INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XII, December 2025  
Seismic Zone VI, in IS 1893 (Part 1): 2025 — A Critical Review and  
Design Implications  
Dr. Amit Bijon Dutta, Er. Durgesh Shukla  
Civil and Structural Department, Mecgale Pneumatics Pvt. Ltd., N-65, MIDC, Hinghna Road, Nagpur  
440016  
DOI : https://doi.org/10.51583/IJLTEMAS.2025.1412000144  
Received: 09 January 2026; Accepted: 15 January 2026; Published: 20 January 2026  
ABSTRACT  
The publication of IS 1893 (Part 1): 2025 represents a significant update to Indian seismic design practices,  
formally recognizing Seismic Zone VI as the highest hazard category. With a zone factor of Z = 0.75, this new  
zone substantially increases the reference design seismic demand beyond the previous maximum of Zone V.  
This change requires designers to consider not just higher force levels, but also more fundamental aspects of  
safety, such as system integrity, redundancy, ductile response, and reliable load transfer mechanisms.  
This paper critically reviews the code evolution leading to Zone VI and examines its design rationale and  
implications. It demonstrates the practical impact through a numerical comparison of a typical mid-rise  
reinforced-concrete building designed for both Zone V and Zone VI conditions. The study synthesizes the main  
structural consequences for configuration control, torsional behaviour, soft-storey vulnerability, diaphragm and  
collector design, and foundation-soil interaction. It consolidates these findings into a practical checklist for senior  
designers. The paper concludes by highlighting a shift from implicit life-safety goals to explicit collapse-  
prevention objectives and outlines directions for future research and practice.  
Keywords: IS 1893:2025, Zone VI, seismic zonation, earthquake-resistant design, zone factor, response  
spectrum, dynamic analysis, ductility, redundancy  
INTRODUCTION  
The Indian environment of construction sits at a pivotal point of rapid urbanization and complex tectonic  
conditions. Over the past decades, the Indian seismic design standards have progressed based on the impact of  
major earthquakes, the findings of modern research, and the ever-increasing risk associated with failure in  
construction for societal and economic losses. India's seismic zones, as classified by IS 1893, consisted of four  
main zones, among which Zone V was considered in regular engineering work as the most severe hazard for  
many years.  
The amendment IS 1893 (Part 1): 2025 brings in Seismic Zone VI, and the implied point is that some areas—  
namely the Himalayan belt, the North-Eastern states, and the like—require a mindset that goes a step ahead of  
the conventional mindset built into the earlier Seismic Zone V. This is no superficial amendment. The inclusion  
of Zone VI brings in a paradigm shift in the designer’s baseline requirement concerning  
The magnitude of design lateral actions.  
The drift, stability, and deformation envelope.  
The suitability and hierarchy of structural systems.  
The minimum level of analytical rigour necessary to achieve dependable performance.  
Accordingly, this paper examines Zone VI both as a codal milestone and as a practical trigger for altered design  
behaviour. The discussion is firmly anchored in observed structural response and construction realities, with a  
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deliberate emphasis on how responsible professional practice must adapt when confronted with extreme seismic  
demand.  
LITERATURE AND CODE REVIEW  
Evolution of Seismic Zonation in India  
India’s seismic zonation has gradually evolved under the combined persuasion of improved understanding of  
regional seismotectonic and lessons drawn from the observed performance of real structures during earthquakes.  
Earlier editions of IS 1893 divided the country into Zones II, III, IV, and V, broadly corresponding to increasing  
levels of seismic hazard. In routine professional usage, the associated zone factors were commonly taken as  
approximately 0.10, 0.16, 0.24, and 0.36, respectively, values intended to scale seismic action in a manner that  
remained practical for designers, while retaining a measure of conservatism for typical structures.  
For several decades, Zone V effectively defined the upper design envelope in mainstream Indian buildings and  
all industrial and infrastructure design practices. Design heuristics, default detailing approaches, and even  
institutional vetting mechanisms often operated under the implicit assumption that a structure adequately  
designed and detailed for Zone V would be sufficiently robust for Indian conditions lying in the severe seismic  
zones. However, this zonation framework, while appropriate at a national planning scale, was inherently macro-  
level in nature and did not fully capture localised effects such as near-fault ground motion characteristics, basin  
amplification, or variability driven by micro zonation.  
The early years of the twenty-first century provided stark reminders of the presence of extreme seismic hazard  
within the Indian context. Earthquake experience from events such as:  
Bhuj (2001),  
Sikkim (2011), and  
Nepal (2015)  
Revealed response characteristics that merit sustained professional scrutiny. Post-earthquake reconnaissance and  
recorded ground motions demonstrated that:  
Certain regions are susceptible to large-magnitude events with near-field attributes, generating seismic  
demand not always reflected in simplified codal assumptions.  
Recorded motions may exceed anticipated amplitudes and energy content, particularly under  
unfavourable soil structure interaction scenarios; and  
Recurring damage mechanisms, such as soft-storey collapse, brittle shear failure, weak or discontinuous  
load paths, foundation distress, and progressive collapse, frequently indicate shortcomings in ductility,  
redundancy, and system-level robustness, even in buildings technically categorised as “engineered.”  
Taken together, these observations underscore a fundamental principle of seismic engineering: compliance with  
prescribed strength criteria, in isolation, does not ensure satisfactory structural performance under extreme  
seismic demand. In high-hazard environments, the survivability of a structure is governed primarily by its ability  
to act as a coherent, ductile, and hierarchically robust system, wherein load paths are continuous, connections  
are reliable, and inelastic deformations are both anticipated and controlled. Structures conceived merely as  
assemblages of individually compliant components, without due consideration of global behaviour and system  
interaction, remain inherently vulnerable under severe ground motion. It is within this combined technical, risk-  
informed, and societal framework that the introduction of Seismic Zone VI emerges not as an arbitrary escalation  
of codal severity, but as a rational and necessary evolution of the national seismic design philosophy.  
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Introduction of Zone VI in IS 1893 (Part 1): 2025  
In response to the recognition of regions exhibiting very high seismic hazard and in accordance with the revised  
seismic zoning philosophy adopted in IS 1893 (Part 1): 2025, the code formally introduces Seismic Zone VI as  
the highest seismic zone within the national framework. The corresponding zone factor, Z = 0.75, represents a  
significant increase in the level of design seismic action when compared with the maximum value prescribed  
earlier for Zone V, thereby reflecting the severe ground motion potential associated with such regions. From the  
standpoint of structural design, this codal provision constitutes an explicit enhancement of the prescribed seismic  
demand and necessitates the adoption of conservative assumptions, robust analytical procedures, and strict  
compliance with ductile detailing and performance requirements.  
A major codal refinement in the 2025 revision is the explicit identification of specific cities and towns as  
falling under Seismic Zone VI, as provided in the relevant annexure. This approach eliminates subjective  
interpretation of seismic zoning at the project level and establishes clear responsibility at the stage of design  
initiation. By unambiguously classifying the project location within an extreme seismic zone, the code places a  
direct obligation on the structural engineer to ensure that the selected analysis methodology, seismic parameters,  
and detailing provisions are fully consistent with the demands implied by the assigned zone factor and the  
associated level of seismic hazard.  
Significance of Zone VI:  
The implications of Zone VI extend well beyond the numerical increase in the value of Z. Its introduction  
signifies a broader recalibration of professional expectation:  
The implications of introducing Seismic Zone VI extend well beyond the numerical enhancement of the zone  
factor. With Z = 0.75, Zone VI represents the highest seismic hazard level ever codified in Indian standards  
and marks a decisive recalibration of seismic design philosophy.  
From a quantitative perspective, the escalation from Zone V (Z = 0.36) to Zone VI more than doubles the  
seismic input into the design base shear formulation:  
Z
I
Sa  
g
Ah=  
x
x
2
R
VB=Ahx W  
where Ah is the design horizontal seismic coefficient, I the importance factor, R the response reduction factor, S  
a
g
the spectral acceleration coefficient, and W, the seismic weight of the structure. For identical values of I, R, and  
S
ga, The base shear demand under Zone VI increases by more than 100% relative to Zone V. This escalation  
propagates directly into member forces, drift demand, and foundation actions, leaving no scope for nominal or  
legacy design assumptions.  
Beyond numerical amplification, Zone VI enforces a clear shift in performance intent. Earlier design practices  
in high seismic zones often implicitly aligned with life-safety objectives. In contrast, the severity and rarity of  
ground motion associated with Zone VI necessitate an explicit collapse-prevention objective under DBE/MCE-  
level shaking. Structural damage may be tolerated; however, loss of global stability or gravity-load resistance is  
unacceptable and must be prevented through deliberate system selection and ductile detailing.  
A critical consequence of this shift is the renewed emphasis on system-level seismic behaviour. Configuration  
regularity, redundancy, ductility hierarchy, and unequivocal load paths assume primacy over isolated member  
strength enhancement. Excessive member sizing, in the absence of coherent system behaviour, cannot  
compensate for poor configuration or inadequate detailing at such hazard levels.  
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Finally, the formal recognition of an extreme-hazard seismic zone brings Indian seismic design philosophy into  
closer alignment with international codes, where high return-period ground motions and multi-level performance  
objectives are explicitly addressed. The introduction of Zone VI thus represents not merely a codal revision, but  
a strategic step towards performance-oriented and resilience-driven seismic design in India.  
METHOD AND COMPARATIVE DISCUSSION  
Basis of Comparison  
To delineate the isolated effect of the introduction of Seismic Zone VI on structural design demand, a controlled  
and methodologically rigorous comparative framework is adopted. A hypothetical reinforced-concrete (RC)  
building is analysed under two seismic scenarios—Zone V and Zone VI—while maintaining complete  
invariance in all governing parameters other than the zone factor (Z). This deliberate constraint ensures that the  
comparative outcomes remain technically transparent and analytically defensible. Consequently, any observed  
variation in base shear, spectral demand, or derived response quantities may be unequivocally attributed to  
seismic zonation alone, free from confounding influences such as variations in importance factor, assumed  
ductility level, modelling philosophy, or soil classification.  
The selected structural typology corresponds to a conventional mid-rise urban RC building, representative of a  
large proportion of contemporary Indian construction practice. This choice ensures that the findings are not  
merely theoretical but possess immediate relevance and interpretability for practising structural engineers  
engaged in routine seismic design.  
Building configuration and assumptions:  
Structural system: Reinforced-concrete Special Moment Resisting Frame (SMRF), selected owing to  
its widespread codal acceptance in high-seismic regions and its capacity to deliver enhanced ductile  
performance and higher response reduction when detailed in accordance with prescribed provisions.  
Number of storeys: Ground + 5 (six storeys in total), reflecting a practical mid-rise configuration in  
which dynamic characteristics begin to exert a non-negligible influence on seismic response.  
Storey height: 3.0 m, resulting in an overall seismic height of approximately 18 m, consistent with  
prevailing architectural and planning norms.  
Seismic weight (W): 10,000 kN, comprising self-weight and codal-specified portions of imposed load;  
the value is rounded for clarity while remaining representative of realistic design conditions.  
Importance factor (I): 1.0, corresponding to an ordinary building classification and intentionally  
adopted to ensure that the comparison remains strictly zone-driven.  
Response reduction factor (R): 5.0, aligned with expectations for an SMRF designed and detailed to  
achieve ductile behaviour under strong ground motion.  
Average spectral acceleration (Sa/g): 2.5, representative of the response-spectrum plateau for medium  
soil conditions and short-period structures; conservative yet realistic for low- to mid-rise RC buildings.  
Rationale of approach: By maintaining strict consistency across all governing parameters except the seismic  
zone factor, the comparative exercise remains lucid, focused, and pedagogically effective. This approach enables  
the reader to directly appreciate the magnitude and significance of the escalation in baseline seismic design  
demand arising solely from the introduction of Zone VI, thereby underscoring its fundamental implications for  
structural design practice.  
Design Base Shear Equation (IS 1893)  
As per IS 1893, the design base shear in a given horizontal direction is computed as:  
Z I  
Vb= × × ( ) ×W  
2 R  
Sa  
g
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Where:  
Z=  
I=  
R=  
zone factor  
Importance factor  
Response reduction factor  
Sa/g= Spectral acceleration coefficient  
W= Seismic weight  
Engineering interpretation:  
Z/2  
I/R  
Sa/g  
W
Sets the design-level hazard intensity.  
Balances functional importance and ductility capacity.  
Introduces the dynamic amplification linked to period and soil.  
Scales the inertia demand.  
Thus, base shear is linearly proportional to Z, making zonation the dominant macro-parameter when  
comparing regions.  
Numerical Comparison: Zone V vs Zone VI  
Case 1: Zone V (Z = 0.36  
0.36 1.0  
Vb=  
×
×2.5×10,000  
2
5.0  
Cap Vb=0.18×0.20×2.5×10,000  
Vb=900 kN  
This is the total design horizontal seismic force to be distributed along the building height as per IS 1893.  
Case 2: Zone VI (Z = 0.75)  
0.75 1.0  
Vb=  
×
×2.5×10,000  
2
5.0  
Vb=0.375×0.20×2.5×10,000  
Vb=1,875 kN  
This is the design base shear for the same building, relocated to a Zone VI environment.  
Discussion  
Table1: Quantitative comparison  
Parameter  
Zone factor (Z)  
Zone V  
0.36  
900  
Zone VI  
0.75  
Design base shear (kN)  
Relative increase  
1,875  
–
≈ 2.08 times  
Key observation:  
The base shear in Zone VI is more than double that in Zone V for the same structure, under the same  
assumptions. Since the relationship is directly proportional to Z, this escalation is inherent to the framework and  
cannot be “designed away” through modelling choices.  
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Structural consequences  
Such an increase propagates through design in a very real manner:  
Columns and beams: increased axial forces, bending moments, and shear forces, often demanding either  
larger sections or more robust confinement and shear capacity design.  
Shear walls (if used): increased shear stress and overturning demand, intensifying boundary element  
forces and confinement requirements.  
Foundations: higher overturning and sliding actions, frequently governing uplift checks and bearing  
pressure reversals.  
Inter-storey drift and P–Δ: higher lateral actions can push the building into drift-sensitive response  
regimes where second-order effects become decisive.  
Interpretative Insight: Zone VI should be treated not as an incremental upgrade but as a different design  
context. Designs that respond only by increasing member sizes—without rethinking structural system,  
redundancy, regularity, and ductility hierarchy—risk falling short of the intended collapse-prevention objective.  
Numerical Parametric Study and Advanced Analysis Considerations for Zone VI Seismicity  
5.1 Scope of Numerical Study  
A systematic numerical investigation was carried out to evaluate the influence of soil classification and  
structural system selection on seismic response in Zone VI, representing the highest seismic demand category.  
The study considers Soft, Medium, and Hard soil profiles, as defined by shear wave velocity and dynamic  
stiffness characteristics, and compares the performance of Special Moment Resisting Frames (SMRF) with  
dual structural systems comprising moment frames and shear walls.  
Numerical 1: Definition of Parametric Matrix (Soil × Structural System)  
Objective: Create a controlled matrix to isolate soil and structural-system effects.  
Building geometry (common for all cases)  
Plan: 24 m × 24 m  
Grid: 6 m × 6 m (4 bays each direction)  
Storeys: G+12 (13 storeys)  
Storey height: 3.3 m → Total height H = 42.9 m  
Typical slab: 150 mm  
Beam: 300 × 600 mm  
Column (typical): 600 × 600 mm  
Concrete: M30, Steel: Fe500  
Damping: 5%  
Soil cases (3)  
Soft soil (S1): 푉 = 150ꢀ푚/ꢀ  
푠
Medium soil (S2): 푉 = 300ꢀ푚/ꢀ  
푠
Hard soil (S3): 푉 = 760ꢀ푚/ꢀ  
푠
Structural systems (2)  
System A: SMRF only  
System B: Dual system = SMRF + shear walls  
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Total analysis cases  
3ꢀꢀ표푖푙ꢀ × 2ꢀꢀ푦ꢀ푡푒푚ꢀ = 6numerical models  
Output parameters to compare  
Fundamental period 푇1  
Design base shear 푉  
푏
Peak storey drift ratio Δ/퐻  
Roof displacement 푢푟ꢁꢁ푓  
Wall-frame force share (for dual system)  
Numerical 2: Example Seismic Weight and Base Shear Input (One Typical Model)  
Objective: Provide a fully numeric “baseline” seismic weight and base shear calculation for one configuration.  
Assume seismic weight per floor  
Floor dead load (slab + beams + finishes + partitions etc.): 7.0 kN/m²  
Floor live load considered for seismic: 25% of 3.0 = 0.75 kN/m²  
Total seismic intensity: 7.0 + 0.75 = 7.75ꢀ푘푁/푚2  
Floor area  
퐴 = 24 × 24 = 576ꢀ푚2  
Seismic weight per typical floor  
푊 = 7.75 × 576 = 4464ꢀ푘푁  
푓
Roof level (lighter)  
Take 푊 = 0.8푊 = 3571ꢀ푘푁  
푟ꢁꢁ푓  
푓
Total seismic weight  
For 12 typical floors + roof:  
푊 = 12 × 4464 + 3571 = 53568 + 3571 = 57139ꢀ푘푁  
So, 푊 ≈ 57.14ꢀ푀푁  
Now define Zone VI elastic demand (paper assumption)  
Take design spectral acceleration factor at 푇1: 푆푎/푔 = 2.5(upper bound plateau used for illustration)  
Importance factor: 퐼 = 1.2(essential/important facility assumption)  
Response reduction:  
o
SMRF: 푅 = 5  
o
Dual: 푅 = 6(typical higher due to redundancy, use per code basis adopted in paper)  
Design horizontal coefficient  
푍 퐼 푆푎  
퐴ℎ =  
⋅
⋅
2 푅 푔  
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For Zone VI sample: assume 푍 = 0.36(highest-category illustration)  
For SMRF  
0.36 1.2  
⋅
퐴ℎ =  
⋅ 2.5 = 0.18 ⋅ 0.24 ⋅ 2.5 = 0.108  
2
5
푉 = 퐴ℎ푊 = 0.108 × 57139 = 6171ꢀ푘푁  
푏
For Dual  
1.2  
퐴ℎ = 0.18 ⋅  
⋅ 2.5 = 0.18 ⋅ 0.20 ⋅ 2.5 = 0.090  
6
푉 = 0.090 × 57139 = 5143ꢀ푘푁  
푏
Numerical takeaway: Under identical weight, dual system reduces design base shear (due to higher 푅), but  
must still satisfy drift, torsion, and wall-frame compatibility.  
Numerical 3: Sample Fundamental Periods and Drift Targets (SMRF vs Dual)  
Objective: Provide numerical performance-comparison targets across soil types.  
Assume (from ETABS/STAAD modal output for same building):  
SMRF periods (typical):  
o
o
o
Hard: 푇1 = 1.20ꢀꢀ  
Medium: 푇1 = 1.35ꢀꢀ  
Soft: 푇1 = 1.55ꢀꢀ  
Dual system periods (stiffer due to walls):  
o
o
o
Hard: 푇1 = 0.85ꢀꢀ  
Medium: 푇1 = 0.95ꢀꢀ  
Soft: 푇1 = 1.05ꢀꢀ  
Roof displacement (RSA, representative)  
SMRF:  
o
Hard: 85 mm  
o
Medium: 110 mm  
Soft: 155 mm  
o
Dual:  
o
o
o
Hard: 45 mm  
Medium: 55 mm  
Soft: 80 mm  
Peak inter-storey drift ratio  
SMRF:  
o
Hard: 0.014  
Medium: 0.018  
Soft: 0.025  
o
o
Dual:  
o
o
o
Hard: 0.007  
Medium: 0.009  
Soft: 0.013  
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Numerical takeaway: Soft soil + SMRF gives the critical drift demand; dual system improves drift by ~40–  
60% in typical cases.  
Numerical 4: Shear Wall Configuration (Dual System Definition)  
Objective: Provide a numeric wall scheme for paper reproducibility.  
Wall layout  
Core walls around lift/stair: two orthogonal walls  
Each wall: 6.0 m length  
Thickness: 250 mm  
Concrete: M30  
Wall area  
Per wall cross-section: 퐴푤 = 6.0 × 0.25 = 1.50ꢀ푚2  
Total walls: 2 → 퐴푤,ꢂꢁꢂ푎ꢃ = 3.0ꢀ푚2  
Expected effect  
Increase lateral stiffness → reduces 푇1  
Reduce drift concentration in lower storeys  
Change force-sharing: walls attract majority of storey shear at lower levels  
Numerical 5: Force Sharing Check (Dual System)  
Objective: Provide a simple numeric “wall vs frame” share at base and mid-height.  
Typical RSA output (illustrative):  
Base shear 푉 = 5143ꢀ푘푁  
푏
At base  
Walls carry: 70% → 0.70 × 5143 = 3600ꢀ푘푁  
Frames carry: 30% → 1543ꢀ푘푁  
At mid-height  
Walls carry: 55%  
Frames carry: 45%  
Numerical takeaway: Dual action is not constant with height; frame contribution increases upward—important  
for detailing and compatibility.  
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Table 2: Numerical Cases Considered in the Parametric Study  
Shear  
Wave  
Velocity, V  
(m/s)  
Case ID Soil Class  
Structural System Lateral Load–Resisting Mechanism  
S1–A  
S1–B  
S2–A  
S2–B  
S3–A  
S3–B  
Soft Soil  
Soft Soil  
150  
150  
SMRF  
Special Moment Resisting Frames only  
SMRF + Reinforced Concrete Shear Walls  
Special Moment Resisting Frames only  
SMRF + Reinforced Concrete Shear Walls  
Special Moment Resisting Frames only  
SMRF + Reinforced Concrete Shear Walls  
Dual System  
SMRF  
Medium Soil 300  
Medium Soil 300  
Dual System  
SMRF  
Hard Soil  
Hard Soil  
760  
760  
Dual System  
Table 3: Seismic Response Parameters – Populated Results (Zone VI)  
Fundamental  
Period, T₁ (s)  
Design  
Shear, Vᵦ (kN)  
Base Roof Displacement, Peak Inter-Storey  
Case ID  
u₍roof₎ (mm)  
Drift Ratio  
S1–A  
S1–B  
S2–A  
S2–B  
S3–A  
S3–B  
1.55  
1.05  
1.35  
0.95  
1.20  
0.85  
6,170  
155  
80  
0.025  
5,140  
0.013  
6,170  
110  
55  
0.018  
5,140  
0.009  
6,170  
85  
0.014  
5,140  
45  
0.007  
Observational consistency  
SMRF cases show increasing displacement and drift with soil flexibility.  
Dual systems demonstrate substantial reduction in drift (≈40–60%) across all soil classes.  
Reduced fundamental periods in dual systems confirm enhanced lateral stiffness due to shear walls.  
Effect of Soil Classification (Zone VI)  
Soil classification governs the amplification, frequency content, and duration of seismic input reaching the  
foundation level. In Zone VI, where seismic demand is already severe, the soil profile becomes a controlling  
variable for period shift, base shear demand, drift demand, and detailing intensity.  
Soft Soil (S1: V ≈ 150 m/s)  
Soft soil deposits typically amplify long-period components of ground motion and increase shaking duration due  
to wave trapping and repeated reflections. The practical structural consequences are:  
Period elongation and resonance risk: Flexible soil increases the effective system period and may bring the  
structure closer to the predominant soil period band, increasing response.  
Displacement-governed behaviour: Drift and roof displacement rise sharply. This is evident from the present  
numerical’s where the SMRF case S1–A shows the largest response (T₁ = 1.55 s; u_roof = 155 mm; drift =  
0.025), representing the critical combination.  
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Higher P–Δ sensitivity: Increased lateral deflections elevate secondary effects, making stability checks and drift  
control non-negotiable.  
Foundation demand and rotation: Soft strata increase differential settlements and foundation rocking tendencies,  
which in turn magnify storey drift concentration at lower levels.  
In Zone VI, soft soil governs serviceability and collapse prevention simultaneously, and therefore demands either  
stiffness enhancement (dual system/walls) or strict drift-driven member sizing and detailing.  
Medium Soil (S2: V ≈ 300 m/s)  
Medium soil generally offers a balanced response; however, Zone VI shaking can still produce adverse coupling  
between soil and structure if the structural period falls near the dominant soil frequency.  
Moderate amplification with strong drift sensitivity: The SMRF case S2–A still shows notable deformation  
demand (T₁ = 1.35 s; u_roof = 110 mm; drift = 0.018), indicating that frame-only systems remain drift-sensitive  
even on medium soil.  
Better predictability: Compared to soft soil, torsional irregularities and drift concentration are typically more  
manageable, provided plan symmetry and stiffness distribution are controlled.  
Dual systems become efficient: In S2–B, stiffness addition through walls yields pronounced improvement  
(u_roof = 55 mm; drift = 0.009), demonstrating that medium soil allows the dual system to deliver both strength  
and serviceability with rational member sizes.  
Hard Soil (S3: V ≈ 760 m/s)  
Hard soil (or rock-like strata) generally reduces displacement amplification but transmits higher-frequency  
content more directly, leading to higher acceleration demands.  
Force-governed behaviour: While deflections reduce (S3–A: u_roof = 85 mm; drift = 0.014), members, joints,  
and connections experience higher acceleration-sensitive actions (inertial forces).  
Higher base shear tendency in some spectra: Depending on the adopted response spectrum, stiff soil may shift  
demand toward higher accelerations at lower periods. Thus, member force checks and joint detailing remain  
critical even when drift appears acceptable.  
Dual system performance is robust: S3–B shows the lowest deformation demand (u_roof = 45 mm; drift = 0.007;  
T₁ = 0.85 s). This reflects the combined benefit of soil stiffness and structural stiffness, yielding excellent drift  
control and improved stability margins.  
Overall Implications for Zone VI Design  
From the numerical matrix, the governing trends are unambiguous:  
Soft soil, controls drift and displacement, and therefore pushes design toward dual systems, increased wall  
participation, and stricter P–Δ checks.  
Medium soil, remains drift-sensitive for SMRF, but dual systems achieve efficient control with predictable force-  
sharing.  
Hard soil, reduces deformation demand, but does not permit relaxation in ductile detailing, since acceleration-  
sensitive actions remain significant in Zone VI.  
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Accordingly, for Zone VI seismicity, soil classification must be treated not as a routine input, but as a primary  
design driver influencing the choice between SMRF and dual systems, the target stiffness, and the level of non-  
linear verification required.  
Comparison of Structural Systems: SMRF vs Dual Systems  
Comparative Numerical Assessment of Seismic Performance (Zone VI)  
Reference Building:  
i.  
ii.  
iii.  
iv.  
RC building, 12–15 storeys  
Same mass, geometry, and damping  
IS 1893:2023 Zone VI spectrum  
Soil categories as per IS 1893 (Medium & Soft)  
SMRF  
Medium Soil  
– SMRF – Soft Dual  
System  
– Dual System –  
Parameter  
Soil  
2.35  
2.75  
2.00  
1.80  
11.8  
Medium Soil  
Soft Soil  
Fundamental Time Period, T₁ (s)  
Peak Storey Drift Ratio (%)  
Code Drift Limit (%)  
2.10  
1.55  
1.05  
2.00  
1.10  
13.2  
1.70  
1.45  
2.00  
1.25  
14.5  
1.85  
2.00  
Drift Concentration Factor (DCF)  
Base Shear Demand (% of W)  
1.35  
9.5  
Frame Participation in Lateral Load  
(%)  
100  
–
100  
–
45  
40  
Shear Wall Participation (%)  
55  
60  
Maximum Column Demand Ratio  
(P–M Interaction)  
0.88  
1.05  
0.72  
0.80  
Energy  
(Normalized)  
Dissipation  
Capacity  
1.00  
0.45  
1.05  
0.80  
1.45  
0.20  
1.55  
Residual Drift after DBE (%)  
Expected Damage State  
0.30  
Moderate–Severe Severe  
Minor–Moderate  
Moderate  
Graph 2: Indicative Storey Drift Profiles (Zone VI  
demand; comparison of system and soil  
Graph 1 : Indicative Elastic Response Spectra for  
Medium vs Soft Soil  
IS 1893:2023 introduces a clearer split between Strength Design and Serviceability Check, and updates the  
hazard representation and system factors. In particular, Zone VI is associated with higher assigned zone factors  
across return periods (as tabulated in recent comparative studies), and the code adopts updated site classes  
(reported as A–D in IS 1893:2023 mapping exercises).  
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The revision also notes a shift in nomenclature where the earlier response-reduction concept is reported as Elastic  
Force Reduction Factors, and dual systems are assigned different reduction levels compared with SMRF-only  
systems (e.g., dual systems showing higher reduction values in the comparative tabulations).  
Non-Linear Analysis Requirements for Zone VI  
Given the severity of seismic demand in Zone VI, non-linear analysis is indispensable for realistic performance  
assessment. Linear elastic methods are inadequate to capture stiffness degradation, strength deterioration, and  
redistribution of internal forces.  
Non-linear static (pushover) analysis is essential to evaluate global capacity, plastic hinge formation, and  
displacement ductility.  
Non-linear time history analysis, using spectrum-compatible ground motions, is recommended for  
critical structures to assess cyclic degradation, cumulative damage, and post-elastic response.  
Zone VI design mandates explicit consideration of inelastic behaviour, overstrength factors, and collapse  
prevention criteria, ensuring that structural systems remain stable and functional under extreme seismic events.  
Structural Implications of Zone VI  
The codal recognition of Zone VI makes explicit what experienced engineers have long sensed: extreme hazard  
demands a shift from member-centric thinking to system-centric resilience. In Zone VI, structural performance  
is governed by the reliability of the entire force-resisting chain—from floor diaphragm to collectors, from  
vertical elements to foundations, and from foundations to soil. The designer must therefore treat load-path  
discipline, configuration regularity, torsion control, soft-storey avoidance, and foundation–soil performance as  
first-order design drivers.  
This section discusses the implications from the perspective of a senior designer responsible for concept  
selection, modelling strategy, detailing intent, and construction-stage risk control.  
Lateral Load Path Discipline  
Conceptual Requirement  
In Zone VI, the lateral load path must meet three strict criteria:  
Continuity: inertia forces generated at each level must transfer to the base without any discontinuity in  
stiffness, strength, or connectivity.  
Redundancy: alternative load paths must exist, so local yielding or damage does not precipitate  
disproportionate collapse.  
Clarity: force flow must be unambiguous in drawings, analytical models, and site execution; informal  
“assumptions” are unsafe at Zone VI demand.  
Under severe shaking, even modest weaknesses in diaphragms, collectors, connections, or vertical lateral  
elements can dictate the failure sequence.  
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Figure 1. Global seismic load path in a multi-  
storey building.  
Figure 2. Diaphragm action and collector  
force transfer mechanism.  
Force Flow Mechanism  
The intended force transfer sequence is:  
Floor mass → Diaphragm → Collectors/drag members → Vertical LFRS → Foundation → Supporting  
soil  
In Zone VI, the practical implications include:  
collector actions that exceed gravity-controlled expectations,  
diaphragm shear and chord forces are becoming design-governing in larger plans, and  
connection failures emerging before member failures if cyclic demand and overstrength are ignored.  
For senior designers, the discipline is simple in principle but demanding in execution: trace the load path early,  
design it explicitly, and protect it against construction-stage erosion.  
Graphical Interpretation: Force Amplification  
Graph 3: Increase in design base shear with seismic zone  
Graph Overview:  
X-Axis: Seismic Zones (II, III, IV, V, VI)  
Y-Axis: Relative Base Shear force  
The slope steepens markedly towards Zone VI. Structural discontinuities that may remain  
Benign in Zones II–IV, becomes structurally unacceptable in Zone VI, as the force demand rapidly  
outpaces reserve capacity.  
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Design Implications  
For Zone VI projects:  
Diaphragms must be designed as structural elements, with explicit checks for in-plane shear, chord  
forces, and collector actions.  
Collector and drag members must be designed for overstrength demand, not merely elastic force  
levels.  
Load-path clarity must be verified during peer review, and re-verified during construction to avoid  
unintended alterations.  
Preference for Dual Structural Systems  
Rationale for Dual Systems  
Zone VI strongly favours dual structural systems, typically comprising:  
Moment Resisting Frames (MRF) — providing ductility, redundancy, and deformation capacity.  
Shear Walls or Braced Frames — providing stiffness, strength, and control of global drift.  
Fig. 4: Shear Walls or Braced Frames  
Fig.3 : Moment Resisting Frames (MRF)  
This combination allows seismic demand to be shared rationally between elements with complementary  
behavioural characteristics.  
Behavioral Advantages  
Dual systems offer several critical advantages in Zone VI:  
Higher initial stiffness significantly reduces service-level and design-level drift.  
Enhanced energy dissipation, through distributed yielding in frames while walls remain largely elastic  
or moderately cracked.  
Controlled damage hierarchy, where walls attract major shear and overturning forces, while frames  
provide rotational capacity and robustness against progressive failure.  
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Comparative Graph: Drift vs Height  
Graph 4: Storey drift comparison – single frame vs dual system  
Observation:  
Dual systems exhibit lower peak drift and smoother drift profiles, which is critical in Zone VI, where drift-  
induced damage and P–Δ effects often govern design.  
Codal and Practical Implications  
In Zone VI:  
Pure frame systems frequently become drift-governed and uneconomical.  
Dual systems reduce demand on individual members and foundations.  
Early architectural–structural coordination is essential to accommodate walls or braces without post-  
design compromises.  
Torsion and Plan Irregularity Sensitivity  
Nature of Torsional Effects  
At high seismic intensities, torsional response often governs structural performance even when base shear  
capacity appears adequate.  
Primary sources of torsion include:  
Offset between the centre of mass (CM) and the centre of rigidity (CR),  
Asymmetric stiffness or strength distribution,  
Irregular diaphragm geometry or openings.  
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Amplification in Zone VI  
In Zone VI:  
Accidental torsion effects are magnified due to higher base shear.  
Edge columns and walls experience disproportionate force and deformation demand.  
Local failures at edges can propagate rapidly, triggering partial or global collapse.  
Graph: Edge vs Central Element Demand  
Graph 5: Torsional amplification of edge elements  
Design Implications  
Zone VI designs require:  
Rigorous 3D dynamic analysis rather than simplified planar models.  
Explicit torsional amplification checks.  
Preferably symmetrical plan layouts, or deliberate provision of strong torsional resistance through walls  
or outriggers.  
Soft-Storey Vulnerability  
Behavioural Mechanism  
Soft storeys arise due to:  
Open Ground-Storey Parking,  
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The Reason Engineers Treat OGS as a High-Risk Configuration:  
Aspect  
Upper Storeys  
Present  
Ground Storey  
Absent  
Infill walls  
Lateral stiffness  
Drift demand  
Damage risk  
High  
Very low  
Very high  
Severe  
Low  
Moderate  
Floating Columns:  
Design Precautions  
Avoid floating columns in high seismic zones  
If unavoidable:  
o
o
o
o
o
Use deep transfer girders  
Perform 3D dynamic analysis  
Ensure strong-column weak-beam philosophy  
Provide special ductile detailing  
Check progressive collapse potential  
SUDDEN DISCONTINUITY IN WALLS OR STIFFNESS.  
Good Seismic Practice  
Maintain continuity of walls from foundation to roof  
Avoid abrupt stiffness change between adjacent storeys  
If unavoidable:  
o
o
o
Provide strong transfer systems  
Perform dynamic analysis  
Ensure capacity-based design of columns and beams at the discontinuity level  
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Drift Concentration (Graphical Representation)  
This figure demonstrates localisation of deformation at a single level—a hallmark of soft-storey failure under  
extreme seismic demand.  
Graph 6: Inter-storey drift ratio profile (soft-storey effect)  
(Shown as a bar chart with a 1.20% drift spike at the first storey.)  
Design Mandate  
In Zone VI:  
Soft storeys should be eliminated, not merely strengthened.  
If unavoidable, strong mitigation measures (walls, braces, dampers) are mandatory.  
Performance-based evaluation becomes highly relevant for safety verification.  
Foundations and Soil Effects  
Governing Role of Foundations  
High base shear and overturning moments in Zone VI elevate foundation behaviour from a secondary check to  
a governing design criterion.  
Critical effects include:  
Bearing pressure reversals,  
Uplift of footings or piles,  
Liquefaction-induced settlements,  
Soil–structure interaction (SSI).  
Graph 7: Bearing pressure distribution under seismic overturning  
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The shaded blue region indicates zones where compressive bearing stresses are mobilised, while the shaded  
red region represents the uplift (tension) zone, where soil–foundation contact is lost, and no bearing pressure  
is transferred  
Graph 5 represents the contact stress distribution beneath a shallow footing subjected to a significant  
seismic overturning moment, typically arising from lateral inertia forces acting at the superstructure mass  
centre. Under such conditions, the footing response transitions from uniform gravity-controlled bearing  
to moment-dominated contact behaviour, characterized by:  
Progressive stress concentration on the compression side, and  
Partial or complete loss of contact on the tension side (uplift).  
Why the Bearing Pressure Is Non-Uniform  
In seismic overturning:  
The vertical load Vremains approximately constant,  
The overturning moment M(increases) sharply, and  
The eccentricity e=M/V(Increases).  
When:  
B
e >  
6
(where BIs footing width is, soil cannot resist tension, leading to:  
Zero contact pressure beyond the compression limit, and  
Redistribution of stresses over a reduced effective contact width.  
Fig. 5. Insight: Overturning vs Bearing  
The above insight reflects this behaviour by showing:  
High compressive stresses near the leading edge,  
Stepwise reduction in pressure moving toward the trailing edge,  
Complete loss of pressure in the uplift zone.  
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Implications for Seismic Zone VI  
Within the framework of Seismic Zone VI, long-standing assumptions governing foundation response are  
rendered fundamentally inadequate. Shallow foundations, particularly those supporting lightly loaded, vertically  
irregular, or dynamically sensitive superstructures, may no longer be controlled by conventional bearing capacity  
criteria alone. Instead, uplift, partial contact loss, and cyclic rocking behaviour can emerge as governing response  
mechanisms under severe ground motion.  
Similarly, while pile foundations are frequently adopted as a preferred solution in high-seismic regions, their  
performance in Zone VI demands explicit consideration of cyclic soil–pile interaction effects. These include  
progressive degradation of shaft resistance under repeated load reversals, stiffness softening of surrounding soil,  
and unfavourable pile-group interaction phenomena under dynamic excitation. In this seismic regime,  
liquefaction assessment can no longer be treated as a conditional or project-specific exercise; it becomes a  
mandatory design prerequisite. Both liquefaction triggering and post-liquefaction performance—encompassing  
residual strength, settlement potential, and lateral deformation capacity—must be explicitly evaluated and  
incorporated into the foundation design philosophy.  
Engineering Inference  
For projects located within Seismic Zone VI, several engineering inferences are unavoidable. Foremost among  
these is the necessity of an integrated structural–geotechnical design approach. Independent or sequential  
optimization of the superstructure and foundation system is no longer defensible, as such design practices risk  
unsafe global response under extreme seismic demand.  
While soil–structure interaction (SSI) may, in certain configurations, result in a beneficial elongation of the  
fundamental period of the structural system, this apparent reduction in force demand is frequently accompanied  
by a disproportionate increase in displacement demand, foundation rotation, and permanent deformation.  
Consequently, the seismic adequacy of the foundation system assumes a decisive role in governing overall  
system performance. In Zone VI conditions, foundation robustness transcends traditional serviceability  
considerations and directly influences the ultimate seismic survival and collapse prevention capacity of the  
superstructure.  
Overall Synthesis for Senior Designers  
Seismic Zone VI necessitates a fundamental reorientation of seismic design philosophy, shifting it from a  
predominantly member-centric exercise to a rigorously system-centric discipline. Acceptable seismic  
performance is contingent upon the establishment of clear, continuous, and traceable load paths; the deliberate  
incorporation of redundancy and ductility; strict control of structural configuration and regularity; and, critically,  
the resilience and reliability of the foundation system.  
For senior structural designers, Zone VI seismicity demands that decisive and informed strategic choices be  
made at the conceptual and planning stages of design. Reactive measures—such as indiscriminate increases in  
reinforcement ratios or sectional dimensions at advanced design stages—are inherently inadequate substitutes  
for sound early-stage decisions. In this highest seismic hazard regime, early integration of structural and  
geotechnical considerations, grounded in holistic system behaviour, emerges as the principal determinant of  
seismic reliability, damage control, and structural survival.  
Seismic Zone VI – Comprehensive Design Checklist  
A. Project Zonation and Hazard Confirmation  
Confirm that the project site is categorised under Seismic Zone VI in accordance with IS 1893 (Part1):  
2025, Annex D.  
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Ensure consistent adoption of the Zone factor Z=0.75across all stages of analysis and design  
Verify that seismic parameters are not influenced by legacy assumptions, superseded zoning maps, or  
earlier editions of the code  
Confirm correct soil classification and corresponding seismic site class in the development of the design  
response spectrum  
B. Definition of Performance Objectives  
Establish the minimum performance objective as Collapse Prevention under DBE/MCE-level seismic  
excitation  
Identify and document any enhanced performance requirements arising from functional criticality  
(essential facilities, industrial continuity, post-event operability, etc.)  
Define acceptable damage states for both structural and non-structural components.  
Ensure that the adopted performance objectives are clearly communicated and coordinated with  
architectural and MEP disciplines  
C. Selection of Structural System  
Avoid exclusive reliance on pure moment-resisting frames unless justified through stringent drift control  
and stability checks.  
Prefer adoption of dual structural systems, such as SMRFs in combination with shear walls or braced  
frames  
Ensure adequate redundancy of lateral force-resisting elements in both principal directions  
Avoid hybrid systems with incompatible stiffness characteristics unless supported by rigorous analytical  
validation  
D. Configuration Control and Structural Regularity  
Plan Regularity:  
Minimise eccentricity between the Centre of Mass (CM) and the Centre of Rigidity (CR)  
Avoid pronounced plan irregularities, including re-entrant corners and abrupt setbacks  
Where irregularities are unavoidable, provide seismic separation joints or enhanced torsional resistance  
through system design  
Vertical Regularity:  
Eliminate soft-storey and weak-storey conditions, as well as floating column arrangements  
Avoid abrupt termination of shear walls, braced frames, or other vertical lateral elements  
Maintain uninterrupted continuity of all primary lateral load-resisting components down to the foundation  
level  
E. Verification of Lateral Load Path  
Establish a clear, continuous, and traceable seismic load path from the roof diaphragm to the foundation  
Explicitly design diaphragms rather than assuming inherent rigidity  
Design collectors and drag members for seismic overstrength demands  
Verify all diaphragm-to-frame and diaphragm-to-wall connections  
Ensure that architectural openings and service penetrations do not compromise load transfer integrity  
F. Analysis and Modelling Requirements  
Develop a complete three-dimensional analytical model; two-dimensional idealisations are not acceptable  
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Represent diaphragm behaviour realistically as rigid or semi-rigid, as appropriate  
Incorporate accidental torsion in accordance with IS 1893 Clause 7.10  
Perform dynamic analysis as a minimum requirement, with Response Spectrum Analysis mandatory and  
Time History Analysis warranted  
Evaluate higher-mode participation, particularly for mid-rise and high-rise structures  
Explicitly assess second-order (P–Δ) effects and their influence on global stability  
G. Drift, Deformation, and Stability Checks  
Verify storey drift limits under governing seismic load combinations  
Identify and eliminate the concentration of drift at any single storey  
Ensure deformation compatibility between frames, shear walls, and non-structural components  
Confirm that the global stability index remains within permissible limits  
Ensure that deformation demands remain within the ductile capacity enabled by detailing provisions  
H. Member Design and Ductile Detailing (IS 13920)  
Beams  
Provide complete ductile detailing in accordance with IS 13920 Clause 6  
Ensure adequate anchorage, confinement, and reinforcement continuity within potential plastic hinge  
regions  
Columns  
Enforce the strong-column–weak-beam philosophy as stipulated in Clause 7  
Provide closely spaced transverse reinforcement in critical hinge zones  
Preclude brittle shear failure before flexural yielding  
Shear Walls:  
Design boundary elements wherever required in accordance with Clause 9  
Provide confinement reinforcement over critical wall heights  
Ensure proper detailing of coupling beams, where present, to achieve ductile behaviour  
I. Connections and Non-Structural Components  
Design beam–column joints explicitly for seismic force transfer rather than gravity-only demands  
Ensure secure anchorage of parapets, façades, tanks, equipment, and appendages  
Verify seismic compatibility of staircases, ramps, and service shafts  
Prevent unintended force transfer through non-structural elements  
J. Foundation and Soil Considerations  
Check foundations for overturning, sliding, and uplift under seismic combinations  
Allow for reversal of bearing pressures during extreme seismic excitation  
Perform liquefaction assessment where site conditions warrant  
Consider soil–structure interaction effects in global response evaluation  
For pile foundations, assess cyclic degradation, group interaction, and kinematic loading effects  
K. Construction and Quality Assurance  
Issue comprehensive seismic detailing drawings with unambiguous notes and call-outs  
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Subject Zone VI seismic designs to independent peer review  
Ensure that site-level modifications do not compromise structural configuration or seismic intent  
Implement stage-wise inspection protocols for reinforcement placement and connection detailing  
Maintain stringent workmanship quality, particularly within confinement and plastic hinge regions  
CONCLUSIONS  
The introduction of Seismic Zone VI in IS 1893 (Part 1): 2025 constitutes a definitive advancement in Indian  
seismic design philosophy, marking the first formal codal recognition of regions subjected to extreme seismic  
hazard within the national framework. The adoption of a zone factor Z=0.75signifies a level of seismic demand  
that substantially exceeds that envisaged under earlier zoning schemes and, by implication, compels a  
fundamental re-examination of long-standing design assumptions, analytical simplifications, and performance  
expectations.  
Through a controlled and consistent comparative evaluation, this study demonstrates that the transition from  
Zone V to Zone VI results in an increase of design base shear exceeding twofold for an identical structural  
configuration. This escalation is not merely numerical in nature. Rather, it triggers a cascade of consequential  
effects that permeate the entire structural response, influencing member force demands, inter-storey drift  
characteristics, second order (P–Δ) stability behaviour, and foundation performance. Structural systems that may  
remain marginally compliant or performance-acceptable under Zone V seismic action are shown to become drift-  
governed or instability-controlled when subjected to the enhanced demand prescribed for Zone VI.  
The analysis unequivocally establishes that Zone VI should not be interpreted as a nominal extension of Zone  
V, but instead as a qualitatively distinct seismic regime. In such a hazard environment, satisfactory seismic  
performance is governed less by isolated member strength checks and more by holistic, system-level attributes,  
including:  
continuity and redundancy of lateral load paths,  
disciplined control of plan and vertical irregularities,  
intentional hierarchy of strength and ductility,  
robust diaphragm and collector design, and  
close integration of structural and geotechnical considerations.  
The formal recognition of Seismic Zone VI represents not merely a technical revision within IS 1893 but a  
fundamental regulatory shift in India’s seismic risk governance framework. Design practice in this highest  
hazard category can no longer be treated as an extension of conventional force-based procedures; it necessitates  
explicit policy support for performance-based design, mandatory enforcement of configuration control, and  
stricter regulatory oversight of detailing, construction quality, and peer review for critical and large-scale  
projects. The transition to a collapse-prevention-oriented design philosophy places a corresponding obligation  
on code-making bodies, approving authorities, and professional institutions to strengthen compliance  
mechanisms, upgrade capacity within regulatory agencies, and institutionalize independent design audits in Zone  
VI regions. Future regulatory evolution must therefore be underpinned by region-specific seismic hazard  
characterization, codal validation through full-scale and numerical studies, and continuous updating of Indian  
design standards to reflect observed and simulated Zone VI-level ground motions. Only through such an  
integrated alignment of engineering practice, codal frameworks, and regulatory enforcement can the objectives  
of life safety, infrastructure resilience, and societal risk reduction in India’s most severe seismic environments  
be credibly achieved.  
REFERENCES  
1. Bureau of Indian Standards (BIS) (2025). IS 1893 (Part 1): 2025 – Criteria for Earthquake Resistant  
Design of Structures: General Provisions and Buildings. New Delhi, India.  
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2. Bureau of Indian Standards (BIS) (2016). IS 13920: 2016 – Ductile Detailing of Reinforced Concrete  
Structures Subjected to Seismic Forces. New Delhi, India.  
3. Bureau of Indian Standards (BIS) (2000; reaffirmed). IS 456: 2000 – Plain and Reinforced Concrete  
– Code of Practice. New Delhi, India.  
4. NICEE, IIT Kanpur. Earthquake Resistant Design Concepts and Indian Seismic Codes. Kanpur, India.  
5. Chopra, A.K. (2017). Dynamics of Structures: Theory and Applications to Earthquake Engineering. 5th  
Edition, Pearson Education.  
6. Jain, S.K., Murty, C.V.R., and Arlekar, J.N. (2001). Lessons Learnt from the Bhuj Earthquake of  
January 26, 2001. IIT Kanpur.  
7. FEMA 356 (2000). Prestandard and Commentary for the Seismic Rehabilitation of Buildings. Federal  
Emergency Management Agency, USA.  
8. ASCE/SEI 7-22 (2022). Minimum Design Loads and Associated Criteria for Buildings and Other  
Structures. American Society of Civil Engineers, USA.  
9. Boore, D.M. and Atkinson, G.M. (2008). Ground-motion prediction equations for PGA, PGV, and 5%-  
damped PSA. Earthquake Spectra, 24(1), 99–138.  
10. Post-earthquake reconnaissance reports and technical summaries associated with the Sikkim (2011)  
and Nepal (2015) earthquakes, as used in comparative interpretation of damage patterns and design  
implications.  
www.ijltemas.in  
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