Admissible Stress for Steel and Concrete

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 f_y = 500 MPa. With FS ≈ 2.3 that puts the allowable tensile stress at roughly 215 MPa. Class C25 concrete works the same way: f_ck = 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.