Analysis of Flat Slab in ETABS and Design of Flat Slab & Footing in Safe

Abstract

A flat slab is a reinforced conc rete floor system that is supported directly by columns without the use of beams or girders. Unlike conventional systems where loads travel from slab to beam to column, a flat slab transfers loads directly to the supporting columns. This design is primarily used to achieve minimum construction depth, reduce storey height, and provide architectural flexibility for open floor plans in buildings like malls, hotels, and IT parks.

Introduction

This study evaluates the seismic performance of a G+20 reinforced concrete flat slab building using ETABS for structural analysis and SAFE for slab and foundation design. Flat slab systems are increasingly used in modern construction because they eliminate beams, providing architectural flexibility, reduced floor height, faster construction, and lower costs. However, their lack of beams makes them more vulnerable to larger lateral displacements, storey drifts, and punching shear during earthquakes, requiring careful seismic analysis.

The study models and analyzes a 20-storey reinforced concrete flat slab building under earthquake loading using Response Spectrum Analysis and Nonlinear Pushover Analysis in accordance with IS 456:2000 and IS 1893:2016. Structural performance is evaluated through base shear, storey displacement, storey drift, and flat slab design, while SAFE is used to verify the adequacy of isolated footings.

Literature Review

Previous studies indicate that:

• Flat slab and flat beam systems reduce structural depth, material consumption, and construction cost while improving architectural flexibility.
• The primary limitations of flat slabs are reduced lateral stiffness and punching shear vulnerability.
• Post-tensioned flat slab systems and Mivan construction improve seismic performance through increased stiffness and reduced lateral displacement.
• Flat slab systems are generally most economical for spans between 5 m and 9 m, although appropriate design optimization is essential for taller buildings.

Objectives

The study aims to:

• Evaluate changes in seismic demand between Seismic Zone II and Zone III.
• Compare the lateral performance of conventional RCC and Mivan structural systems.
• Verify compliance with the IS 1893 storey drift limit (0.004 × storey height).
• Assess isolated footing design using SAFE.
• Compare the structural efficiency of flat slab buildings using different structural systems.

Methodology

A G+20 RCC flat slab building was modeled in ETABS using:

• Concrete: M40
• Steel: Fe500
• Column size: 800 × 800 mm
• Slab thickness: 200 mm
• Drop panels: 3.0 × 3.0 × 0.3 m
• Storey height: 4 m

Applied loads included:

• Floor finish: 1 kN/m²
• Live load: 4 kN/m²
• Wall load: 13.7 kN/m
• Parapet load: 3.6 kN/m

The building was analyzed under gravity and seismic loads according to Indian Standards, and SAFE was used to design flat slabs and isolated footings.

Results

1. Storey Displacement

Storey displacement increased progressively from the base to the roof, with maximum displacement at the top floor.

Key observations include:

• The Mivan structural system experienced substantially lower displacement than the conventional RCC building in both X and Y directions.
• Displacements increased from Zone II to Zone III because of greater seismic intensity.
• The higher stiffness of the Mivan system significantly improved lateral stability.

For example, at the roof level:

• RCC (Zone III):
  • X-direction: 207.9 mm
  • Y-direction: 186.4 mm
• Mivan (Zone III):
  • X-direction: 26.9 mm
  • Y-direction: 32.8 mm

2. Storey Drift

Maximum storey drift occurred in the middle storeys and decreased toward the base and roof.

Results showed:

• Conventional RCC buildings experienced higher drift than Mivan buildings.
• All models satisfied the IS 1893 allowable drift limit.
• Increased seismic intensity in Zone III produced larger drifts than Zone II.

The improved lateral stiffness of the Mivan system resulted in better control of earthquake-induced deformation.

3. Flat Slab Design

SAFE design results confirmed that all flat slab sections satisfied flexural and shear design requirements.

The maximum reinforcement demand increased under Zone III loading, particularly for the Mivan structure, but every slab achieved an "OK" design status, confirming compliance with IS 456:2000.

4. Base Shear

Base shear increased significantly with seismic intensity.

For the X-direction:

• Conventional RCC
  • Zone II: 1724 kN
  • Zone III: 2709 kN
• Mivan
  • Zone II: 4271 kN
  • Zone III: 6719 kN

The increase from Zone II to Zone III was approximately 57% for both structural systems.

Although the Mivan building developed higher base shear because of its greater stiffness, it exhibited much smaller lateral displacements and drifts, indicating superior seismic resistance.

Conclusion

This chapter presents the results obtained from ETABS and SAFE analysis & design of four 80 m high flat slab RCC buildings with different structural systems and seismic zones. The results include base shear, storey displacement, and isolated footing design data. The conclusions are drawn based on the comparative study of conventional frame system and RCC wall system in Zone II and Zone III as per IS 1893:2002 • Base shear increases by 57.1% for conventional frame and 57.3% for Mivan system when seismic zone changes from II to III. This is directly proportional to the increase in zone factor Z from 0.10 to 0.16 as per IS 1893:2002. Therefore, seismic zone is a critical parameter in design of tall buildings. • The RCC wall system reduces top storey displacement by 86.9% in Zone II and 87.1% in Zone III compared to conventional frame system in X-direction. In Y-direction, reduction is 83.3% and 82.4% respectively. The continuous RCC walls around the perimeter provide significantly higher lateral stiffness and reduce torsional effects. • All models satisfy the storey drift limit of 0.004 times storey height (80000mm X 0.004 = 320 mm) as per IS 1893:2002 Cl. 7.11.1. The maximum average drift observed is 1207.90 for M2, which is 35% below the permissible limit. Mivan models show lesser drift values of 32.78, indicating excellent serviceability performance. • Isolated footings of size 3000 mm × 3000 mm × 1200 mm with SBC 500 kN/m² are found safe for all four models. SAFE analysis confirms that bearing pressure and punching shear remain within limits even for G+20 Mivan model in Zone III. This indicates that isolated footing is viable up to 80 m height for soil with moderate bearing capacity. • Conventional flat slab frame without shear wall exceeds comfortable serviceability limits for displacement in Zone III. The top displacement of 207.90 mm may cause non-structural damage and occupant discomfort. RCC wall system keeps displacement below 33 mm even in Zone III, making it suitable for high-rise construction. • For 80 m high flat slab buildings in Zone II and Zone III, the RCC wall system is recommended over conventional frame system. It provides superior seismic performance, better drift control, and improved occupant comfort with manageable increase in base shear. Isolated footings are adequate provided soil SBC is ? 500 kN/m².

References

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Copyright

Copyright © 2026 Raghu K J, Dr. K. Shreenivas Reddy Shahapur, Madhukaran . This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.