Admissible Stress for Steel and Concrete
Computes admissible stress for steel and concrete given characteristic strength and safety factor.
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Allowable stress for steel and concrete
To get the allowable stress you take the material's characteristic strength and divide it by a safety factor: σ_perm = f / FS. Take Brazilian rebar CA-50, which has a yield strength fy = 500 MPa. With FS ≈ 2.3 that puts the allowable tensile stress at roughly 215 MPa. Class C25 concrete works the same way: fck = 25 MPa gives an allowable compressive stress near 18 MPa, and tension is usually ignored altogether. The partial-safety coefficients behind today's limit-state design come from two Brazilian standards, NBR 6118 for concrete and NBR 8800 for steel.
Applications
It comes up in structural sizing of beams, columns and slabs. It's handy on preliminary projects, where you want a fast check before committing to detailed analysis. And it shows up when you have to verify the ABNT code-required limits in technical reports and ART/RRT submissions.
FAQ
Allowable stress vs limit-state design? These days NBR 6118 runs on limit states (ULS/SLS), applying partial factors to both loads and resistances. The older allowable-stress method hasn't disappeared, though — people still reach for it as a quick sanity check.
Why neglect concrete tension? A concrete's tensile strength is only about 10% of its compressive strength, it varies a lot from one pour to the next, and it's gone the moment the section cracks. So the design hands the whole tensile load to the reinforcement.
What FS should I use? Common figures are 1.15 for steel yield (ULS) and 1.4 for concrete compression. If you're using the simplified allowable-stress approach, something in the 2.0-2.5 range is typical.
Related Tools
Edge Stress from Prestressing
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Simplified estimate of longitudinal rebar area for a reinforced concrete rectangular beam.
Vessel Allowable Stress (ASME)
Calculate the design allowable stress of a pressure-vessel material by the ASME criterion, S = σ_uts ÷ n, from the material minimum tensile strength σ_uts (MPa) and the safety factor n (3.5 in the current ASME VIII Div. 1 edition for tensile strength). The allowable stress S is the MAXIMUM stress permitted in the vessel material in service, and is the basis of all thickness and MAWP calculations — it embeds the safety margin against failure. The ASME code sets the allowable stress as the SMALLEST among several criteria: a fraction of the TENSILE strength (σ_uts/3.5 in the current edition — formerly /4.0, reduced as materials and inspection advanced), a fraction of the YIELD strength (2/3 of σ_yield), and, at high temperatures, criteria based on CREEP and creep rupture (since at high temperature the material deforms slowly under constant load). For each material and temperature, the code TABULATES the S value — this formula shows the tensile-strength criterion, often governing at moderate temperatures. Using the correct allowable stress (from the code, for the right material and temperature) is absolutely essential: it is the safety margin protecting against vessel explosion. Enter the tensile strength and the safety factor.
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.
Cable Tension in Accelerated Lift
Calculate the dynamic tension in a cable while lifting a load with acceleration, T = W·(1 + a/g), from the load weight W (N), the vertical lift acceleration a (m/s²) and gravity g. When a load is lifted with ACCELERATION (at lift start, when accelerating the rise), the cable must provide not only the force to support the weight (W) but ALSO the force to accelerate the mass upward — by Newton's second law, the total tension is the weight times the factor (1 + a/g). This means the DYNAMIC tension is GREATER than the static weight: an acceleration of g/2 (5 m/s²) raises the tension by 50%! That is why ABRUPT lifts (fast start, or worse, lifting an already-moving load or stopping abruptly) generate dangerous dynamic OVERLOADS in the cable, which can break it even with the static load within capacity. The effect is worse in abrupt STOPS and in loads 'snatching off the ground' (cable slack suddenly removed, generating an impact). So experienced operators lift SMOOTHLY (low acceleration), and the cable safety factors (5 or more) exist precisely to cover these inevitable dynamic overloads. This calculation quantifies the tension increase due to acceleration, essential in the safety analysis of dynamic lifts. Enter the load weight and the lift acceleration.
Initial Prestress Stress
Calculate the allowable initial stress in prestressing steel, σ_pi = coef·f_ptk, from the code coefficient (fraction of strength) and the steel characteristic tensile strength f_ptk (MPa). Prestressing steel is tensioned to a VERY HIGH stress — a significant fraction of its tensile strength — possible because these are HIGH-STRENGTH steels (strands with f_ptk of 1900 MPa, versus ~500 MPa for ordinary reinforcing steel). But there is a LIMIT to the initial stress, set by code for safety and to limit relaxation: typically the lesser of about 0.74·f_ptk and 0.82·f_pyk (yield strength) for low-relaxation steels in pretensioning, with slightly different values for post-tensioning and right after anchorage. Applying a high initial stress is DESIRABLE (more effective prestress, less steel needed), but the code limit prevents tensioning the steel too close to yield (which would reduce safety margin and greatly increase relaxation). This calculation gives the jacking stress to apply (before losses), the starting point of all prestress design. Enter the code coefficient and the steel characteristic strength.
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.