How to size an LVL beam, explained

An LVL beam is sized by whichever of three checks runs out of headroom first — strength in bending, strength in shear, or stiffness — all computed against design values the manufacturer publishes for that exact product. The math is ordinary simple-beam theory; the honesty is in whose numbers you feed it.

The three checks

bending: M/S ≤ Fb · shear: 1.5V/(b·d) ≤ Fv · deflection: 5wL⁴/384EI ≤ L/360 (live), L/240 (total)

For a uniform load, M = wL²/8 and V = wL/2; the section properties come from the built-up width (plies × ply width) and depth. The LVL beam calculator runs all three, reports which governs, and inverts the math into the allowable load and minimum depth. The generic-section version of the same theory is the beam deflection calculator, and the deflection limits themselves are unpacked in the L/360 guide.

Worked example — 2-ply 1¾ × 11⅞, 12 ft span

Carrying 320 plf live + 120 plf dead (a typical floor-beam tributary), with maker values Fb = 2,600 psi, Fv = 285 psi, E = 2.0 × 10⁶ psi:

M = 440 × 12²/8 = 7,920 lb·ft → fb = 1,155 psi (44% of Fb) · fv = 95 psi (33%) · δ_live = 0.15 in vs 0.40 limit (38%)

Bending governs at 44% — comfortable. The calculator's inverse outputs tell the sharper story: this beam could carry about 990 plf total before anything hits its limit, and the minimum depth for bending alone is 8.6 in. A 9¼ in LVL would clear strength here (73% bending) with live deflection at 81% — workable, but one span class up and deflection takes over, which is the general pattern.

What the maker's sheet controls

Fb, Fv and E are product data: LVL grades differ by brand, depth class and treatment, and published values like 2,600/285/2.0E6 are typical, not universal. Repetitive-member factors, depth factors and load-duration adjustments also come from the maker's literature. For the permit set, the manufacturer's span tables or sizing software are what the plans examiner expects to see — this math is how you understand and sanity-check them, and how you bracket a size before opening the catalog.

Common mistakes

• Sizing a point-load beam with uniform-load math. A girder picking up another beam mid-span sees M = PL/4-type moments — different formulas, same checks. The beam capacity calculator covers the common cases.
• Using nominal instead of actual width. Plies are 1¾ in each; a "4×" of anything is not 4 inches. Section properties want actual dimensions.
• Forgetting the unbraced compression edge. The Fb on the sheet assumes the top edge is laterally restrained (sheathing, joists). A beam loaded on an unbraced edge needs the maker's stability reduction.
• Checking the beam and skipping the bearing. The reaction has to crush neither the LVL end nor the plate under it — bearing length is its own check at each support.

Frequently asked questions

How do you size an LVL beam?

Three checks against the manufacturer’s design values: bending (M/S vs Fb), shear (1.5V/A vs Fv) and deflection (5wL⁴/384EI vs L/360 live, L/240 total). A 2-ply 1¾ × 11⅞ LVL on a 12 ft span carrying 440 plf passes all three with bending governing at 44% utilization.

What LVL design values should I use?

The ones on the maker’s sheet for the exact product — they are manufacturer data, not code constants. Commonly published values run around Fb = 2,600–3,100 psi, Fv ≈ 285 psi and E = 1.9–2.1 × 10⁶ psi, but the stamped product literature governs, and the permit set usually cites the maker’s span tables or software.

Does doubling LVL plies double the strength?

For bending and shear, yes in proportion to width: plies multiply b, and both S = bd²/6 and I = bd³/12 scale linearly with b. Depth is the lever though — going from 9¼ to 11⅞ at the same width raises stiffness about 2.1× because I grows with depth cubed.

Why does deflection govern long LVL spans?

Strength scales with d² but stiffness needs d³ and fights L⁴: double the span at the same load and bending stress rises 4× while deflection rises 16×. Short spans fail the strength checks first; long ones drift past L/360 while stresses still look comfortable.