Australian Braced Frame Design — AS 4100 and AS 1170.4

Complete reference for concentrically braced frame (CBF) design for lateral load resistance in steel buildings. Covers brace configuration types, ductility classes per AS 1170.4:2007, capacity design philosophy with overstrength factors, gusset plate design, and seismic detailing requirements.

Related pages: AS 4100 Moment Frames | AS 4100 Column Design | Gusset Plate Design | Bolt Group Capacity

Braced Frame Function

In a steel building, braced frames provide lateral stability by forming vertical trusses that resist wind and earthquake loads. The braces (diagonals) act as axial members in tension or compression, while the beams and columns of the frame carry gravity loads in addition to participating in the lateral system. Braced frames are substantially stiffer than moment frames and are the default lateral system for Australian buildings up to approximately 40 storeys.

Brace Configuration Types — AS 4100 and AS 1170.4 Recognition

K-bracing is prohibited for seismic-resisting systems under AS 1170.4 because buckling of the compression brace imposes a transverse force on the column at mid-height, potentially precipitating column failure before the brace can dissipate energy.

Ductility Classes — AS 1170.4 Table 6.5

Australian seismic design uses ductility classes D1 through D4, with increasing ductility (mu factor) allowing a reduction in the design seismic force in exchange for more stringent detailing:

For Australian conditions, D2 moderate ductility is typical for CBFs in commercial buildings. D3 and D4 are reserved for essential facilities (hospitals, emergency response centres) in moderate-to-high seismic zones. Most of Australia is classified as low seismicity (hazard factor Z <= 0.08, corresponding to AS 1170.4 Site Hazard), and D1 or D2 is adequate.

Capacity Design — Clause 14.3

Capacity design ensures that the ductile yielding mechanism (brace buckling in compression, or yielding in tension) occurs BEFORE brittle failure of connections. The hierarchy of failure modes is deliberately ordered: brace yielding/buckling > connections > columns > foundations.

Overstrength factor for connection design: phi_o = Ry x omega x Sh

Where:

• Ry = ratio of expected to nominal yield strength (1.10 for Grade 300, 1.15 for Grade 350)
• omega = strain hardening factor (1.10 for compact sections at moderate ductility)
• Sh = system overstrength factor accounting for the difference between actual and design seismic forces (1.10-1.30 depending on the number of storeys and redundancy)

Typical phi_o values for Australian CBF design:

• D2 moderate ductility: phi_o = 1.10 x 1.10 x 1.20 = 1.45, use 1.50
• D3 high ductility: phi_o = 1.15 x 1.15 x 1.25 = 1.65, use 1.70

Connection design force: N*_conn = phi_o x N*_brace

This capacity-designed connection force is typically 50-70% higher than the elastic analysis force, which governs the bolt count, weld size, and gusset plate dimensions.

Brace Slenderness Limits

Tightening slenderness with increasing ductility ensures the brace section is sufficiently stocky to undergo multiple cycles of inelastic buckling without excessive degradation of compressive resistance.

Gusset Plate Design — Whitmore Section

The gusset plate connecting the brace to the beam-column joint must be designed for the capacity-designed brace force. The effective width of gusset plate for compression is determined by the Whitmore section:

Whitmore width: b_e = b_brace + 2 x d_gusset x tan(30 degrees)

Where b_brace is the brace width and d_gusset is the gusset plate length from the end of the brace to the beam/column connection line. The 30-degree spread angle gives an effective width approximately equal to the brace width plus 1.15 times the gusset length.

Block shear: Check the rectangular path from the bolt line along the gusset edge, accounting for both shear yield/fracture along the load direction and tension fracture perpendicular to it. Block shear commonly governs gusset plate capacity for thin plates.

Buckling: The gusset plate free edge between the brace and the beam/column lines can buckle in compression. The unbraced length L_g along the free edge, with an effective length factor k = 0.65 (fixed at the beam/column, free at the brace edge), must satisfy the column buckling check per Clause 6.3.

Worked Example — 3-Storey V-Braced Frame

Problem: A 3-storey office building in Melbourne (hazard factor Z = 0.08, site class C_e) uses V-braced frames in both directions. Brace at ground level carries N*_EQ = 620 kN (compression) from seismic load case. Design for D2 moderate ductility (mu = 2.0, Sp = 0.67). Grade 300 steel, E48XX electrodes, M20 Grade 8.8/S bolts.

Step 1 — Brace selection: Required phi_Nc >= 620 kN. Try 200UC52 (hot-rolled UC, Grade 300). Ag = 6,670 mm^2, ry = 51.8 mm. Brace length L = sqrt(4.5^2 + 4.0^2) = 6.02 m (bay width = 4.5 m, storey = 4.0 m, V-brace).

Le = 0.85 x L = 5.12 m (Clause 6.3.2 for one end pinned, one end fixed gusset). Le/r = 5120 / 51.8 = 98.8. lambda_n = 98.8 x sqrt(300/250) = 98.8 x 1.095 = 108.2. alpha_b = 0.0 (UC), alpha_c ~ 0.52 (interpolated). phi_Nc = 0.90 x 0.52 x 6,670 x 300 / 1000 = 936 kN > 620 kN. OK.

Slenderness = 98.8 < 100 (D2 limit). OK.

Step 2 — Capacity design connection force: phi_o = 1.5 (D2 moderate). N*_conn = 1.5 x 620 = 930 kN.

Step 3 — Gusset plate design: Brace width = 204 mm (UC 200UC52 flange width). Gusset length from end of brace to beam/column line = 250 mm.

Whitmore width: b_e = 204 + 2 x 250 x tan(30) = 204 + 2 x 250 x 0.577 = 204 + 289 = 493 mm.

Try 16 mm Grade 300 gusset plate: phi_Ns_gusset = 0.90 x 493 x 16 x 300 / 1000 = 2,130 kN > 930 kN. OK (squash).

Buckling check on free edge: L_g = 250 mm, t = 16 mm, k = 0.65. Le_g = 0.65 x 250 = 162.5 mm. r = 16 / sqrt(12) = 4.62 mm (weak axis of rectangular section). Le/r = 162.5/4.62 = 35.2. lambda_n = 35.2 x 1.095 = 38.5, alpha_c ~ 0.93. phi_Nc_gusset = 0.90 x 0.93 x 16 x 493 x 300 / 1000 = 1,981 kN > 930 kN. OK.

Step 4 — Bolt design. Connection uses a bolted splice at the brace end. 6-M20 Grade 8.8/S bolts in double shear (brace flange to gusset plate on each side).

Double shear capacity per bolt: phi_Vfn = 0.80 x 0.80 x 830 x 314 x 2 / 1000 = 333 kN per bolt (shank in shear, X category).

6 bolts: Total = 6 x 107 (conservative single shear check) = 642 kN < 930 kN capacity design force. Upgrade to M24.

M24 bolt: As = 353 mm^2, phi_Vfn = 0.80 x 0.80 x 830 x 353 / 1000 = 187 kN (single shear, shank). In double shear: 374 kN per bolt. Double shear total for 6-M24: 6 x 187 (conservative) = 1,122 kN > 930 kN. OK.

Alternatively use 8-M20 (8 x 107 = 856 kN < 930 kN still marginal). 6-M24 is the economic choice.

Step 5 — Gusset-to-beam and gusset-to-column weld: 930 kN force must be developed in the weld group. Using 10 mm fillets each side along 300 mm gusset edge: 2 x 300 x 1.66 = 996 kN > 930 kN. OK.

Final specification: 200UC52 brace, 16 mm Grade 300 gusset plate, 6-M24 Grade 8.8/S bolts at each brace end, 10 mm GP E48XX fillet welds.

P-Delta Effects in Braced Frames

For braced frames, the second-order P-Delta effect is evaluated through the storey stability coefficient theta:

theta = P_sum x delta / (V_storey x h_storey)

Where P_sum is the total gravity load at and above the storey, delta is the first-order interstorey drift, V_storey is the storey shear, and h_storey is the storey height.

If theta <= 0.10, second-order effects can be accounted for by amplifying design actions by 1/(1 - theta). If theta > 0.10, a second-order analysis is required — but this is unusual for CBFs because the braced frame stiffness typically keeps theta below 0.05.

Frequently Asked Questions

Why is K-bracing prohibited in seismic frames? Under seismic loading, the compression brace in a K-brace buckles, imposing a large transverse force at the column mid-height. This lateral kick can precipitate column instability before the tension brace engages. The failure is brittle and non-ductile. Both AS 4100 and AISC 341 prohibit K-bracing for seismic resistance.

What is the difference between capacity design in D2 vs D3 ductility classes? D2 requires connections to be designed for 1.5-2.0 times the brace expected strength (phi_o x N*). D3 requires 2.0-2.5 times, reflecting the higher ductility demand and the need for the connection to survive larger brace overstrength during repeated inelastic cycling. D3 also requires closer brace slenderness limits (Le/r <= 80 vs 100) and weld access hole detailing per AS 4100 Appendix N.

How does AS 4100 handle brace buckling for reversed cyclic loading? The brace is checked for both tension (yield) and compression (buckling) under the seismic load case. The compression capacity is based on the monotonic buckling curve (alpha_c), which is conservative for cyclic loading because repeated buckling degrades the compression resistance below the monotonic value. For D3-D4, a further 20% reduction in compression capacity is sometimes applied based on research at the University of Sydney and University of Auckland.

Is tension-only bracing permitted in Australian practice? Yes, for non-seismic frames and for low-rise structures in low-seismicity zones (Z <= 0.05). The brace must satisfy Le/r <= 300 and the connections must still be capacity-designed. However, the resulting frame has lower lateral stiffness (only one brace active) and is prone to "snap-through" effects under load reversal. For buildings exceeding 3 storeys, compression-resisting braces are strongly recommended.

This page is for educational reference. Braced frame design per AS 4100:2020, AS 1170.4:2007, and ASI design guides. Verify seismic hazard factors and ductility classes for the specific site. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.