AS 4100 Bolt Group Design — M20 8.8 Worked Example

Designing a bolted connection to AS 4100:2020 means checking multiple limit states systematically: bolt shear, ply bearing, tear-out, block shear, and net section fracture. This guide walks through every check with a fully worked M20 8.8/S example, including real numbers you can verify with our free bolt group calculator.

What you will learn

• How to set up an AS 4100 bolt group design from scratch
• The five limit states that govern bolted connections
• A complete worked example with M20 Grade 8.8/S bolts in a lap splice
• How to identify the controlling failure mode
• Comparison of 8.8/S, 8.8/TB, and 8.8/TF bolt categories

Copyright and standards notice

This site does not reproduce copyrighted code clauses or proprietary tables verbatim. Discussion of AS 4100 here is high-level and intended to help you understand verification workflows. Always consult the official published standard (AS 4100:2020) for authoritative requirements.

Step 1 — Define the problem

Before touching a calculator, record everything that defines the connection:

The connection transfers 240 kN in shear through four M20 8.8/S bolts in a single lap splice. Threads are assumed to be in the shear plane (conservative for snug-tight installation).

Step 2 — Identify all limit states

AS 4100 requires you to check multiple failure modes for any bolted connection. Do not assume bolt shear governs — in many thin-plate or short-edge-distance configurations, bearing or block shear controls. The limit state families are:

1. Bolt shear capacity (Cl. 9.2.2.1) — bolt shank shearing through the bolt cross-section
2. Ply bearing capacity (Cl. 9.2.2.4) — local crushing of the plate around the bolt hole
3. Plate tear-out (Cl. 9.2.2.4) — bolt pulling through the end of the plate
4. Block shear (Cl. 9.2.2.5) — combined tearing along shear and tension paths
5. Net section fracture (Cl. 7.2) — tension rupture through the bolt holes

List all five explicitly before computing anything. This prevents the common error of checking only one mode.

Step 3 — Capacity reduction factors

AS 4100 assigns different phi factors depending on the failure mode:

These values are critical — using the wrong phi factor is one of the most common errors in AS 4100 connection design. Always record which phi you used for each check.

Step 4 — Worked example: check every limit state

Check 1: Bolt shear capacity

For a single M20 8.8 bolt in single shear with threads in the shear plane:

• Bolt shank area: A_s = 314 mm² (nominal)
• Core area (threads in shear plane): A_c = 225 mm² (AS 1275 minor diameter area for M20)
• Ultimate tensile strength: f_uf = 830 MPa (Grade 8.8)
• Shear factor: 0.62 (AS 4100 Cl. 9.2.2.1)

Single bolt shear capacity: V_f = phi x 0.62 x f_uf x A_c (threads included) V_f = 0.80 x 0.62 x 830 x 225 / 1000 V_f = 92.6 kN per bolt

Group capacity (4 bolts): V_f,group = 4 x 92.6 = 370.5 kN

Utilization: 240 / 370.5 = 0.65 (OK)

Check 2: Ply bearing capacity

Bearing is checked at each bolt location against the connected plate:

• Plate thickness: tp = 10 mm
• Bolt diameter: df = 20 mm
• Plate ultimate strength: fu = 440 MPa

Bearing capacity per bolt (Cl. 9.2.2.4): V_b = phi x 3.2 x df x tp x fu V_b = 0.90 x 3.2 x 20 x 10 x 440 / 1000 V_b = 253.4 kN per bolt

Group bearing capacity: 4 x 253.4 = 1013.8 kN

Utilization: 240 / 1013.8 = 0.24 (OK — bearing does not govern here)

Check 3: Plate tear-out

Tear-out is critical at the end bolts where the edge distance is shortest:

• End distance: ae = 35 mm
• Hole diameter: dh = 22 mm
• Available tear-out length: ae - dh/2 = 35 - 11 = 24 mm

Tear-out capacity per end bolt: V_tp = phi x ae x tp x fu (using effective end distance) V_tp = 0.90 x 24 x 10 x 440 / 1000 V_tp = 95.0 kN per end bolt

For 2 end bolts + 2 interior bolts (interior governed by pitch): Interior tear-out length = p - dh = 70 - 22 = 48 mm V_tp,int = 0.90 x 48 x 10 x 440 / 1000 = 190.1 kN per interior bolt

Group tear-out: 2 x 95.0 + 2 x 190.1 = 570.2 kN

Utilization: 240 / 570.2 = 0.42 (OK)

Check 4: Block shear

Block shear (Cl. 9.2.2.5) considers combined shearing and tearing of the plate. For this 2x2 bolt pattern loaded in one direction:

• Shear path length (Lv): along two bolt rows from the edge = 2 x (35 + 70) - 1.5 x 22 = 177 mm per side
• Tension path length (Lt): across the gauge = 100 - 22 = 78 mm

Block shear capacity: R_bs = phi x (0.6 x fu x Lv x tp + fu x Lt x tp) (simplified)

Using the net areas: Shear net area: A_nv = (177) x 10 = 1770 mm² (per shear plane) Tension net area: A_nt = 78 x 10 = 780 mm²

R_bs = 0.75 x (0.6 x 440 x 1770 + 440 x 780) / 1000 R_bs = 0.75 x (467.3 + 343.2) R_bs = 607.9 kN

Utilization: 240 / 607.9 = 0.39 (OK)

Check 5: Net section tension

The net section is checked across the critical bolt line (Cl. 7.2):

• Gross width: 170 mm
• Hole deductions: 2 x 22 = 44 mm
• Net width: 170 - 44 = 126 mm
• Net area: A_n = 126 x 10 = 1260 mm²

Net section capacity: phi x 0.85 x fu x A_n = 0.90 x 0.85 x 440 x 1260 / 1000 = 424.1 kN

Utilization: 240 / 424.1 = 0.57 (OK)

Summary of results

Controlling limit state: Bolt shear at 65% utilization.

This is typical for snug-tight M20 8.8/S bolts in single shear with threads in the shear plane. If threads were excluded from the shear plane, bolt shear capacity would increase significantly, and net section would likely become the controlling mode.

Try this example yourself: Bolted Connection Calculator -- enter the values above and compare outputs.

Bolt category comparison: 8.8/S vs 8.8/TB vs 8.8/TF

Understanding the three AS 4100 bolt installation categories is essential for getting design right:

Key insight: 8.8/S and 8.8/TB have the same ultimate shear capacity because both rely on bolt bearing at ULS. The difference is that 8.8/TB bolts are tensioned to control fatigue and vibration loosening. 8.8/TF connections resist load through friction, not bearing, and have lower design capacity but prevent slip at SLS.

Step 5 — Sensitivity analysis

Understanding how capacity shifts with parameter changes helps you optimize connections:

This type of sensitivity check is what turns a calculator output into an engineering judgment.

Common mistakes in AS 4100 bolt design

1. Confusing bolt categories. Using 8.8/S capacity for a connection that specifies 8.8/TF underestimates the required bolt size because friction capacity is lower than bearing capacity.

2. Ignoring thread condition. The difference between threads-included and threads-excluded in the shear plane is roughly 40% of shear capacity. Always confirm the thread condition for your connection detail.

3. Using gross area for net section checks. AS 4100 Cl. 7.2 requires net area through the bolt holes. Using gross area overestimates capacity.

4. Checking only bolt shear. In thin-ply or short-edge-distance configurations, bearing, tear-out, or block shear often controls. Always check all five limit states.

5. Mixing factor philosophies. Actions must be factored per AS/NZS 1170 combinations before comparison with phi-reduced capacities. Never compare unfactored loads against design capacities.

6. Ignoring prying action. In connections with flexible flanges or end plates loaded in tension, prying amplifies bolt tension well beyond the applied load. This is particularly important for 8.8/TF connections where bolt tension directly affects slip resistance.

Frequently Asked Questions

What is the minimum edge distance for M20 bolts to AS 4100? AS 4100 Cl. 9.5.2 requires a minimum edge distance based on the bolt diameter and hole type. For standard holes, this is typically 1.5 x hole diameter. For M20 bolts with 22 mm holes, the minimum is approximately 33 mm. Always verify against the current edition.

When should I use 8.8/TF instead of 8.8/S? Use 8.8/TF (friction-type) when slip at serviceability is unacceptable — for example, connections with oversize or slotted holes, connections subject to fatigue, or joints where slip would cause a serviceability problem.

Can I mix bolt sizes in one connection? AS 4100 does not prohibit it, but mixing sizes complicates fabrication and increases error risk. If different bolt sizes are used, each bolt must be checked individually.

How do I handle combined shear and tension? AS 4100 Cl. 9.2.2.3 provides an interaction equation. Compute individual utilizations for shear and tension, then check combined interaction. The elliptical interaction curve means both demands must be well within capacity.

Does bolt hole size affect capacity? Yes. Larger holes reduce net section area, bearing length, and tear-out length. Oversize and slotted holes also affect slip resistance for 8.8/TF connections.

Key Takeaways

• Always check all five limit states: bolt shear, bearing, tear-out, block shear, and net section. Never assume one mode governs.
• Record the phi factor for each check — they range from 0.75 (block shear) to 0.90 (bearing/net section).
• Thread condition matters. Threads-included vs threads-excluded changes shear capacity by ~40%.
• Bolt category (S/TB/TF) changes the design model, not just the installation method.
• Edge distance is the most sensitive parameter for tear-out and block shear. Always check minimum requirements per Cl. 9.5.2.
• Sensitivity analysis identifies the controlling mode and reveals how much margin you have.

Run This Calculation

Bolted Connection Calculator — eccentric bolt group shear, block shear, and bearing checks per AS 4100, AISC 360, EN 1993, and CSA S16.

Base Plate & Anchors Calculator — anchor bolt tension and shear checks for base plate connections.

Further Reading

• Bolted connection checklist — step-by-step verification guide
• Bolt hole sizes reference — standard, oversize, and slotted
• Bolt capacity table — shear and tension for common bolt grades
• Bolt spacing and edge distance requirements
• Bolt torque chart — A325, A490, Metric 8.8/10.9
• Steel Fy and Fu reference — yield and tensile strength by grade
• AS 4100 code notes
• EN 1993-1-8 steel connections guide
• How to verify calculator results
• AS 4100 fillet weld design guide

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.