IS 1893:2016 → IS 1893:2025 — Z values, Importance Factor & Calculator
Executive summary
IS 1893-1:2025 revises how seismic potential for structures is expressed. The code removes the explicit Importance Factor (I) and instead uses return-period–based probabilistic zone factors Z. Practically, the 2025 Z for a chosen return period matches the 2016 Z×I values for the corresponding importance category — hence the same engineering intent is preserved while moving to a probabilistic framework. The uploaded note by Dr. Ashok K. Jain (Nov 2025) compares the values and concludes that the new Z values validate the earlier Z×I approach while the base-shear formulation yields about a +33% difference (see section on base-shear change below). 0
Why Importance Factor I is removed (clear technical note)
The Importance Factor in IS 1893:2016 was a deterministic multiplier applied on the code zone factor to reflect higher safety/longer return period for important structures (hospitals, emergency installations). IS 1893:2025 replaces this two-step approach with a single probabilistic framework:
- Return-period-based Z: Instead of multiplying by I, the 2025 code assigns Z based on the return period (e.g., 475, 975, 2475, 4975 years) that implicitly captures the risk tolerance for different importance classes.
- Simplified design workflow: Using a single Z value avoids an extra multiplicative step, reduces confusion, and ties the design value directly to probabilistic hazard levels used in modern hazard mapping.
- Validation of previous practice: Numerical comparison shows the product Z×I (2016) and Z (2025) for the matched return period are close — confirming the older approach is validated by the probabilistic approach.
Table: 2016 (Z×I) → 2025 (Z) mapping
| Zone | Return Period: 475 yr | Return Period: 975 yr | Return Period: 2475 yr | Return Period: 4975 yr | ||||
|---|---|---|---|---|---|---|---|---|
| 2016 Z×I (I=1) | 2025 Z | 2016 Z×I (I=1.2) | 2025 Z | 2016 Z×I (I=1.5) | 2025 Z | 2016 Z×I (I=1.9) | 2025 Z | |
| Zone 2 | 0.10 | 0.075 | 0.12 | 0.10 | 0.15 | 0.15 | 0.19 | 0.20 |
| Zone 3 | 0.16 | 0.15 | 0.19 | 0.20 | 0.24 | 0.30 | 0.304 | 0.33 |
| Zone 4 | 0.24 | 0.30 | 0.29 | 0.36 | 0.36 | 0.45 | 0.456 | 0.44 |
| Zone 5 | 0.36 | 0.40 | 0.43 | 0.48 | 0.54 | 0.60 | 0.684 | 0.625 |
| Zone 6* | 0.50 | 0.50 | 0.60 | 0.60 | 0.76 | 0.75 | 0.95 | 0.94 |
*Zone 6 values are given/assumed for comparison within Dr. Jain's note (see reference). All values in table are taken from the comparison table in the referenced note. 1
Interactive calculator — pick zone and return period
Select a seismic zone and a return period to get the IS 1893:2025 Z value and the comparable IS 1893:2016 Z×I values shown in the mapping table above.
Base-shear formula note (practical impact)
As highlighted in the referenced note, the base-shear expression used in the 2025 formulation effectively replaces the previous multiplicative seismic load factor (1.5) and 1/2 term with a single treatment that increases the calculated code-level base shear by roughly +33% relative to the 2016 expression when comparing like-for-like variables. Design engineers must therefore carefully compare the new spectral factors, response reduction factors (R), and other parameters when transitioning projects between versions.
- For a given project, identify the correct seismic zone and select the return period aligned with the structure's importance (e.g., hospitals → longer return period).
- Use the 2025 Z values directly (they already account for importance class via return period) — do not multiply by an extra Importance Factor.
- Compare base-shear outputs from both approaches during transition projects to check sensitivity and safety margins; document the change for audit/approval.
Posts
0 Comments
If you have any doubts, suggestions , corrections etc. let me know