Wood Column (ASD)

How it calculates

The Wood Column (ASD) calculator checks sawn lumber, glulam and structural composite lumber (SCL) columns and studs to NDS 2024 Allowable Stress Design. Sawn lumber covers dimension lumber, MSR lumber, posts and timbers, and beams and stringers. Columns can be single or built up from up to five plies. Each check divides demand by adjusted capacity, and the column passes when every ratio is 1.0 or less.

Compression

Compression is checked about each axis with its own unbraced length:

• Sawn lumber: F'c = Fc × CD × CM × Ct × CF × Ci × Cp
• Glulam and SCL: F'c = Fc × CD × CM × Ct × Cp

Cp comes from the Euler buckling stress FcE = 0.822 E'min / (Le/d)² and F*c, which is F'c without Cp. The factor c is 0.8 for sawn lumber and 0.9 for glulam and SCL. Supports and strong-axis or weak-axis braces divide the column into segments. K defaults to 1.0, or 2.0 for cantilevers, and you can override it per segment.

Strong-axis capacity is the lesser of the buckling capacity and the crushing capacity of the net section at the largest hole you enter. Setting continuous weak-axis bracing to Yes takes Cp,y as 1.0. For built-up columns, Kf (0.6 nailed, 0.75 bolted) reduces Cp for weak-axis buckling.

Bending and shear

Bending, shear and deflection are checked on each axis that carries lateral loads or moments. Each axial load carries an eccentricity, which defaults to the value you enter for that axis.

• Sawn lumber, strong axis: F'b = Fb × CD × CM × Ct × CF × Ci × Cr × CL
• Sawn lumber, weak axis: F'b = Fb × CD × CM × Ct × CF × Ci × Cr × Cfu
• Glulam, strong axis: F'b = Fb × CD × CM × Ct × the lesser of CL and CV
• Glulam, weak axis: F'b = Fb × CD × CM × Ct × Cfu
• SCL: Fb × CD × CM × Ct × Cr, with CL and CV on the strong axis and Cfu on the weak axis

Cr is 1.15 for repetitive sawn members and 1.04 for SCL. CL comes from the critical buckling moment using the AWC TR14 method. CL is 1.0 when you set continuous lateral-torsional bracing, or when the section is at least as wide as it is deep.

Shear capacity is V' = (2/3) × F'v × A on each axis. F'v = Fv × CD × CM × Ct × Ci for sawn lumber. Glulam takes Cvr = 0.72 when live load is set to impact.

Combined loading

With lateral loads or moments present, compression and biaxial bending are checked together:

(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

A second check, fc / FcE,y + (fb,x / FbE)² ≤ 1.0, covers compression with lateral-torsional buckling. The summary is the larger of the two, taken for the critical load combination rather than an envelope.

When the column goes into tension, tension capacity is F't × An on the net section. Combined tension and bending checks ft / F't + fb,x / Fb,x + fb,y / Fb,y ≤ 1.0, where F*b excludes CL. A second check limits (fb - ft) / F**b, the net compression side, to 1.0.

Bearing

Bearing checks are optional and run on the bearing area you enter, for the critical axial load:

• Parallel to grain: F*c × Ab, using the compression value without Cp
• Perpendicular to grain: F'c⊥ = Fc⊥ × CM × Ct × Cb, times Ab

The bearing area factor Cb is conservatively taken as 1.0. Perpendicular bearing gets no load duration factor. It assumes the column bears on a member of the same species and grade, unless you pick a different one.

Deflection

Lateral deflection is checked on each loaded axis for short-term, long-term and simplified dead plus live or snow combinations. The long-term combination is kD + L + Lr, where k adds creep to dead load (0.5 dry, 1.0 wet by default). Each limit is a span ratio you set, capped by an absolute limit. Second-order effects are not included in the deflection.

Load duration and service conditions

• CD is set by the shortest-duration load in each combination, from 0.9 for dead load alone to 1.6 for wind and seismic.
• CM applies in wet service, above 19% moisture content (16% for glulam). For SCL in wet use, you enter the manufacturer's factor.
• Ct reduces capacity above 100°F, in two bands up to 150°F.
• Ci applies to incised sawn lumber. Timbers with a nominal width over 4 in. are not reduced.

Stud walls and load linking

Axial and moment loads from linked beam calculations feed the column's load tables. In stud wall mode, line loads are converted to stud loads using the stud spacing, and can link from beams above. Column reactions link on to US spread, pole and combined footings. Stud wall reactions link to wall footings and masonry walls as line loads.

Assumptions

The calculator states its assumptions on every sheet. The ones that most often matter:

• Members are straight, prismatic and not notched.
• Glulam has at least four laminations.
• In built-up columns, strong and weak axes refer to the individual plies.
• Impact live loads conservatively assume treated wood, with CD = 1.6.