Journal of the Civil Engineering Forum.
September 2025, 11.
:285-296 DOI 10.
22146/jcef.
Available Online at https://jurnal.
id/v3/jcef/issue/archive Seismic Performance of Reinforced Concrete Buildings in Darchula.
Nepal:
A Fragility-Based Approach Birendra Kumar Bohara1* .
Benbokhari Abdellatif2 .
Jyoti Deupa1 .
Nirmal Mani Joshi3 .
Sangam Jagari3 1 Assistant Professor.
School of Engineering.
Far Western University.
Kanchanpur-10400.
NEPAL 2 Laboratoire LTPiTE.
Ecole Nationale Supyrieure des Travaux Publics.
ALGERIA 3 School of Engineering.
Far Western University.
Mahendranagar.
Kanchanpur-10400.
NEPAL *Corresponding author: birendra.
bohara@fwu.
SUBMITTED 30 April 2025 REVISED 27 June 2025 ACCEPTED 30 June 2025 ABSTRACT This research evaluated the seismic vulnerability of non-engineered reinforced concrete (RC) buildings compared with that of engineered RC structures in the Darchula region of Far-Western Nepal, an area prone to high seismic risk.
This study emphasizes the seismic performance of buildings under various loading conditions by examining construction practices and identifying structural deficiencies in RC buildings in Darchula.
Nepal.
Linear elastic and nonlinear pushover analyses are used to assess periods, mass participation, base shear, inter-story drift, capacity curves, nonlinear drift demand, and fragility curves.
Models designed according to national and international standards are compared with non-engineered buildings (S1 Ae S.
to highlight the discrepancies in seismic resilience.
The study further provides a probabilistic fragility framework to quantify damage likelihood at varying seismic demand levels.
The results show that engineered buildings exhibit significantly greater resistance to seismic forces, with greater flexibility and higher base shear capacities.
In contrast, non-engineered buildings, particularly shorter structures, are more prone to damage under moderate seismic Research indicates that ground floors in non-engineered buildings consistently exhibit the most significant inter-story drift as a result of softstory impacts, highlighting them as crucial failure points.
Fragility curves derived from spectral displacement values reveal that non-engineered buildings reach critical damage states at lower levels of seismic demand, indicating greater vulnerability.
This research underscores the importance of enforcing seismic design standards and retrofitting non-engineered buildings to improve their earthquake resilience in seismic hotspots such as Darchula.
Nepal.
These findings provide a foundation for future seismic risk reduction strategies and highlight the urgent need for improved building practices to mitigate earthquake-related damage.
KEYWORDS Engineered buildings.
Non-engineered Buildings.
Seismic vulnerability.
Pushover analysis.
Fragility curve.
Darchula A The Author.
This article is distributed under a Creative Commons Attribution-ShareAlike 4.
0 International license.
1 INTRODUCTION
Nepal is situated in the seismically active Himalayan Ae Hindukush region, where the convergence of the Indian and Eurasian tectonic plates results in a high risk of This area is notable for its intricate seismotectonic conditions, regular seismic activity, and geological instability, with some of the most catastrophic earthquakes (Chaulagain et al.
, 2.
The countryAos susceptibility is amplified by various fault systems, including the Himalayan megathrust fault, which significantly shapes its geological and topographical characteristics (Parajuli et al.
, 2.
NepalAos seismic past is marked by severe incidents such as the Nepal-Bihar earthquake .
Gorkha earthquake .
, and the Jajarkot earthquake in the western region (Poudel and Chaulagain, 2.
These disasters have caused extensive loss of life and widespread damage, exposing significant shortcomings in structural resilience, especially in non-engineered and inadequately constructed buildings (Bhagat et al.
, 2.
In Nepal, especially in western areas such as Darchula.
Achham, and Dadeldhura, evaluating seismic risk is es- sential for protecting lives and shaping policies aimed at risk reduction and structural resilience.
These regions, which are characterized by mountainous terrain and regular seismic occurrences resulting from the Himalayan megathrust fault, are vulnerable because of substandard construction practices and poor adherence to modern building regulations (Bohara, 2023.
Poudel and Chaulagain, 2.
Darchula district lies in a highly seismically active zone due to its proximity to the main Himalayan thrust fault system.
According to the recent study (Maharjan et al.
, 2.
and Indian Standard IS 1893:2016 (Bureau of Indian Standards, 2.
Darchula falls within Seismic Zone V, which is assigned a zone factor (Z) of 0.
36, the highest seismic classification, indicating potential for very severe ground shaking.
Despite NepalAos high seismic risk, there is a lack of studies focusing on the seismic vulnerability of non-engineered reinforced concrete (RC) structures in Darchula.
Limited field-based modeling and fragility assessments exist for this region, highlighting the need Journal of the Civil Engineering Forum Vol.
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for localized seismic analysis to inform risk mitigation strategies.
When both analytical and empirical methodologies are used to assess seismic vulnerability.
Nepal might benefit from the use of comparable strategies, including mechanical modeling to analyze its building inventory.
Implementing advanced techniques such as deriving fragility curves through displacement-based or capacity spectrum methods could improve the comprehension of structural behavior under seismic forces (Gondaliya et al.
, 2.
1 Objectives of the Study The primary objective of this research is to assess the seismic vulnerability of RC structures in Far-Western Nepal, with a particular focus on non-engineered and inadequately designed structures in the Darchula district.
To achieve this, the study pursues the following specific objectives:
Evaluate the seismic vulnerability of the RC buildings in Darchula using nonlinear static analysis and fragility-based analysis.
Develop lognormal fragility curves for different types of RC building based on design standards and construction periods.
Classify the likelihood of damage in various types of RC building under seismic loading conditions.
Identify high-risk RC building types.
Figure 1 Flowchart of Seismic Vulnerability Assessment Methodology.
2 METHODOLOGY
The overall methodology adopted in this study is illustrated in Figure 1, which outlines the sequential process from field survey and model development to seismic analysis and fragility assessment.
1 Modelling of Common Construction Practices for RC Buildings Based on the surveyed data, three-story RC buildings, which are typically constructed in Darchula, are selected.
The spacing of the bays, story height, seismic property, material property, size of the columns, and story of the buildings were selected based on the observations of the buildings as shown in Table 1.
The plan and three-dimensional view of the studied buildings can be seen in Figures 2 and 3.
Eight types of models of RC buildings are used in the study, in which the beam and column sizes of the buildings are changed based on the data obtained in the surveys.
Table 2 shows the 9 models with different column beams and the area of rebar used in the columns.
Table 2 compares the SMRF structure of NBC and IS models to the other building Models S1 to S6 are OMRFs, which are generally noticed in field surveys.
The model NBC is designed based on the Nepal building code, and the IS models Figure 2 Plan of the building .
imensions in m.
are designed based on Indian standards.
Other models are based on non-engineered buildings observed in Darchula.
The typical characteristics of each building structure studied are presented in sections 2.
2 to 2.
2 Nepal Building Codes (NBC.
In 2010, the Department of Urban Development and Building Construction published further suggestions for earthquake-resistant building construction in Nepal, with the help of UNDP.
This document is based on the National Building Code (NBC) (Mahal and Kathmandu, 2.
and defines the minimum dimensions of columns needed for structures up to three stories.
Vol.
11 No.
3 (September 2.
Journal of the Civil Engineering Forum 3 Newly Revised Building COdes (RUD) The national building code for Nepal.
NBC 205:2024 (DUDBC, 2.
, is a RUD guideline for low-rise threestory buildings made of reinforced concrete without masonry infill.
The primary objective of this RUD Guideline is to provide practical details and dimensions for various elements in RC-framed and residential buildings up to three stories tall, typically constructed by owner-builders in Nepal, and for popular non-engineered construction practices to meet the basic seismic safety requirements.
Figure 3 3D view of the RC buildings.
4 Indian Standards (IS) Table 1.
Material properties, loads, and seismic parameters of the case study buildings Material properties Grade Modulus of elasticity Loads Live load on floor level Live load on roof level Finishing in roof load Weight of the wall on each floor Weight of the parapet wall in the roof Seismic factor Seismic zone according to the Indian standard Zone factor (Z) according to the Indian standard Importance factor for all models (I) Type of soil .
Response reduction factor R (SMRF) Response reduction factor R (OMRF) M20 415 MPa 36 GPa 200 MPa 3 kN/m2 5 kN/m2 1 kN/m2 2 kN/m 4 kN/m 5 Common Constructed Buildings (S1.
S2.
S3.
S4.
S5.
Table 2.
Model information for the case study RC buildings NBC Size of .
300 y300
RUD
350 y 350 300 y 300 300 y 300 300 y 300 300 y 300 230 y 230 230 y 230 250 y 230 Models Reinforcement used in 4-12 yo 4-16 yo 8-20 yo 8-16 yo 8-16 yo 4-12 yo 4-16 yo 6-12 yo 4-12 yo 6-12 yo 4-16 yo Size of .
m y 250 y 300 250 y 380 250 y 355 250 y 300 230 y 250 230 y 250 230 y 250 230 y 230 230 y 250 230 y 250 Tie bar used in The structure was designed by following Indian standard codes for seismic zones, specifically Zone V, which has high earthquake risk.
The buildingAos low height and regularity allowed seismic analysis via the seismic coefficient method (IS 1893-2.
(Bureau of Indian Standards, 2.
The columns and beams are designed according to IS 13920:1993 (Bureau of Indian Standards, 1.
for ductile detailing.
The IS 875 standard was used to calculate the dead and live load, while the IS 456-2000 standard was used for the load combinations as shown in Table 1.
This sort of structure reflects the prevailing construction techniques in Darchula.
Nepal.
Buildings are typically constructed using low-rise RC frames without masonry infill.
As urbanization increases and land prices rise, owners often add extra stories to existing buildings without planning for these additional floors or considering seismic safety.
As the height increases, high occupancy levels pose a significant risk in urban areas during an earthquake.
In this study, non-engineered RC buildings refer to RC frame structures constructed without formal structural design, typically found in rural and urban areas (Darchul.
These buildings are not designed or reviewed by qualified engineers and often fail to meet national as well as Indian seismic code requirements.
The models S1 through S6 represent typical field-observed variations in column cross-sectional dimensions and reinforcement ratios .
ee Table .
, based on detailed surveys conducted in the region.
Previous earthquakes in adjacent regions have highlighted the devastating repercussions of such structural collapse, resulting in severe loss of life and extensive property destruction (Poudel and Chaulagain, 2.
The structural and seismic parameters used in this study were derived from field data, national/international codes, and engineering best practices.
A summary of the key parameters and their justifications is presented in Table 3.
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6 Nonlinear Static Analysis Modeling ure was not explicitly represented in the nonlinear response due to software limitations and scope constraints.
This simplification is deemed acceptable for an initial seismic vulnerability assessment but should be improved upon in future studies to more accurately represent potential brittle failure modes.
Nonlinear pushover analysis is a commonly used technique in seismic vulnerability assessment to analyze the structureAos performance under lateral loads, particularly during earthquake events (Prajwal et al.
, 2.
Numerous guidelines and standards, such as Eurocode 8 .
(Tenchini et al.
, 2.
ATC-40 .
, and FEMA 356 .
(FEMA 356, 2.
emphasize the necessity to comprehend how structures react beyond their elastic limit and encourage the use of nonlinear static processes, such as pushover analysis.
Pushover nonlinear static analysis was used to assess the seismic capabilities of the nine buildings.
For nonlinear analysis in each model, the finite element method software ETABs .
was used.
To assess the seismic performance of the structure, displacement-control pushover analysis is a sophisticated nonlinear static approach (Chopra and Goel.
Dya and Oretaa, 2.
This analysis in ETABS entails applying increasing displacement or lateral displacement until the structure achieves the desired displacement.
Nonlinear modeling of the columns and beams was performed.
Plastic hinges were assigned at the ends of the beam and column.
ETABS allows the selection of predefined hinge properties based on ASCE 41 (ASCE/SEI 41-17, 2.
The analysis is made simpler by using plastic hinges, which assume that inelastic behavior only occurs in these hinges while the rest of the element stays elastic.
These hinges represent the deformation-controlled behavior .
uctile respons.
of the element, allowing for large rotations without immediate failure.
Flexural hinges are assigned near the ends of columns and beams to simulate the inelastic rotation that occurs during seismic events.
For columns, a coupled degree of freedom (P OeM OeM ) hinge is defined, accounting for the interaction between axial forces (P ) and moments (M ) in two directions.
For beams, an uncoupled (M .
degree of freedom hinge is used, considering only the moment about the strong axis .
ending in one directio.
Beam-column joints were modeled as rigid connections in ETABS.
The study does not explicitly model joint shear failure.
however, the effects of poor joint detailing, common in non-engineered RC buildings, were indirectly accounted for through reduced material strength and reinforcement assumptions.
In ETABS, plastic hinges can be defined via the auto hinge option, which simplifies the modeling process by automatically assigning hinge properties based on predefined parameters from codes such as ASCE/SEI 41-17 .
In this research, a nonlinear static pushover analysis was carried out using plastic hinges to represent flexural behavior in line with FEMA-356 guidelines.
Checks on shear capacity were performed to confirm that structural elements satisfied the minimum shear strength criteria under lateral forces.
Nevertheless, shear fail288 Table 3.
Summary of the modeling parameters and their corresponding sources Parameter Column .
Column size .
Reinforcement ratios Beam size Seismic zone Soil type Load combinations Seismic Value/Range Used 230 mm y 230 mm to 300 mm y 300 mm Ou300 mm y 300 mm (NBC), 320 mm y 320 mm (RUD) 7% to 1.
230 mm y 300 mm to 300 mm y 450 mm Zone V Medium to soft As per IS 456 Linear and nonlinear Source/Justification Based on field measurements of existing buildings in Darchula Nepal National Building Code (NBC 205:2.
, and Indian standard IS 456:2016 Field data and code Common practices observed and code Indian Standard IS 1893 Based on site reconnaissance Standard code-based design practice As per the code and 3 RESULTS AND DISCUSSION 1 Analysis of Survey Data on Building Practices and Seismic Vulnerability Most of the buildings in rural areas, such as Darchula, fall under the non-engineered category, including adobe, stone, block, and mud structures.
These areas are highly vulnerable to seismic events, such as the 2015 Gorkha earthquake (Varum et al.
, 2.
and the Jajarkot earthquake (Poudel and Chaulagain, 2.
, which severely impacted traditionally built homes.
However, a large portion of the population continues to rely on non-engineered buildings, especially in remote and economically disadvantaged areas.
Buildings with more than 1,000 sq.
The plinth area has more than three stories and a structural span greater than 5 m, must follow NBC 101 to NBC 114 and NBC 206 to NBC 207 (DUDBC, 1.
Smaller buildings, under 1,000 sq.
, fewer than three stories, and spans under 4.
5 meters, follow thumb rules in NBC 201, 202, and 205, which provide provisions for beam and column dimensions and structural detailing (DUDBC, 2.
To increase the earthquake resistance of buildings.
NBC 205:2070 introduced revisions, including a recommen- Vol.
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dation that the columnAos minimum size should be 300 mm y 300 mm for structures up to three stories, with column-to-column spans of less than 4.
5 meters.
Recently, the new Ready to Use (RUD) code NBC 205: 2024 was introduced for low-rise reinforced concrete buildings for up to 3-story regular buildings.
This code introduced a new column size of 320 y 320 mm .
inimum Siz.
and 4-16 yo 4-12 yo minimum number of reinforcements in the buildings (DUDBC, 2.
Despite clear guidelines, compliance remains low.
The public views the permit process as costly and timeconsuming due to fees and taxes.
A study of column sizes in 238 buildings revealed that only 43% of the structures followed the recommended column size of 300 mm y 300 mm, whereas approximately 48% of featured columns are smaller than this standard, and 18% of buildings that are taller than two stories have undersized columns, which is due to a lack of awareness of building laws as shown in Figure 4.
A survey revealed that many buildings were unregistered or built in compliance with established codes and bylaws during the last six years, highlighting gaps in public knowledge and enforcement.
Journal of the Civil Engineering Forum The data reveals that some buildings consist of small column sections .
mm y 230 m.
with less than a minimum of 0.
8% rebar and almost the same beam section, violating the code rule.
This problem is observed in old and new buildings of up to 5 stories, in which some columns are too small and contain less steel than required by the building codes.
Insufficient stirrup spacing and weak confinement of the RC section are also observed in the RC buildings in the Mahakali The beams and columns lack the proper reinforcement during observation and against the MRT Nepal building codes.
The detailed requirements for lap-splice, beam, and columns, and the number of main reinforcements are under the requirements.
As per the code, the number of longitudinal rebars is limited to more than 16 mm diameter bars ranging from 4 Ae 8 bars, but it is observed that 10 mm and 12 mm bars are only provided in columns, as shown in Figure 5.
Moreover, 6 mm diameter bars are used as a stirrup with a spacing 15 m or more.
Some step-back buildings, buildings with open lower stories and infilled brick masonry wall upper stories, resulted in soft stories as shown in Figures 6.
About 11% are soft-story buildings, and during several earthquakes, soft-story failure causes building collapse (Gautam et al.
, 2015.
Bohara et al.
, 2.
The soft-story buildingAos columns are 230 mm y 230 mm and 230 mm y 300 mm in size, having a minimum amount of steel, indicating that the structures are seismically dangerous.
In beams, columns, and eventually foundations, the lateral load is not efficiently transferred due to the discontinuity of the load path during earthquakes shown in Figures 6.
Figure 4 Relationship between the .
Percentage of RC building data and the number of stories, .
cross-section of columns based on the number of stories.
In RC buildings, the connection between beams and columns is the most critical part of the design, especially in earthquake-prone areas (Bohara et al.
, 2.
If these joints are not properly designed to be strong and flexible, the entire building could collapse during a quake.
In the 2015 Gorkha earthquake, columnbeam joint failure was observed, which was due to insufficient transverse reinforcement and detail (Gautam et al.
, 2.
In the Mahakali Municipality, many buildings have poorly constructed column-beam joints, where the column and beam joints are not cast together, which weakens the connection.
Figure 5 .
No proper anchorage in column-beam joints and stirrup spacing .
, .
, and .
poor column-beam joint spacing.
Journal of the Civil Engineering Forum Figure 6 .
Poor column-beam joints, a soft story.
poor beam column, soft story, floating columns.
Load accumulation in the upper floor.
floating columns.
In hilly areas such as Darchula, there are notable difficulties concerning the quality and accessibility of construction materials, which frequently result in substandard building practices.
Whereas excessive filler materials such as sand are incorporated because of the use of incorrectly shaped and oversized coarse aggregates, generally ranging from 40 mm to 70 mm, as illustrated in Figures 7.
Additionally, improper mixing techniques, unsuitable water-cement ratios, and inadequate vibration during the construction process lead to problems such as segregation and bleeding of concrete, especially in columns, further jeopardizing structural stability shown in Figures 7.
As a result of the honeycombing effects observed in the beams, columns, and slabs, the primary reinforcements are easily visible, which leads to corrosion.
Improper covers are observed in the columns, and owing to this visible patching, plaster is applied to the columns to cover them.
As seen in Figure 7.
, nearly all of the columnAos height .
ore than 5 .
is cast in a single stage, causing various types of defects in the columns.
As seen in Figure 7.
, the cement used in construction is three months old, demonstrating the inferior quality of the materials.
2 Numerical Analysis 1 Model Mass Participation Ratio.
Periods, and Design Base Shear Table 4 shows the key seismic parameters across multiple models (S1 to S.
compared with codes such as NepalAos NBC.
RUD, and IS 1893.
According to Indian Standard 1893 (Part I): 2016, the data derived from lin290 Vol.
11 No.
3 (September 2.
Figure 7 .
An improper aggregate size is used.
Cement outflow from the lowest section of the formwork.
A cement lump has been found at the site.
ear elastic analysis establishes seismic design forces.
A structureAos global seismic requirements are determined by its fundamental period, mode shape, and design base shear (Bohara, 2.
The first mode periods range from 0.
602 to 1.
144 sec across different models, showing variability based on structural characteristics such as the height, stiffness, and mass distribution of the structures.
Buildings with periods of approximately 75 sec Ae 0.
79 sec (NBC.
RUD, and IS) might represent typically designed or engineered buildings.
Whereas models with longer periods (S4 at 1.
144 se.
could indicate non-engineered buildings and more flexible structures, it is due to their low stiffness, resulting from small column sections, poor reinforcement detailing, absence of infill walls.
Buildings with longer periods, such as models S4-S6, might require more robust lateral reinforcement to control deflection.
Table 4 also shows that all the models achieve high mass participation ratios close to 0.
90 in the first and second modes, which is favorable per IS 1893.
For seismic design.
IS 1893 recommends ensuring mass participation ratios above 0.
85 in the first few modes to capture an adequate dynamic response under seismic loading.
The high mass participation values here indicate efficient dynamic response and resonance control in the models.
The base shear values were high in models S1.
S2, and S3, whereas lower values were observed in models NBC and IS.
In linear analysis, it is difficult to understand the strength capacity of buildings.
While linear analysis is suitable for preliminary design and compliance with basic code requirements, pushover analysis provides a detailed understanding of the inelastic behavior and true strength of the structure.
Vol.
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Journal of the Civil Engineering Forum Table 4.
Summary of the modeling parameters and their corresponding sources Seismic parameters Periods of the building models .
n Second.
Mass participation ratio Base shear .
N) of the building Ratio of base shear to seismic weight .
ead load reduced live loa.
N) Mode/axis 1st mode 2nd mode 3rd mode 1st mode 2nd mode X-axis Y-axis X-axis Y-axis NBC 2 Capacity Curve and Inter-Story Drift (ISD) The building capacity curve or pushover curve is an essential tool in seismic analysis, offering insight into how a building responds to lateral loads, particularly in an earthquake scenario.
It is essentially a forcedisplacement plot, where the buildingAos resistance to lateral loads is plotted against a characteristic lateral displacement, usually the displacement of the roof or other significant points on the structure (Ali and Sanghai, 2021.
Bohara and Saha, 2.
This curve typically passes through several key stages.
Elastic Range: Initially, the structure behaves elastically, meaning that the deformation is directly proportional to the applied Yield Point: At this point, the structure begins to exhibit nonlinear behavior, with some elements, such as beams and columns, forming plastic hinges.
Postyield behavior: After yielding, the structure still resists additional loads, but its stiffness gradually decreases as more plastic hinges form and deformation localizes in certain structural elements.
Ultimate capacity: This is the maximum strength the structure can achieve before experiencing significant degradation in load-carrying capacity, possibly leading to structural By transforming the base shear into spectral acceleration (Sa ) and the roof displacement into spectral displacement (Sd ), the base shear and displacement are converted into spectral coordinates.
These changes make it possible to compare the capacity curve and the earthquake response spectrum directly, giving an indication of the seismic demand on the structure at various intensities.
For each X and Y loading direction, the findings of the nonlinear pushover analysis for the examined building models can be seen in Figure 8.
In this study, the base shear force of the model RUD building was greater than that of the other models in both the X and Y directions of loading.
The results show that the capacity strengths of the RUD and IS models were greater than those of the NBC model.
Compared with engineered buildings, other non-engineered buildings have lower strength capacities.
The graph illustrates the interstory drift (ISD%) across different story levels for mul-
RUD
tiple building models.
All the models consistently exhibit a greater ISD at the ground story than at the upper levels, indicating that the ground floor experiences the most lateral deformation under seismic loads, likely due to increased flexibility or a soft story effect.
The IS drift rate of change is regular and consistent across the upper stories for all the models, suggesting a uniform distribution of stiffness and controlled deformation throughout the structures above ground level.
This consistency across models indicates that all buildings maintain a similar drift pattern, although specific ISD values vary by model, as shown in Figure 9.
Figure 8 Capacity curves of models in the .
X-direction and .
Y-directio.
An improper aggregate size is used.
Cement outflow from the lowest section of the formwork.
A cement lump has been found at the site.
Journal of the Civil Engineering Forum Vol.
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where Sdy and Sdu represent the yielding and ultimate spectral displacement of the idealized capacity curve.
Fragility curves are created by connecting seismic demand .
uch as PGA or Sd ) with structural capacity derived from the capacity curve.
The fragility curve is expressed by the following equation:
P [DS Ou DSi |D] = ln A .
i Figure 9 Inter-story drift curves of the models in the .
Xdirection and .
Y-direction.
3 Fragility Analysis In an earthquake, fragility curves show the likelihood that a building may reach or surpass specific damage levels according to its capacity and seismic demand (Gautam et al.
, 2021.
Chapagain and Chaulagain, 2.
These curves are derived via probabilistic models that consider both the variability in the building capacity and the intensity of ground shaking (Ahmad et al.
The risk-UE methodology provides a standardized approach for generating fragility curves based on extensive empirical and analytical data.
HAZUS FEMA
classifies damage into four categories: slight damage, minor cracking, and little or no permanent deformation.
Moderate damage involves cracking and minor yielding.
extensive damage includes plastic hinge formation and element failure.
complete damage indicates collapse or imminent failure of critical components.
The damage state thresholds are determined as a function of the spectral displacement (Sd ) yielding and ultimate values of the pushover curve after being transformed into acceleration and displacement response spectra (ADRS.
Sd1 = 0.
7 Sdy .
Sd2 = Sdy .
Sd3 = Sdy 0.
25 (Sdu Oe Sdy ) .
Sd4 = Sdu where P [DS Ou DSi |D] is the probability that the damage state DSi is exceeded for a given demand D.
D is the seismic demand .
ypically represented by displacement or drif.
Si is the median capacity of the building corresponding to damage state DSi .
where i is the dispersion parameter .
tandard deviatio.
, which represents the uncertainty in the buildingAos capacity and demand and is the normal distribution function.
Each fragility curve corresponds to a different damage state .
light, moderate, extensive, and complet.
, and the fragility function calculates the likelihood that a building will surpass a specific damage condition given a specific seismic demand.
The standard deviation (E) for each damage state was calculated by fitting a normal cumulative distribution function (CDF) to the probabilities provided in Barbat (Barbat et al.
, 2.
Table 5 displays spectral displacement values (Sd1AeSd4 ) that indicate average points for various damage states.
optimization process reduced squared errors by comparing observed probabilities to normal CDF values, adjusting E iteratively while keeping means constant, for each structure and damage state separately.
Table 5.
Probabilities for each fragility curve to fit the data (Barbat.
Pujades and Lantada, 2.
Condition P .
P .
P .
P .
P .
P .
P .
P .
Fragility curves are constructed and presented in Figure 10 for all the damage states and structures.
The means and standard deviations of the structures are summarized in Table 6.
1 Discussion and Results of the Fragility Assessment Table 6 displays the mean and standard deviation values for each damage state, reflecting spectral displacements with a 50% exceedance probability.
Increased mean values imply enhanced seismic resistance, acting as vital measures of structural sensitivity among the buildings assessed (Liu et al.
, 2023.
Rc et al.
, 2.
The fragility curves in Figure 10, combined with the data from Table 6, highlight the significant differences in seismic performance between buildings designed according to modern codes (NBC.
RUD.
IS) and non- Vol.
11 No.
3 (September 2.
Journal of the Civil Engineering Forum engineered structures (S1 Ae S.
Structures designed according to standards (NBC.
RUD, and IS) present significantly higher mean values across all damage states than non-engineered buildings do (S1 Ae S.
For example, for the complete damage state, the mean values for NBC.
RUD, and IS are 61.
5, 63.
1, and 72.
1, respectively, whereas for noncompliant structures, the values range 08 (S.
5 (S.
This highlights the enhanced seismic resilience of engineered buildings, which are better able to withstand seismic demands before sustaining significant damage (Anggraini et al.
Fauzan et al.
, 2.
Structures such as S1 and S2 present lower mean values across all damage states, indicating greater fragility.
For example, the means for moderate damage are 10.
for S1 and 17.
64 mm for S2, which are significantly lower than those of structures such as S5 and S6.
This is primarily due to their reduced flexibility, which makes them more susceptible to higher acceleration demands during seismic events.
Buildings, such as S5 and S6, show slightly improved performance, particularly for higher damage states.
For example, the mean values for complete damage are 55.
5 (S.
19 (S.
, which are greater than those of shorter structures such as S1 .
This improvement is attributed to the greater flexibility of buildings, allowing them to dissipate seismic energy more effectively.
However, they remain significantly more fragile than engineered buildings are, primarily because of inadequate detailing and insufficient lateral resistance.
Table 6.
Means and standard deviations of each damage state for all the buildings Structure NBC
RUD
Slight Moderate Extensive Complete Figure 10 Fragility curves of the studied structures via the RISK-UE LM2 method: .
slight damage, .
moderate damage, .
extensive damage, and .
complete damage.
Journal of the Civil Engineering Forum The standard deviation provides insight into the variability of the damage states.
Structures with lower standard deviations .
S1: 0.
006522 for extensive damag.
show less dispersion in their fragility behavior, suggesting a more consistent response under seismic loading.
Conversely, higher standard deviations .
IS: 0.
008291 for extensive damag.
reflect greater uncertainty, potentially due to differences in design assumptions and construction practices.
For all the structures, the mean values increase progressively from slight to complete damage, reflecting the expected increase in the spectral displacement required to reach higher damage states.
However, non-engineered buildings consistently present lower mean values for each damage state than their engineered counterparts do (Gautam et al.
, 2.
This underscores their vulnerability, as they reach critical damage thresholds at much lower levels of seismic demand.
The fragility curves (Figure .
illustrate a significant difference in seismic behavior between engineered (NBC.
RUD.
IS) and non-engineered (S1 Ae S.
reinforced concrete buildings.
Engineered structures display higher spectral displacement thresholds across all damage levels, with the IS model demonstrating the highest degree of resilience.
On the other hand, non-engineered buildings, especially S1 and S3, exhibit the lowest thresholds for catastrophic damage due to inadequate construction practices and poor detailing.
Taller non-engineered structures (S5.
perform somewhat better owing to their increased flexibility.
These findings underscore the vital impact of construction quality, detailing, and adherence to codes, highlighting the pressing necessity for retrofitting and strict regulation in Darchula.
Nepal.
Although fragility curves were developed using a lognormal distribution fitted to spectral displacement values from pushover analysis, this approach simplifies the representation of uncertainty (Nemutlu et al.
The dispersion parameter () alone does not fully account for variability in real building performance.
Several sources of uncertainty influence the results.
First, due to the lack of site-specific ground motion records for Darchula, generalized spectral shapes were used, introducing epistemic uncertainty in seismic input.
Second, field observations revealed considerable variability in concrete quality, reinforcement detailing, and column sizes, especially in non-engineered These contribute to aleatoric uncertainty that is not easily quantifiable.
Third, modelling simplifications such as idealized plastic hinges and symmetric layouts do not fully capture the complex behavior of actual buildings.
Finally, fitting cumulative distribution functions to damage states assumes smooth transitions that may not reflect observed fragility trends (Rc et al.
, 2.
Future studies could improve the reliability of fragility Vol.
11 No.
3 (September 2.
analysis by employing incremental dynamic analysis (IDA), probabilistic Monte Carlo simulations, and sensitivity analyses to better capture input variability and modeling uncertainty.
4 CONCLUSION
The survey and analysis of building practices in rural areas of Darchula.
Nepal, reveal significant seismic vulnerabilities in residential and public structures.
Most buildings, including recently constructed ones, fail to comply with NepalAos NBC requirements.
Key deficiencies include undersized columns, inadequate reinforcement, poor-quality materials, improper beamcolumn joint construction, and widespread use of nonengineered practices.
The analysis of seismic performance for buildings designed according to various standards (NBC.
RUD.
IS) and non-engineered structures (S1 Ae S.
reveals significant differences in their Both linear and nonlinear pushover analyses indicate that structures with natural periods between 0.
75 and 0.
79 seconds exhibit appropriate engineering practices, whereas those with extended periods, like 1.
144 seconds for model S4, demonstrate increased flexibility and a demand for enhanced lateral support.
Non-engineered models S1.
S2, and S3 recorded higher base shear values, while engineered models such as NBC and IS showed comparatively lower The nonlinear pushover outcomes indicated that RUD and IS models achieved the greatest base shear capacities, confirming that structures built in compliance with codes exhibit superior strength and resilience in comparison to non-engineered buildings, which are more vulnerable during seismic events.
analysis of inter-story drift (ISD%) revealed that the ground floors consistently undergo the highest levels of deformation due to potential soft-story impacts, while upper floors displayed a more consistent distribution of stiffness.
The fragility assessment further highlighted those non-engineered buildings (S1 Ae S.
are considerably more prone to damage, particularly shorter structures like S1, because of their limited flexibility and insufficient energy dissipation.
Additionally, non-engineered models (S5.
showed considerable fragility primarily due to inadequate seismic detailing.
Conversely, engineered buildings (NBC.
RUD.
IS) exhibited greater resilience, which can be attributed to improved ductility, lateral strength, and timely reinforcement detailing as required by contemporary seismic codes.
1 Recommendation The findings highlight the urgent necessity of adhering to seismic design regulations and upgrading buildings that do not meet these standards to reduce risks and improve safety in areas susceptible to earthquakes.
Vol.
11 No.
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Structures that comply with current codes should act as models for enhancing the seismic resilience of future Considering the results, we suggest implementing compulsory seismic evaluations, reinforcing at-risk buildings, ensuring adherence to building codes, offering training for local contractors, and incorporating seismic risk considerations into city planning.
These actionable steps are vital for bolstering resilience and minimizing losses associated with earthquakes, especially in high-risk regions like Darchula, where inadequate structural conditions significantly endanger lives and infrastructure.
FUNDING
No funding agency has supported this project.
DISCLAIMER
The authors declare no conflict of interest.
REFERENCES