International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
LATERAL RESPONSE DIFFERENTIATION BETWEEN
GEOMETRICALLY UNIFORM AND PLAN-DISTORTED REINFORCED CONCRETE BUILDING CONFIGURATIONS
Ravi Kumar Gupta1, Mr. Ushendra Kumar2
1Master of Technology, Civil Engineering, Lucknow Institute of Technology, Lucknow, India
2Head of Department, Department of Civil Engineering, Lucknow Institute of Technology, Lucknow, India
Abstract - Rapid urbanization has led to the widespread adoption of high-rise reinforced concrete (RC) buildings with complex architectural configurations. While geometrically uniform buildings exhibit predictable seismic behavior, plan-distorted structures such as L-, T-, and Ushaped configurations introduce significant irregularities that influence their lateral response. This study investigates the effect of plan irregularity on the seismic performance of G+20 RC buildings by conducting a comparative analysis under identical structural and loading conditions. Four models rectangular (regular), L-shaped, T-shaped, and Ushaped are developed and analyzed using ETABS software in accordance with IS 1893 (Part 1): 2016 provisions for Seismic Zone III. Both equivalent static and response spectrum methods are employed to evaluate key response parameters, including base shear, storey displacement, storey drift, natural period, and overturning moment. The results indicate that plan irregularity significantly affects structural behavior, particularly in terms of torsional response and force distribution. The T-shaped configuration exhibits the highest base shear and overturning moment, whereas the rectangular model shows maximum storey displacement. Among irregular configurations, the L-shaped model demonstrates relatively better performance. All models satisfy permissible drift limits; however, irregular buildings exhibit amplified torsional effects. The findings highlight the importance of considering plan configuration during seismic design and provide practical insights for improving the safety and performance of high-rise RC structures.
Keywords: Seismic response, plan irregularity, reinforced concrete buildings, storey drift, base shear, torsional effects, response spectrum analysis, highrise structures
1. INTRODUCTION
1.1 Background
1.1.1 Urbanization and High-Rise RC Structures
Rapid urbanization, particularly in developing countries like India, has significantly increased the demand for vertical construction due to limited land availability and growing population density. Reinforced concrete (RC)
structures have emerged as the most preferred structural system for high-rise buildings because of their strength, durability, and adaptability to complex architectural forms. Multi-storey buildingssuchasG+20configurations are now common in urban landscapes, necessitating a deeper understanding of their structural behavior under lateral loads, especially seismic forces (Paulay and Priestley,1992;Chopra,2017).
1.1.2 Use of Irregular Geometries (L, T, U Shapes)
Modern architectural practices often prioritize aesthetics and functional efficiency, leading to the adoption of irregular building configurations such as L-shaped, Tshaped, and U-shaped plans. While these geometries optimizespaceutilizationandenhancevisualappeal,they introduce discontinuities in mass and stiffness distribution. Such irregularities significantly influence the seismic response of structures by inducing eccentricity betweenthecenterofmassandcenterofstiffness,thereby increasing susceptibility to torsional effects (Taranath, 2016).
1.2 Problem Statement
1.2.1
Torsion and Stress Concentration in Irregular Buildings
Irregular buildings exhibit complex structural behavior under seismic loading due to non-uniform distribution of stiffness and mass. One of the most critical issues associated with plan irregularity is torsion, which arises when lateral loads do not align with the structural stiffnessaxis.Thisleadstorotationalmotioninadditionto translational movement, increasing internal forces in structural members. Furthermore, re-entrant corners present in L-, T-, and U-shaped buildings create zones of stress concentration, making these regions highly vulnerable to damage during earthquakes (Chopra, 2017; IS1893-1,2016).
1.2.2
Lack of Predictable Seismic Behavior
Unlike geometrically uniform structures, irregular buildingsdonotfollowsimpleandpredictablepatternsof loaddistribution.Theirdynamicresponseisinfluencedby multiple interacting factors such as geometry, stiffness
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
variation, and modal coupling. This unpredictability complicates both analysis and design, often leading to conservativeassumptions orpotential underestimation of critical forces. As a result, ensuring seismic safety in such buildings remains a major challenge for structural engineers(Taranath,2016).
2. LITERATURE REVIEW
2.1 Seismic Behavior of RC Buildings
2.1.1
Lateral Forces and Response Parameters
Reinforced concrete (RC) buildings are designed to resist both gravity and lateral loads; however, seismic forces govern the design of structures in earthquake-prone regions. These forces act predominantly in the horizontal direction and induce critical structural responses such as storey displacement, inter-storey drift, base shear, and overturning moment. Storey displacement generally increases with height and is maximum at the top level, while storey drift represents relative deformation betweenadjacentfloorsandisakeyindicatorofstructural damage potential. Base shear reflects the total seismic force transferred to the foundation and is influenced by mass distribution and dynamic characteristics of the structure.TheoverallseismicperformanceofRCbuildings depends on their ability to dissipate energy and maintain stabilityundertheselateraldemands(Chopra,2017).
2.2 Structural Irregularity
2.2.1 Plan vs Vertical Irregularity (IS 1893 Classification)
Structural irregularity plays a significant role in influencingtheseismicresponseofbuildings.Accordingto IS1893(Part1):2016,irregularitiesarebroadlyclassified into plan and vertical types. Plan irregularity arises when there is asymmetry in the horizontal layout, leading to eccentricity between the center of mass and center of stiffness. This results in torsional effects and uneven distributionoflateralforces.CommonexamplesincludeLshaped, T-shaped, and U-shaped configurations with reentrantcorners.
Vertical irregularity, on the other hand, occurs due to discontinuities along the height of the structure, such as sudden changes in stiffness (soft storey), mass irregularity, or geometric discontinuity. These irregularities cause concentration of forces at specific levels, increasing the likelihood of structural damage. The presence of either type of irregularity complicates structural analysis and necessitates the use of advanced dynamic methods for accurate assessment (Bureau of IndianStandards,2016).
2.3 Previous Comparative Studies
2.3.1
Regular vs Irregular Performance
Numerous studies have compared the seismic performance of regular and irregular building configurationstounderstandtheinfluenceofgeometryon structural behavior. Regular buildings, characterized by uniform mass and stiffness distribution, tend to exhibit stableandpredictableresponsesunderseismicloading.In contrast, irregular buildings demonstrate complex dynamic behavior due to uneven stiffness and load paths, resulting in increased torsion and localized stress concentrations.
2.3.2 Reported Increase in Displacement (~50–70%)
Empirical and analytical studies have consistently reported that plan irregularity significantly amplifies lateral response parameters. In many cases, irregular configurations such as L-, T-, and U-shaped buildings exhibit 50–70% higher storey displacement compared to their regular counterparts. This increase is primarily attributed to torsional coupling and disruption in load transfer mechanisms. Additionally, irregular buildings often show higher base shear variation and drift concentration in critical zones, making them more vulnerableduringseismicevents(Taranath,2016).
2.4 Research Gaps
2.4.1
Scarcity of High-Rise Studies
Despite extensive research on seismic behavior, most studies focus on low- and mid-rise buildings, with limited attention given to high-rise structures such as G+20 configurations. The dynamic behavior of tall buildings differssignificantlydue to increased flexibility and higher mode participation, which necessitates dedicated investigation.
2.4.2
Lack of Unified Comparative Analysis
Anothermajorgapinexistingliteratureistheabsenceofa unified comparative framework. Many researchers analyzeindividualirregularconfigurationsindependently, making it difficult to establish direct performance comparisons across multiple plan geometries under identicalconditions.
2.4.3
Limited Indian Code-Based Studies
Although international research is abundant, there is relatively limited work based on Indian seismic codes, particularly IS 1893 (Part 1): 2016. Considering the regional variations in seismicity and construction practices, studies aligned with Indian standards are essential for practical design applications and reliable
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
implementationin thelocal context(PaulayandPriestley, 1992).
3. METHODOLOGY
3.1 Overview
3.1.1
Comparative Analytical Study
The present study adopts a comparative analytical approachtoevaluatethelateralresponseofgeometrically uniform and plan-distorted reinforced concrete (RC) buildings under seismic loading. The methodology is structuredtoensurethatallbuildingmodelsareanalyzed under identical geometric, material, and loading conditionssothatthevariationinresultscanbeattributed solely to differences in plan configuration. This approach enablesaclearandunbiasedassessmentofhowstructural irregularityinfluencesseismicbehavior.
3.1.2
Software: ETABS
Thestructuralmodelingandanalysisarecarriedoutusing ETABS (Extended Three-Dimensional Analysis of Building Systems), a widely used software for the analysis and design of multi-storey buildings. It provides advanced capabilities for modeling complex geometries, applying loads,andperformingbothstaticanddynamicanalysesin accordancewithrelevantdesigncodes.
3.2 Structural Models
3.2.1 Description of Building Configurations
Four different G+20 RC building models are developed to represent both regular and irregular plan configurations commonly observed in modern construction. Each model is designed with identical structural parameters to maintainconsistencyincomparison.
3.3 Building Specifications
3.3.1 Geometric Details of Structure
All models are considered as G+20 storey structures to represent typical high-rise buildings. The total height of the building is 63.5 meters, with uniform storey heights throughout the structure. Structural elements such as beams, columns, and slabs are designed with consistent dimensionsacrossallmodels.Thisuniformityensuresthat differences in results arise due to plan geometry rather thandimensionalvariations.
3.4 Material Properties
3.4.1 Concrete and Steel Grades
The material properties selected for the study reflect standardconstruction practices.Concrete of gradeM25 is
used for all structural components, providing adequate compressive strength for high-rise construction. Reinforcement steel of grade Fe500 is adopted to ensure sufficienttensilestrengthandductility.Theuseofuniform material properties across all models ensures a fair basis forcomparison.
3.5 Loading Conditions
3.5.1
Applied Loads on Structure
The loading conditions are defined in accordance with Indian Standard provisions. Dead loads are calculated based on self-weight and other permanent structural components as per IS 875. A live load of 3 kN/m² is appliedtorepresenttypicaloccupancyconditions.Seismic loads are considered as per IS 1893:2016 to evaluate the response of the structure under earthquake forces. These standardizedloadingconditionsensurerealisticandcodecompliantanalysis.
3.6 Seismic Parameters
3.6.1
Design Seismic Inputs
The seismic analysis is performed for Zone III, correspondingto a zonefactor(Z)of 0.16. An importance factor of 1.2 is considered to account for the functional significanceofthebuilding.Theresponsereductionfactor istakenas5,assumingaSpecialMomentResistingFrame (SMRF)systemwithadequateductility.Theseparameters are selected in accordance with IS 1893 provisions to simulaterealisticseismicconditions.
3.7 Analysis Methods
3.7.1 Equivalent Static Method
The Equivalent Static Method is used to estimate seismic forces by converting dynamic effects into equivalent lateral static loads applied at different storey levels. This methodissuitableforpreliminaryanalysisandprovidesa simplifiedrepresentationofseismicresponse.
3.7.2 Response Spectrum Method
The Response Spectrum Method is employed for a more accuratedynamic analysis.Itconsiders multiple modes of vibration and captures the influence of structural properties such as mass and stiffness distribution. This method is particularly suitable for high-rise and irregular buildings,wheredynamiceffectsplayasignificantrole.
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4. RESULTS
4.1 Base Shear Comparison
4.1.1 Variation and Percentage Difference
The base shear values obtained from seismic analysis indicate a clear variation among the different plan configurations. The T-shaped model (M3) exhibits the highest baseshear,followed bytheL-shaped (M2)and Ushaped (M4) models, while the rectangular model (M1) showsthelowestvalues.Thisvariationisprimarilydueto differences in stiffness distribution and dynamic characteristics introduced by plan irregularity. A simplified comparison of peak base shear values is presentedbelow.
Table-1: Base Shear Comparison
Model
4.2 Natural Time Period
4.2.1 Comparative Behavior
The natural time period reflects the flexibility of the structure. The rectangular model (M1) shows the highest fundamental time period, indicating greater flexibility compared to irregular configurations. In contrast, irregularmodelsexhibitslightlylowertimeperiodsdueto stiffnessvariationsintroducedbytheirgeometry.
Table-2: Natural Time Period
5. CONCLUSION
This study presents a comprehensive comparative evaluation of the seismic response of geometrically uniform and plan-distorted reinforced concrete (RC) building configurations. Four G+20 models rectangular, L-shaped, T-shaped, and U-shaped were analyzed under identicalconditionstoisolatetheeffectofplanirregularity on lateral response. The results demonstrate that structural configuration plays a significant role in governingseismicperformance.
The rectangular model, due to its uniform distribution of mass and stiffness, exhibited the highest storey displacement and drift, indicating greater flexibility. However,itsbehaviorremainedpredictableandfreefrom significant torsional effects. In contrast, irregular configurations showed comparatively lower displacement but experienced pronounced torsional response due to eccentricity between the center of mass and stiffness. Among all models, the T-shaped configuration was identified as the most critical, exhibiting the highest base shearandoverturningmoment,makingitmorevulnerable underseismic loading.The L-shapedmodel demonstrated relatively better performance among irregular configurations, while the U-shaped model showed moderatebehavior.
All models satisfied the permissible drift limits as per codalprovisions,indicatingoverallstructuralsafetyunder theconsideredloadingconditions.However,theamplified torsional effects in irregular buildings highlight potential risks that are not fully reflected through displacementbased parameters alone. The study emphasizes the importance of considering plan geometry in seismic design and suggests that symmetrical configurations are generally more reliable for high-rise buildings in seismic regions.
5.1. Future Scope
The present study is limited to linear elastic analysis; therefore, future research can incorporate nonlinear time history analysis to capture inelastic behavior and damage mechanisms more accurately. The inclusion of soilstructure interaction effects can further enhance the realism of the analysis. Additionally, studies can be extended to higher seismic zones (IV and V) to evaluate theperformanceofirregularbuildingsundermoresevere conditions.Theeffectofdifferentstructuralsystems,such asshearwalls,bracings,andoutriggersystems,canalsobe investigated to improve seismic resistance. Furthermore, optimization techniques and performance-based design approaches can be explored to develop safer and more efficient irregular structures. Experimental validation and real-time case studies would provide valuable support to analyticalfindings.
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
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