Prestressed Concrete Bridge Design
Learning Objectives
- Distinguish pretensioned and post-tensioned bridge systems and their construction implications.
- Select a preliminary girder depth, spacing, concrete strength, strand/tendon layout, and deck system.
- Compute prestress force and distinguish initial, transfer, effective, and ultimate design stages.
- Organize immediate and time-dependent prestress losses without double counting.
- Check concrete stresses at transfer and service at critical fibers.
- Explain ultimate flexural and shear resistance checks and the role of prestressing steel development.
- Evaluate camber, deflection, composite section behavior, and construction-stage demand.
- Produce a traceable design workflow linked to the bridge-analysis envelopes.
Prestressed bridge design is a staged problem. The girder experiences different section properties, prestress levels, loads, and concrete strengths at transfer, erection, deck placement, and final service. A reliable workflow therefore tracks each stage explicitly rather than collapsing the bridge into one final-section calculation.
Use project-specific provisions
Concrete stress limits, resistance factors, loss models, strand properties, development requirements, shear equations, service criteria, and section limits must be taken from the governing project criteria and adopted bridge specification edition.
1. Select the Prestressed Bridge System
Pretensioned Girder
Strands are stressed before concrete placement and released after the concrete reaches the required transfer strength. Force enters the concrete by bond. Pretensioned I-girders and bulb-tee forms are widely used for repetitive highway spans.
Post-Tensioned Girder or Box
Tendons are stressed after concrete hardens and transfer force through anchorages. Post-tensioning enables curved tendon profiles, continuity, segmental construction, and long-span box-girder systems.
One-Pass Girder Design Map
From bridge demand to a buildable girder
- Establish girder spacing, span arrangement, material properties, and a trial section.
- Generate staged dead-load and live-load effects for the girder.
- Select prestressing force, strand or tendon layout, eccentricity, and release strength.
- Estimate losses and evaluate transfer, handling, deck-placement, and service stresses.
- Check service behavior: stress, cracking where applicable, deflection, and camber.
- Check flexural strength, shear, interfaces, anchorage or development, and end regions.
- Confirm fabrication, transportation, erection, diaphragm, and continuity details.
- Iterate the section and prestress layout until all stages work together.
2. Define the Design Stages
Minimum stage model
- Jacking / stressing: establish tendon force before transfer losses.
- Transfer: apply initial effective prestress to the young concrete section and check transfer stresses/camber.
- Erection: include girder self-weight and handling/support conditions.
- Deck placement: add wet deck, forms, diaphragms, and construction loading using the stage-appropriate section.
- Composite service: use the hardened deck-girder composite section for later superimposed dead load and live load as applicable.
- Ultimate: use factored demand and nominal resistance for strength checks.
3. Prestress Force and Eccentricity
Uniform plus bending stress from prestress
Conceptual elastic stress at a section caused by concentric compression plus the moment generated by prestress eccentricity.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Prestress force at the stage under consideration | - | |
| Concrete section area | - | |
| Tendon eccentricity relative to the section centroid | - | |
| Section modulus to the fiber being checked | - |
External load stress is superimposed on prestress stress using the section properties appropriate to the stage. Sign convention must remain consistent throughout the calculation.
4. Prestress Losses
Immediate Losses
Depending on the system, immediate losses can include elastic shortening, anchorage seating, and friction or wobble effects in post-tensioning ducts.
Time-Dependent Losses
Creep, shrinkage, and relaxation reduce effective prestress after transfer. Their interaction depends on material properties, member age, environmental conditions, construction sequence, and the loss method permitted by the governing basis.
Loss bookkeeping
Do not mix a simplified total-loss percentage with separately computed component losses unless the adopted method explicitly permits that combination. The effective prestress used for service checks must be traceable to one consistent loss model.
Prestress Force Is a Timeline
Jacking or Initial Force
The force introduced at stressing establishes the starting tendon state and must stay within the governing stressing and material limits.
Transfer Force
For pretensioned girders, force is transferred into the concrete after release. This stage often combines high prestress with relatively low concrete strength and therefore deserves its own stress and stability checks.
Effective Service Force
After time-dependent and immediate losses, the remaining effective prestress combines with composite self-weight, superimposed dead load, live load, temperature, and other service actions.
5. Transfer Stress Checks
Transfer check
- Use concrete properties and allowable stresses appropriate to transfer age.
- Use the prestress force after applicable immediate losses.
- Include girder self-weight if it acts at transfer/release as modeled.
- Compute top- and bottom-fiber stresses at critical locations.
- Check compression and permitted tension limits and revise strand pattern, debonding, concrete strength, or girder geometry if required.
6. Service Stress Checks
Service checks use effective prestress after long-term losses together with the appropriate permanent and live-load effects. Composite section properties are used only for loads applied after composite action develops.
Stage-specific elastic fiber stress
Combines prestress, self-weight, and external bending for a selected construction or service stage.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Effective prestress at the selected stage | - | |
| Bending moment applied to the section properties used for that stage | - |
7. Flexural Strength
Ultimate flexural design compares factored moment demand from the verified bridge-analysis envelope with the factored nominal resistance. The resistance model must account for prestressing steel area and strength, concrete compression behavior, bonded/unbonded tendon rules where relevant, nonprestressed reinforcement, flange participation, and applicable ductility/section limits.
Development and Transfer Length
Prestressing steel cannot be assumed fully effective at every section. Transfer and development requirements become especially important near girder ends, hold-down points, debonded strands, and regions where high shear and moment interact.
8. Shear and Web Design
Prestressing changes the diagonal cracking state and shear response of the girder. Shear design must use the governing code model for concrete contribution, prestressing effects, transverse reinforcement, web geometry, and minimum/maximum reinforcement limits. Critical regions commonly include supports, concentrated loads, and changes in tendon force or geometry.
9. Camber and Deflection
Prestress Camber
Eccentric prestress generally produces upward curvature, while self-weight and subsequent loads produce downward deflection. The final profile depends on time-dependent losses, creep, deck placement, continuity, and construction timing.
Camber is not merely a serviceability number. It affects deck thickness, haunch geometry, grade control, bearing elevations, erection fit-up, and drainage. Predicted and measured camber should be compared during construction where project procedures require it.
Handling, Shipping, and Erection
Checklist
- Use actual lifting-point and support locations when checking temporary girder behavior.
- Verify release strength and lateral stability before handling long slender girders.
- Check transportation geometry, support spacing, sweep, and permit constraints where applicable.
- Confirm temporary bracing and diaphragm sequence before the deck provides composite restraint.
- Coordinate predicted camber with seat elevations, haunch range, deck thickness, and field tolerances.
- Compare measured plant or field camber with assumptions when project procedures require reconciliation.
10. Composite Deck-Girder Behavior
Composite-stage workflow
- Analyze precast girder loads using the non-composite precast section.
- Add wet-deck construction demand before composite action develops.
- Transform or otherwise define the composite section according to the governing method after the deck reaches the required condition.
- Apply later superimposed dead load and live load to the composite section.
- Accumulate stage-specific stress increments at the correct fibers.
11. Design Iteration
Checklist
- Span, spacing, deck thickness, and girder type are established.
- Required concrete strengths at transfer and service are defined.
- Strand/tendon pattern is constructible and symmetric where appropriate.
- Prestress losses use one consistent method.
- Transfer stresses pass at critical sections.
- Service stresses pass for governing combinations.
- Flexural strength and ductility requirements pass.
- Shear and transverse reinforcement checks pass.
- Transfer/development requirements pass.
- Camber/deflection and deck-haunch implications are acceptable.
- Bearing reactions and end-zone details are transferred to downstream design.
12. Strand Pattern, End Regions, and Local Effects
Bridge-girder design must resolve the end region, not only midspan flexure. Strand transfer/development, debonding where permitted, hold-down or draped-tendon geometry, bursting/splitting forces, anchorage-zone reinforcement for post-tensioning, bearing-zone stresses, diaphragm connections, and end-block geometry can govern detailing.
Checklist
- Strand/tendon geometry is constructible and compatible with cover and spacing.
- Transfer/development requirements are checked at critical sections.
- Debonding or draping assumptions are explicitly documented.
- End-zone reinforcement follows the governing force-transfer model.
- Bearing reactions and local bearing-zone demands are coordinated with support details.
- Prestress force and concrete strength at release/stressing are tied to construction acceptance requirements.
13. Composite Interface and Continuity
Where the deck is intended to act compositely, interface shear transfer, surface preparation, shear-friction or connector provisions as applicable, diaphragm behavior, continuity details, negative-moment reinforcement, differential shrinkage, and construction sequence must be coordinated. The final composite section cannot be assumed until the physical interface can develop the required action.
14. Prestressed Girder Design Deliverable
Checklist
- Section geometry and stage-specific properties are tabulated.
- Strand/tendon coordinates, jacking force, transfer force, and effective force are stated.
- Loss method and assumptions are documented.
- Transfer and service fiber-stress envelopes are reported by critical section.
- Flexural and shear strength checks identify governing combinations.
- Transfer/development and end-zone checks are reported.
- Camber/deflection predictions include construction ages/stages.
- Deck-haunch, bearing-seat, diaphragm, and continuity interfaces are coordinated.
Interactive visualization
Adjust prestress force, eccentricity, losses, and external moment to see how transfer and service stresses change across the girder depth. The visualization separates initial and effective prestress to reinforce stage-based design.
Controls
Prestress timeline
Purposeful camera
Elevation
Effective force after the stated total service loss assumption.
- Prestressed bridge design is fundamentally stage-dependent.
- Prestress losses must be tracked with one consistent, code-approved method.
- Transfer and service stresses can control strand pattern even when ultimate flexural strength is adequate.
- Composite section properties apply only after composite action develops.
- Camber, development, shear, end-zone behavior, and construction sequence are part of girder design—not afterthoughts.