Beam Load Calculator

Given a W-shape and span — how much load can it carry? Returns both the bending-governed and deflection-governed allowable load, so you see which criterion controls.

Beam

ft

Maximum allowable load

Governing UDL capacity

— lb/ft

UDL — bending limit

— lb/ft

UDL — deflection limit

— lb/ft

Point load at centre — capacity

— lb

Maximum moment (M_max)

— kip·ft

Allowable deflection

— in

The "governing UDL capacity" is the lesser of bending-limited and deflection-limited capacity. For service load (unfactored). For LRFD, divide ultimate load by 1.4 – 1.6 to compare with this service-level result.

Informational only. Lateral-torsional buckling, shear, web crippling, bearing and connection checks are NOT included. Full AISC 360 verification by a licensed engineer is required for any structural beam in real construction.

How capacity is computed

Two independent limits constrain a simply-supported beam: bending stress (the section's modulus times allowable stress) and deflection (the section's moment of inertia times stiffness). Either can govern. Compute both and pick the smaller.

Bending: Mallow = Fb · Sx
UDL bending: w = 8·Mallow / L²
UDL deflection: w = 384·E·I·δallow / (5·L⁴)
Point at centre: P = 4·Mallow / L  ·  P = 48·E·I·δallow / L³

Worked example — W12×16, 20 ft span, A992, L/360

  • Sx = 17.1 in³, Ix = 103 in⁴, Fb = 30 ksi, E = 29,000 ksi
  • Mallow = 30 × 17.1 = 513 kip·in = 42.75 kip·ft
  • UDL bending: w = 8 × 513 / (240)² × 12 (unit conversion) = 855 lb/ft
  • δallow = 240 / 360 = 0.667 in
  • UDL deflection: w = 384 × 29,000 × 103 × 0.667 / (5 × 240⁴) = 0.546 kip/in = 6.55 kip/ft = 6,547 lb/ft (would govern if higher than bending)
  • Bending governs at 855 lb/ft

Frequently asked questions

How much weight can a W8x10 beam support?
Depends on the span. For a 10 ft simply-supported span, A992: bending allows about 1,950 lb/ft UDL; deflection (L/360) allows about 5,500 lb/ft — bending governs. For a 20 ft span, bending allows ~488 lb/ft, deflection ~344 lb/ft — deflection governs.
How much weight can a W12x26 beam support?
At 20 ft span, A992: bending allows 1,670 lb/ft UDL; deflection (L/360) allows 1,090 lb/ft — deflection governs at about 1,090 lb/ft. At 10 ft span the same shape carries 6,680 lb/ft (bending governs because deflection is small at short spans).
Why do bending and deflection have different limits?
Bending is a strength limit — the steel reaches yield stress and fails. Deflection is a serviceability limit — the beam stays elastic but visibly sags or causes cracking in non-structural finishes. For short spans bending governs; for long spans deflection governs. The crossover depends on the section.
What is L/360 deflection?
Span divided by 360 = maximum allowable midspan deflection. For a 20 ft (240 in) span: 240/360 = 0.667 in max. This is the standard for floor live loads. L/240 is used for floor total load (live + dead) or for roof live load. L/480 is for vibration-sensitive uses.
Does this calculator factor for self-weight?
No — the calculator gives the total uniformly distributed load the beam can carry. If you need to subtract self-weight, the W-shape weight per foot is encoded in the tag (W12×26 = 26 lb/ft). For most cases this is a few percent and within the calculation tolerance.
What is the maximum span for a W8x10 floor beam?
For 100 lb/ft floor live load + 50 lb/ft dead load (150 lb/ft total) at L/360: deflection-limited span is about 18 ft. At 200 lb/ft total: ~16 ft. Span tables in AISC Manual give code-prescriptive values; this calculator computes the same numbers with full transparency.