NBC 105:2025 Seismic Coefficient + MRSM Calculator

Code-aligned workflow for Equivalent Static Method (ESM) and Modal Response Spectrum Method (MRSM), with automated soil / importance helpers, adopted-period logic, optional story-force distribution, and MRSM modal-combination checks.

NBC 105:2025 ULS + SLS workflow MRSM integrated Units: m, s, kN

Project setup

Set the ULS analysis method. SLS is still evaluated by ESM, which is allowed for all buildings.

The main correction in this version is that ULS method selection now drives the spectrum and base-shear logic properly: ESM for all SLS work, and MRSM for ULS modal response when selected.

MRSM is intended for structures where ESM is not sufficient, or whenever you want modal ULS evaluation.
Used for ESM ULS applicability check.
If yes and MRSM is used, a 3D analysis is required.
Automatically fills the importance factor I. Class II shelters can be escalated to 1.5.
Applies only to Importance Class II.
Editable override.
You can classify from Vs,30 or NSPT below, then the spectrum parameters update automatically.
If present, this governs the soil classification helper.
Used only when Vs,30 is not available.
Rμ, Ωu and Ωs are auto-filled from Table 5-2.
Enter the project-specific PGA zoning factor.
Height from base to top of roof level, as defined in Figure 5-1.
Used for ESM deflection scale factor kd.
Optional but recommended if you want base shear forces, not only coefficients.
Editable after auto-fill.
Ultimate limit state overstrength factor.
Serviceability limit state overstrength factor.

Period model and spectrum parameters

Empirical / Rayleigh adopted period logic, auto kt, and editable Table 4-1 parameters.

The calculator now separates the modified empirical period 1.25·kt·H3/4 from the adopted ESM comparison period, which is the lesser of the modified empirical value and the Rayleigh period when Rayleigh data is provided.

Auto-filled from the structural system. You can still override it.
Optional. If entered, the adopted period becomes the lesser of Rayleigh and modified empirical.
Used only for systems with concrete shear walls. The calculator computes Aw = Σ Awi{0.2 + (Lwi/H)2} with Lwi/H ≤ 0.9, then kt = 0.075 / √Aw.
For ESM, the calculator internally uses Ta = 0.
Upper limit of constant spectral acceleration branch.
Start of constant displacement range.
Peak spectral acceleration normalized by PGA.

Story data and drift checks

Optional story weights / heights for ESM distribution and advanced MRSM force derivation, plus drift / displacement checks.

Optional
Enter one level per line. This lets the calculator build the ESM vertical force distribution and, if modal shapes are also supplied, derive MRSM effective modal weights and story forces.
Before applying Rμ. For ESM, the tool also shows the optional kd reduction.
Normally from the SLS analysis model.
Before applying Rμ.
Directly compared with the SLS limit after optional ESM kd adjustment.
Optional. Used for the separation gap check: Δgap = √(Δ12 + Δ22).

MRSM modal data

Enter modal periods and either effective modal weights directly, or mode shapes for full equation-based story-force derivation.

Modes are combined using SRSS, or SRSS with automatic close-mode grouping where frequencies lie within 15%. Modes above 33 Hz are flagged because the code requires missing-mass correction for them.

Basic mode is enough for combined modal base shear. Shape mode also derives story forces using Eq. 7.1(2) and 7.1(3).
Close-mode grouping directly sums close modes before SRSS across groups.
Enter Wi directly, or leave it blank and provide massRatio as a fraction or percentage. If both are entered, Wi is used.
Built in plain HTML + CSS + JS for easy deployment. Review project inputs and office assumptions before issuing final design values.