Enhancing the Seismic Performance of Flat Slab Buildings: Comparative Evaluation of Conventional Structural Strengthening Systems

Abstract

This study investigates the seismic performance of reinforced concrete flat slab buildings strengthened with conventional structural elements, including drop panels, edge beams, shear walls, and coupled shear walls. Unlike previous works that examined these elements independently, this research provides an integrated comparative evaluation of several common strengthening approaches under identical modeling and seismic loading conditions, offering clear guidance for practical design optimization. A comparative finite element analysis was conducted using ETABS v20 in accordance with ACI 318-19 and ASCE 7-22 seismic design provisions. Five ten-story building models were developed to assess key response parameters such as story displacement, inter-story drift, column axial forces, diaphragm deformation, and punching shear resistance under gravity and earthquake loading. Results reveal that models incorporating coupled shear walls achieve the greatest improved seismic performance, with up to 50% reduction in story displacement compared to other configurations, while also minimizing column over-compression and lateral drift. Drop panels alone showed a localized improvement in punching resistance, but their global impact on lateral stiffness was limited. However, the combination of drop panels and edge beams produced a synergistic effect, significantly enhancing overall stiffness and controlling drift. Coupled shear walls efficiently redirected lateral forces away from critical slab–column joints, thereby mitigating the risk of punching shear failure. These findings offer practical guidance for structural engineers seeking to optimize the seismic design of flat slab buildings, emphasizing the importance of integrated strengthening strategies in achieving both stiffness and ductility in seismic regions. The findings underline the significance of systematically evaluating conventional strengthening techniques within a unified modeling framework, offering engineers practical insights for improving the seismic behavior of flat slab buildings at the early stage of design.

1. Introduction

Rapid urbanization and increasing population density in developing countries have intensified the demand for mid-rise structures that efficiently utilize limited land while providing both functional and aesthetic architectural solutions. Among various structural systems, flat plate structures, where slabs directly transfer loads to columns without intermediate beams, have become increasingly popular due to their architectural flexibility, reduced floor heights, cost-effectiveness, and ease of construction. However, these advantages are counterbalanced by significant structural challenges in seismically active regions, where earthquakes remain one of the most destructive natural hazards. Between 1998 and 2017, earthquakes caused nearly 750,000 fatalities and displaced more than 125 million people worldwide [1]. According to the European Commission for Humanitarian Aid, every dollar spent on disaster preparedness can prevent approximately four dollars in potential losses, emphasizing the need for improving the seismic performance of building systems [2].

Despite these architectural advantages, flat slab buildings exhibit inherent structural vulnerabilities in seismic zones. The absence of deep beams reduces global lateral stiffness, leading to increased inter-story drifts, greater slab deformation, and higher risks of punching shear failure at slab–column connections. Consequently, flat slab structures are generally more flexible and less capable of dissipating seismic energy compared to traditional moment-resisting or shear wall–frame systems. These weaknesses pose significant challenges for structural engineers seeking to ensure both safety and economy in earthquake-prone regions.

Numerous research efforts have explored conventional methods to enhance the seismic performance of flat slab buildings. Previous investigations demonstrated that adding shear walls and perimeter beams significantly improves lateral stiffness and reduces story displacements by up to 40% [3]. Similarly, drop panels have been shown to mitigate inter-story drift in large-span slabs, though their contribution diminishes for shorter spans [4]. Shear walls, on the other hand, have consistently proven to be the most effective solution for global stiffness enhancement and reduction in lateral deformation [5,6]. Comparative analyses between flat slabs, waffle slabs, and conventional beam–slab systems revealed that flat slabs are the most flexible configuration and require supplemental lateral stiffening to perform adequately under seismic excitation [7]. Similarly, an experimental investigation [8] confirmed that flat slab structures are significantly more flexible than conventional framed systems.

However, the literature findings remain inconsistent regarding the optimal strengthening configuration. Some studies concluded that combining drop panels and shear walls significantly improves both stiffness and ductility, while others emphasized that core shear walls integrated with edge beams provide superior drift control [9,10]. These differences suggest that seismic performance depends not only on the type of strengthening element but also on building geometry, height, and loading conditions.

Recent research has highlighted the significant influence of floor deformability on the seismic behavior and fragility of reinforced concrete buildings. Ruggieri et al. [11] demonstrated that negleting floor flexibility in numerical models may lead to uncoservative seismic vulnerability assessments, as the rigid-floor assumption often underestimates inter-story drifts and torsional effects, their findings show that flexible diaphragms modify the redistribution of base shear and increase the probability of exceeding damage states under strong motion. These insights are particularly relevant for flat slab structures, where the absence of deep beams further reduces in-plane stiffness and increases the risk f brittle punchong failures. Such observations reinforce the need to evaluate conventional strenghthening strategies that can enhance both the local and global seismic performance of flat slab buidings.

Recent investigations (2020–2025) have expanded this understanding by introducing coupled shear wall systems—two shear walls connected by ductile coupling beams—which demonstrate superior stiffness, enhanced energy dissipation, and improved reparability compared to isolated walls [12,13,14]. Additionally, parametric studies on punching shear behavior in flat slab–column connections showed that standard code provisions may underestimate unbalanced moment effects, potentially leading to unsafe predictions unless wall–slab interaction is explicitly modeled [15]. Moreover, experimental studies have shown that slab–column connections subjected to biaxial lateral loading have significantly lower punching shear capacity than under uniaxial loading, underscoring the heightened vulnerability of flat slab systems under realistic multi-directional earthquakes [16]. Other research findings confirm that combining drop panels and edge beams yields better drift control and torsional resistance, though the efficiency of these measures decreases with building height [17]. In addition, a recent parametric evaluation [18] of flat plate buildings designed under different seismic code provisions found that incorporating drop panels can increase ductility by over 35% compared to flat plates with edge beams or conventional beam–column systems. Finally, comprehensive reviews have emphasized that torsional irregularities remain a major weakness in flat slab structures, and that the inclusion of perimeter or core stiffening elements is crucial for ensuring adequate seismic response [19]. Recent advances in computational modeling have emphasized the integration of optimization and uncertainty quantification techniques to enhance prediction accuracy and design efficiency. Yu et al. [20] proposed a hybrid Bayesian model updating and non-dominated sorting genetic algorithm framework for the intelligent mix design of steel fiber reinforced concrete, demonstrating how probabilistic updating can improve the reliability of model-based predictions.

Although numerous studies have investigated individual strengthening techniques for improving the seismic behavior of flat slab structures, such as shear walls, drop panels, or perimeter beams, existing literature rarely addresses the combined and synergistic effects of multiple conventional strengthening systems evaluated under a unified modeling framework. Furthermore, limited attention has been devoted to assessing how integrated systems influence not only global lateral stiffness and drift control but also local mechanisms such as punching shear demand and axial force redistribution under seismic excitation.

To address these gaps, the present study provides a systematic comparative evaluation of five flat slab configurations, including isolated and combined strengthening systems, using consistent geometry, material properties, diaphragm modeling, and seismic provisions. A comprehensive comparative analysis is conducted using ETABS v20 finite element modeling for the following configurations: (i) flat plate, (ii) flat slab with drop panels, (iii) flat slab with edge beams, (iv) flat slab with shear walls, and (v) flat slab with coupled shear walls. All models are designed and analyzed in accordance with ACI 318-19 [21] and ASCE 7-22 [22] seismic provisions.

The present study addresses the following research questions: (i) how different conventional strengthening configurations modify global stiffness, torsional balance, and mass participation when geometric and material variables are held constant; (ii) how global drift control interacts with local demand redistribution in slab–column connections and vertical elements; and (iii) whether combined strengthening systems exhibit synergistic performance advantages compared with isolated interventions.

The methodological contribution of this study lies in establishing a controlled and unified evaluation framework for assessing the seismic performance of flat slab buildings strengthened using conventional structural systems. Rather than proposing new structural components or advanced nonlinear modeling techniques, the study deliberately isolates the influence of commonly adopted strengthening measures by maintaining identical geometry, material properties, diaphragm modeling, and seismic input across all configurations. This approach enables direct performance-based ranking and facilitates the identification of stiffness–demand and global–local response trade-offs that are often obscured in studies employing heterogeneous assumptions.

2. Materials and Methods

2.1. Numerical Modeling Approach

Three analytical approaches can be employed to evaluate the seismic response of flat slab systems: the Direct Design Method (DDM), the Equivalent Frame Method (EFM), and the Finite Element Method (FEM). The DDM provides an initial approximation for calculating the total static moment of a slab system. According to ACI 318-19 specifications, negative and positive bending moments are distributed across column and middle strips, making this method suitable for preliminary design but limited in its ability to capture dynamic behavior. The EFM refines this estimation by modeling each slab span as an equivalent frame. Fixed-end moments are calculated at critical joints to determine positive mid-span and negative support moments. Although more detailed than the DDM, the EFM remains a simplified analytical representation and does not fully account for complex load interactions or three-dimensional effects. In contrast, the FEM, adopted in this study and implemented using ETABS v20, offers a robust and comprehensive analysis framework. The slab and shear wall elements were discretized into finite elements connected at nodes, enabling accurate simulation of force–displacement relationships. This method was selected for its superior capacity to represent structural irregularities, realistic boundary conditions, and coupled lateral–vertical load interactions, which are essential for evaluating seismic performance.

2.2. Description of Structural Models

The selected 10-story reinforced concrete flat slab building represents a typical mid-rise commercial structure commonly encountered in moderate to high seismic regions such as the Mediterranean basin and the Middle East. The chosen plan dimensions, story heights, and material strengths were based on standard design practice and aligned with parameters reported in previous studies on flat slab seismic behavior. This selection ensures that the analysis reflects a realistic and representative structural configuration. Five structural configurations (Figure 1) were developed to evaluate the effects of different conventional structural elements that can be used as strengthening elements. Each model consisted of a 10-story reinforced concrete commercial building with a total floor area of approximately 470 m² per story. The models were created to compare the structural response under seismic loading conditions, ensuring that variations in performance were solely attributed to the introduced structural elements. All configurations (

Table 1. Structural configurations considered in the comparative study.

Model ID	Description
M1	Flat Slab + Shear Wall
M2	Flat Slab + Coupled Shear Wall
M3	Flat Slab + Shear Wall + Drop Panels
M4	Flat Slab + Shear Wall + Edge Beam
M5	Flat Slab + Shear Wall + Central Drop Panels + Edge Beam

) were designed following standard concrete design and seismic provisions to maintain consistency and reliability in the analysis. The main geometric parameters of the models are presented in

Table 2. Geometric parameters of the structural models.

Model ID	Description
Plan dimensions	25 m × 20 m
Number of stories	10
Ground floor height	4.5 m
Typical floor height	3 m
Span length (X-axis)	7 m
Span length (Y-axis)	4 m
Opening size	0.7 m × 0.7 m

. The floor slabs were modeled using shell elements with an average mesh size 0.5 m × 0.5 m, while columns, beams, and shear walls were represented by frame elements incorporating rigid end offsets to accurately capture joint behavior. The shear walls were modeled as reinforced concrete wall elements with a uniform thickness of 250 mm and a wall length of 4 m, extending continuously over the full building height. Concrete compressive strength for the walls was taken as 35 MPa, which is consistent with the column material properties. The coupling beams connecting adjacent shear walls were modeled as reinforced concrete beams with a width of 300 mm, and a depth of 600 mm. Rigid end offsets were assigned to capture realistic joint behavior between coupling beams and wall piers. Fixed-base boundary conditions were applied at the foundation level, and a 5% Rayleigh damping ratio was assigned to the first two vibration modes, which is consistent with ASCE 7-22 recommendations. Material properties followed ACI 318-19 provisions with linear-elastic assumptions and cracked-section stiffness modifiers. Lateral loads were applied according to the Response Spectrum Method and the mass source included self-weight and superimposed dead loads, and lateral as well as gravity load combinations generated in ETABS v20. It is noted that geometric and mechanical parameters were intentionally fixed across all models to ensure a consistent basis for comparison. This approach isolates the effect of the strengthening elements and eliminates confounding variables related to geometry or material strength. While this method provides a clear assessment of relative performance, further parametric studies are recommended to extend these findings to different building heights, plan dimensions, and material grades to confirm the general applicability of the conclusions.

2.3. Material Properties

Table 3. Concrete properties.

Structural Element	f’c (MPa)	E (t/m2)
Columns and Shear Walls	35	27,805.5
Slab and Beams	30	25,742.9

and

Table 4. Steel properties.

Property	Value (MPa)
Yield Strength (fy)	420
Ultimate Strength (fu)	525

show the material properties of reinforced concrete (compressive strength f’_c and modulus of elasticity E) and deformed steel bars, respectively, used in this study. Higher compressive strength was assigned to columns and shear walls, reflecting their critical role in seismic resistance.

2.4. Loading Conditions and Seismic Analysis

Two categories of loads were applied to the models:

• Gravity loads: These included the self-weight of structural members, a superimposed dead load of 3.3 kN/m², a live load of 4.9 kN/m², and a partition load of 6.75 kN/m.
• Seismic loads: Seismic actions were determined using the Response Spectrum Method in ETABS, based on the parameters in

  Table 5. Seismic design parameters.

  Parameter	Symbol	Value
  Spectral acceleration at short period	Ss	1.20
  Spectral acceleration at 1 sec	S1	0.40
  Site coefficient	Fa	1.0
  Site coefficient	Fv	1.6
  Seismic design category	SDC	D
  Response modification factor	R	5
  Overstrength factor	Ω	2.5
  Deflection amplification factor	Cd	5

  . Modal analysis was performed to ensure that at least 90% of the total mass was captured in both horizontal directions. The response spectrum method captures peak structural responses for elastic systems under dynamic excitation and the modal analysis determines the building’s natural frequencies and dominant mode shapes.

Both ultimate limit states (ULS) and serviceability limit states (SLS) were verified using ASCE 7-22 load combinations.

2.5. Performance Evaluation Criteria

To assess and compare the seismic performance of the five flat slab building configurations, several evaluation criteria were adopted, following provisions of ASCE 7-22 [22] and ACI 318-19 [21]. These criteria ensure that results are reported against recognized code-based performance limits. The selected response parameters are treated as performance indicators rather than isolated output quantities. They provide a multi-dimensional assessment of seismic behavior, capturing global stiffness and drift control, torsional balance, force redistribution in vertical elements, diaphragm deformation, and local punching shear safety. This performance-oriented interpretation enables systematic evaluation of strengthening efficiency and highlights inherent design trade-offs between stiffness enhancement and force-demand amplification.

2.5.1. Modal Analysis

In seismic analysis, it is not necessary to consider all higher vibration modes for the superposition process. According to Section 12.9.1.1 of the ASCE 7-22 code [22], the analysis must include a sufficient number of modes to capture at least 90% of the total structural mass participation in each horizontal direction considered. The modal participation factor measures the relative contribution of each mode to the overall response.

2.5.2. Diaphragm Modeling Approach

In all numerical models, the floor diaphragms were defined as semi-rigid to capture realistic in-plane deformation and load transfer between lateral force-resisting elements. This approach accounts for the stiffness contributions of the slab geometry, material properties, and boundary conditions. The semi-rigid diaphragm option in ETABS v20 was adopted, enabling the program to automatically discretize the floor into finite shell elements that represent the actual flexural and membrane stiffness of the slab system.

The envelope load combinations were applied to evaluate the distribution of lateral forces to the vertical elements (columns and shear walls) and to verify the diaphragm deformation consistency across different structural configurations. The modeling procedure follows the recommendations of ASCE 7-22 Section 12.3.1 [22], which requires explicit evaluation of in-plane stiffness to distinguish between flexible, semi-rigid, and rigid diaphragm behavior.

2.5.3. Dynamic and Static Base-Shear Scaling

Dynamic base shear values, V_dyn, were compared with those from the Equivalent Lateral Force (ELF) static procedure, V_stat. If V_dyn ≥ 0.85 × V_stat, scaling is adequate; otherwise, results are adjusted using a scaling factor SF:

$$\mathrm{S}\mathrm{F}=0.85{\displaystyle \frac{{\mathrm{V}}_{\mathrm{stat}}}{{\mathrm{V}}_{\mathrm{dyn}}}}$$

(1)

This scale factor is applied to all dynamic results to maintain code-compliant seismic design forces.

2.5.4. Dynamic Accidental Torsion

Dynamic accidental torsion was evaluated following ASCE 7-22, Section 12.8.4.2. [22]. If the ratio of the maximum displacement at level x, δ_max, over the average of the displacements at the extreme points of the structure at level x, δ_avg exceeds 1.2, the eccentricity must be multiplied by an amplification factor A_x as follows:

$${\mathrm{A}}_{\mathrm{x}}={\left({\displaystyle \frac{{\mathsf{\delta }}_{\max }}{1.2{\mathsf{\delta }}_{\mathrm{avg}}}}\right)}^2$$

(2)

The torsional amplification factor shall not be less than 1 and is not required to exceed 3.0. This adjustment accounts for irregular mass distribution and inherent torsional effects.

2.5.5. Seismic Story Drift

The design earthquake displacement, δ_DE, shall be determined at the location of an element or component using the following equation:

$${\mathsf{\delta }}_{\mathrm{D}\mathrm{E}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{d}}{\mathsf{\delta }}_{\mathrm{e}}}{{\mathrm{I}}_{\mathrm{e}}}}$$

(3)

where C_d is the deflection amplification factor, I_e is the importance factor and δ_e is the elastic displacement computed under design earthquake forces, including the effects of accidental torsion and torsional amplification as applicable.

The design story drift, Δ, shall be computed as the difference in the design earthquake displacements at the centers of mass at the top and bottom of the story under consideration. The design story drift, Δ, shall not exceed the allowable story drift, Δ_a, which is in this case equal to 0.020h_sx, where h_sx is the story height below level x.

2.5.6. Axial Force Distribution in Columns

Axial forces in ground-floor columns were extracted under seismic load combinations to evaluate the influence of overturning moments and gravity loads. Compression control ensured no tensile forces developed, and the effects of coupled shear walls, drop panels, and edge beams were examined to quantify their role in force redistribution.

2.5.7. Punching Shear at Slab–Column Connections

The punching shear capacity, ${\mathrm{v}}_{\mathrm{c}},$ was determined in accordance with ACI 318-19, Section 22.6 [21], and supplemented by a refined empirical model [23] to account for size effect and reinforcement ratio $\rho$:

$${\mathrm{v}}_{\mathrm{c}}=\mathrm{A}{\left(100\mathsf{\rho }\right)}^{1/3}{\left({\mathrm{f}}_{\mathrm{c}}^{\prime }\right)}^{1/3}{\left(1+\mathrm{d}/\mathrm{B}\right)}^{-1/2}{\left({\mathrm{b}}_{\mathrm{l}}/{\mathrm{b}}_{\mathrm{s}}\right)}^{-1/4}$$

(4)

where A = 0.55 and B = 1000 mm, d is the effective depth of the slab, b_l and b_s are the lengths of the critical sections

3. Results

A comparative analysis was performed on the different flat slab building models using ETABS and response spectrum methods to evaluate their performance under seismic loading. The comparison focuses on several key elements, including the dynamic and static base shear ratio, minimum number of required modes, dynamic accidental torsion, seismic story drift, column axial forces, cracking analysis, seismic force distribution on diaphragms, and the comparison of interior slab-column connection shear strength. Each sub-section presents the results obtained for these elements, interprets their significance, and highlights the relative performance of the different structural configurations.

3.1. Dyamic Characteristics and Modal Behavior of the Models

Figure 2 and

Table 6. Summary of dynamic response parameters for all models.

Model	Fundamental Period (s)	Dominant Frequency (Hz)	Base Shear Ratio (Vdyn/Vstat)	Total Mass Participation (%)	Remarks
M1	2.10	0.48	3.74	91.2	Reference flat slab
M2	1.37	0.73	1.50	89.8	Coupled shear wall
M3	2.02	0.50	3.73	90.5	Drop panels
M4	1.67	0.60	3.42	92.0	Edge beams
M5	1.54	0.65	3.24	93.1	Combined system

summarize the dynamic characteristics of the five analyzed flat slab configurations, including their modal mass participation, fundamental period, and base shear ratios. Each structure exhibits distinct vibration characteristics, with every mode corresponding to a specific deformation pattern. The first mode, representing the lowest natural frequency, typically governs the global structural response and the maximum seismic demand.

Among the analyzed models, Model M2 (Flat Slab + Coupled Shear walls) demonstrates the lowest first-mode mass participation (44.32%), indicating that a smaller portion of the total mass is mobilized in this mode. This behavior reflects greater overall rigidity, as a larger fraction of the structure mass resist deformation in the fundamental vibration mode. In contrast models M1, M4, and M5 mobilize higher mass portions, signifying more flexible behavior. The consistently higher excited mass values observed in the Y direction across all models are attributed to the longer plan dimension and the corresponding increase in lateral stiffness along that axis.

The dynamic parameters for all models are summarized in Table 6. These results capture the fundamental period, dominant frequency, dynamic-to-static ratio, and total mass participation percentage, which collectively define the stiffness and overall seismic behavior of the investigated systems.

When the Equivalent Lateral Force (ELF) procedure is used, the building fundamental period is typically limited, resulting in conservative accelerations. In contrast, the modal response spectrum analysis captures the contribution of higher vibration modes, though the first mode generally dominates the response of flat slab systems.

As shown in Table 6, Model M2 exhibits the shortest fundamental period (1.37 s) and lowest base shear ration (1.50 in X and 1.27 in Y), confirming its superior stiffness and effective coupling action. The remaining models display longer fundamental periods and higher base shea ratios, indicating more flexible global behavior and the need for scaling adjustment under ASCE 7-22 Section 12.9.1.4.1 [21].

During seismic excitation, the structural response is largely governed by the mass-stiffness relationship. Longer periods correspond to more flexible systems that attract smaller seismic forces but experience larger displacements, while shorter periods represent stiffer systems with higher force demands. These results reaffirm that coupled shear walls significantly enhance global stiffness and reduce vibration periods, whereas drop panels and edge beams primarily improve local slab-column performance rather than the overall structural stiffness.

3.2. Dynamic Accidental Torsion Results

Figure 3 illustrates the calculated center of mass shift for all structural configurations, reflecting the degree of torsional irregularity under dynamic seismic loading. Inherent torsion originates from the geometric eccentricity between the center of mass (CM) and the center of rigidity (CR), while accidental torsion is introduced through a prescribed offset of the CM to account for mass distribution uncertainties, as required by ASCE 7-22 Sections 12.8.4.2 and 12.8.4.3. [22]. Among all analyzed models, Model M5 (flat slab with drop panels and edge beams) exhibits the smallest center of mass shift (1.38 m), indicating that its diaphragm experiences the lowest torsional stress. This improvement results from the combined effect of drop panels and edge beams, which enhance perimeter stiffness and improve torsional balance. Model M2 (flat slab with coupled shear walls) also shows reduced torsional response (1.50 m), confirming that coupling beams efficiently redistribute lateral forces between walls. By contrast, Models M1 and M3 show the largest torsional shifts (1.60 m), reflecting a less uniform stiffness distribution and higher diaphragm rotation tendencies.

3.3. Seismic Story Drift Results

Except for Model M2, the story drift profiles follow a parabolic trend, reaching their maximum values around the fifth story (Figure 4 and Figure 5). For Models M1, M3, M4, and M5, additional column stiffness is therefore required at mid-height to better control lateral deformation. Model M2, representing the flat slab system with coupled shear walls, demonstrates the lowest drift values in both X and Y directions, confirming its superior lateral stiffness. Conversely, the incorporation of drop panels alone shows no significant influence on drift reduction. In Model M1, the relatively high drift and displacement values result in more pronounced cracking, as load reversals cause cyclic opening and closing of cracks, leading to progressive stiffness degradation at the slab–column connections. Consequently, Model M1 is expected to experience the largest crack widths, as confirmed in later sections. For Models M1 and M3, an increase in shear wall thickness by approximately 5 cm would improve drift performance. Additionally, the presence of edge beams in Model M4 notably reduces story drift, highlighting their importance in buildings with architectural facades such as glazing systems. The combined use of drop panels and edge beams, as implemented in Model M5, provides the most effective solution, achieving a substantial reduction in drift and enhancing overall structural stability.

3.4. Column Axial Forces

The variation in axial forces of ground floor columns under seismic loading is illustrated in Figure 6. As expected, compression develops in reinforced concrete columns as gravity loads are transferred through the structure to the foundation, producing equal and opposite reactions. However, the introduction of seismic loading amplifies these axial forces, especially on the outer frames, while reducing them on the opposite side due to the overturning moment effect.

Among all models, Model M2 (Flat Slab + Coupled Shear Wall) shows the lowest increase in column axial forces, with an average rise of approximately 3 to 12% across all monitored columns (A1–D5). This behavior confirms the significant contribution of coupled shear walls in redistributing seismic demands and stabilizing the overall system.

In contrast, Model M1 (Flat Slab) and Model M3 (Flat Slab + Drop Panels) show a much higher increase, with columns A1 and D5 reaching up to 44%, while the internal columns (B1, C1, D1) record smaller increases around 6 to 13%.

Model M4 (Flat Slab + Edge Beams) and Model M5 (Flat Slab + Drop Panels + Edge Beams) exhibit the highest axial force amplification, where corner columns (A1, D5) reach 50–52%, and central columns (B1, C1) reach 8 to 12%. This indicates that although edge beams enhance slab confinement, they also channel larger portions of seismic load into the vertical elements, thus increasing compression forces.

These results emphasize that the coupled shear wall configuration (M2) remains the most effective solution for minimizing column over-compression under seismic excitation, providing a balanced lateral stiffness and improved overall structural response.

3.5. Seismic Force Distribution on Diaphragm

As shown in

Table 7. Diaphragm deflection and classification.

Model	Maximum Diaphragm Deflection (mm)	Average Inter-Story Drift of Vertical Elements
M1	0.0137	0.0135
M2	0.0053	0.0052
M3	0.0130	0.0129
M4	0.0096	0.0090
M5	0.0086	0.0080

, all diaphragms demonstrated semi-rigid behavior in accordance with ASCE 7-22 Section 12.3.1. [22]. Noticeable differences, however, were observed in their deformation capacities and force-distribution efficiency.

Model M2 (Flat Slab + Coupled Shear Walls) exhibited the smallest diaphragm deflection (0.0053 mm) and inter-story drift (0.0052 mm), confirming its superior lateral stiffness and efficient in-plane shear transfer through the coupled wall mechanism. This configuration minimizes lateral distortion and ensures uniform load redistribution to the vertical elements.

By contrast, Models M1 (Flat Slab) and M3 (Flat Slab + Drop Panels) recorded the largest diaphragm deflections 0.0137 mm and 0.0130 mm, respectively, indicating limited lateral stiffness and greater in-plane flexibility under seismic loading. Models M4 (Flat Slab + Edge Beams) and M5 (Flat Slab + Drop Panels + Edge Beams) showed intermediate performance, with deflections of approximately 0.0096 mm and 0.0086 mm, respectively. While edge beams enhance diaphragm stiffness by confining perimeter columns, they also intensify lateral-force transfer to these columns, leading to higher axial demands as previously discussed.

Overall, the results confirm that introducing coupled shear walls (Model M2) is the most effective strategy for controlling diaphragm deformation and optimizing seismic force distribution, thereby improving the global lateral performance and reducing stress concentrations in both central and perimeter columns.

3.6. Shear Strength

The comparison of punching shear ratios calculated according to ACI 318-19 provisions [20] using ETABS v20 and the proposed empirical equation (Equation (4)) is presented in Figure 7. The ratios represent the relationship between the applied punching shear demand and the corresponding punching shear capacity for each model, reflecting the relative safety margin against potential punching failure at interior slab–column connections. According to the ACI-based ETABS v20 evaluation, Model M4 (Flat Slab + Edge Beams) recorded the highest punching shear ratio (0.99), indicating the most critical condition among all configurations, whereas Model M5 (Flat Slab + Drop Panels + Edge Beams) showed the lowest ratio (0.65), suggesting improved shear resistance due to the combined stiffening effect of drop panels and edge beams. When the empirical equation was applied, all models showed slightly lower ratios, reflecting its more conservative estimation of punching capacity. The lowest value was observed in Model M3 (0.63), while the highest corresponded again to Model M4 (0.79).

Overall, the comparison highlights that incorporating drop panels and edge beams substantially enhances the punching shear resistance of flat slab systems. Meanwhile, coupled shear walls (Model M2) moderately reduce the punching demand on the slab–column joints by redistributing seismic loads away from the slab interior, yielding ratios of 0.77 (ACI) and 0.71 (empirical). The results also confirm that the proposed empirical model provides a more conservative yet realistic prediction of punching behavior, aligning well with the deformation and cracking observations reported earlier. Figure 8 further illustrates the distribution of punching shear ratios for each individual model, showing the localized effects of edge beams, drop panels, and coupled shear walls on the slab–column connection behavior.

4. Discussion

The comparative analysis of the five flat slab models provides valuable insight into how conventional structural elements influence seismic behavior, stiffness distribution, and potential failure mechanisms. The discussion below integrates the numerical results with theoretical expectations and previously reported findings to establish a comprehensive understanding of the observed structural performance. Beyond quantitative differences, the results reveal clear behavioral patterns that explain how each strengthening strategy modifies the global and local response mechanisms of the structure.

The results confirmed that introducing coupled shear walls produced the most substantial improvement in global stiffness and lateral resistance. The fundamental period of the coupled wall model (M2) decreased by nearly 50% relative to the reference structure (M1), consistent with the findings of Gong et al. [24] who demonstrated that wall coupling significantly enhances lateral rigidity. The coupled wall system transforms the lateral load path from a frame-dominated flexural mechanism into a wall-dominated shear-flexure mechanism, thereby increasing lateral stiffness and reducing deformation compatibility demands on slab–column connections. The efficiency of coupling beams in linking adjacent wall segments results in a dual benefit: increased stiffness under moderate seismic excitation and improved ductility under severe shaking due to controlled yielding of the coupling elements. This behavior reflects classical capacity-design principles, where coupling beams act as sacrificial energy-dissipation components that limit force concentration at wall boundaries and stabilize global response. In contrast, drop panels primarily contribute to local stiffness around column zones, with negligible impact on global deformation.

The observed relationship between base shear, fundamental period, and displacement behavior reinforces the stiffness–demand correlation documented in earlier studies. Models with higher stiffness (M2, M5) mobilized greater base shear forces but experienced reduced lateral displacements and story drifts. This trend reflects fundamental dynamic equilibrium principles: increased lateral stiffness shortens the natural period, shifting the structure toward higher spectral accelerations while simultaneously reducing displacement demand. This inverse relationship between stiffness and lateral displacement, as discussed by Li et al. [25] and Dupuis et al. [26], confirms that shear wall–dominated systems attract larger seismic forces yet sustain smaller overall deformations. This trend underscores a key design trade-off: stiffer systems effectively control deformation but must be detailed to accommodate higher internal force demands without premature brittle failure. Such trade-offs highlight the need for balanced design strategies that combine stiffness control with adequate ductility, especially in regions of elevated seismicity where post-elastic deformation capacity governs structural performance. The balanced performance of the combined model (M5) exemplifies an optimal compromise between stiffness enhancement and force distribution efficiency.

The introduction of edge beams and coupled shear walls substantially reduced the center of mass shift across the evaluated models, indicating a more balanced stiffness distribution. As shown in the results, Model M5, combining drop panels and edge beams, achieves the smallest center of mass shift, confirming its enhanced perimeter stiffness and improved diaphragm equilibrium. Similarly, Model M2 demonstrates reduced center of mass offset due to the efficient redistribution of lateral forces through the coupled shear walls. Similar observations were reported by Georgoussis and Mamou [27] and Rashidi et al. [28], who found that perimeter stiffening elements reduce the eccentricity between the centers of mass and rigidity, leading to a more uniform torsional response. This consistency between numerical prediction and theoretical expectation confirms the validity of the adopted modeling approach and the reliability of the observed torsional trends. These findings are particularly relevant for irregular or open-plan structures, where torsional modes may dominate the seismic response. Incorporating edge beams in such systems ensures a continuous load path and limits out-of-plane slab deformations.

While global stiffness improvements were primarily achieved through shear walls and edge beams, drop panels proved highly effective in mitigating punching shear at slab–column connections. The numerical results showed up to a 25% reduction in punching shear demand for drop-panel configurations, which aligns with experimental studies in which drop panels improved punching shear capacity [29]. However, this local enhancement did not translate into a notable increase in overall lateral stiffness. The combined configuration (M5), which integrates drop panels and edge beams, achieved the most balanced performance by simultaneously improving both local punching resistance and global lateral response. This configuration aligns with the recommendations of ACI 318-19 [21], which emphasize that punching shear resistance must be addressed in conjunction with lateral deformation control for seismic design. The synergy observed in M5 also demonstrates the advantage of distributing strengthening interventions across both vertical and horizontal load-resisting components, which enhances redundancy and robustness in the seismic response.

The findings of this study are further supported by experimental research on reinforced concrete flat slabs subjected to corner-column loss scenarios. Qian and Li [30] demonstrated that incorporating drop panels significantly increased the first peak capacity, initial stiffness, and energy dissipation capacity by up to 124.7%, 117.4%, and 85.4%, respectively, compared with specimen without drop panels. Although their work focused on progressive collapse conditions rather than seismic loading, both studies reveal a similar strengthening mechanism: drop panels enhance the local stiffness of the slab-column region, delay punching failure, and improve overall energy absorption capacity. This experimental evidence provides strong qualitative validation for the numerical observations reported in the present work.

Additionally, diaphragm behavior significantly affects load transfer: without coupled shear walls, central and perimeter columns attract higher shear forces, whereas coupled shear walls reduce shear-induced axial forces. From a practical standpoint, these findings demonstrate that combining conventional strengthening techniques can substantially enhance seismic performance without requiring advanced or costly retrofitting systems. Such integration of conventional measures provides an efficient and economically feasible approach for upgrading flat slab systems in existing buildings. The following design implications emerge from the study:

• Coupled shear walls should be prioritized in mid-rise flat slab buildings to improve global stiffness and control story drift.
• Edge beams contributed to a more uniform stiffness distribution along the building perimeter, which led to a modest reduction in the computed center-of-mass shift.
• Drop panels remain essential for local punching resistance but should be combined with lateral stiffening systems for comprehensive performance enhancement.

Overall, the combination of coupled shear walls, drop panels, and edge beams results in improved seismic performance by enhancing stiffness, controlling displacements and torsion, increasing column stability, and reducing the likelihood of shear failures in slab-column connections. Collectively, these results highlight that a rational combination of conventional strengthening strategies can achieve substantial improvements in seismic response with minimal design complexity, providing a practical foundation for future performance-based design methodologies.

Recent advances in resilience-oriented seismic design emphasize the importance of not only limiting immediate earthquake damage but also facilitating rapid post-event recovery. For example, fuse-based systems demonstrate how deliberately designed replaceable components can localize damage and simplify repair after strong shaking [31,32]. While the present study focuses on conventional strengthening measures evaluated under elastic response spectrum analysis, the underlying goal of improving overall performance and reducing demand concentrations is conceptually aligned with resilience objectives.

Although the present analysis captures the essential dynamic behavior of flat slab systems under design-level seismic loads, it does not include nonlinear material degradation or post-yield redistribution effects. Future studies should employ nonlinear time-history analyses to quantify energy dissipation and investigate progressive collapse mechanisms, and cyclic degradation under severe earthquakes. Experimental validation using large-scale structural testing would further substantiate these results and refine design recommendations for performance-based seismic codes.

5. Conclusions

This study examined the seismic response of five ten-story reinforced concrete flat slab buildings strengthened with conventional structural elements, drop panels, edge beams, shear walls, and coupled shear walls using linear response spectrum analysis in ETABS v20. The results provide a quantitative and qualitative understanding of how these measures affect stiffness, drift control, punching shear, and overall structural stability under seismic loading. The following conclusions can be drawn:

• Coupled shear walls were the most effective strengthening measure, reducing story displacements by up to 50% and achieving the lowest inter-story drifts. Their coupling action enhanced both stiffness and ductility, leading to improved seismic performance.
• Edge beams provided significant improvements in torsional control and diaphragm rigidity. They reduced displacement irregularities and ensured a more uniform lateral load distribution across the structure.
• Drop panels, while limited in their contribution to global stiffness, effectively enhanced local punching shear resistance at slab–column connections, reducing punching demand by approximately 20–25%.
• The combined configuration (drop panels + edge beams + coupled shear walls) demonstrated the most balanced performance across all evaluated parameters, simultaneously improving global stiffness and reducing local punching shear demand.
• The findings reaffirm that conventional strengthening methods, when strategically integrated, can significantly improve the seismic performance of flat slab systems without requiring advanced or high-cost retrofitting techniques.

From a design perspective, the study provides a practical framework for engineers seeking to enhance the seismic behavior of flat slab buildings in moderate to high seismic zones. The results support the prioritization of coupled shear walls and perimeter stiffening as primary measures for improving lateral stability, while drop panels serve as complementary local reinforcements for punching shear control. The present conclusions are limited to elastic response indicators derived from linear response spectrum analysis. Nonlinear material behavior, cyclic degradation, damage accumulation, and repairability metrics were not explicitly modeled. Future studies should incorporate nonlinear time-history simulations and experimental validation to evaluate ductility, energy dissipation capacity, and cumulative damage behavior, contributing to the development of performance-based design guidelines for flat slab systems in seismic regions.