Design codes
Structural design to AS/NZS 1170, AS 4100 and AS 3600
How Struxt3D designs to Australian and New Zealand standards: AS/NZS 1170 combinations, wind and earthquake inputs, AS 4100 steel and AS 3600 concrete checks.
Australia and New Zealand share the AS/NZS 1170 series for structural actions: 1170.0 for combinations, 1170.1 for imposed loads, 1170.2 for wind and 1170.4 for earthquake (Australia). Steel members are designed to AS 4100 and concrete to AS 3600. Choosing AS/NZS in Struxt3D applies these, with Australian UB/UC sections and grades. Below is exactly what the app uses, a worked example on a portal frame shed, and what you still need to check.
Standards used
| Standard | Used for |
|---|---|
| AS/NZS 1170.0 §4.2.2 | Strength load combinations (ψc = 0.4, ψE = 0.3) |
| AS/NZS 1170.1 | Imposed actions by use |
| AS/NZS 1170.2 (input) | Regional 3-second gust wind speed |
| AS 1170.4 | Earthquake: hazard kpZ, site sub-soil class, μ/Sp |
| AS 4100:2020 | Steel members and cables (7.2) |
| AS 3600-2018 | Concrete beams, columns, slabs, walls, footings |
Load combinations
| Combination | Purpose |
|---|---|
| 1.35D | Permanent action only |
| 1.2D + 1.5L | Imposed action |
| 1.2D + W + 0.4L | Wind, ψc = 0.4 |
| 0.9D + W | Wind uplift and overturning |
| D + 0.3L + E | Earthquake, ψE = 0.3 |
| D + L (service) | Deflections |
Wind combinations are made for +X, −X, +Y and −Y; earthquake ones for ±(EX + 0.3EY) and ±(0.3EX + EY). For a light portal shed, 0.9D + W is often the combination that governs the rafters and the holding-down bolts, because wind suction lifts the roof. The seismic mass includes 30 % of the imposed load.
Imposed loads
| Use | Q (kPa) |
|---|---|
| Office | 3.0 |
| Residential | 1.5 |
| Retail | 4.0 |
| Storage | 5.0 |
| Roof (non-trafficable) | 0.25 |
| Footbridge deck | 5.0 |
AS/NZS 1170.1 also gives the roof load as 0.12 + 1.8/A kPa (at least 0.25 kPa) and concentrated loads; Struxt3D uses the uniform 0.25 kPa.
Wind
AS/NZS 1170.2 starts from a regional 3-second gust speed VR (45 m/s by default, region A at a 1-in-500-year return). Struxt3D converts it to a pressure with an open-terrain profile and a gust factor:
This is a simplified profile, not the full AS/NZS 1170.2 method. Direction (Md), terrain-height (Mz,cat), shielding (Ms) and topographic (Mt) multipliers, roof Cp,e zones and internal pressure are not applied. For cyclonic regions C and D, or for roofs where suction governs, enter a site design speed Vsit,β and check the roof pressures by hand.
Earthquake
Enter the hazard factor kpZ (in g), the sub-soil class (Ae to Ee) and the structural system. Struxt3D uses an EN 1998-shaped spectrum with the soil parameters of the matching EN 1998 ground type, divided by μ/Sp:
| Structural system | μ/Sp |
|---|---|
| Ductile moment frame (μ = 4, Sp = 0.67) | 6.0 |
| Ordinary moment frame | 2.6 |
| Braced frame | 4.5 |
| Walls or core | 4.5 |
| Elastic | 1.0 |
The default kpZ of 0.22 g suits a more active region than most of Australia. For Sydney or Melbourne at a 1-in-500-year return, Z is about 0.08–0.09 and kp = 1.0, so enter about 0.08 g. AS 1170.4's own spectral shape Ch(T) differs from the EN 1998 shape used here; compare the base shear with the equivalent static method.
Materials and sections
| Material | Grades offered (default first) |
|---|---|
| Steel | Grade 300, Grade 350 (AS/NZS 3679.1; E = 200 GPa) |
| Concrete | N32, N25, N40 (Ec = 30.1, 26.7, 32.8 GPa) |
| Reinforcement | fsy = 500 MPa (D500N) |
| Glulam | GL13, GL10 |
Beams come from 250UB31.4 to 610UB101 and then 700WB130 and 900WB175; columns from 150UC30.0 upwards. Steel uses φ = 0.9 with the AS 4100 member compression curves. Concrete uses the AS 3600 capacity reduction factors and the rectangular stress block α2f′c.
Member and element checks
- Steel: AS 4100:2020 section capacity, member moment capacity with αm and αs (lateral-torsional buckling), compression, shear and combined actions. See steel beam design.
- Concrete: AS 3600-2018 bending, shear, columns, slabs with punching, walls and footings. See RC beam design.
- Connections: bolted and welded steel connections, base plates and anchors. See steel connection design.
Worked example: a portal frame shed
- Click Open the template to start the Portal frame shed.
- In Materials, choose AS/NZS. Rafters and columns switch to UB sections in Grade 300.
- In Supports & loads, keep wind on at 45 m/s. Earthquake rarely governs a light shed; leave it off.
- Press F5 and F6. In the results, choose Utilization and the load selector to see which wind direction governs.
Check 1: wind pressure at eaves height 6 m
Check 2: gravity on a floor (for a mezzanine)
Check 3: earthquake plateau with kpZ = 0.08 g
How AS/NZS changes the design
The Australian gravity combination 1.2G + 1.5Q sits between the Eurocode and the US one, and 1.35G on its own governs only when the imposed load is small compared with the dead load — common for roofs. With ψc = 0.4, wind combinations carry less imposed load than in other codes, so for light steel sheds the uplift case 0.9G + W is often what sizes the rafters, purlin cleats and holding-down bolts.
AS 4100 treats lateral-torsional buckling through the moment modification factor αm and the slenderness reduction factor αs. Struxt3D takes the effective length as the largest gap between restraints along the member (where other members frame in), so purlins and fly braces matter: an unrestrained rafter bottom flange under uplift can govern. Check that your real restraints match the modelled ones.
In AS 3600 the capacity reduction factor depends on the ductility of the section (kuo), similar in spirit to ACI. Grade N32 concrete and D500N bars are the defaults; N40 helps heavily loaded columns more than beams.
What you still need to check
- Full AS/NZS 1170.2 wind: wind region, multipliers, roof and local pressure zones, internal pressure from dominant openings.
- Snow (AS/NZS 1170.3) for alpine sites.
- Earthquake: the AS 1170.4 spectral shape and earthquake design category, the importance level, and NZS 1170.5 for New Zealand projects (which this simplified spectrum does not reproduce).
- NZS 3404 and NZS 3101 for New Zealand steel and concrete; Struxt3D uses the Australian AS 4100 and AS 3600.
See accuracy and limitations for the analysis assumptions.