A concrete girder bridge carries its deck on beams supported by bearings and abutments or piers. In a pretensioned girder, steel strands are tensioned before concrete is cast around them. After the concrete has reached the required strength, releasing the strands from the bed transfers force into the concrete through bond.
Concrete Bridge Interactive Calculator
Explore the jacking-stress ratio used in prestressing steel. The bridge view shows a deck carried by precast I-girders, while the animation below explains the stages of pretensioning. The two inputs calculate steel stress only, not bridge capacity.
Equation Used
- The entered strength is the ultimate tensile strength of the strand.
- The ratio is a user-selected fraction of that strength.
- The pretensioning sequence is conceptual; it is not a construction procedure.
- Transfer, immediate losses and time-dependent losses are not calculated.
- No span, section, load rating or design-code compliance is established.
The slider range is for exploring ratios, not a statement of permissible jacking limits. Bridge proportions, strand number and sequence timing are schematic.
Deck, girders and prestressing strands
The upper view shows a simple concrete deck supported by parallel precast I-girders. Bearings transmit the girder reactions to the abutments. This is one bridge arrangement, not a drawing of a particular bridge or a complete reinforcement layout.
The lower animation follows a single girder conceptually: tension straight strands against a casting bed, cast concrete around them, allow the concrete to develop the required strength, and transfer prestress by releasing the bed restraint. The timing is compressed for explanation and is not a production instruction.
Pretensioning differs from post-tensioning, where tendons are stressed after the concrete has hardened and their forces are introduced through anchorages. The illustration follows the pretensioning sequence.
Where precast prestressed girders are used
Precast girders can be manufactured in a controlled plant and transported to a bridge site for erection. Their cross-section, reinforcement, strand layout and erection arrangements depend on the project.
The PCI plant-tour reference below shows stressing equipment, production beds and completed bridge girders. The interactive drawing is an original simplified illustration of that system; it does not reproduce a particular girder specification.
Jacking stress from a selected ratio
The calculator uses f_jack = r × f_pu, where f_pu is strand ultimate tensile strength in MPa and r is the selected ratio. The other outputs are f_pu − f_jack and 100r percent.
The difference from ultimate strength is a numerical comparison, not a design safety margin. Actual permissible stresses depend on the applicable requirements and the stage being checked. The slider limits are not permissible design limits.
Jacking stress, stress after transfer and effective stress in service are distinct. Elastic shortening, creep, shrinkage, steel relaxation and other applicable losses can change the prestress. This two-input calculator does not solve those effects.
Example: 1860 MPa strand at a 0.70 ratio
For strand ultimate strength 1860 MPa and ratio 0.70, the selected jacking stress is 0.70 × 1860 = 1302 MPa. The arithmetic difference from ultimate strength is 558 MPa, and the selected level is 70 percent.
Changing the ratio to 0.60 gives 1116 MPa, a difference of 744 MPa and a level of 60 percent. Neither result states the prestress remaining in the completed bridge.
Total strand force would additionally require the total steel area. Bridge stress and capacity would require section properties, material properties, reinforcement, loads, losses and the relevant design checks. No values for those missing quantities are inferred from the animation.
Bridge components and limits of the stress comparison
The view shows a deck, separate I-girders, bearings, abutments, a casting bed and straight strands. The animated stages explain the difference between tensioning steel and transferring its force to concrete.
It does not show a calculated neutral axis, tension zone or compression zone. Their locations cannot be inferred from strand strength and jacking ratio alone. Prestressing also does not mean every part of every bridge section remains in compression under every loading condition.
Girder geometry, strand count and time are illustrative. The selected stress bar responds to both inputs, while the bridge stays the same size because no dimensional input is provided.
Concrete bridge calculator questions
Does the displayed stress remain in the completed bridge?
No. It is the selected jacking stress. Transfer and subsequent losses are not calculated.
Is the difference from ultimate strength a safety margin?
No. It is simply the ultimate strength minus the selected jacking stress. It does not establish an allowable stress or a design factor.
Why does changing strength not resize the bridge?
Strength changes the stress calculation. The calculator has no span or cross-section inputs, so its bridge geometry is illustrative.
Is the animated process pretensioning or post-tensioning?
Pretensioning: the strands are tensioned against a bed before concrete is cast around them.
Can this calculate bridge capacity?
No. Capacity requires a structural model, dimensions, reinforcement, material properties, loads, losses and applicable design checks.
Technical and visual references
PCI Mid-Atlantic, Virtual Plant Tour handout shows strand jacks and anchorage equipment on PDF page 10 and precast I-girders in bridge construction on PDF page 22.
FHWA, Prestressed Concrete Bridge Design Example: Prestressing Force distinguishes jacking, transfer and service conditions and discusses prestress losses. It supports those distinctions, rather than supplying universal limits for this calculator.
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