MIDAS Civil Construction Stage Analysis: Sequential Casting, Creep, and Camber Control

MIDAS Civil Construction Stage Analysis: Sequential Casting, Creep, and Camber Control

Construction stage analysis in MIDAS Civil is something I consider essential for any segmental or cast-in-place bridge. I learned this the hard way on a precast segmental bridge where the camber calculations from a single-stage analysis didn't match what we saw in the field. Once I ran a proper construction stage analysis with creep and shrinkage, the predictions lined up. Let me walk you through the workflow.

Why Construction Stage Analysis for Bridges

Sequential Construction Methods

• Cast-in-place segmental: Segments cast sequentially on falsework
• Precast segmental: Segments erected sequentially with post-tensioning
• Balanced cantilever: Segments cast alternately on each side of pier
• Incremental launching: Bridge pushed out from one abutment
• Span-by-span: Temporary supports used for each span

Effects of Sequential Construction

• Force redistribution: As new segments are added, forces in existing segments change
• Creep deformation: Sustained loads cause continued deformation over time
• Shrinkage: Concrete volume reduction causes shortening and cracking
• Prestress redistribution: Tendon forces redistribute as structure changes
• Camber requirements: Each segment must be cast with camber to compensate for future deformation

Defining Construction Stages

Stage Definition

1. Load > Construction Stage > Define Stage
2. Create stages per the construction schedule:

| Stage | Duration | Activity | |-------|----------|----------| | 1 | 7 days | Cast Pier 1 | | 2 | 28 days | Cast Pier 2 | | 3 | 7 days | Cast Segment 1L (left of Pier 1) | | 4 | 7 days | Cast Segment 1R (right of Pier 1) | | 5 | 1 day | Stress Tendon Group 1 | | 6 | 7 days | Cast Segment 2L | | 7 | 7 days | Cast Segment 2R | | 8 | 1 day | Stress Tendon Group 2 | | ... | ... | ... | | N | 365 days | Service (long-term) |

Element Activation

1. For each stage, select elements to activate:
   • New segments: Activate with concrete age = 1 day
   • New tendons: Activate with jacking stress
2. Set activation properties:
   • Initial age: 1 day (fresh concrete)
   • Support condition: Fixed to existing structure
3. MIDAS Civil adds elements to the active model

Element Deactivation

1. For temporary supports and formwork:
   • Activate at the construction stage
   • Deactivate at a specified stage (when no longer needed)
2. MIDAS Civil removes the element and redistributes forces

Load Activation

1. For each stage, specify loads:
   • Self-weight: Automatic for activated elements
   • Construction loads: Equipment, formwork, workers
   • Prestress: Tendon jacking force at stressing stage
   • Superimposed dead: Applied after all segments are erected
2. Loads accumulate across stages

Time-Dependent Material Properties

Creep Model

1. Model > Property > Time-Dependent Material (Creep)
2. Select model:
   • CEB-FIP Model Code 1990: Most common for bridges
   • ACI 209R: American standard
   • JSCE: Japanese standard
   • B3: Bazant model
3. Set parameters (CEB-FIP):
   • Concrete strength: fck = 40 MPa
   • Humidity: 70% (typical)
   • Notional size: h = 2Ac/u (perimeter)
   • Cement type: Normal, rapid, or slow
4. MIDAS Civil calculates creep coefficient:
   • φ(t, ta) at any time t for loading age ta

Shrinkage Model

1. Model > Property > Time-Dependent Material (Shrinkage)
2. Select model (same options as creep)
3. Set parameters (CEB-FIP):
   • Humidity: 70%
   • Notional size: h = 2Ac/u
   • Cement type: Normal
4. MIDAS Civil calculates shrinkage strain:
   • εcs(t) at any time t

Compressive Strength Development

1. Model > Property > Time-Dependent Material (Strength)
2. Set:
   • fck: 40 MPa (28-day strength)
   • fck(t): Strength at age t per CEB-FIP:
     • fck(t) = fck × exp(s × (1 - √(28/t)))
     • s: Coefficient for cement type (0.25 for normal)
3. At each stage, MIDAS Civil uses the appropriate concrete strength for the element's age

Running Construction Stage Analysis

1. Analysis > Analysis Control > Construction Stage
2. Set:
   • Number of stages: As defined
   • Time steps: Per stage duration
   • Save results: At each stage and at final time
3. Analysis > Run Analysis
4. MIDAS Civil performs:
   • For each stage:
     • Activate/deactivate elements
     • Apply stage loads (including prestress)
     • Update time-dependent properties (strength, creep, shrinkage)
     • Calculate displacements and forces
   • Continue through all stages to service life

Camber Calculation

What Is Camber

Camber is the upward deflection built into each segment during construction to compensate for:

• Dead load deflection: Self-weight causes downward deflection
• Prestress camber: Prestress causes upward deflection
• Creep deflection: Long-term creep causes additional deflection
• Shrinkage deflection: Shrinkage causes shortening

Camber Output

1. Results > Construction Stage > Camber
2. MIDAS Civil calculates camber per segment:
   • Elastic camber: Immediate (at construction)
   • Creep camber: Long-term (over 10-30 years)
   • Total camber: Elastic + Creep + Shrinkage
3. For each segment:
   • Camber value: e.g., +15mm (upward) at midspan
   • Setting elevation: Design elevation + camber
4. Contractor casts each segment at the setting elevation

Camber Diagram

1. Results > Construction Stage > Camber Diagram
2. View camber along the bridge:
   • At each segment joint: Camber value
   • At midspan: Maximum camber
   • At supports: Zero camber (fixed points)
3. Export camber table for construction

Force Redistribution

Prestress Force Redistribution

1. Results > Construction Stage > Prestress Force
2. View tendon force over time:
   • At stressing: fpj (jacking stress)
   • After immediate losses: fpj - ES - friction - anchorage
   • After long-term losses: fpe (effective prestress)
   • Due to sequential construction: Force redistributes as new segments are added
3. Check that all tendons maintain required effective prestress

Moment Redistribution

1. Results > Construction Stage > Moment
2. View moments at each stage:
   • Stage 1: Moments due to first segment only
   • Stage N: Cumulative moments from all segments
   • Service: Final moments including all loads
3. Observe redistribution:
   • As new segments are added, existing moments change
   • Creep causes additional redistribution over time
   • Final moments may differ significantly from single-step analysis

Shear Redistribution

1. Results > Construction Stage > Shear
2. View shear at each stage:
   • Shear forces redistribute as structure becomes continuous
   • Temporary supports carry shear until removed
   • Final shear may differ from single-step analysis

Balanced Cantilever Construction

Stage Sequence for Balanced Cantilever

1. Pier construction: Cast pier and pier table segment
2. Segment 1L and 1R: Cast simultaneously on both sides (balanced)
3. Stress tendons: Post-tension tendons for segments 1L and 1R
4. Segment 2L and 2R: Cast next pair
5. Stress tendons: Post-tension for segments 2L and 2R
6. Continue until cantilevers meet at midspan
7. Closure pour: Cast closure segment at midspan
8. Continuity tendons: Stress continuity tendons across closure
9. Remove temporary supports

Key Checks During Cantilever Construction

1. Stability during construction:
   • Each cantilever must be stable against overturning
   • Check wind load during construction
   • Check unbalanced construction load
2. Stresses at each stage:
   • Concrete stresses within limits at every stage
   • Tendon stresses within limits
3. Camber accuracy:
   • Each segment cast at correct elevation
   • Closure segment fits between cantilevers
   • Maximum mismatch: ±10mm (typical)

Long-Term Effects

Creep-Induced Redistribution

1. Over 10-30 years, creep causes:
   • Moment redistribution: From negative (support) to positive (midspan)
   • Up to 15% redistribution possible
   • Camber change: Initial upward camber may reduce over time
2. Design must account for final (long-term) forces, not just construction forces

Shrinkage Effects

1. Shrinkage causes:
   • Shortening: Bridge deck shortens by 10-30mm over 100m length
   • Bearing movement: Bearings must accommodate shrinkage
   • Crack risk: If restrained, shrinkage can cause cracking
2. Provide expansion joints and flexible bearings to accommodate

Common Issues

Excessive Camber

Cause: Overestimated prestress or underestimated creep. Fix: Adjust prestress force, use higher creep coefficient, or add counter-weight.

Insufficient Camber

Cause: Underestimated prestress or overestimated creep. Fix: Increase prestress force, use lower creep coefficient, or adjust camber calculation.

Closure Mismatch

Cause: Inaccurate camber calculation or construction tolerance. Fix: Adjust closure segment dimensions. Use jacking force to align cantilevers before closure pour.

Excessive Long-Term Deflection

Cause: High creep, high sustained load, or insufficient prestress. Fix: Increase prestress, use higher strength concrete (lower creep), add reinforcement, or use thicker sections.

Wrapping Up

If you're doing segmental bridge construction, don't skip the construction stage analysis. I learned this the hard way — the camber predictions from a single-stage analysis were off by enough that the contractor was struggling to match segments. Once I ran a proper staged analysis with creep and shrinkage, the camber values lined up with what we saw in the field. The camber output is what the contractor needs most, so get it right.