Timber Properties and ASD Design Values

Timber Properties and ASD Design Values

For the July 2026 PE Civil: Structural exam, wood design is ASD only. Use the 2018 NDS and Supplement, the 2015 SDPWS where a lateral system is involved, and the current NCEES PE Civil Reference Handbook. Do not import NDS LRFD format-conversion, resistance, or time-effect factors, and do not substitute a later NDS edition.

Reference values begin with identity

Wood is orthotropic and naturally variable. A reference value is not selected by nominal dimensions alone. Establish:

For sawn lumber, common reference values include bending Fb, tension parallel to grain Ft, shear parallel to grain Fv, compression perpendicular Fc⊥, compression parallel Fc, modulus E, and stability modulus Emin. They answer different limit states. Do not use Fb to check bearing or use average E where the NDS stability procedure calls for Emin.

Nominal size identifies the commercial member; stress calculations use actual dressed dimensions unless the problem states otherwise. A 2×10 is not 2 in by 10 in. Species, grade, and seasoning marks must agree with the specified member rather than being inferred from appearance.

ASD adjustment workflow

Write an adjusted value as

F' = F × (product of factors applicable to that property).

Use the 2018 NDS applicability table, not one memorized string. Important factors include:

• CD, load duration, for the ASD properties to which the table assigns it; it does not generally adjust E or compression perpendicular to grain;
• CM, wet service, whose value depends on the property and tabulated conditions;
• Ct, elevated-temperature service;
• CF, size, where applicable to dimension lumber values;
• Cfu, flat use, when bending orientation triggers it;
• Ci, incising, for affected properties of incised lumber;
• Cr, repetitive member, for qualifying bending members;
• CL, beam stability, and CP, column stability, determined from bracing and slenderness rather than chosen as generic environmental factors;
• Cb, bearing area, for eligible compression-perpendicular checks.

A factor absent from the property's row does not become applicable because its name sounds relevant. Determine the governing ASD load combination and its permitted CD; then apply moisture, temperature, geometry, treatment, system, and stability factors required for that property. Compare the stress or force effect from the same ASD load basis with the adjusted allowable value or capacity.

Load duration and repetitive action

The reference condition corresponds to normal load duration. Shorter-duration maximum loads can permit a larger adjusted strength through CD, while permanent loading does not receive that benefit. Determine duration from the load causing the maximum combined stress, not from the project calendar. CD changes allowable strength for eligible properties; it does not reduce calculated load.

For sawn-lumber bending, Cr recognizes load sharing among three or more qualifying parallel members, in contact or spaced no more than 24 in on center, connected by a floor, roof, or other load-distributing element adequate to support the design load. A single beam, isolated header, or pair of joists does not qualify merely because similar lumber exists elsewhere. Engineered products use their applicable NDS or manufacturer provisions rather than automatically inheriting the sawn-lumber factor.

Worked adjusted bending value

A repetitive floor-joist problem gives reference Fb = 1,000 psi. The stated applicable factors are CD = 1.15, CM = 0.85, CF = 1.10, Cr = 1.15, and CL = 1.00; all other applicable factors are 1.0. Then

Fb' = 1,000(1.15)(0.85)(1.10)(1.15)(1.00) = 1,237 psi.

For an actual 1.5 in × 9.25 in rectangular joist,

S = bd²/6 = 1.5(9.25²)/6 = 21.39 in³.

The ASD bending moment corresponding to this stress check is

Mallow = Fb'S = 1,237(21.39) = 26,460 lb-in = 2.21 kip-ft.

If analysis gives M = 2.00 kip-ft, the bending stress is 2.00(12)(1,000)/21.39 = 1,122 psi, which is less than Fb'; this bending check passes. That result does not complete shear, bearing, deflection, connection, or stability checks.

Do not apply Cr unless the stated joist group and sheathing meet its conditions. Do not apply CD again to the 2.00-kip-ft demand. The factors adjust the allowable property once.

Moisture, orientation, and engineered wood

Wet service is not synonymous with rain during construction. Use NDS service-condition definitions and the property's tabulated CM; bending, compression, and stiffness can receive different factors. Moisture also causes shrinkage and swelling primarily across grain, affecting bearing, connections, and differential movement.

For glulam, verify the bending axis, tension side, combination symbol, and whether a beam can be inverted; laminations may be arranged for a specified stress pattern. For laminated veneer lumber or other structural composite lumber, use product-specific reference values and orientation. For wood structural panels, align the marked strength axis with the assumed span and diaphragm direction. A correctly graded product installed about the wrong axis can fail the analytical assumption without any material defect.

Finish with four questions: Did I select the correct product table and grade? Did I use actual dimensions and axes? Did I apply each factor only to an eligible property? Did I compare ASD demand with ASD capacity? If any answer is uncertain, an attractive numerical ratio is not yet a valid wood design.