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# Calculate Seismic Loads (NBCC 2020)
> Determine design base shear to NBCC 2020 (Art. 4.1.8) with the Equivalent Static Force Procedure, and distribute forces story by story.
Determine the design base shear of a building using the Equivalent Static Force Procedure of NBCC 2020 (Art. 4.1.8). Interpolates the six-period design spectral acceleration, screens for structural irregularities, checks base shear against code limits, and distributes forces story by story.
## Method & scope
| Property | Detail |
| ---------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Design standards | NBCC 2020, Cl. 4.1.8 |
| Regions | Canada |
| Who it's for | Canadian structural engineers who need to establish earthquake design base shear and story force distribution per NBCC 2020 before designing beams, walls, or connections. Available now to Core Calcs users (Beta). |
| What it checks | Site parameters, Design spectral acceleration, Key building properties and period, Seismic design parameters, Design base shear, Structural irregularity screening, Seismic force distribution |
#### What it calculates
Canadian structural engineers who need the earthquake design base shear and story force distribution to NBCC 2020 before designing beams, walls, and connections. Interpolates the design spectral acceleration from site parameters, screens for structural irregularities, and distributes the base shear story by story using the Equivalent Static Force Procedure.
Replaces roughly 2-4 hours per building of hand calculation to interpolate the six-period hazard spectrum, classify the Seismic Category, screen structural irregularities, evaluate base shear against code limits, distribute forces story-by-story, and (when triggered) repeat the run as an elastic analysis.
#### 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 2020 (Article 4.1.8), then distributes that base shear over the height of the structure. The workflow moves from site seismic hazard, through the design spectral acceleration and system parameters, to the base shear and its story-by-story distribution.
##### Site parameters
The site seismic hazard is defined by the design spectral response accelerations Sa at the six periods 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 seconds (Cl. 4.1.8.4), together with the peak ground acceleration and peak ground velocity. The site is classified by site class or by the average shear wave velocity over the top 30 m, Vs30, per Table 4.1.8.4.-A. These values set the amplified spectral accelerations used to build the design spectrum.
##### Design spectral acceleration
The design spectral acceleration S(T) is interpolated from the six-period spectrum for the building's fundamental period. The site condition factors are applied to the spectral values, and the calculator builds the full design spectrum so that S(T) at the relevant period is available for the base shear equation.
##### Key building properties and period
The importance category sets the importance factor IE from Table 4.1.2.1. The fundamental lateral period Ta is either taken from the code empirical expression for the selected system type or entered as a custom period, subject to the maximum period cap of Cl. 4.1.8.11. Separate periods can be handled where the building responds differently in each direction.
##### Seismic design parameters
The seismic force-resisting system (SFRS) sets the ductility-related force modification factor Rd and the overstrength-related factor Ro from Table 4.1.8.9. These can be selected from the system list or entered as custom values. The product Rd·Ro reduces the elastic force demand to the design level and is central to the base shear calculation.
##### Design base shear
The design base shear is V = S(Ta)·Mv·IE·W / (Rd·Ro), where W is the seismic weight and Mv the higher-mode factor. The result is checked against the NBCC 2020 limits of Cl. 4.1.8.11: the base shear need not be taken greater than two-thirds of the value computed at 0.2 s for systems with Rd of at least 1.5, and must not be less than the code minimum. The governing value is reported.
##### Structural irregularity screening
Before the base shear is accepted, the calculator screens the building against the structural irregularities of Table 4.1.8.6, including a soft-story check for any story whose lateral stiffness is less than that of the story above. Where an irregularity or a gravity-induced lateral demand condition (Qy relative to QG) is present, the calculator flags where a dynamic or elastic analysis is required rather than the static procedure alone.
##### Seismic force distribution
Once the base shear is established, it is distributed over the height of the building story by story per Cl. 4.1.8.11, including the concentrated force Ft applied at the top for taller or longer-period structures. When the configuration triggers it, the run is repeated as an elastic analysis so the distributed forces can be compared against the static result.
## 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 of a building using the Equivalent Static Force Procedure of the National Building Code of Canada 2020, Article 4.1.8. Forces are then distributed over the height of the building per Cl. 4.1.8.11.
The design spectral response accelerations Sa at 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 seconds, the site class or average shear wave velocity Vs30, PGA and PGV, the building importance category, the seismic force-resisting system with its ductility and overstrength factors Rd and Ro, and the fundamental period of the building.
It returns the design spectral acceleration S(T), the design base shear V, the story-by-story seismic force distribution, and screening results for structural irregularities. When the configuration triggers it, the run is repeated as an elastic analysis.
Yes. The base shear is evaluated against the NBCC 2020 minimum and maximum limits of Cl. 4.1.8.11, including the upper bound that need not be taken greater than two-thirds of the short-period spectral value for systems with Rd of at least 1.5, and the minimum base shear provisions.
You select the seismic force-resisting system, which sets Rd and Ro from Table 4.1.8.9, or enter custom values. The calculator screens for the structural irregularities of Table 4.1.8.6, including a soft-story check for any story with lateral stiffness less than the story above, and flags where a dynamic or elastic analysis is required.
## Next steps
Complete list of Calcs.com design calculators available for Canadian projects (CSA O86, CSA S16, NBCC, and related standards).
Pull the reference wind pressure and climatic data for any Canadian site from the NBCC 2020 tables (Appendix C) as the basis for design wind loads.
Get balanced, unbalanced, drift, and sliding roof snow loads to NBCC 2020 Cl. 4.1.6 from the 1-in-50-year ground snow basis, with each code clause shown.
Design simple and continuous wood beams, joists and girders to CSA O86:19 with unlimited supports and loads. Checks moment, shear and deflection.
Design wood columns and studs to CSA O86:19, with load linking between beams and columns.
