Wood Beam

How it calculates

Structural model and load combinations

The calculator models the beam as a one-dimensional element and computes bending moment, shear, and deflection diagrams under each load combination. Loads are entered by type: dead (D), live (L), roof live (Lr), snow (S), wind (W) and earthquake (E). The calculator then applies the NBCC 2015 strength (ULS) and serviceability (SLS) combinations from your project defaults, for example:

• 1.25D + 1.5L
• 1.25D + 1.5S + 1.0L
• 0.9D - 1.4W (wind uplift)

Importance factors are not applied to the load combinations. Short-term serviceability combinations cover live, snow, wind and wind uplift. Long-term combinations cover dead load alone and dead plus sustained live load.

Bending moment resistance

Factored bending resistance M_r uses the specified bending strength f_b adjusted by CSA O86:19 modification factors, with phi_b = 0.9:

• K_D - load-duration factor, taken from the governing load combination
• K_H - system factor for repetitive-member, built-up, glulam and LVL assemblies
• K_S - service-condition factor
• K_T - treatment factor
• K_Zb - size factor for sawn lumber and LVL; glulam bent about the strong axis uses the lesser of K_Zbg and K_L

Lateral-torsional buckling is checked for strong-axis bending when d/b exceeds 2.5 and the beam is not braced on both edges. Top-edge bracing prevents it in positive bending and bottom-edge bracing in negative bending. For each unbraced segment, the effective length is 1.92 times the unbraced length, and the slenderness ratio is C_B = sqrt(L_e × d / b²). K_L is 1.0 up to C_B = 10, then reduces with C_B up to C_K, with an Euler-type form beyond. The calculator flags a failure if C_B exceeds 50.

moment utilization = M_f / M_r ≤ 1.0

Shear resistance

Factored shear resistance is V_r = phi_v × F_v × 2A/3 × K_Zv, with phi_v = 0.9. F_v applies the load-duration, system, service-condition and treatment factors to the specified shear strength f_v. LVL takes no system factor for shear, and K_Zv is 1.0 for LVL. Glulam bent about the strong axis uses the CSA O86 simplified shear method without K_Zv, which is valid for members under 2 m³. Shear demand is taken at the location of peak shear, which may conservatively be directly over the support rather than at distance d.

shear utilization = V_f / V_r ≤ 1.0

Bearing capacity

For each support, the factored reaction Q_f is checked against Q_r = phi_cp × f_cp × K_D × K_Scp × K_Tcp × A_b × K_B × K_Zcp, with phi_cp = 0.8. The bearing area A_b is the bearing length entered for that support times the member width. The length-of-bearing factor K_B applies to bearings under 150 mm that are more than 75 mm from the member ends. The check is skipped for supports with zero bearing length, and bearing at point loads is not checked.

Deflection checks (CSA O86:19 Cl.5.4.2 / NBCC Commentary D)

Deflection is computed with E_S × I, where E_S applies the service-condition and treatment factors to E. Two deflection limits are checked per span by default:

1. Live/short-term deflection - the maximum from the short-term serviceability combinations, checked against a user-defined span/n ratio
2. Long-term deflection - the maximum from the dead and sustained live combinations, checked against a separate span/n ratio

A third DL+LL deflection check runs when your project defaults define DL+LL combinations. An optional absolute deflection limit in mm applies alongside the span ratios. Cantilever limits can use twice the cantilever length, and shear deflection can be added as an option.

Load linking

Support reactions are exported as linked outputs, split by load type. Wood Column and Steel Column calculations in the same project can import them, and other beams can take them as point loads. When the beam inputs change, the linked reactions update with no manual re-entry of loads.