MIDAS Civil Bridge Analysis: Modeling, Moving Loads, and Bridge Design per AASHTO and Eurocode

MIDAS Civil Bridge Analysis: Modeling, Moving Loads, and Bridge Design per AASHTO and Eurocode

MIDAS Civil is my go-to tool for bridge analysis. I've used it on highway bridges, railway bridges, and pedestrian structures, and the moving load analysis and code-based design features are excellent. Whether you're working with AASHTO LRFD or Eurocode, the workflow is well-structured. Let me walk you through the complete bridge analysis process.

Bridge Modeling

Project Setup

1. File > New Project
2. Set units: Metric (kN, m, mm) or Imperial (kip, ft, in)
3. Set design code: AASHTO LRFD, Eurocode, IS, KS, GB
4. Set bridge type: Girder, arch, cable-stayed, suspension, or custom

Bridge Modeler (Wizard)

1. Model > Bridge Modeler > Bridge Wizard
2. Step through the wizard:
   • Bridge type: I-girder, box girder, PSC, steel composite
   • Spans: Number and lengths (e.g., 3 spans: 30m, 40m, 30m)
   • Width: Bridge deck width (e.g., 12m)
   • Girders: Number and spacing (e.g., 5 girders at 2.5m)
   • Deck: Thickness and material
   • Supports: Abutments and piers
3. MIDAS Civil generates the complete model:
   • Nodes, elements, sections, materials
   • Supports, bearings
   • Grid and geometry

Manual Modeling

For non-standard bridges:

1. Model > Node: Create nodes at key points
2. Model > Element: Create beam, truss, or plate elements
3. Model > Section: Define cross-sections
4. Model > Material: Define materials
5. Model > Boundary: Define supports and bearings

Section Properties

1. Model > Section
2. Add sections:
   • Steel I-girder: Top flange, web, bottom flange dimensions
   • PSC Box girder: Multi-cell box with top and bottom slabs
   • Composite: Steel girder + concrete deck (effective width)
   • AASHTO I-beam: Standard Type I-IV
   • Predefined: Select from international section databases
3. Calculate section properties:
   • Area (A), Moment of inertia (Iy, Iz), Torsion constant (J)
   • Section modulus (Zy, Zz), Shear area (Ay, Az)

Materials

1. Model > Material
2. Add:
   • Concrete C40: f'c = 40 MPa, Ec = 32,500 MPa, density = 25 kN/m³
   • Steel A709: fy = 345 MPa, Es = 200,000 MPa, density = 78.5 kN/m³
   • Prestressing Steel: fpu = 1860 MPa, Eps = 195,000 MPa
   • Rebar Grade 420: fy = 420 MPa

Supports and Bearings

Abutments

1. Model > Boundary > Support
2. At abutment nodes:
   • Fixed abutment: All DOF restrained (rare)
   • Expansion abutment: Translation restrained, rotation free
   • Elastomeric bearing: Spring with specified stiffness
     • Vertical: k = 500,000 kN/m
     • Horizontal: k = 50,000 kN/m

Piers

1. At pier nodes:
   • Fixed pier: All DOF restrained
   • Expansion pier: Translation restrained, rotation free
   • Elastomeric bearing: Spring support

Bearing Modeling

1. Model > Boundary > Elastic Link
2. Create bearing link:
   • Type: Rigid or elastic
   • Stiffness:
     • Vertical: 500,000 kN/m (stiff)
     • Horizontal: 50,000 kN/m (flexible)
     • Rotational: As specified by manufacturer

Loading

Dead Load

1. Load > Self-Weight: Automatic from section and material
2. Load > Beam Load: Additional dead load (wearing surface, barriers, utilities)
   • Uniform: e.g., 8 kN/m (asphalt + barriers distributed to girders)

Live Load (Moving Load)

Lane Definition

1. Load > Moving Load > Lane
2. Create lanes:
   • Lane 1: Centerline at X = 2.0m, width = 3.6m
   • Lane 2: Centerline at X = 8.0m, width = 3.6m
3. Set lane type:
   • Lane width: 3.6m (AASHTO standard)
   • Lane offset: From bridge centerline
4. Lanes define the vehicle path

Vehicle Definition

1. Load > Moving Load > Vehicle
2. Select standard vehicles:
   • AASHTO HL-93: Design truck (72 kip) + design tandem (50 kip) + lane load (9.3 kN/m)
   • AASHTO Permit: Overweight permit vehicles
   • Eurocode LM1: Tandem system (TS) + uniform load (UDL)
   • Eurocode LM2: Single axle for short spans
   • Custom: User-defined axle loads and spacings

Moving Load Case

1. Load > Moving Load > Moving Load Case
2. Set:
   • Lanes: Select active lanes
   • Vehicles: Select vehicles to analyze
   • Direction: Forward and/or backward
   • Number of positions: How many stops along the lane
3. MIDAS Civil moves the vehicle along each lane at incremental positions
4. At each position: calculates all response quantities

Wind Load

1. Load > Wind Load
2. Set per AASHTO 3.8:
   • Base wind speed: 100 km/h (typical)
   • Exposure category: Per site conditions
   • Drag coefficient: Per girder type
3. Apply to exposed surfaces

Temperature Load

1. Load > Temperature Load
2. Set:
   • Uniform temperature:
     • Rise: +30°C (per AASHTO 3.12.2)
     • Fall: -20°C
   • Temperature gradient:
     • Top hotter than bottom (per AASHTO 3.12.3)
     • Type: Positive (top hot) or negative (bottom hot)

Braking Force

1. Load > Braking Force
2. Per AASHTO 3.6.4:
   • BR: 25% of axle weights of design truck in one lane
   • Multiple lanes: 25% × (number of loaded lanes)
3. Apply in longitudinal direction

Analysis

Running Analysis

1. Analysis > Run Analysis
2. MIDAS Civil performs:
   • Static analysis for each load case
   • Moving load analysis (thousands of positions)
   • Influence line calculation
3. Check analysis log for warnings

Influence Lines

1. Results > Moving Load > Influence Line
2. Select a response quantity:
   • Reaction: At any support
   • Moment: At any section
   • Shear: At any section
   • Deflection: At any point
3. View influence line:
   • X-axis: Position of unit load along the lane
   • Y-axis: Value of response
4. Identify critical positions for vehicle placement

Moving Load Envelope

1. Results > Moving Load > Envelope
2. Create envelopes:
   • Maximum: Maximum value over all positions
   • Minimum: Minimum value over all positions
   • Absolute max: Maximum absolute value
3. Use envelopes for design:
   • Maximum positive moment
   • Maximum negative moment
   • Maximum shear
   • Maximum reaction

AASHTO LRFD Load Combinations

Strength I

1.25 × DC + 1.50 × DW + 1.75 × (LL + IM + BR + PL) + 1.00 × WA

Strength II (Permit)

1.25 × DC + 1.50 × DW + 1.35 × (LL + IM) + 1.00 × WA

Service I

1.00 × DC + 1.00 × DW + 1.00 × (LL + IM) + 1.00 × WA

Fatigue

0.75 × (LL + IM)  [single HL-93 truck]

Bridge Design

Steel Girder Design

1. Design > Steel Girder Design > AASHTO LRFD
2. Set parameters:
   • Fy: 345 MPa (girder)
   • Fc': 30 MPa (deck)
   • Composite action: Yes
   • Effective flange width: Per AASHTO 4.6.2.6
3. Design checks:
   • Flexural strength: Positive and negative moment capacity
   • Shear strength: Web shear capacity
   • Fatigue: Stress range under fatigue truck
   • Serviceability: Deflection limit (L/800)
   • Constructability: Check during construction stages

PSC Girder Design

1. Design > PSC Girder Design > AASHTO LRFD
2. Set parameters:
   • Concrete: f'c = 40 MPa (girder), f'ci = 35 MPa (at transfer)
   • Prestressing: 7-wire strand, fpu = 1860 MPa
   • Loss method: AASHTO refined or lump sum
3. Design checks:
   • Stresses at transfer: Compression and tension limits
   • Stresses at service: Compression, tension, and crack control
   • Flexural strength: Nominal moment capacity
   • Shear strength: Concrete + transverse reinforcement
   • Prestress loss: Elastic shortening, creep, shrinkage, relaxation

Deck Design

1. Design > Concrete Deck Design
2. Set:
   • Deck thickness: 225mm
   • Reinforcement: #16 @ 150mm (main), #13 @ 200mm (distribution)
3. Design checks:
   • Flexural capacity: Per AASHTO 5.7.3
   • Shear capacity: Per AASHTO 5.8.3
   • Crack control: Per AASHTO 5.7.3.4
   • Empirical design: Per AASHTO 9.7.2 (if applicable)

Post-Processing

Girder Forces

1. Results > Beam Forces
2. View:
   • Moment envelope: Maximum and minimum moment per girder
   • Shear envelope: Maximum and minimum shear per girder
   • Axial force: For arch and cable-stayed bridges
3. Export to Excel for report preparation

Reactions

1. Results > Reactions
2. View:
   • Dead load reactions: For bearing and abutment design
   • Live load reactions: Maximum and minimum
   • Wind reactions: For lateral design
3. Export for foundation design

Deflections

1. Results > Displacements
2. Check:
   • Dead load deflection: For camber calculation
   • Live load deflection: L/800 (vehicular), L/1000 (pedestrian)
   • Long-term deflection: Including creep and shrinkage

Wrapping Up

MIDAS Civil is my preferred tool for bridge analysis, and the moving load workflow is a big reason why. The influence line approach is efficient, the AASHTO and Eurocode design checks are comprehensive, and the envelope generation captures the worst-case effects automatically. My tip: always check your deflection limits against the code — I've seen engineers get so focused on strength that they forget serviceability. A bridge that's strong enough but bounces too much under traffic isn't a good bridge.