Wood Member (Design Only - ASD, NDS 2018)

How it calculates

Design method

This calculator uses Allowable Stress Design (ASD) per the AWC National Design Specification for Wood Construction, 2018 edition (NDS 2018). Unlike the Wood Beam or Wood Column calculators - which are purpose-built for single load-path elements - this design-only calculator accepts any combination of axial force and bending moment as direct inputs or via a linked Analysis Module element. It is the standard tool for checking truss chords and frame members where combined loading governs.

The governing requirement across all limit states is that every utilization ratio must satisfy:

actual stress / adjusted allowable stress ≤ 1.0

Only sawn lumber sections are supported in this version, and only strong-axis (X-X) bending is evaluated for combined loading.

Adjustment factors

Reference design values (Fc, Fb, Ft, Fv, Fc⊥, Emin) from the NDS 2018 Supplement are multiplied by all applicable adjustment factors before any stress check is performed:

• CD (load duration factor) - the single most influential factor for ASD; short-term loads (wind, seismic at CD = 1.6) allow higher allowable stresses than long-duration dead loads (CD = 0.9)
• CM (wet service factor) - reduces allowable values when in-service moisture exceeds 19% for sawn lumber; dry service (≤19%) uses CM = 1.0 for most properties
• Ct (temperature factor) - reduces reference values for members in sustained high-temperature environments (above 100°F); at normal conditions Ct = 1.0
• CF (size factor) - adjusts Fb, Ft, and Fc for sawn lumber based on nominal cross-section depth and width; for members deeper than 12 in. the factor falls below 1.0
• Ci (incising factor) - reduces allowable values when preservative treatment requires incising the wood fibers; Ci = 0.80 for Fb, Ft, Fc and 0.67 for E and Emin
• Cr (repetitive member factor) - increases Fb by 1.15 when three or more parallel sawn lumber members at 24 in. or less center-to-center spacing share load through adequate sheathing or decking
• CP (column stability factor) - reduces Fc to account for slenderness and the risk of buckling; computed per NDS Eq. 3.7-1 as a function of the Euler critical stress FcE and the adjusted reference stress F*c

Column stability factor

The column stability factor CP controls the allowable compression stress and is computed independently for each principal axis:

FcE = 0.822 × E'min / (Le / d)²

Where Le is the effective column length for the axis being checked and d is the corresponding cross-section dimension. A separate slenderness ratio and FcE is calculated for each axis. The governing (lower) FcE is used in the combined interaction equation.

CP is then found from NDS Eq. 3.7-1:

CP = (1 + FcE / Fc) / (2c) - sqrt[((1 + FcE / Fc) / (2c))² - FcE / (F*c × c)]

Where c = 0.8 for sawn lumber and F*c = Fc × CD × CM × Ct × CF × Ci (all factors except CP). Higher slenderness ratios produce lower FcE and lower CP values, increasing the risk of buckling governing over material crushing.

Individual stress checks

Before evaluating the interaction equations, each stress component is checked individually:

Compression: fc / F'c ≤ 1.0, where F'c = Fc × CD × CM × Ct × CF × Ci × CP

Tension: ft / F't ≤ 1.0, where F't = Ft × CD × CM × Ct × CF × Ci

Bending: fb / F'b ≤ 1.0, where F'b = Fb × CD × CM × Ct × CF × Ci × CL × Cr

The beam stability factor CL accounts for lateral-torsional buckling; for members with continuous bracing on the compression edge, CL = 1.0.

Shear: fv / F'v ≤ 1.0, where F'v = Fv × CD × CM × Ct × Ci

Combined axial compression and bending interaction

For members under simultaneous compression and bending - the governing case for most truss chords and frame rafter members - the calculator evaluates NDS 2018 Eq. 3.9-3 for strong-axis bending:

(fc / F'c)² + fb,x / (F'b,x × [1 - fc / FcE,x]) ≤ 1.0

The denominator term [1 - fc / FcE,x] is a P-delta amplification factor: as axial compression approaches the Euler buckling stress FcE, lateral deflection under bending increases and the effective bending demand rises. When fc approaches FcE the denominator approaches zero, correctly capturing the instability limit.

If biaxial bending is also present, the full NDS Eq. 3.9-4 is applied:

(fc / F'c)² + fb,x / (F'b,x × [1 - fc / FcE,x]) + fb,y / (F'b,y × [1 - fc / FcE,y - (fb,x / FbE)²]) ≤ 1.0

Where FbE = 1.20 × E'min / RB² is the critical buckling stress for lateral-torsional buckling. The nested term (fb,x / FbE)² represents the interaction between strong-axis bending and lateral buckling on the weak-axis bending capacity.

Combined tension and bending interaction

For truss bottom chords and other tension members with simultaneous bending, NDS 2018 Eq. 3.9-1 governs:

ft / F't + fb / F'b ≤ 1.0

This linear interaction does not include a P-delta amplification term because tension stiffens the member rather than amplifying lateral displacement. NDS Eq. 3.9-2 provides a complementary check at the extreme fiber for the combined stress state.

Assumptions and scope

The calculator assumes members are straight, prismatic (not tapered), and not notched. For built-up members where multiple plies increase the depth beyond the width, the strong and weak axes refer to the axes of individual plies rather than the composite section. Only sawn lumber is supported - engineered lumber (LVL, PSL, LSL) and glulam require the appropriate section properties and different reference values.