About Timber Design Calculator
The timber design calculator checks a solid timber or glulam beam, column, or joist for bending, shear, bearing, and deflection per Eurocode 5, NDS, and AS 1720. It is used by structural and timber engineers to verify section sizes under service-class and load-duration conditions.
Enter the width, depth, and span, choose the support and load type, set the design and service loads, and pick the timber grade, moisture class, and load-duration class. The tool returns the applied and allowable bending and shear stresses, the deflection against its limit, and utilization ratios that update in real time.
How It Works
- Compute the bending moment, shear, and deflection coefficient from the support condition (simple, cantilever, continuous), load type, span, and loads.
- Apply modification factors: under EC5 the design strength is f_d = kmod * f_k / gammaM; under NDS the allowable value is multiplied by CD, CM, and CF.
- Calculate the applied bending stress M/W and shear stress 3V/(2A), then divide by the design strengths to get utilization ratios.
- Compute the service deflection from 5wL^4/(384 EI) (or the relevant coefficient) and compare it to the L/300 (EC5) or L/360 (NDS) limit for a pass/fail verdict.
Worked Example
A 75 x 225 mm C24 beam (fb = 24, fv = 4.0 MPa, E = 11000 MPa) spanning 3.5 m simply supported under a 4 kN/m UDL, service class 1, medium-term (kmod = 0.80, gammaM = 1.3). Design strengths: fm,d = 0.80*24/1.3 = 14.77 MPa, fv,d = 0.80*4.0/1.3 = 2.46 MPa. With W = 75*225^2/6 = 632813 mm3, M = 4*3.5^2/8 = 6.13 kN.m gives a bending stress of 9.68 MPa (utilization 0.66). Deflection 5*4*3500^4/(384*11000*I) with I = 71.19e6 mm4 is 9.98 mm, within the L/300 = 11.67 mm limit, so the beam passes.
Formulas
- Design strength (EC5)
f_d = kmod * f_k / gammaM- Bending stress
sigma_m = M / W, W = b * h^2 / 6- Shear stress (rectangular section)
tau = 3 * V / (2 * b * h)- Deflection (simply supported UDL)
delta = 5 * w * L^4 / (384 * E * I)
Standards & References
- Eurocode 5 (EN 1995-1-1)
- NDS
- AS 1720
Frequently Asked Questions
Which timber design codes are supported?
The calculator supports Eurocode 5 (EN 1995-1-1), the US NDS, and AS 1720. EC5 and AS 1720 use the kmod / gammaM approach, while NDS applies adjustment factors CD, CM, and CF to allowable stresses.
What is the kmod factor in Eurocode 5?
kmod modifies the characteristic strength for load duration and service (moisture) class. It ranges from about 0.50 for permanent loads in wet conditions up to 1.10 for instantaneous loads in dry conditions, taken from EC5 Table 3.1.
What deflection limit does the tool apply?
It uses L/300 under Eurocode 5 and L/360 under NDS by default, comparing the computed service deflection to that limit and reporting the actual span-to-deflection ratio.
Can I design glulam as well as solid timber?
Yes. The grade library includes glulam grades (for example GL24h, GL28h, GL32h); for glulam the EC5 material factor gammaM is 1.25 instead of 1.3 for solid timber.