Beam Load Capacity Calculator

Calculate the maximum allowable load for a steel beam based on both bending stress and deflection limits. The calculator determines which criterion governs and shows the controlling capacity.

How Beam Capacity is Determined

A beam's load capacity is limited by two criteria:

• Bending stress — The beam must not yield under the applied moment
• Deflection — The beam must not deflect more than the serviceability limit

The actual capacity is the lower of these two values. Short, deep beams are typically stress-governed. Long, shallow beams are typically deflection-governed.

Stress-Based Capacity

The maximum bending stress in a beam is:

Where M is the bending moment and S is the section modulus. For the stress to not exceed the allowable value (F_b = 0.6F_y for typical ASD), we rearrange to find the maximum allowable moment, then back-calculate the load.

For a simply supported beam with UDL: M_max = wL²/8, so the allowable load is:

Deflection-Based Capacity

The maximum deflection must not exceed the serviceability limit (e.g., L/360). Working backwards from the deflection formula:

Rearranging for the allowable load:

Input Values Explained

Section Modulus (S)

The elastic section modulus in mm³ or in³. For symmetric sections, S = I / (h/2) where h is the total depth. Look this up in section tables, or calculate it using our section properties calculator.

Yield Stress (F_y)

The yield strength of the steel. Common values:

• 250 MPa / 36 ksi — Mild steel (Grade 250, A36)
• 300 MPa / 44 ksi — Grade 300 steel
• 345 MPa / 50 ksi — High-strength steel (A992, S355)

The calculator uses 0.6 × F_y as the allowable bending stress, which is typical for Allowable Stress Design (ASD). For LRFD design, different factors apply.

Important Limitations

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