Wood Timber Calculator

Wood and timber structural design per NDS. Bending, shear, deflection, and connection design for sawn lumber and glulam members with load duration factors. Educational use only.

This page documents the scope, inputs, outputs, and computational approach of the Wood Timber Calculator on steelcalculator.app. The interactive calculator runs in your browser; this documentation ensures the page is useful even without JavaScript.

What this tool is for

Preliminary bending, shear, and deflection screening of sawn lumber and glulam members.
Understanding how NDS adjustment factors (CD, CM, Ct, CL, CF, Cfu, Ci, Cr) modify reference design values.
Comparing member sizes and species/grade combinations for typical framing conditions.

What this tool is not for

It does not handle all NDS provisions including notch effects, bearing at supports, or lateral stability of unbraced deep members.
It does not cover engineered wood products (I-joists, LVL, CLT) beyond basic glulam.
It does not replace project-specific engineering analysis for fire-rated assemblies, high-moisture environments, or preservative-treated lumber.

Key concepts this page covers

NDS reference design values and adjustment factors
load duration factor CD
beam stability factor CL
size factor CF for sawn lumber

Inputs and outputs

Typical inputs: species and grade, member dimensions (nominal or actual), span, unbraced length, load type and magnitude, moisture and temperature conditions.

Typical outputs: adjusted design values (Fb', Fv', E'), applied stress vs. allowable stress ratios, deflection check (L/240, L/360), and the controlling limit state.

Computation approach

The calculator retrieves reference design values for the selected species/grade from NDS Supplement tables, applies the chain of adjustment factors per NDS Section 4, computes actual bending stress fb = M/S and shear stress fv = 3V/(2A) for rectangular sections, and compares these to the adjusted allowable stresses. Deflection is computed using standard elastic beam formulas with the adjusted modulus of elasticity.

Frequently Asked Questions

What is the load duration factor CD in NDS? CD accounts for the fact that wood can sustain higher stresses for short-duration loads. For permanent (dead) loads CD = 0.90, for 10-year occupancy loads CD = 1.00, for 2-month snow CD = 1.15, for 7-day construction CD = 1.25, and for wind/seismic CD = 1.60. The shortest-duration load in the combination determines which CD applies. This is unique to wood design and has no direct equivalent in steel or concrete codes.

What is the difference between sawn lumber and glulam design values? Sawn lumber design values depend on species, grade, and size category (dimension lumber, timbers, decking). Glulam (glued-laminated timber) is manufactured in controlled conditions with graded laminations, so it achieves higher and more reliable design values than equivalent-size sawn timber. Glulam also uses different adjustment factors (volume factor CV instead of size factor CF) and has separate values for positive and negative bending in unbalanced layups.

Why are there so many adjustment factors in NDS? Wood is a natural material whose strength depends on moisture content, temperature, size, duration of load, and manufacturing process. Each adjustment factor accounts for a specific condition that modifies the reference design value established under standard test conditions. While this creates complexity, it also means wood design can be optimized more finely for actual service conditions than a single factor approach would allow.

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.