Edge Stress from Prestressing
Calculate the normal stress at an extreme fiber of a prestressed concrete section, σ = P/A + (P·e)/W, from the prestressing force P (MN), the section area A (m²), the tendon eccentricity e (m) and the section modulus W (m³). Prestressed concrete is one of the great structural engineering inventions of the 20th century: high-strength steel tendons are tensioned (prestressed) and anchored in the member, COMPRESSING the concrete before it even receives service loads. Since concrete is strong in compression but weak in tension, this pre-compression 'cancels' the tensions external loads would cause, allowing much longer spans and slenderer members than conventional reinforced concrete. The tendon is placed with ECCENTRICITY (below the centroid), so prestressing generates not only uniform compression (P/A) but also a moment (P·e) producing stresses opposite to the loading — compressing exactly the fiber that would tend to crack. This formula computes the resulting edge stress, summing axial compression and prestress bending; design verifies stresses stay within limits in all phases (at transfer, empty, and in service, loaded). Enter the prestressing force, area, eccentricity and section modulus.
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Tensão na borda por protensão
A tensão normal em uma fibra extrema de uma seção de concreto protendido é σ = P/A + (P·e)/W, a partir da força de protensão P, da área da seção A, da excentricidade do cabo e e do módulo de resistência da seção W. O concreto protendido é uma das maiores invenções da engenharia estrutural do século XX: cabos de aço de alta resistência são tracionados (protendidos) e ancorados na peça, comprimindo o concreto antes mesmo de ele receber as cargas de serviço. Como o concreto é forte na compressão mas fraco na tração, essa pré-compressão 'cancela' as trações que as cargas externas causariam, permitindo vencer vãos muito maiores e usar peças mais esbeltas que no concreto armado convencional. O cabo é colocado com excentricidade (abaixo do centroide), de modo que a protensão gera não só uma compressão uniforme (P/A) mas também um momento (P·e) que produz tensões opostas às do carregamento — comprimindo justamente a fibra que tenderia a tracionar. Esta fórmula calcula a tensão resultante numa borda, somando a compressão axial à flexão da protensão; o projeto verifica que as tensões fiquem dentro dos limites em todas as fases (na protensão, com a peça vazia, e em serviço, carregada). Informe a força de protensão, a área, a excentricidade e o módulo de resistência.
Related Tools
Prestress Moment
Calculate the moment generated by eccentric prestressing at a section, M_p = P·e, from the prestressing force P (kN) and the tendon eccentricity e (m). When the prestressing tendon is positioned with ECCENTRICITY relative to the section centroid (usually below, in the region tensioned by loads), the prestressing force, besides axially compressing the section (P/A), generates a BENDING MOMENT equal to force times eccentricity. This prestress moment is the key to prestressed concrete's efficiency: it is OPPOSITE to the moment from external loads (self-weight, live loads), 'bowing' the member upward (camber) and producing top-fiber tension and bottom-fiber compression — exactly the opposite of what the load does. So eccentric prestressing 'pre-loads' the member against the service loading, so that when loads act, they must first CANCEL the prestress effects before tensioning the concrete. That is why prestressed beams often show camber (upward curvature) when still unloaded. The prestress moment is fundamental in computing edge stresses, camber and the optimal tendon profile along the member (which roughly follows the load moment diagram, with varying eccentricity). Enter the prestressing force and the eccentricity.
Steel Relaxation Loss
Calculate the prestress loss from steel relaxation, Δσ = (ψ/100)·σ_pi, from the relaxation coefficient ψ (% of initial stress) and the initial tendon stress σ_pi (MPa). Relaxation is a STEEL phenomenon analogous to concrete creep: when a steel wire or strand is held under CONSTANT tension (fixed elongation, as in an anchored prestressing tendon), its stress DECREASES slowly over time, even without length change. It is as if the steel 'yields' microscopically under prolonged load, losing part of its tension. Relaxation depends on the steel type (LOW-relaxation steels — LR —, thermomechanically treated, relax much less, ~2.5% in 1000h at 0.7·fptk, than normal-relaxation — NR —, ~12%), the initial stress level (the higher, the more relaxation) and temperature. The coefficient ψ is tabulated as a function of these factors and time. Relaxation is one of the TIME-DEPENDENT prestress losses, along with concrete shrinkage and creep, and design sums them all for the total loss and the effective final prestressing force. Enter the relaxation coefficient and the initial tendon stress.
Final Prestressing Force
Calculate the final (effective) prestressing force after losses, P_∞ = P_0·(1 − losses/100), from the initial prestressing force P_0 (kN) and the total loss percentage (%). A member's prestressing force is NOT constant: it starts at an initial value (the jacking force) and DECREASES due to the various losses — immediate (elastic shortening, friction, anchorage set) and time-dependent (concrete shrinkage and creep, steel relaxation). The effective final force, after all losses stabilize (after years), is what actually acts in the structure in service and must ensure performance. Total losses typically sum 15-25% of the initial force in post-tensioned structures and can reach 20-30% in pretensioned ones. This simple calculation applies the total loss percentage to the initial force, giving the effective force — fundamental to check service stresses, cracking and member deflection. The designer works with TWO critical situations: maximum INITIAL force (right after transfer, member still unloaded — risk of excess compression or top-fiber tension) and minimum FINAL force (after all losses, member loaded — risk of decompression and cracking). Enter the initial force and the total loss percentage.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.