Structural Modelling using Etabs 2018

This is a comprehensive structural modeling and design challenge for a G+7 residential building in Seismic Zone V, with an emphasis on integrating architectural functionality and seismic performance. Let’s break this down step-by-step, from conceptual framing to ETABS modeling and analysis verification, as per IS 1893:2016 and IS 13920:2016.

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🏗️ Step 1: Conceptual Framing Plan Layout

📌 Inputs from Architectural Drawings:

• Floor Plan (repeated for 7 floors): includes brick wall layout and usage (for live load estimates)

• Section through Staircase & Lift Core: defines vertical circulation and potential shear wall zone

• Ground Floor Parking Layout: defines clear zones—columns/shear walls cannot obstruct vehicle movement

🔍 Initial Design Strategy:

✅ Column Placement:

• Start with column grid shown in floor plan

• Adjust column positions in parking level to ensure:

  • No obstructions at entry/exit ramps

  • Avoid columns in drive aisles

• Ensure symmetry (or near-symmetry) to reduce torsion

• Use minimum eccentricity in column layout w.r.t. building mass center

✅ Shear Wall Placement:

• Place shear walls in:

  • Stair core

  • Lift shaft walls

  • End walls or side walls (if architecturally acceptable)

• Extend from foundation to mumty level

• Use C-shaped or L-shaped walls at stair/lift core for added stiffness

✅ Beam Layout:

• Align beams with wall lines where possible

• Connect all columns with beams for rigid frame action

• Consider beam-free areas over parking where flat slab or transfer beams may be used

✅ Slab Type:

• Use waffle slab or conventional RCC slab, depending on span and architectural needs

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📏 Step 2: Load Calculations

🧱 Dead Load:

• AAC Block Walls (assumed 150 mm thick): ~3.5 to 4 kN/m

• Floor Finish: 75 mm tiles → ~1.8 kN/m²

• RCC slab (150 mm): 3.75 kN/m²

🚶 Live Load: as per IS 875 Part 2

• Residential: 2.0 kN/m²

• Corridors/staircase: 3.0 kN/m²

• Parking: 2.5–5.0 kN/m² depending on type

🛢️ Other Loads:

• Water Tank: 20 KL = 200 kN → Apply point load on roof slab over staircase

• Lift Machine Room: Apply extra UDL/point loads at mumty level

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🧠 Step 3: Model in ETABS

3.1 Define Geometry:

• Create floor levels (Plinth + 7 Storeys + Mumty)

• Define gridlines as per architectural plan

3.2 Define Materials:

• Concrete: M30 (or M35 if preferred)

• Steel: Fe500

• AAC block: user-defined material for wall loading

3.3 Define Load Patterns:

• DEAD

• LIVE

• WALL LOAD

• FINISHES

• SEISMIC_X

• SEISMIC_Y

3.4 Assign Section Properties:

• Beams: 300×450 mm (adjust later)

• Columns: 600×600 mm or 750×750 mm (adjust based on axial load)

• Shear Walls: 250–300 mm thickness (core walls, external L/C walls)

• Slab: 150 mm thick

3.5 Load Assignments:

• Use uniform line loads for walls

• Assign area loads for finishes/live loads

• Use shell area loads for water tank/lift room as additional superimposed dead loads

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🧪 Step 4: Response Spectrum Analysis (RSA)

4.1 Define Seismic Parameters (IS 1893:2016 – Zone V):

• Zone factor (Z) = 0.36

• Importance Factor (I) = 1.0 (residential)

• Response Reduction Factor (R) = 5 (SMRF)

• Soil Type: Site-specific (assume Medium - Type II)

• Damping: 5%

4.2 Load Combinations:

Use auto-generated combinations with RSA:

• DL + LL

• DL + EQx

• DL – EQx

• DL + EQy

• DL – EQy

• DL + LL + EQx...

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🧮 Step 5: Check Code Compliance & Design Output

✔ Inter-storey Drift:

• Check that inter-storey drift ≤ 0.004h (h = storey height)

✔ Irregularities:

• Plan irregularity: Check eccentricity between center of mass and stiffness < 0.15

• Vertical irregularity:

  • Check for sudden stiffness/strength reduction in vertical elements

  • Ensure consistent column/shear wall layout floor to floor

✔ Shear Wall Requirements:

• Ensure shear wall area ≥ 0.0025 of plan area per floor

• Shear wall height-to-length ratio within limits

✔ Column Axial Load Ratio:

• Confirm P/fckAg < 0.4–0.5 (for compression-controlled columns)

• Adjust column sizes where axial demand is high

✔ Moment of Inertia:

• Use effective moment of inertia:

  • Beams: I_eff = 0.35 I_gross

  • Columns: I_eff = 0.7 I_gross

  • Shear walls: as per stiffness modifiers in IS 1893

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📐 Step 6: Refine Model

• Resize elements based on ETABS outputs (deflections, forces, utilization ratios)

• Add or shift shear walls if torsional irregularity or drift too high

• Avoid placing heavy walls asymmetrically

• Rerun analysis each time and iterate until IS code compliance is met

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🧾 Final Deliverables

✅ Structural Model Checks:

• ✅ All code checks (IS 456, IS 13920, IS 1893) passed

• ✅ Acceptable drift, stiffness irregularities

• ✅ All elements within design limits

• ✅ Proper shear wall placement & ductile detailing

✅ Drawings/Exports (from ETABS):

• Plan & elevation views of beams/columns/walls

• Reinforcement detailing (AutoCAD or ETABS export)

• ETABS .EDB model file with saved load combinations and design results