ETABS

ETABS is specialised for building analysis and design. Learn grid- and storey-based modelling, rigid/semi-rigid diaphragms, shells for slabs and walls, auto wind/seismic loads, modal and response spectrum analysis, drift and irregularity checks, and concrete/steel/shear-wall design.

Multi-storey buildings have repeating floors, cores, shear walls and floor diaphragms. ETABS models them storey by storey, generating code wind and seismic loads and checking drift, irregularities and member design automatically.

Its power makes checking essential: a single wrong assumption (diaphragm, mass source, cracked sections) changes periods, forces and drifts dramatically.

An analogy

ETABS treats a building like a stack of plates (floors) connected by legs (columns and walls) — and asks how the stack sways and shakes.

Model setup

Define grids and storeys, materials, frame sections (beams, columns), shell sections (slabs, walls), draw objects on plan with similar-storey replication, assign supports, diaphragms, loads and mass source (DL + fraction of LL).

Lateral loads and analysis

Auto-generate wind and seismic loads per code, run modal analysis (enough modes for ≥ 90 % mass participation), response spectrum analysis, and scale results to code minimum base shear. Use cracked section properties for concrete (e.g. 0.7I columns, 0.35I beams).

Checks

Storey drifts, torsional irregularity (max/avg drift), soft/weak storey, mass and stiffness irregularities, P-Δ, diaphragm forces, and foundation reactions. Review mode shapes — first modes should be translational, not torsional, for a well-behaved building.

Design and advanced analysis

Concrete frame design with seismic detailing (IS 13920/ACI 318), shear-wall and core design, composite beams, steel frame design. Non-linear pushover and time-history, construction sequence and creep/shrinkage for tall buildings. Export to SAFE for slabs/foundations and to Revit for drawings.

Formulas

• Column lateral stiffness (fixed–fixed): k = 12 E I / h³
• Period (single storey): T = 2π √(m / Σk)
• Drift: Δ = V / k_storey; check Δ/h ≤ 0.004 (IS 1893)
• Response spectrum scaling: scale RSA base shear to ≥ equivalent static V_B (per code)

Worked example: Single-storey period check

Given: 4 columns 400 × 400 mm, h = 4 m, E = 25 GPa (fixed–fixed); Floor mass 200 t

Find: Storey stiffness and period.

• I = 0.4⁴/12 = 2.13 × 10⁻³ m⁴
• k = 12 × 25 × 10⁹ × 2.13 × 10⁻³/64 = 10 MN/m each → Σk = 40 MN/m
• T = 2π√(200 000/4 × 10⁷) = 0.44 s

Answer: T ≈ 0.44 s — compare with ETABS mode 1

Worked example: Drift check

Given: Storey shear 20 kN, Σk = 40 MN/m, h = 4 m

Find: Drift and ratio.

• Δ = 20 000/4 × 10⁷ = 0.5 mm
• Δ/h = 0.000125 ≪ 0.004 ✓

Answer: 0.5 mm, OK

Common mistakes

• Wrong: Using gross (uncracked) stiffness for concrete in seismic analysis. — Right: Apply code cracked-section modifiers.
• Wrong: Too few modes. — Right: Include enough modes for ≥ 90 % mass participation in each direction.

Applications

• Residential & commercial towers: RC frames with shear walls and cores.
• Seismic design: Response spectrum, irregularity checks and ductile design.
• Retrofit & assessment: Pushover analysis of existing buildings.

Key takeaways

• Model storeys, diaphragms, mass source and cracked sections correctly.
• T = 2π√(m/k); check with hand estimates.
• Check drift, irregularities and mode shapes.
• Design with seismic detailing and verify.

Going further

• Semi-rigid diaphragms: Needed for floors with large openings or long plans.
• Performance-based design: Non-linear time-history with acceptance criteria per ASCE 41/TBI.
• Construction sequence: Staged analysis avoids unrealistic gravity forces in tall buildings.