Australian Bolt Bearing and Tearout Resistance — AS 4100 Clause 9.3.2.4

Complete reference for bearing and tearout limit states in bolted steel connections per AS 4100:2020 Clause 9.3.2.4. The bearing resistance of a bolted connection is governed by two distinct failure modes: bearing yielding of the plate material ahead of the bolt in the direction of load, and tearout fracture where the end distance is insufficient to develop the full bearing capacity. Both checks are mandatory for every bolted connection.

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Bearing Capacity Formula — Clause 9.3.2.4(1)

The design bearing capacity of a single bolt on a connected ply is given by:

phi V_b = phi x 3.2 x d_f x t_p x f_up

where:

• phi = 0.90 (capacity factor for bearing, Table 3.4)
• d_f = nominal bolt diameter (mm)
• t_p = thickness of the connected ply (mm)
• f_up = tensile strength of the connected ply material (MPa)

For Grade 300 steel plate (f_up = 440 MPa), the nominal bearing capacity per millimetre of bolt diameter and plate thickness is:

V_bn / (d_f x t_p) = 3.2 x 440 = 1408 MPa

So for a single M20 bolt in a 10 mm Grade 300 plate:

phi V_b = 0.90 x 3.2 x 20 x 10 x 440 = 0.90 x 281.6 = 253.4 kN

Bearing Capacity per Bolt — Grade 300 Plate

The table below gives the design bearing capacity phi V_b (kN) for standard plate thicknesses and common bolt diameters in Grade 300 material (f_up = 440 MPa):

For Grade 350 plate (f_up = 480 MPa), multiply the above values by 480/440 = 1.091.

Bearing Capacity for Grades Other Than 300

Tearout Resistance — Clause 9.3.2.4(2)

When the end distance (a_e) measured from the bolt centre to the edge of the plate in the direction of the applied force is less than a critical value, the failure mode transitions from bearing yielding to tearout fracture. The critical end distance for full bearing capacity is:

a_e_crit = 3.2 x d_f x f_up / (2 x f_up) = 1.6 d_f

For end distances a_e < 1.6 d_f, the bearing capacity is linearly reduced:

phi V_b_reduced = phi V_b x (a_e / a_e_crit) = phi V_b x (a_e / (1.6 d_f))

Where a_e is the actual end distance provided.

This reduction reflects the smaller volume of plate material available to resist the bearing force through shear along two planes parallel to the direction of load. At a_e = 1.6 d_f, the shear area on the two tearout planes provides exactly enough capacity to match the bearing yielding capacity ahead of the bolt.

Tearout Reduction Factors

For standard end distances commonly used in Australian practice:

At a_e = 1.5 d_f, which is the minimum edge distance for a sheared edge (a_e = 1.5 d_h, and d_h ~ d_f + 2 mm, so a_e ~ 1.5 d_f + 3 mm), the tearout reduction is small but non-zero, and must be checked where the end distance is tight.

Combined Bearing and Tearout Check — Clause 9.3.2.4(3)

For a bolt group subjected to a force inclined at an angle to the plate edge, the bearing and tearout capacity shall be checked in the component directions. The vector sum of the demand-to-capacity ratios in orthogonal directions must satisfy:

(N_x* / phi V_bx)^2 + (N_y* / phi V_by)^2 <= 1.0

where N_x* and N_y* are the bolt force components in two orthogonal directions, and phi V_bx and phi V_by are the corresponding bearing/tearout capacities.

This interaction check is particularly important for connections at beam-to-column junctions where the bolt group carries combined shear and axial forces, and the end distance differs in the two directions.

Bearing in Slotted Holes — Clause 9.3.2.4(4)

When bolts are installed in slotted holes, the bearing capacity is reduced to account for the reduced contact area between the bolt shank and the hole wall at initial loading:

• Short slotted holes (SSL) with slot perpendicular to load: phi V_b_ssl = 0.80 x phi V_b
• Long slotted holes (LSL) with slot perpendicular to load: phi V_b_lsl = 0.65 x phi V_b
• Slotted holes with slot parallel to load: phi V_b_slot_parallel = phi V_b (no reduction, as full bearing contact is maintained)

The reduction factors account for the bolt initially seating at one end of the slot and the reduced bearing contact length at the beginning of load application. Once the bolt has slipped to the end of the slot and fully engaged the hole wall, the bearing capacity approaches the standard hole value, but the code conservatively maintains the reduction.

Bearing of Bolt on Ply versus Ply on Bolt

It is important to note that AS 4100 Clause 9.3.2.4 checks the bearing capacity of the connected ply (plate material), not the bolt itself. The bolt is generally of higher strength material (f_uf = 830 MPa for Grade 8.8) and its bearing capacity is almost never the controlling limit state.

However, when the bolt bears against a plate of higher strength than the bolt material (an unusual configuration), the bearing capacity should be checked using the bolt tensile strength f_uf rather than the plate tensile strength f_up. This situation can arise when high-strength quenched and tempered plate (Grade 400 or 450, f_up = 540-600 MPa) is used with Grade 4.6 bolts (f_uf = 400 MPa).

Worked Example: Bearing and Tearout Check

Problem: A 310UB40.4 Grade 300 beam is connected to a column using a 10 mm thick Grade 300 end plate with four M20 Grade 8.8 bolts in standard holes. The bolts are arranged in two vertical rows of two bolts. End distance from the last bolt centre to the plate edge in the direction of the vertical shear force is 35 mm. Factored shear force on the connection is V* = 280 kN. Check the bearing and tearout capacity of the end plate.

Given:

• Bolt diameter: d_f = 20 mm
• Hole diameter: d_h = 22 mm
• Plate thickness: t_p = 10 mm
• Plate material: Grade 300, f_up = 440 MPa
• End distance: a_e = 35 mm
• Number of bolts: n_b = 4
• phi = 0.90

Solution:

Step 1: Full bearing capacity per bolt

phi V_b = 0.90 x 3.2 x 20 x 10 x 440 x 10^(-3) = 253.4 kN

Step 2: Tearout check

Critical end distance: a_e_crit = 1.6 x 20 = 32 mm

Since a_e = 35 mm >= 32 mm, no tearout reduction applies. The full bearing capacity is available.

Step 3: Total bearing capacity of the bolt group

phi V_b_total = 4 x 253.4 = 1013.6 kN

Step 4: Check adequacy

V* = 280 kN / phi V_b_total = 280 / 1013.6 = 0.276 -- OK (27.6% utilisation)

Step 5: Check governing limit state

The bolt shear capacity must also be checked. For M20 Grade 8.8 bolts in single shear (threads in the shear plane), phi V_f = 0.80 x 0.62 x 830 x 245 x 10^(-3) = 100.8 kN per bolt (Clause 9.3.2.1).

Total shear capacity: 4 x 100.8 = 403.2 kN > 280 kN -- OK.

The bolt shear controls (403 kN) rather than plate bearing (1014 kN), which is typical for end plate connections.

Step 6: Block shear check

Check block shear failure of the end plate around the bolt group. The block shear path includes the end distance (35 mm), the gauge between bolt rows, and the pitch between bolts in each row. This is a separate limit state check per Clause 9.1.4.

Result: Bearing capacity adequate. Tearout does not govern at a_e = 35 mm. Bolt shear controls the design at 27.6% of bearing capacity. The connection is satisfactory.

Edge Distance Optimisation for Bearing Connections

For bearing-type connections where bolt shear strength governs the design (the most common case in Australian practice), the end distance can often be set to the minimum per Clause 9.6.3 (a_e = 1.5 d_h) without affecting the connection capacity.

For connections where plate bearing or tearout is the governing limit state (typically in thin plate or high-strength plate where f_up is significantly higher than the bolt f_uf), increasing the end distance beyond 1.6 d_f provides no further bearing capacity increase, but may be desirable for fabrication tolerance and to ensure full bearing capacity is reliably achieved.

Frequently Asked Questions

What is the difference between bearing and tearout failure in bolted connections?

Bearing failure occurs when the plate material ahead of the bolt yields in compression from the bolt bearing force, resulting in hole elongation but not complete separation. Tearout failure occurs when the end distance is too short and the plate material shears along two planes parallel to the load direction, resulting in complete separation of the plate from the bolt hole to the edge. Tearout is a brittle failure mode whereas bearing yielding is ductile, giving warning before complete failure.

How is the end distance measured for bearing/tearout calculations per AS 4100?

The end distance a_e is measured from the centre of the bolt hole to the nearest edge of the plate in the direction of the applied force component, per AS 4100 Clause 9.3.2.4. For forces at an angle to the plate edge, the end distance is measured in the direction of each force component. The controlling end distance for tearout is the shortest distance from the bolt centre to any free edge in the direction of any applied force component.

Does the bearing capacity formula apply to Grade 350 and Grade 400 plates?

Yes. The bearing formula phi V_b = phi x 3.2 x d_f x t_p x f_up applies to all structural steel grades covered by AS/NZS 3679.1. The tensile strength f_up is the material-specific value: 440 MPa for Grade 300, 480 MPa for Grade 350, and 540 MPa for Grade 400. The constant 3.2 is calibrated against Australian test data on bolted connections in hot-rolled structural steel.

Is bearing capacity checked for slip-critical connections?

In slip-critical (TF category) connections per AS 4100 Clause 9.3.8, the connection is designed to resist the serviceability limit state loads without slip. At the ultimate limit state, the connection is assumed to have slipped into bearing, and therefore the bearing and tearout checks of Clause 9.3.2.4 must still be satisfied for the factored ultimate loads. The slip-critical design governs at serviceability; the bearing design governs at ultimate.

What plate thickness is required to develop the full bolt shear capacity of an M20 Grade 8.8 bolt?

For an M20 Grade 8.8 bolt in single shear (threads excluded), phi V_f = 0.80 x 0.62 x 830 x 245 x 10^(-3) = 100.8 kN. Equating this to the bearing capacity: 100.8 = 0.90 x 3.2 x 20 x t_p x 440 x 10^(-3). Solving: t_p = 100.8 / (0.90 x 3.2 x 20 x 440 x 10^(-3)) = 100.8 / 25.34 = 3.98 mm. Therefore a 4 mm thick Grade 300 plate has adequate bearing capacity to develop the full bolt shear capacity of an M20 Grade 8.8 bolt. For plates thinner than 4 mm, bearing/tearout will control over bolt shear.

Educational reference only. All design values must be verified against the current edition of AS 4100:2020 and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.