral ratio S(0.2)/S(5.0) and on the seismic force-resisting system category. The calculator evaluates Mv at 1.0, 2.0 and 5.0 seconds by interpolating on the spectral ratio within the code bands, then selects or interpolates the value appropriate to Ta. For short-period structures with Ta at or below 0.5 seconds, Mv is unity.
##### Design base shear
The calculated base shear is
**V,calc** = S(Ta) × Mv × IE × W / (Rd × Ro)
where W is the total effective seismic weight summed from the story table, Rd the ductility-related force modification factor and Ro the overstrength-related factor, both set by the selected system or entered as custom values. The result is bounded by a maximum and a minimum:
**V,max** = max( 2/3 × Sa(0.2) × IE × W / (Rd × Ro), Sa(0.5) × IE × W / (Rd × Ro) )
**V,min** = S(4.0)Mv × IE × W / (Rd × Ro)
For systems with Rd of at least 1.5 the governing value is max( min(V,calc, V,max), V,min ), and the calculator reports which limit governed. Where a Type 6 weak-story irregularity is present, the governing shear is amplified by Rd·Ro per the requirement of Sentence 4.1.8.10.(1).
##### Irregularity and system restrictions
The building is screened against the structural irregularities of Table 4.1.8.6. The calculator distinguishes irregularities that merely trigger a dynamic analysis from restricted irregularities that are not permitted for the given importance category and hazard level, and it checks the minimum Rd permitted for high importance and post-disaster buildings. Where the height, period or irregularity combination puts the building outside the scope of the static procedure, that limitation is flagged rather than silently accepted.
##### Force distribution and elastic analysis
Once the base shear is established it is distributed over the height of the building in proportion to the weight-height product Wx·hx, with a concentrated force Ft applied at the top for structures with Ta above 0.7 seconds. The story table reports the lateral force Fx and the cumulative story shear Vx at each level, and these forces are exported for diaphragm design.
A parallel elastic analysis is reported alongside the design result, using Rd of 1.3 and Ro of 1.0 with unit importance factor, and its own hazard spectrum at the required probability of exceedance. This gives the elastic base shear and force distribution next to the design-level values without a second run.
## How to use it
Open it from **Run calc** in the About this calculator panel above.
Work through the input sections from top to bottom. Click any input label to see its reference explanation, clause, conditions and assumptions. See [Checks, References, Conditions and Assumptions](/docs/running-calculations/checking-results/checks-references-conditions-assumptions).
Check the utilization of each governing check in the summary, then export a PDF report. See [Views and Export](/docs/running-calculations/exporting-reports/views-and-export).
## Common questions
It determines the design base shear using the Equivalent Static Force Procedure of the National Building Code of Canada 2025, Article 4.1.8. The design spectral acceleration is built per Cl. 4.1.8.4 and the base shear and its vertical distribution follow Cl. 4.1.8.11.
NBCC 2025 keeps the six-period hazard format but adds the X450 spectral accelerations at 0.2s and 2.0s and a site coefficient Fs derived from the site condition, rather than the 2020 site coefficient tables. The elastic analysis spectrum is also tied to a stated probability of exceedance, 5 percent in 50 years for post-disaster buildings. The two editions are kept as separate calculators because the hazard basis is not interchangeable.
Spectral accelerations at 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 seconds plus the X450 values at 0.2s and 2.0s, peak ground acceleration and velocity, site class or average shear wave velocity, the building importance category, the seismic force-resisting system with its Rd and Ro factors, and a story table giving each story height and effective seismic weight.
The seismic category, the design spectral acceleration at the fundamental period, the higher mode factor, the design base shear and the governing base shear limit, the concentrated force at the top, and a story-by-story table of lateral force and story shear. An elastic analysis is reported in parallel, and the story forces are exported for diaphragm design.
The period is taken from the code empirical expression for the selected system type, capped at the maximum permitted period Tmax for that system, or entered as a custom period. Separate periods can be given for the X and Y directions. For single-story buildings with a steel deck or wood roof diaphragm on a moment or braced frame, the diaphragm-specific period expression is used instead.
Yes. The calculated base shear is bounded by a maximum, taken as the larger of two-thirds of the 0.2 second value and the 0.5 second value, and by a minimum based on the spectral product at 4.0 seconds. It screens the Table 4.1.8.6 irregularities, flags restricted irregularities and Rd limits that are not permitted for the importance category and hazard level, and applies the Type 6 weak-story amplification of Sentence 4.1.8.10.(1) where that irregularity is present.
## Next steps
Determine design base shear to NBCC 2020 (Art. 4.1.8) with the Equivalent Static Force Procedure, and distribute forces story by story.
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