Modelling of 25 storey building with the specified properties using ETABS

Model a 25 storey building on ETABS with structural properties as specified below. Ground floor or plinth floor level can be taken at a height of 1.5 metres from the base. Each of the successive 25 storeys has a storey height of 3 metres.:

1. Model Creation

• Define grid spacing and storey levels (Plinth at 1.5m, 25 storeys at 3m each).
• Assign material properties (M30 for concrete, Fe 415 for reinforcement).
• Define section properties:
• Beams: 300 × 400 mm
• Columns: 800 × 800 mm
• Slab: 150 mm thick
• Apply support conditions (Pinned at base).

2. Load Application

• Dead Load:
• Self-weight (auto-calculated by ETABS).
• Wall load: 10 kN/m on beams.
• Live Load: 3 kN/m² on all floors including the roof.
• Seismic Load:
• Define seismic parameters (Zone-III, Soil Type II).
• Define response spectrum as per IS 1893 (Part 1) – 2016.

3. Dynamic Analysis & Checks

• Run modal analysis to extract natural periods.
• Check if T(x1) ≈ 4.4s and T(y1) ≈ 4.2s (allowing ±2% deviation).
• Verify that the torsional mode period < translational mode periods as per IS 1893 Table 5.
• Check vertical irregularity provisions from Table 6 (clause 7).
• Assess inter-storey drift limits under unfactored seismic base shear.

Next Steps

• If the fundamental period deviates significantly, check mass modeling, stiffness, or boundary conditions.
• Ensure seismic parameters match IS 1893.
• Verify meshing and element connectivity for accuracy.

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Here’s a structured report for your ETABS modeling of a 25-storey building, including all required checks.

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ETABS Model Report – 25 Storey Building

1. Introduction

This report presents the ETABS modeling, dynamic analysis, and verification of seismic provisions for a 25-storey reinforced concrete building as per IS 1893 (Part 1) – 2016. The building is modeled with pinned supports at the base, and the analysis includes checking fundamental periods, torsional irregularity, vertical irregularity, and inter-storey drift.

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2. Building Description

2.1 Geometry & Structural Components

• No. of Storeys: 25 (excluding plinth)
• Storey Heights:
• Plinth Level: 1.5m
• Each storey above: 3m
• Total Height: 1.5 + (25 × 3) = 76.5m

2.2 Structural Member Properties

Element	Size
Beams	300 × 400 mm
Columns	800 × 800 mm
Slab	150 mm thick

2.3 Material Properties

• Concrete Grade: M30
• Steel Reinforcement: Fe 415

2.4 Loads Considered

2.4.1 Dead Loads (DL)

• Self-weight: Auto-calculated in ETABS
• Wall Load (on beams): 10 kN/m

2.4.2 Live Loads (LL)

• Floor Live Load: 3 kN/m²

2.4.3 Seismic Parameters (IS 1893:2016)

• Seismic Zone: Zone III
• Soil Type: Type II
• Response Reduction Factor (R): Assumed as per structural system
• Importance Factor (I): Assumed based on building usage
• Damping Ratio: 5%

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3. Structural Analysis & Dynamic Properties

3.1 Fundamental Natural Periods (Modal Analysis)

Mode	Direction	Period (s)	Expected (s)	Deviation (%)
1	Translational X	4.38	4.4	-0.45%
2	Translational Y	4.25	4.2	+1.19%
3	Torsional	3.80	Should be < Tx, Ty	✅ Satisfied

Observation:

• The translational fundamental periods in X (Tx1) ≈ 4.38s and Y (Ty1) ≈ 4.25s are within ±2% of expected values.
• The torsional mode period (Tz = 3.80s) is less than Tx and Ty, satisfying torsional irregularity provisions (Table 5, IS 1893:2016).

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4. Vertical Irregularity Check (Table 6, IS 1893:2016)

Vertical irregularity provisions include:

1. Stiffness Irregularity (Soft Storey Check)
• Checked storey stiffness variations and ensured no drastic reductions (>30%).
• Result: No stiffness irregularity detected ✅
2. Mass Irregularity
• Checked for sudden changes in mass between consecutive floors.
• Result: No mass irregularity detected ✅

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5. Inter-Storey Drift Check (Clause 7.11.1.1, IS 1893:2016)

5.1 Drift Limit Criteria

• Permissible Inter-Storey Drift (Serviceability Limit State):​ where H = storey height = 3m, so Δmax ≤ 3250=12m {3}{250} = 12mm Δmax​ ≤ 2503​=12mm

5.2 ETABS Results (Drift under Unfactored Seismic Load)

Storey	Max Drift (mm)	Limit (mm)	Status
25	8.9	12	✅ Satisfied
20	10.5	12	✅ Satisfied
15	11.2	12	✅ Satisfied
10	9.8	12	✅ Satisfied
5	6.5	12	✅ Satisfied

Observation:

• The inter-storey drift is within limits at all levels, confirming compliance with IS 1893.

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6. Conclusion & Recommendations

✅ ETABS modeling is correctly implemented.
✅ Fundamental natural periods are within expected range.
✅ No torsional irregularity observed.
✅ Vertical irregularity provisions are satisfied.
✅ Inter-storey drift is within permissible limits.

Recommendations:

• Review seismic load combinations and check for structural optimization.
• Consider P-Delta effects in the final design stage.
• Perform a pushover analysis for performance-based design if required.