Course · lesson 1 of 6

Timber beams: grades, duration and two routes

What makes timber different from steel and concrete: grades, moisture, how long a load lasts, and creep, with the two design routes the STATIX timber beam calculator offers, EN 1995-1-1 and SANS 10163-1.

1. Learning outcomes

After this lesson you can:

choose a grade and read its characteristic stresses;
set the service class and the load duration, and say what they do;
check a joist for bending, shear, bearing and deflection;
allow for creep in the final deflection;
tell the EN 1995 route from the SANS 10163-1 route and keep them apart.

2. Before you start

Loads 1, lessons 2 and 3 (floor loads) and lesson 1 (combinations).
The free timber beam calculator.
A pocket calculator and about 45 minutes.

3. Timber as a material

Timber grades. South African pine is stress graded S5, S7 and S10; SANS 10163-1 Table N.1 gives their characteristic bending, shear and bearing stresses and stiffness. Imported timber is often graded to EN 338 (C24). Glulam strengths come from the maker. The STATIX timber beam calculator fills the properties from the grade and lets you change them.

Service class. Timber is weaker and creeps more when it is wet. Service class 1 is dry inside a heated building, class 2 covered but humid, class 3 exposed. The class sets k_mod for strength and k_def for creep in the EN 1995 route.

Load duration. Timber carries a short load better than a permanent one. In the EN 1995 route k_mod multiplies the strength by the duration of the shortest action in the combination (3.1.3(2)): permanent load alone 0.6, with imposed floor load (medium-term) 0.8, with wind 0.9 or more.

Two routes. The timber beam calculator can check the same beam by EN 1995-1-1 (with the SANS or EN load combinations) or by SANS 10163-1:2003. The routes use different factors and do not give the same answer; never mix their factors.

4. The joist

A timber joist. Floor joists carry the boards and the load on them over a spacing; the line load is the floor load times the spacing. Bending and deflection usually decide a timber joist; shear and bearing rarely do unless the span is short or the bearing small.

Load sharing and restraint. Joists at close centres joined by boarding share the load, so a weak joist is helped by its neighbours (k_sys 1.1); the boards also hold the compression edge, so lateral-torsional buckling does not govern. Give the calculator the restraint spacing.

Bending and shear stresses in a rectangle. The bending stress is the moment over the section modulus; the shear stress is 1.5 times the average, on a width reduced for drying cracks (k_cr) in the EN 1995 route.

Bearing. At the support the reaction presses across the grain, where timber is weakest (f_c,90). The calculator checks it over the bearing length plus a small spread (EN 1995-1-1 6.1.5).

Deflection. Timber creeps under lasting load. The calculator checks the instantaneous deflection and the final deflection, where the permanent part (and the quasi-permanent part of the imposed load, ψ₂) grows by k_def. The limits are page inputs, span/300 and span/250 by default.

From floor loads to the joist: the permanent and imposed floor loads (Loads 1 lessons 2 and 3) times the joist spacing give the line loads the calculator takes.

5. The course

The course: three timber lessons with the timber beam calculator, two masonry lessons with the masonry wall calculator, and lintels with the lintel calculator.