Timber member design (AS 1720.1)

CivilKit Studio can design timber beams, posts and rafters to AS 1720.1 (the Australian timber structures standard) - the same model you analyse for steel can carry timber members, checked automatically when you solve.

First-pass / maturing feature

Timber design is an early, honest slice and is still being matured. Read Scope & assumptions at the bottom before relying on a number - the section library is not yet engineer-verified, stability handling is simplified, and timber connections are not designed (the connection tools are steel-only for now).

A "member" is one stick of timber in the model (a beam, post or rafter). A "section" is its size and grade - e.g. 240x45 MGP10, meaning 240 mm deep, 45 mm wide, machine-graded pine grade 10.

Assigning a timber section

Timber lives in its own catalogue, separate from steel, in the same section picker.

1. Select a member and open its inspector (the panel on the right).
2. Click Browse sections to open the section library.
3. At the top, change the Catalogue dropdown from a steel country to Timber (AU, AS 1720). The title switches to Timber section library.
4. Pick a size from the list (e.g. 190x45 F17) and apply it.

Choosing a timber section does three things automatically:

• Routes the member to AS 1720 - it will be checked to the timber standard, not AS 4100 steel.
• Sets timber stiffness - the member is pointed at a timber material whose stiffness (E, the modulus of elasticity) comes from the grade, so the analysis bends it like wood, not steel. Timber is far less stiff than steel, so deflections will be larger and realistic.
• Renders it in a wood tone - the member is drawn in a timber colour in the 3D view, so you can see at a glance which members are timber.

The grade is part of the name

A timber section name carries both the size and the stress grade - the strength class. The library covers visually/machine-stress-graded sawn timber (F5, F7, F8, F11, F14, F17, F22, F27 and MGP10, MGP12, MGP15) and glue-laminated timber (GL8, GL10, GL12, GL13, GL17). Higher numbers are stronger and stiffer.

Seeing the design check

The check appears in the inspector after you solve.

1. Assign a timber section (above).
2. Solve the model (press S) with Linear static or P-Delta.
3. Select the timber member. Its Timber member (AS 1720.1) panel sits in the inspector's Connection & material checks group. For a timber member that group opens automatically and the timber panel leads it, so the check is in front of you the moment you select the member - no hunting through folds.

The demands (the forces the member must carry - bending, compression and shear) are pulled straight from the analysis, so you don't type them in. They refresh each time you re-solve.

Inputs and outputs

The panel shows what it used and what it found.

Field	What it is
Grade	The stress grade (F5...GL17). Set by the section, editable here.
Breadth b (mm)	Section width.
Depth d (mm)	Section depth (the tall direction).
M* (kN·m)	Design bending moment from the analysis (the bending demand).
N*c (kN)	Design compression force from the analysis.
V* (kN)	Design shear force from the analysis.
k1 (duration)	Load-duration factor - shorter loads (wind) allow more, long loads (permanent) less.
k4 (moisture)	Moisture-condition factor - dry vs damp service.
k9 (sharing)	Strength-sharing factor - for grids of members sharing load (e.g. closely spaced joists).
k12 (stability)	Stability factor - reduces capacity for slender members that could buckle sideways.

The k-factors are AS 1720.1 modification factors: multipliers that adjust the raw timber strength for the real service conditions. They all default to 1.0 (the neutral, mostly conservative value). You set them for your situation - the panel footnote spells out which is which.

What the panel reports:

Output	Meaning
Bending M*/φM	Bending utilisation - demand ÷ capacity (a bar; green ≤ 1, red > 1).
Compression N*/φN	Compression utilisation.
Shear V*/φV	Shear utilisation.
Governing	The worst of the three ratios, with OK (≤ 1) or OVER (> 1) and which action drives it.
φM / φN / φV	The design capacities (kN·m / kN / kN) - bending, compression, shear.

"Utilisation" is demand divided by capacity: under 1.0 the member is adequate, over 1.0 it is overloaded. "φ" (phi) is the capacity-reduction factor already built into the capacity, as the standard requires.

Show your working

The panel includes a clause-referenced AS 1720.1 working that substitutes your k-factors into the capacity formula, with a Copy button - so you can paste the calculation, with the standard's clauses, into your own notes. The capacities come from the validated AS 1720.1 calculation core (the same engine behind the calculators).

Try the timber pergola sample

The quickest way to see it working is the built-in sample.

1. Open the model gallery and load Timber pergola (AS 1720) (under Frames).
2. It is a 4.8 x 3.0 m, 2.4 m-high timber frame: glulam posts (90x90), glulam eave beams (90x190) and MGP10 rafters (45x140) under a light roof load. The whole frame renders in the timber tone.
3. Solve (S), then select any member - posts, beams or rafters - to see its AS 1720.1 check populated with the real forces from the analysis.

There is also a simpler Timber floor beam (AS 1720) sample (under Beams): a 4.5 m simply-supported F17 hardwood floor beam (65x300) under a floor load - good for seeing a single bending-governed member.

Scope & assumptions (read before professional use)

This is a first-pass tool. Be aware of these limits:

• Section library not yet verified. The timber catalogue is flagged verified: false - the geometry is exact (computed from breadth x depth) but the grades and stiffness values are pending an engineer cross-check against the standard and manufacturer data. Confirm grade and properties before relying on a result.
• Density is nominal - a softwood/hardwood default is used for self-weight only, not a graded value.
• Stability is simplified. Out-of-plane buckling is handled through the k12 stability factor, which you set; the panel does not derive an effective length from intermediate restraints. With the default k-factors of 1.0 there is no stability reduction beyond what you enter, so set k12 (and the duration/moisture factors) deliberately for your member - do not assume the defaults suit your case.
• Connections are not designed. Timber joints (bolts, coach screws, nail plates, joist hangers) are not checked - the connection designer is steel-only at present. Design timber connections separately.
• The combined bending-plus-compression interaction and full member-stability derivation are planned refinements.

Verify before professional use

Always confirm the grade, section properties, k-factors and the governing load case against AS 1720.1 and a manufacturer's data before relying on these numbers. For the underlying maths and other materials, see the design checks overview, sections and the concrete member design page.