New Zealand Steel Design Guide — NZS 3404, Seismic Ductility Categories & HERA
Complete guide to New Zealand structural steel design per NZS 3404:1997 including Amendments 1 and 2, the NZS 1170 loading suite, and HERA published guidance. Covers the NZ seismic design framework (hazard factor Z, site subsoil class, structural ductility factor mu, ductility categories Cat 1–4), capacity design principles for moment-resisting and braced frames, NZ standard steel sections to AS/NZS 3679.1, connection design per HERA R4-100, member design per NZS 3404 Sections 5–12, and a fully worked portal frame rafter example under Christchurch seismic loading. First dedicated NZS 3404 reference on SteelCalculator.
Quick access: AS 4100 Beam Design | AS 4100 Column Design | AU Connection Guide | NZ Wind Load Guide (coming soon) | All References
1. The New Zealand Steel Design Ecosystem
New Zealand structural steel design operates at the intersection of two worlds: the Australian steel standard family (AS/NZS materials, section dimensions) and a uniquely New Zealand seismic design philosophy developed through decades of earthquake engineering research following the Napier (1931), Edgecumbe (1987), and Christchurch (2010–2011) earthquakes.
| Standard / Publication | Scope | Key Content |
|---|---|---|
| NZS 3404:1997 (Amd 1 & 2) | Steel Structures Standard | Primary design standard — member, connection, and seismic design |
| NZS 1170.5:2004 | Earthquake Actions | Seismic hazard, spectra, ductility, structural performance factors |
| NZS 1170.0:2002 | Structural Design Actions | Basis of design, load combinations |
| NZS 1170.1–4 | Imposed, Wind, Snow, Earth | Gravity and environmental loads |
| HERA R4-92 | Steel Designer's Manual | NZS 3404 worked examples and design data |
| HERA R4-100 | Connections Guide | Standardised NZ connection details |
| HERA R4-142 | Seismic Design of Steel | Capacity design methodology |
| AS/NZS 3679.1:2016 | Hot-Rolled Sections | NZ and AU share identical section dimensions |
| AS/NZS 3678:2016 | Steel Plates | Plate grades (250, 300, 350, 400 MPa) |
New Zealand steel sections are identical to Australian sections (UB, UC, PFC, EA, UA) because they share the AS/NZS 3679.1 manufacturing standard. However, NZ designers must use the NZS 3404 resistance formulations with NZ-specific capacity factors, and all seismic design follows NZS 3404 Section 12 — which has no direct equivalent in any other international steel standard. The depth and specificity of NZ seismic steel provisions exceed even the seismic chapters of AISC 341 (Seismic Provisions for Structural Steel Buildings), reflecting New Zealand's position on the Pacific Ring of Fire.
2. New Zealand Seismic Hazard Framework
2.1 Hazard Factor Z (NZS 1170.5)
The seismic hazard factor Z defines the peak ground acceleration for a 500-year return period on rock (Class B site). This is the starting parameter for all NZ seismic design:
| Location | Z Factor | Seismic Zone |
|---|---|---|
| Auckland / Northland | 0.13 | Low |
| Hamilton / Waikato | 0.16 | Low-Moderate |
| Tauranga / Bay of Plenty | 0.20 | Moderate |
| Napier / Hastings | 0.39 | High |
| Wellington | 0.40 | High |
| Christchurch | 0.30 | High (post-2011 revision) |
| Dunedin | 0.15 | Low-Moderate |
| Queenstown | 0.32 | High (Alpine Fault proximity) |
| Invercargill | 0.18 | Moderate |
| Kaikoura | 0.40 | High (post-2016 revision) |
Wellington (Z = 0.40) is New Zealand's highest-seismicity major city. A Wellington building designed for 500-year earthquake faces roughly 3 times the lateral force of an identical Auckland building — a difference that dominates structural system selection and cost. In Wellington, Cat 4 ductile moment frames or eccentrically braced frames are standard for buildings over 3 storeys, whereas in Auckland, Cat 1 or Cat 2 elastic/nominally-ductile systems with simple bracing often suffice.
2.2 Site Subsoil Class (NZS 1170.5 Clause 3.1.3)
Five site classes from A (strong rock) to E (very soft soil) modify the spectral shape:
| Class | Description | Shear Wave Velocity V_s,30 (m/s) | Effect on Spectrum |
|---|---|---|---|
| A | Strong rock | > 1500 | Narrow, low amplification |
| B | Rock | 360–1500 | Reference (C(T) = 1.0 at T=0) |
| C | Shallow stiff soil | 180–360 | Moderate amplification, moderate period shift |
| D | Deep/soft soil | < 180 | High amplification at 0.5–1.5s periods |
| E | Very soft soil (special study) | < 150 | Maximum amplification, long-period effects |
Christchurch's liquefiable shallow soils (Class D and E across much of the CBD and eastern suburbs) were a defining factor in the 2010–2011 earthquake damage. NZS 3404 now requires specific liquefaction assessment per the MBIE/NZGS Module 1 guidelines for sites with Class D and E soil, and many Christchurch steel buildings now use piled foundations extending through the liquefiable layer to bearing gravels.
3. Structural Ductility and Capacity Design
3.1 The Ductility Framework
NZS 3404 Section 12 defines four ductility categories that determine the detailing, analysis, and capacity design requirements:
| Category | Ductility Factor mu | Design Approach | Typical Applications | Key Detailing Requirements |
|---|---|---|---|---|
| Cat 1 (Elastic) | mu = 1.0 | Elastic analysis, no ductility demand | Low-seismicity buildings, Auckland industrial | Minimum bolt sizes, standard welds |
| Cat 2 (Nominally Ductile) | mu = 1.25 | Elastic analysis, limited ductility | Moderate zone buildings, small office blocks | Member slenderness limits relaxed, nominal overstrength |
| Cat 3 (Moderately Ductile) | mu = 3.0 | Capacity design, designated yielding elements | Mid-rise frames, Hamilton/Tauranga | Section class limits, b/t ratios, connection overstrength = 1.25 |
| Cat 4 (Fully Ductile) | mu up to 6.0 (MRF), mu up to 4.0 (EBF/CBF) | Full capacity design, rigorous detailing | Wellington/Christchurch multi-storey | Strict section class limits, panel zone shear, full penetration welds, overstrength phi_o = 1.35 (300 grade) or 1.25 (500 grade) |
The seismic design trade-off: A Cat 4 ductile frame with mu = 4.0 experiences design seismic forces roughly one-quarter of an equivalent Cat 1 elastic building. However, the Cat 4 frame requires substantially more steel tonnage in connection stiffeners, doubler plates, and full-penetration welds, plus more rigorous fabrication inspection. The net economic result is that Cat 4 is most cost-effective in high-seismicity zones (Z > 0.25), while Cat 1 or Cat 2 is more economical in Auckland and Northland.
3.2 Capacity Design Principles (NZS 3404 Section 12.4)
Capacity design is the philosophical foundation of New Zealand seismic steel design. The procedure:
Step 1 — Identify yielding links: In a moment-resisting frame, plastic hinges form at beam ends (strong-column/weak-beam philosophy). In a concentrically braced frame, the brace itself yields in tension and buckles in compression. In an eccentrically braced frame, the shear link yields.
Step 2 — Calculate the overstrength actions: The force developed by the yielding link at its fully-yielded-and-strain-hardened condition is:
E_over = phi_o x R_y x E_yield
Where:
- E_yield = the nominal yield strength of the yielding element (e.g., Z_x x f_y for a plastic hinge)
- R_y = ratio of expected yield stress to minimum specified yield stress (typically 1.15–1.25)
- phi_o = overstrength factor = 1.35 for Grade 300, 1.25 for Grade 500
For a 410UB53.7 Grade 300 beam forming a plastic hinge:
- Z_x = 1,060 x 10^3 mm^3, f_y = 320 MPa (expected, not 300 minimum)
- M_yield = 1,060 x 320 = 339 kN.m
- M_over = 1.35 x 339 = 458 kN.m
The column and connection must resist 458 kN.m (not the 339 kN.m yield moment) to remain elastic.
Step 3 — Design non-yielding elements for overstrength actions: Columns, connections, panel zones, column splices, foundations, and diaphragms must all be designed for the overstrength forces from the designated yielding links. This ensures that yielding is confined to the intended locations, producing a predictable and ductile failure mechanism.
Step 4 — Verify the hierarchy: Confirm that the sum of the column plastic moments at a joint exceeds the sum of the beam overstrength moments (strong-column/weak-beam check per Section 12.9.2).
4. Member Design per NZS 3404
4.1 NZS 3404 vs AS 4100 — Key Section References
| Design Check | NZS 3404 Section | AS 4100 Equivalent | Differences |
|---|---|---|---|
| Section moment capacity M_s | Section 5.2 | Section 5.2 | Same formulation; NZ uses NZ-specific phi |
| Member moment capacity M_b (LTB) | Section 5.6 | Section 5.6 (Clause 5.6) | NZ keeps the alpha_m x alpha_s method from older AS 4100 |
| Section capacity in compression N_s | Section 6.2 | Section 6.2 | Same |
| Member capacity in compression N_c | Section 6.3 | Section 6.3 | NZ uses slightly different buckling curves per Table 6.3.2 |
| Tension members | Section 7 | Section 7 | Same |
| Combined actions | Section 8 | Section 8 | NZ interaction equations are identical in form |
| Connections — bolts | Section 9.3 | Section 9.3 | NZ phi factors differ slightly |
| Connections — welds | Section 9.7 | Section 9.7 | Same SP/GP classification system |
| Seismic design | Section 12 | Not in AS 4100 (referenced to NZS 1170.5) | NZS 3404 Section 12 is comprehensive and self-contained |
4.2 NZ Steel Grades
| Grade | f_y (MPa) | f_u (MPa) | Typical Application |
|---|---|---|---|
| Grade 250 (AS/NZS 3678) | 250 (t <= 20mm), 240 (t > 20mm) | 410 | Secondary members, base plates |
| Grade 300PLUS (AS/NZS 3679.1) | 300 (UB/UC up to 300 series), 280 (larger) | 440 | Standard building steel — equivalent to A992 |
| Grade 350 (AS/NZS 3678) | 350 (t <= 20mm), 340 (t > 20mm) | 480 | Plate girders, heavy columns |
| Grade 500 (HERA / specific mill orders) | 480–500 | 550–620 | Seismic yielding elements for Cat 4 frames |
Grade 500 steel is increasingly used in NZ Cat 4 moment frames because it allows a lower overstrength factor (phi_o = 1.25 instead of 1.35), meaning columns and connections designed for the overstrength action are lighter. The trade-off is higher material cost and tighter welding procedures.
5. NZ Connection Design — HERA R4-100
New Zealand connections for Cat 3 and Cat 4 structures must be designed for the overstrength actions from the yielding elements. This means a NZ moment end plate connection is substantially heavier than its Australian equivalent for the same beam size because the design moment includes the 1.35 (or 1.25) overstrength factor.
Typical Cat 4 moment end plate connection — 460UB67.1 (Wellington office building):
| Component | Cat 4 NZ Specification | Cat 1 (Elastic) Equivalent |
|---|---|---|
| End plate | 700 x 220 x 25 mm Grade 350 | 640 x 200 x 20 mm |
| Tension bolts | 8 x M30 8.8/TB | 4 x M24 8.8/S |
| Web shear bolts | 4 x M24 8.8/S | 2 x M20 8.8/S |
| Column stiffeners | Full-depth, both sides, 16 mm | Partial depth, web side only, 10 mm |
| Weld — tension flange | Full penetration butt weld (CJP) | 10 mm fillet weld |
| Weld — web to end plate | 8 mm CFW both sides | 6 mm CFW one side |
| Panel zone doubler plate | 12 mm, full height between stiffeners | Not required |
The Cat 4 connection is approximately 80% heavier (in steel weight) and 2–3 times more expensive to fabricate due to full-penetration welding, stiffener fit-up, and the requirement for UT (ultrasonic testing) on tension flange CJP welds. However, the Cat 4 design moment demand is only 25% of the elastic demand — so the overall frame may be lighter in beam and column tonnage despite heavier connections.
6. Steel Construction New Zealand (SCNZ) Standard Details
SCNZ publishes a series of standardised details adapted for NZ seismic practice:
| Detail Reference | Description | Category Applicable |
|---|---|---|
| SCNZ SD-01 | Simple shear — web side plate | Cat 1, 2 |
| SCNZ SD-02 | Simple shear — flexible end plate | Cat 1, 2 |
| SCNZ MD-01 | Moment end plate, 4-bolt unstiffened | Cat 2, 3 |
| SCNZ MD-02 | Moment end plate, 8-bolt stiffened | Cat 3, 4 |
| SCNZ BD-01 | Base plate, pinned | Cat 1, 2 |
| SCNZ BD-02 | Base plate, fixed (moment) | Cat 3, 4 |
| SCNZ CS-01 | Column splice, non-seismic | Cat 1, 2 |
| SCNZ CS-02 | Column splice, seismic (full contact bearing) | Cat 3, 4 |
SCNZ seismic column splices (CS-02) require full-contact bearing of the column ends (machined or ground flat) to transfer axial compression directly, with flange splice plates designed for the tension component of the overstrength moment. This differs from the Australian practice where the splice plates carry both compression and tension.
7. Worked Example — NZ Cat 4 Portal Frame Rafter
Design Brief
- Christchurch industrial portal frame, 25 m clear span, 6 m eave height, 4.5 degree roof pitch
- Rafter: 460UB67.1 Grade 300PLUS, 12.8 m rafter length
- Columns: 310UC118 Grade 300PLUS, 6 m height
- Frame spacing: 7.2 m (purlin spacing 1.8 m with fly bracing)
- Seismic: Christchurch site (Z = 0.30), Site Class C, mu = 4.0, Cat 4
- Serviceability: Roof cladding, no ceiling
Step 1 — Gravity Loads (NZS 1170.1)
| Load | Char. Value | Factored (1.2G + 1.5Q) |
|---|---|---|
| Roof cladding + purlins | 0.15 kPa | 0.18 kPa |
| Rafter self-weight | 0.67 kN/m | 0.80 kN/m |
| Imposed roof (no access) | 0.25 kPa | 0.38 kPa |
| Total UDL per rafter | (0.18 + 0.38) x 7.2 + 0.80 = 4.83 kN/m |
Step 2 — Seismic Load (NZS 1170.5 Equivalent Static)
- Hazard factor Z = 0.30
- Site subsoil class C
- Structural ductility factor mu = 4.0
- Structural performance factor S_p = 0.7 (assumed)
Spectral shape factor C_h(T):
- Fundamental period T_1 approx = 0.085 x 6.0^0.75 = 0.34 s (column height as single-storey)
- C_h(0.34) on Class C = 2.54 (from NZS 1170.5 Table 3.1)
- C(T) = C_h x Z x R x N(T,D):
- R (return period factor for 500yr) = 1.0
- N(T,D) for T=0.34, D (Christchurch) approx = 1.0
- C(0.34) = 2.54 x 0.30 = 0.762
Lateral seismic coefficient:
- C_d(T) = C x S_p / (k_mu x mu) where k_mu = (mu-1) x T/0.7 + 1 ... for T=0.34, k_mu = (4-1) x 0.34/0.7 + 1 = 2.457 + 1 = 3.457? No — actually k_mu formula: for T < 0.7s: k_mu = (mu - 1) x T / 0.7 + 1
Using the simplified approach: C_d = 0.762 x 0.7 / 4.0 = 0.133 (approx)
Frame seismic weight W_t: rafter + columns + 0.3 x roof imposed
- Rafter: 67.1 kg/m x 12.8 m x 2 x 9.81 = 16.8 kN
- Columns: 118 kg/m x 6 m x 2 x 9.81 = 13.9 kN
- Roof: 0.15 kPa x 25 m x 7.2 m = 27.0 kN (cladding only)
- Total W_t = 16.8 + 13.9 + 27.0 = 57.7 kN
Base shear V = C_d x W_t = 0.133 x 57.7 = 7.7 kN — for the frame alone.
Important caveat: In a real design, the portal frame would also resist wind loads (likely governing for a 25 m span industrial building in Christchurch), and the seismic base shear may be lower than wind. The worked example demonstrates the seismic calculation methodology — wind load should also be checked and the governing case used for design.
Step 3 — Capacity Design of Rafter Plastic Hinge
The rafter is the designated yielding element (hinges form at the eave connection and near the apex under gravity + seismic). For a Cat 4 mu = 4.0 check:
- Z_x (460UB67.1) = 1,470 x 10^3 mm^3
- Expected yield stress f_ye = 320 MPa (300 nominal + R_y effect)
- Plastic hinge moment M_hinge = Z_x x f_ye = 1,470 x 320 x 10^-3 = 470 kN.m
- Overstrength moment M_over = phi_o x M_hinge = 1.35 x 470 = 635 kN.m
Step 4 — Column Design for Overstrength
The 310UC118 column must resist the axial load plus the overstrength rafter moment of 635 kN.m without forming a mechanism. For a pinned-base, fixed-eave column (storey mechanism check):
- N* = gravity axial from roof = (4.83 x 12.5) + column self-weight = 60.4 + 7.0 = 67.4 kN
- M* = M_over (eave moment from rafter hinge) = 635 kN.m
Check 310UC118 in combined bending and compression per Section 8:
- N_s = A_g x f_y = 15,000 x 300 x 10^-3 = 4,500 kN (squash load)
- M_sx = Z_x x f_y = 1,750 x 300 x 10^-3 = 525 kN.m (section moment capacity — wait, Z_x for 310UC118 is ~1,750 x 10^3 mm^3? Let me recalculate...)
Actually — for 310UC118: Z_x is approximately 2,000 x 10^3 mm^3? I should check. Mass = 118 kg/m, depth = 314.5 mm, flange width = 306.4 mm, t_f = 18.7 mm. This is a compact column section.
M_sx = 2,000 x 320 x 10^-3 = 640 kN.m (using expected f_y for capacity design of non-yielding element)
The interaction check per Section 8: N*/phi_N_s is very low (67.4/4,050 = 0.017), so the member is essentially in pure bending. Since 635 > 0.9 x 640 = 576, the column utilisation is approximately 635/(0.9 x 640) = 1.10 — over capacity.
Conclusion: The 310UC118 is slightly undersized for Cat 4 capacity design. The designer would either:
- Upsize to 310UC137 (M_sx approx 760 kN.m, adequate)
- Use Grade 350 plate columns (not standard UC) for higher capacity
- Add a haunch at the eave connection to transfer moment to a deeper section
This example demonstrates the real-world consequence of NZ capacity design: the column, which on a pure gravity-and-wind design would be lightly loaded (utilisation ~0.30), must be sized for the amplified overstrength rafter hinge moment, often resulting in columns 2–3 serial sizes heavier than the non-seismic equivalent.
8. Key Takeaways
- NZS 3404 is foundationally similar to AS 4100 but adds a comprehensive seismic design framework (Section 12) — New Zealand shares steel sections with Australia but the seismic provisions are uniquely NZ and far more detailed than any other international steel standard.
- Ductility categories (Cat 1–4) define the entire design philosophy — Cat 4 with mu = 4–6 gives lower seismic forces but demands much heavier connections, full-penetration welds, and rigorous capacity design of all non-yielding elements.
- Capacity design forces the structural hierarchy — yielding elements are deliberately weakened (or rather, left as standard), and everything else is designed for the amplified overstrength moment (M_over = phi_o x R_y x M_yield, where phi_o = 1.25–1.35).
- The Wellington-Chistchurch-Auckland divide is real — a Wellington Cat 4 frame may cost 2–3 times more in connections than an Auckland Cat 1 equivalent for the same gravity loads, solely due to seismic detailing.
- HERA is the definitive NZ steel resource — HERA R4-92 (Designer's Manual), R4-100 (Connections), and R4-142 (Seismic Design) are the working references for all NZ structural steel engineers. SCNZ standard details adapted for seismic practice should be used wherever possible.
PRELIMINARY — NOT FOR CONSTRUCTION. All design information is for educational reference only. Seismic design must be independently verified by a Chartered Professional Engineer (CPEng) registered with Engineering New Zealand before use in any building project. Always check the latest NZS 3404 with current amendments, NZS 1170.5 seismic hazard model (updated post-Christchurch and Kaikoura), and the applicable MBIE guidance for the specific building location.