The force jacked into a prestressing tendon is never fully retained. Elastic shortening, creep, drying shrinkage and steel relaxation slowly take part of it away. Change the jacking force or the section dimensions and see the loss breakdown and the effective prestress that finally remains, in real time.
Parameters
Initial jacking force P0
kN
Force the jack applies to the tendon
Tendon area A_ps
mm²
Concrete area A_c
mm²
Modular ratio n = E_ps/E_c
Steel modulus divided by the concrete modulus
Creep + shrinkage loss
MPa
Loss from time-dependent concrete deformation
Steel relaxation loss rate
%
Fraction of the initial steel stress
Results
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Initial steel stress f_p0 (MPa)
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Elastic-shortening loss (MPa)
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Creep + shrinkage loss (MPa)
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Relaxation loss (MPa)
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Effective prestress force Pe (kN)
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Total loss (%)
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Prestress force decay — jacking to long-term state
A concrete beam with the tendon running through it. The force bar shrinks from the initial P0 to the effective Pe. The right side shows the stacked breakdown of the three loss components.
The effective prestress f_pe is the stress that remains after subtracting the elastic-shortening, creep + shrinkage and relaxation losses from the initial steel stress f_p0.
Prestressed concrete is the one where you pull the steel really tight and then cast it into the concrete, right? But what does "prestress loss" mean — does the force you pulled with actually drop?
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Good question to ask. In prestressed concrete (PC), a jack pulls the tendon hard and transfers that force into the concrete, which keeps the beam from cracking. But here is the catch: even if the jack reads "1400 kN", the force that actually stays in the member is always less. That gap is the "prestress loss". You cannot avoid it — typically 15 to 25% of the initial force just disappears.
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15 to 25% — that's a lot! What causes it to drop that much?
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There are two groups. First the "immediate losses", which happen the instant you stress. The main one is elastic shortening: when you pull the tendon and move that force into the concrete, the concrete itself squeezes elastically. The tendon embedded in it shortens by the same amount, so part of its stress just slips away. Raise the "modular ratio n" on the left and you will see that elastic-shortening loss grow. The other group is the "time-dependent losses" — the ones that creep along over years.
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Time-dependent losses — that's creep and shrinkage and so on? I've heard the words but I'm fuzzy on the difference.
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Right, there are three. Creep is concrete slowly continuing to shorten, year after year, while it sits under a sustained compressive load. Shrinkage is the volume loss as moisture escapes the concrete — that one happens with or without load. If the concrete shortens, the tendon shortens with it and the force drops. The third is relaxation, which is a steel thing: a high-strength tendon held at constant strain slowly bleeds off stress over time. In this tool you enter creep + shrinkage together in MPa, and relaxation as a fraction of the initial stress.
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I see. So the force left over after subtracting the total of all of those is the force you can actually use?
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Exactly. That is the "effective prestress f_pe". The strength check, the deflection, the cracking check — all of them use the effective prestress. And this is the part that matters most in practice: underestimate the losses and you design with a prestress that is really too small, so the bridge or slab cracks in service. Overestimate them and you put in extra tendon steel you do not need, which wastes money. So the loss estimate has to be "just right".
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With the default values the total loss came out around 15%. Is that in the "just right" range?
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Yes, 15% is typical. The total loss of a PC member generally falls between 15 and 25%. This tool shows a warning above 25%. Going past 25% is a sign that the section is too small and the concrete stress σ_c is running too high, or that the creep and shrinkage input is excessive. When that happens you revisit the section size, or pick a low-relaxation tendon, so that you still secure a healthy effective prestress.
Frequently Asked Questions
Prestress losses are the gradual (or instantaneous) reduction of the initial jacking force applied to a prestressing tendon. The causes split into immediate losses and time-dependent losses. Immediate losses include elastic shortening — when the prestress is transferred, the concrete shortens elastically and the embedded tendon shortens with it — plus anchorage seating and friction inside the duct. Time-dependent losses are concrete creep (deformation under sustained load), drying shrinkage, and relaxation of the high-strength steel (loss of stress under constant strain). These losses are unavoidable and together remove roughly 15-25% of the initial force.
The elastic-shortening loss is Δf_el = n·σ_c, where n is the modular ratio E_ps/E_c (the steel modulus divided by the concrete modulus, typically 5-7) and σ_c is the concrete compressive stress at the tendon level. When the prestress is applied, the concrete shortens elastically, and the tendon shortens by the same strain, so the steel stress drops by n·σ_c. In pretensioning the full amount is lost; in post-tensioning with several tendons stressed in sequence the average loss is about half. This tool assumes a simplified axial case and evaluates σ_c = P0/A_c.
All three are time-dependent losses, but the mechanisms differ. Creep is the slow, ongoing shortening of concrete under a sustained compressive load over months and years. Drying shrinkage is the volume reduction as moisture leaves the concrete; it proceeds whether or not there is a load. Both shorten the concrete, so the embedded tendon shortens too and the force drops. Relaxation is a steel phenomenon: a high-strength tendon held at constant strain slowly loses stress over time. In this tool, creep and shrinkage are combined into a single MPa input, and relaxation is handled as a fraction of the initial steel stress.
The effective prestress f_pe is the stress that actually remains in the tendon after all losses have been subtracted (f_pe = f_p0 − total loss). Strength checks, deflection calculations and cracking checks of the member must all use this effective prestress. Underestimating the losses means designing with a prestress that is really insufficient, and the member cracks in service. Overestimating them means putting in more tendon steel than necessary, which is uneconomical. The total loss is typically 15-25%; this tool flags a warning above that range.
Real-World Applications
PC bridge girder design: The most common use of prestressed concrete is the main girder of a bridge. In post-tensioned PC box girders and pretensioned T-girders, the spans are long and the bending moments large, so an inaccurate effective prestress leads to cracking or excessive deflection in service. Designers evaluate the immediate losses just after stressing and the time-dependent losses over decades separately, and run the stress checks with the long-term effective prestress.
PC slabs and PC decks: Long-span flat plates in buildings and the PC decks of multi-storey car parks use prestress to control deflection under self-weight. Slabs are thin, with small section areas, so the concrete stress σ_c tends to be high and the creep loss large. Misjudging the loss leaves the slab sagging for years, interfering with partitions and services.
PC tanks, LNG storage and reactor containments: Cylindrical structures that store liquids or gases are prestressed circumferentially to cancel the tension caused by internal pressure. As critical structures that must not leak, they use low-relaxation tendons and are designed with margin so that the long-term effective prestress never drops below zero.
Stressing control and CAE checks: On site, engineers measure both the jack pressure (force) and the tendon elongation to confirm the design jacking force was actually introduced. Before running a detailed time-dependent analysis (staged-construction creep and shrinkage) in FEM, a quick estimate like this tool of "roughly what percentage the total loss is" gives a sanity check when the FEM result comes out an order of magnitude off.
Common Misconceptions and Pitfalls
The biggest misconception is assuming the force from the jack stays in the member unchanged. The initial jacking force P0 and the effective prestress force Pe are different quantities, and the gap between them is the loss. The deflection and cracking checks must use the effective prestress Pe; using P0 overestimates the prestress and leaves you with a design that is really under-prestressed. Always subtract the total of the immediate loss (elastic shortening) and the time-dependent losses (creep, shrinkage, relaxation) before you check the member.
Next is the mistake of lumping immediate and time-dependent losses together. The two occur at different times. Immediate losses finish at the moment of stressing and anchoring, so the construction-stage stress check uses the state with only the immediate loss removed. Time-dependent losses progress over years to decades, so the long-term service condition uses the full immediate + time-dependent loss. Forget that the same member has a different effective prestress "just after stressing" versus "long term", and you get top-fibre cracking during construction, or long-term deflection beyond the prediction.
Finally, the belief that a smaller concrete section is lighter and more economical. Reducing the area A_c raises the concrete stress σ_c = P0/A_c for the same P0. That increases the elastic-shortening loss n·σ_c, and a higher stress also makes creep progress faster, so the total loss percentage jumps. Drop A_c near its minimum in this tool and you will see the total loss spike and switch to the warning state. Squeeze the section too far and the prestress "leakage" is large — and you can end up needing more tendon steel after all.
How to Use
Enter initial jacking stress f_p0 (MPa) — typically 0.75 f_py for unbonded tendons, 0.80 f_py for bonded (input range 800–1900 MPa for 270 ksi strand)
Input concrete strength f_c' (MPa) and tendon area A_ps (mm²) to calculate elastic shortening loss via ΔF_ES = (n × E_p / E_c) × f_ci where n is modular ratio
Set creep coefficient φ and shrinkage strain ε_sh to model long-term losses; simulator sums all four loss mechanisms and returns effective prestress Pe (kN) and total loss percentage
Elastic shortening dominates immediately after transfer; unbonded tendons experience NO elastic shortening loss in companion members (applies only to transfer length in bonded ducts)
Creep and shrinkage losses scale with concrete age — use φ values from ACI 209 or fib Model Code; coastal/marine environments increase shrinkage by 20–40%
Relaxation depends on strand grade and stress ratio; stress-relieved strand loses ~8%, low-relaxation ~3% at 0.75 f_py over 1000 hours