MIDAS Gen Building Analysis: Modeling, Wind, Seismic, and Steel-Concrete Design

MIDAS Gen Building Analysis: Modeling, Wind, Seismic, and Steel-Concrete Design

I started using MIDAS Gen about six years ago when our firm took on a project in Southeast Asia where the local engineers required it. Coming from ETABS, the story-based modeling approach felt different but intuitive. Now I use it regularly for building projects in Asia and the Middle East. Let me walk you through the complete workflow from model creation to steel and concrete design.

Project Setup

Creating a New Project

1. File > New Project
2. Set unit system: Metric (kN, m, mm) or Imperial (kip, ft, in)
3. Set design code:
   • Steel: AISC 360, Eurocode 3, IS 800, KS, GB
   • Concrete: ACI 318, Eurocode 2, IS 456, KS, GB
4. Set building type: Office, residential, hospital (affects live load reduction)

Story Definition

1. Model > Building > Story
2. Define stories:
   • Basement: -3.5m
   • Ground Floor**: 0.0m
   • Level 1: +3.5m
   • Level 2: +7.0m
   • Level 3: +10.5m
   • Roof: +14.0m
3. Set story height for each level
4. Stories control:
   • Floor mass calculation
   • Wind load distribution
   • Seismic load distribution
   • Story shear and drift output

Modeling

Grid and Nodes

1. Model > Grid
2. Set grid lines:
   • X-direction: 0, 5, 10, 15, 20m
   • Y-direction: 0, 6, 12, 18m
   • Z-direction: Per story definition
3. Nodes are automatically created at grid intersections
4. Add additional nodes as needed: Model > Node > Create

Member Creation

1. Model > Element > Beam
2. Click two nodes to create a beam element
3. Or use Quick Draw:
   • Model > Element > Quick Beam
   • Click grid lines to auto-place beams
4. For columns: Model > Element > Column
   • Click a node and the column extends to the story above

Section Assignment

1. Model > Property > Section
2. Add sections:
   • Steel: W-shapes (AISC), H-beams (JIS/KS), custom
   • Concrete: Rectangular (b × h), circular (Ø), custom
3. Assign to elements:
   • Select elements
   • Model > Property > Assign Section

Material Assignment

1. Model > Property > Material
2. Add materials:
   • Steel A992: fy = 345 MPa, E = 200,000 MPa
   • Concrete C30: f'c = 30 MPa, E = 25,000 MPa
   • Rebar Grade 420: fy = 420 MPa
3. Assign to elements

Supports

1. Model > Boundary > Support
2. Select base nodes
3. Set support type:
   • Fixed: All 6 DOF restrained
   • Pinned: 3 translations restrained
   • Spring: Elastic with specified stiffness

Loading

Self-Weight

1. Load > Self-Weight
2. Set direction: Z = -1 (downward)
3. MIDAS Gen calculates self-weight from section and material automatically

Floor Loads

1. Load > Floor Load
2. Define floor load cases:
   • SDL: 5 kN/m² (superimposed dead)
   • LIVE: 2.5 kN/m² (office live load)
3. Select floor area (closed polygon of beams)
4. MIDAS Gen distributes floor load to supporting beams:
   • One-way: Tributary to two beams
   • Two-way: Tributary to four beams (based on aspect ratio)

Beam Loads

1. Load > Beam Load
2. Set:
   • Type: Uniform or trapezoidal
   • Direction: Global Z (downward)
   • Value: e.g., 10 kN/m
3. Select beams to apply

Wind Load (Automatic)

1. Load > Wind Load
2. Set per ASCE 7:
   • Basic wind speed (V): e.g., 45 m/s
   • Exposure category: B, C, or D
   • Building category: II (standard)
   • Wind direction: X or Y
3. MIDAS Gen automatically:
   • Calculates wind pressure at each story height
   • Applies to the building face
   • Distributes to floor levels
   • Creates wind load cases (WX, WY)

Seismic Load (Automatic)

1. Load > Seismic Load
2. Set per ASCE 7:
   • SDS: 0.6g
   • SD1: 0.3g
   • Site class: D
   • Response modification (R): 5 (special steel frame)
   • Importance factor (I): 1.0
   • Seismic weight: Dead + 0.25 × Live
3. MIDAS Gen automatically:
   • Calculates seismic mass per story
   • Calculates base shear: V = Cs × W
   • Distributes lateral force per story: Fx = V × wx × hx^k / Σ(wi × hi^k)
   • Includes accidental eccentricity (5% of building dimension)
   • Creates seismic load cases (SX, SY)

Load Combinations

1. Load > Load Combinations
2. Auto-generate per code:
   • Strength (LRFD): 1.4D, 1.2D+1.6L, 1.2D+1.0W+0.5L, etc.
   • Service (ASD): D+L, D+W, D+0.75L+0.75W
   • Seismic: 1.2D+1.0E+0.5L, 0.9D+1.0E
3. Or create manually

Analysis

Running Analysis

1. Analysis > Run Analysis
2. MIDAS Gen performs:
   • Stiffness matrix assembly
   • Load vector generation
   • Displacement calculation
   • Member force calculation
3. Check analysis results for warnings

P-Delta Analysis

1. Analysis > Analysis Control
2. Enable P-Delta:
   • P-Delta: Include geometric nonlinearity
   • Iterations: 3-5 (typically sufficient)
3. P-Delta increases:
   • Lateral displacements (secondary moments)
   • Column moments (gravity × displacement)
   • Story drift

Modal Analysis

1. Analysis > Analysis Control > Eigenvalue
2. Set number of modes: 15-30
3. Check mass participation:
   • Cumulative ≥ 90% in X and Y
   • If not: increase modes

Response Spectrum Analysis

1. Load > Response Spectrum
2. Define spectrum per ASCE 7
3. Create response spectrum load case
4. Set:
   • Direction: X or Y
   • Modal combination: CQC (recommended)
   • Damping: 5%
5. Run analysis
6. Check base shear scaling (≥ 85% of static)

Post-Processing

Story Shear

1. Results > Story Shear
2. View shear per story:
   • Seismic X: Story shear distribution
   • Seismic Y: Story shear distribution
   • Wind X: Story shear distribution
3. Verify shear decreases with height (triangular distribution)

Story Drift

1. Results > Story Drift
2. View drift per story:
   • Drift ratio: Δ/h per story
   • Allowable drift: Per code (h/400 for wind, h/50 for seismic)
3. If drift exceeds limit:
   • Increase stiffness (larger sections, more bracing)
   • Add shear walls
   • Reduce building height

Center of Mass and Rigidity

1. Results > Center of Mass/Rigidity
2. View per story:
   • Center of mass (CM): Where the mass is concentrated
   • Center of rigidity (CR): Where the lateral stiffness is concentrated
   • Eccentricity: Distance between CM and CR
3. Large eccentricity causes torsion:
   • Redistribute mass or stiffness
   • Add accidental eccentricity (5%)

Member Forces

1. Results > Beam Forces
2. View:
   • Axial (N): Tension/compression
   • Shear (V): Shear in two directions
   • Moment (M): Bending about two axes
   • Torsion (T): Twisting moment
3. View as diagrams or contour plots

Reactions

1. Results > Reactions
2. View support reactions:
   • Vertical: For foundation design
   • Horizontal: For shear key design
   • Moments: For fixed foundation design
3. Export to foundation design software

Steel Design

Setting Up Steel Design

1. Design > Steel Design > Design Code
2. Select: AISC 360-16 (LRFD or ASD)
3. Set parameters:
   • FY: 345 MPa (A992)
   • Unbraced length: Per member or per group
   • K factor: Calculated or manual
4. Design > Steel Design > Run Design
5. Results:
   • Unity ratio: Demand/capacity (≤ 1.0)
   • Critical combination: Which combo governs
   • Critical check: Which failure mode controls

Steel Optimization

1. Design > Steel Design > Auto Design
2. Set section list for each member group:
   • Columns: W12×45 to W12×96
   • Beams: W16×26 to W24×62
3. MIDAS Gen selects the lightest section that passes all checks
4. Re-analyze with new sections
5. Re-design to verify

Concrete Design

Setting Up Concrete Design

1. Design > Concrete Design > Design Code
2. Select: ACI 318-19
3. Set parameters:
   • FC: 30 MPa
   • FY: 420 MPa (main bars)
   • FYH: 420 MPa (stirrups)
   • Clear cover: 40mm
4. Design > Concrete Design > Run Design

Beam Design Output

| Parameter | Value | |-----------|-------| | Section | 300 × 600mm | | Mu (top) | 250 kN·m | | As (top) | 1450 mm² → 4 #22 | | As (bottom) | 980 mm² → 3 #20 | | Stirrups | #10 @ 150mm | | Capacity ratio | 0.85 |

Column Design Output

| Parameter | Value | |-----------|-------| | Section | 500 × 500mm | | Pu | 1800 kN | | Mux | 120 kN·m | | Muy | 80 kN·m | | Reinforcement | 8 #25 (1.96%) | | Ties | #10 @ 100/300mm | | Capacity ratio | 0.78 |

Wrapping Up

MIDAS Gen's automatic lateral load generation is what saves me the most time — wind and seismic loads per multiple codes, generated automatically from the building geometry. The story-based output (shear, drift, center of mass) is exactly what you need for code compliance checks. My tip: always verify the auto-generated wind loads against a hand calculation for the first few projects until you build confidence in the defaults.