Brace Connection Types

Braced frames resist lateral loads (wind and seismic) through diagonal braces that transfer axial forces into the beam-column joints. The brace connection is the critical interface between the brace member and the frame — it must develop the full brace capacity and provide a clear, verifiable load path.

Three common brace connection types in Australian practice:

Gusset Plate Design Philosophy

The gusset plate is a flat steel plate that connects the brace to the beam-column intersection. It must transfer the full brace axial force (tension or compression) from the brace to the frame members. The design follows a three-step check:

1. Yield on the Whitmore section — tension yielding across an effective width at the end of the brace
2. Block shear — combined tension and shear failure at the bolt group
3. Buckling — plate buckling under compression (compression braces only)

Whitmore Effective Width — AS 4100 Adaptation

The Whitmore section defines the effective width of gusset plate that resists the brace force. It is taken as the width of plate intersected by lines spreading at 30 degrees from the first to the last bolt in the brace connection, measured perpendicular to the brace axis at the end of the brace.

Whitmore width: bw = 2 x Lw x tan(30 deg) + nb x s (approximately)

Where Lw is the length from the first bolt to the theoretical end of the Whitmore section (typically the last bolt row centre), nb is the number of bolt rows, and s is the bolt pitch.

Tension yield capacity of Whitmore section: phi_Nt_w = 0.90 x fy_plate x bw x tp

Tension rupture (net section through first bolt row): phi_Nt_r = 0.90 x fu_plate x (bw - nh x dh) x tp x kt

Where kt = 0.85 for bolted connections with multiple bolt rows.

Block Shear — AS 4100 Clause 9.1.10

Block shear is a potential failure mode where a block of plate material tears out around the bolt group. Two modes must be checked:

Mode 1 — Shear yield, tension rupture: phi_Rbs1 = phi x (0.60 x fy x Agv + fu x Ant)

Mode 2 — Shear rupture, tension yield: phi_Rbs2 = phi x (0.60 x fu x Anv + fy x Agt)

The design block shear capacity is the lesser of phi_Rbs1 and phi_Rbs2.

Agv and Anv are the gross and net areas in shear (along the bolt lines parallel to the force). Agt and Ant are the gross and net areas in tension (across the bolt group perpendicular to the force).

Gusset Plate Buckling — Compression Braces

When the brace is in compression, the gusset plate projects into the frame and may buckle before the brace reaches its compressive capacity. Two buckling checks apply:

1. Plate Buckling per AS 4100 Clause 6 (Column Analogy)

Treat the free (unrestrained) length of gusset plate between the brace end and the beam/column attachment as a column strip of width equal to the Whitmore width:

Effective length: Le = k x Lg (where Lg is the free length from brace end to frame attachment line) Radius of gyration: r = tp / sqrt(12) (for a rectangular strip of unit width) Slenderness: Le/r

For the column analogy, k = 0.7 (fixed at the welded edge connection to the beam/column, pinned at the brace end) is typically conservative. A k factor of 0.5 may be justified if both the beam and column connections provide rotational restraint.

2. Thornton Method (Alternative)

The Thornton method treats the gusset plate as a compression member with an effective length equal to the average of three buckling lengths measured from the brace end to the three supported edges (beam, column, and the intersection line). The controlling length is the longest of the three.

Bolt Group Design

Bolts Connecting Brace to Gusset

The bolt group connecting the brace member to the gusset plate must transfer the full brace capacity. For Grade 8.8 M24 bolts (threads excluded from shear plane):

phi_Vfn = 0.80 x 0.80 x 830 x 353 / 1000 = 187.5 kN per bolt (single shear)

Capacity reduction for long joints (Clause 9.3.2.2): For bolt groups with more than 10 bolts in a row, the capacity of each bolt beyond the 10th is reduced by the factor (15 - Lj)/(15 - 10) where Lj is the joint length in metres. This accounts for uneven load distribution in long joints.

Bolts Connecting Gusset to Beam/Column

Typically, the gusset plate is welded to the beam and column flanges at the frame joint. If bolted, the bolts must transfer the horizontal and vertical components of the brace force to the beam and column respectively.

Force Resolution at the Gusset-to-Frame Interface

For a brace at angle theta to the horizontal:

• Horizontal component to beam: H = N x cos(theta)
• Vertical component to column: V = N x sin(theta)

The weld or bolt group to the beam must resist H (shear) plus any eccentricity moment. The connection to the column resists V plus eccentricity.

Worked Example — 200UC59.5 Tension Brace

Problem: Design a gusset plate connection for a 200UC59.5 diagonal brace (Grade 300) in a concentrically braced frame. Brace force N* = +1,050 kN (tension). Brace angle = 45 degrees. Gusset plate: Grade 300 (fy = 300 MPa, fu = 440 MPa). Bolts: M24 Grade 8.8, threads excluded from shear plane.

Section properties — 200UC59.5: d = 209 mm, bf = 207 mm, tf = 14.3 mm, tw = 9.4 mm Ag = 7,580 mm^2 Tension capacity: phi_Nt = 0.90 x 300 x 7,580 / 1000 = 2,047 kN > 1,050 kN. Brace OK.

Step 1 — Number of bolts (brace to gusset): phi_Vfn per bolt = 187.5 kN (single shear). Bolts required = 1,050 / 187.5 = 5.6 → use 8-M24 bolts (2 columns x 4 rows). Check long joint: Lj = 3 x 70 = 210 mm << 1.2 m. No reduction.

Step 2 — Gusset plate thickness: Try tp = 16 mm. Ply bearing check (M24 bolt, 16 mm plate, edge distance 40 mm): phi_Vb = 0.90 x 3.2 x 24 x 16 x 440 / 1000 = 487 kN per bolt > 187.5 kN. OK.

Step 3 — Whitmore width check: Bolt group: 2 columns at 70 mm gauge, 4 rows at 70 mm pitch. Length from first bolt to end of bolt group = 3 x 70 = 210 mm. Whitmore width at end of bolt group: bw = 2 x 210 x tan(30 deg) + 2 x 35 = 2 x 210 x 0.577 + 70 = 243 + 70 = 313 mm.

Step 4 — Tension yield on Whitmore section: phi_Nt_w = 0.90 x 300 x 313 x 16 / 1000 = 1,353 kN > 1,050 kN. OK.

Step 5 — Tension rupture (net section through 2 bolt holes): Hole diameter = 24 + 2 = 26 mm. Net width = 313 - 2 x 26 = 261 mm. phi_Nt_r = 0.90 x 440 x 261 x 16 x 0.85 / 1000 = 1,405 kN > 1,050 kN. OK.

Step 6 — Block shear check: Shear plane: 4 rows x 70 mm pitch, 2 bolt columns → Agv = 2 x (3 x 70 + 40) x 16 = 2 x 250 x 16 = 8,000 mm^2. Anv = 2 x (3 x 70 + 40 - 3.5 x 26) x 16 = 2 x 159 x 16 = 5,088 mm^2. Tension plane: Agt = (70 - 0) x 16 = 1,120 mm^2 (gauge distance between bolt columns). Ant = (70 - 1 x 26) x 16 = 704 mm^2.

Mode 1 (shear yield, tension rupture): phi_Rbs1 = 0.90 x (0.60 x 300 x 8,000 + 440 x 704) / 1000 = 0.90 x (1,440,000 + 309,760) / 1000 = 1,575 kN.

Mode 2 (shear rupture, tension yield): phi_Rbs2 = 0.90 x (0.60 x 440 x 5,088 + 300 x 1,120) / 1000 = 0.90 x (1,343,232 + 336,000) / 1000 = 1,511 kN.

Block shear capacity = min(1,575, 1,511) = 1,511 kN > 1,050 kN. OK.

Step 7 — Gusset to beam/column weld: Brace force at 45 degrees: H = 1,050 x cos(45 deg) = 742 kN, V = 1,050 x sin(45 deg) = 742 kN.

Weld to beam flange (horizontal leg): Length = 400 mm (minimum). 8 mm fillet weld both sides. Weld capacity per mm = 0.80 x 0.60 x 490 x 0.707 x 8 / 1000 = 1.33 kN/mm. Total weld capacity = 800 x 1.33 = 1,064 kN > 742 kN. OK.

Weld to column flange (vertical leg): Length = 500 mm. 8 mm fillet both sides. Total weld capacity = 1000 x 1.33 = 1,330 kN > 742 kN. OK.

Final specification: 16 mm gusset plate Grade 300, 8-M24 Grade 8.8 bolts, 8 mm fillet welds to beam and column. Connection capacity = 1,050 kN in tension. Conforms to AS 4100:2020 Clause 9.

Gusset Plate Detailing Requirements

Edge distances: Minimum 1.5 x bolt diameter from bolt centre to plate edge for sheared edges. Standard edge distance for M24 is 40 mm.

Plate slenderness: For compression braces, the gusset plate free length should not exceed 25 x tp to control plate buckling. For 16 mm plate, maximum free length = 25 x 16 = 400 mm.

Weld access: Provide a minimum 5 mm clearance between the bolt head/nut and the gusset-to-frame weld for tightening access.

Bolt tightening sequence: Tighten bolts from the centre of the group outward to avoid trapping slack.

Frequently Asked Questions

When should a gusset plate have stiffeners? Add stiffeners to the free edge of the gusset plate when the free length exceeds 30 x tp for compression braces, or when the plate thickness would otherwise exceed 25 mm to satisfy buckling requirements. Stiffeners effectively reduce the buckling length by bracing the free edge against out-of-plane displacement. For heavy bracing connections (N* > 2,500 kN), stiffened gussets are standard practice. The stiffener should be a flat bar welded perpendicular to the gusset plate along its free edge.

How does AS 4100 handle gusset plate buckling differently from AISC 360? Both codes use the same fundamental approach (Whitmore section, Thornton buckling method), but AS 4100 applies its specific column buckling curves (alpha_c per Clause 6.3.3) to the gusset plate strip, while AISC 360 uses its column curve. The resistance factor phi differs: AS 4100 uses phi = 0.90 for the plate in compression, while AISC 360 uses phi = 0.90 as well. In practice, designs to both codes produce similar gusset plate thicknesses for typical Australian and US conditions.

What is the typical gusset plate thickness range in Australian practice? For light bracing (N* < 500 kN): 10-12 mm gusset plates with M20 bolts. For medium bracing (N* = 500-1,500 kN): 12-20 mm plates with M24 bolts. For heavy bracing (N* = 1,500-3,500 kN): 20-32 mm plates with M30-M36 bolts. Plates exceeding 32 mm are unusual and may indicate that the brace force is better accommodated by a different brace section or a double-gusset (gusset plates each side of the brace) arrangement.

This page is for educational reference. Brace connection design per AS 4100:2020 Clauses 9.1, 9.3, and 9.5. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer registered with Engineers Australia or the relevant state registration board. Results are PRELIMINARY — NOT FOR CONSTRUCTION.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.