AS 4100 Base Plate Design — Procedure & Worked Example

Base plates transfer column loads into the concrete foundation. Under AS 4100-2020, the steel base plate is designed for bearing, bending, and welding. The concrete bearing capacity is checked per AS 3600-2018. This guide covers the complete procedure with a fully worked numerical example.

What AS 4100 Section 8 Covers

AS 4100 Section 8 deals with connections, including base plates. The key design checks for a concentrically loaded base plate are:

• Bearing pressure on the concrete: the applied pressure must not exceed the concrete's design bearing capacity (AS 3600 Cl 12.6)
• Plate bending: the plate cantilevers beyond the column footprint and must resist bending from the bearing pressure
• Plate thickness: derived from the required bending capacity at the critical section
• Fillet weld to column: the weld transfers the column load into the plate
• Anchor bolts: under axial load only, holding-down bolts; under moment, the tension bolt group resists overturning

The design uses the strength limit state throughout, with factored loads N*, V*, and M*.

Design Procedure — Step by Step

Step 1 — Determine Factored Loads (N*, V*, M*)

Apply AS 1170.1 load combinations to establish the governing strength limit state demands. For a concentrically loaded column, only N* (axial compression) is significant. For moment-base connections, M* and V* must also be resolved.

Step 2 — Select Trial Plate Dimensions (B × D)

Choose a trial plate size based on the column section. A starting point is to extend 50–75 mm beyond the column flange on each side. The plate must fit within the concrete pedestal.

Step 3 — Check Bearing Pressure

The average bearing pressure under the plate:

fp = N* / (B × D)

This must not exceed the design bearing capacity of the concrete. Per AS 3600 Cl 12.6, the concrete bearing capacity with confinement:

phi_Nc = phi × 0.85 × f'c × A1 × sqrt(A2 / A1)  ≤  phi × 1.7 × f'c × A1

Where:

• phi = 0.65 (bearing, AS 3600)
• A1 = bearing area of the plate (B × D)
• A2 = concentric bearing area of the pedestal (larger than A1)
• sqrt(A2/A1) is the confinement factor, capped at 2.0

A simplified check for a plate on a large pedestal: fp ≤ phi × 0.6 × f'c × sqrt(A2/A1).

Step 4 — Determine Plate Cantilever Extension

The critical cantilever length is measured from the column face. For a UC or WB section:

n_flange = (B - 0.80 × bf) / 2   (beyond the effective flange width)
n_web    = (D - 0.95 × dc) / 2   (beyond the effective web depth)

Use the larger of n_flange and n_web as the governing cantilever n.

Step 5 — Check Plate Bending

The bearing pressure fp acts upward on the plate cantilever. The bending moment per unit width at the column face:

M*_pl = fp × n² / 2

Step 6 — Required Plate Thickness

The plate section capacity per unit width (rectangular section, phi = 0.9):

phi × Ms = phi × fy × tp² / 4

Setting M*_pl = phi × Ms and solving for tp:

tp ≥ n × sqrt(2 × fp / (phi × fy))    [phi = 0.9]

Round up to the next standard plate thickness (e.g., 10, 12, 16, 20, 25 mm).

Step 7 — Anchor Bolt Design

Axial compression only: The anchor bolts act as holding-down bolts. Verify minimum embedment for the shear transfer mechanism and check that no net tension occurs under any AS 1170 load combination.

With bending moment M*: The tension bolt force:

T* = M* / lever_arm - N* × e / lever_arm

Where lever_arm is the distance between the tension bolt group and the plate compression resultant. The bolt must be checked for tension capacity per AS 4100 Table 9.3.2 (or AS 3600 for the concrete side: pullout, breakout, side-face blowout).

Step 8 — Weld Design

Fillet welds connect the column to the base plate. For axial compression, design the weld around the full column perimeter or at flanges and web. The weld capacity per unit length (AS 4100 Cl 9.7.3.10, SP category):

phi_vw = phi × 0.6 × fuw × tt

Where tt = 0.707 × leg size, phi = 0.8 (SP weld category), fuw = electrode tensile strength (E48XX → fuw = 480 MPa). The weld demand is N* divided by the total weld length.

Worked Example — 200UC52.2, 500 kN Axial Only

Section: 200UC52.2 (d = 206 mm, bf = 206 mm, tf = 12.5 mm, tw = 8.0 mm) Applied load: N* = 500 kN (factored axial compression, strength limit state) Plate: Grade 350 steel (fy = 360 MPa at t ≤ 17 mm; use 350 MPa conservatively) Concrete: f'c = 25 MPa, pedestal = 500 × 500 mm Trial plate: 350 × 350 mm

Bearing Check

fp = N* / (B × D) = 500,000 / (350 × 350) = 4.08 MPa

A1 = 350 × 350 = 122,500 mm²
A2 = 500 × 500 = 250,000 mm²
sqrt(A2/A1) = sqrt(250,000/122,500) = 1.43

phi × 0.85 × f'c × sqrt(A2/A1) = 0.65 × 0.85 × 25 × 1.43
                                 = 19.8 MPa

fp = 4.08 MPa  ≤  19.8 MPa  ✓

Bearing: PASS

Plate Cantilever

n_flange = (350 - 0.80 × 206) / 2 = (350 - 164.8) / 2 = 92.6 mm
n_web    = (350 - 0.95 × 206) / 2 = (350 - 195.7) / 2 = 77.2 mm

Governing n = 92.6 mm  (flange direction controls)

Required Plate Thickness

tp ≥ n × sqrt(2 × fp / (phi × fy))
   = 92.6 × sqrt(2 × 4.08 / (0.9 × 350))
   = 92.6 × sqrt(8.16 / 315)
   = 92.6 × sqrt(0.02590)
   = 92.6 × 0.1609
   = 14.9 mm

Use 16 mm plate (next standard size above 14.9 mm). Check: 16 mm ≥ 14.9 mm ✓

Plate thickness: PASS

Weld to Column Flange

Use 8 mm fillet weld (E48XX, SP category) to both flanges:

Weld length = 2 × (2 × bf) = 2 × (2 × 206) = 824 mm  (both flanges, each side)

phi_vw = 0.8 × 0.6 × 480 × (0.707 × 8) = 0.8 × 0.6 × 480 × 5.66
       = 1,305 N/mm

Total weld capacity = 1,305 × 824 = 1,075 kN  >>  N* = 500 kN  ✓

Weld: PASS (utilisation = 0.47)

Summary

Common Base Plate Size Reference

The table below gives typical starting-point dimensions for 200UC and 310UC sections under moderate axial loads. Always verify with the procedure above.

Before You Start

Before designing a base plate per AS 4100, gather:

• Factored column demands: N* (axial), V* (shear), and M* (moment) at the column base from AS 1170 load combinations. Know the governing combination and whether it produces net compression, tension, or combined loading.
• Column section: UC or WB section designation with d, bf, tf, tw from the OneSteel tables or AS/NZS catalogue.
• Plate grade: AS/NZS 3678 Grade 250 (fy = 250 MPa), Grade 300 (fy = 300 MPa), or Grade 350 (fy = 360 MPa for t <= 17 mm, 340 MPa for 17 < t <= 40 mm). Note that fy varies with plate thickness for Grade 350.
• Concrete pedestal: f'c (typically 25-40 MPa), pedestal plan dimensions, and reinforcement details. The pedestal must be larger than the plate for the confinement benefit.
• Anchor bolt details: Number, diameter, grade (Grade 4.6 or 8.8), embedment depth, and layout pattern. For combined loading, know the bolt gauge and lever arm.
• Grout specification: Non-shrink grout, thickness (typically 25-50 mm), and whether it needs a separate bearing check.

Common Pitfalls

1. Using fy = 350 MPa for thick plates. AS/NZS 3678 Grade 350 has fy = 360 MPa for t <= 17 mm but only 340 MPa for 17 < t <= 40 mm and 330 MPa for 40 < t <= 80 mm. Using 350 MPa for a 25 mm plate is unconservative by about 3%.

2. Forgetting the 0.80 bf and 0.95 d effective footprint. The cantilever projection is not measured from the column centreline. It is measured from an effective footprint that accounts for the column's ability to distribute load. Using the full column dimension overestimates the footprint and underestimates the cantilever, leading to an unconservative plate thickness.

3. Omitting the confinement cap. The confinement factor sqrt(A2/A1) is capped at 2.0 per AS 3600. For a very large pedestal relative to the plate, the factor exceeds 2.0 but must be limited. Using the uncapped value overestimates the concrete bearing capacity.

4. Ignoring the weld return at re-entrant corners. AS 4100 Section 9.6.3 requires a weld return of at least 2 x weld leg size at re-entrant corners. Omitting this causes stress concentrations and potential cracking at the flange-web junction.

5. Using GP weld category for primary connections. SP category (phi = 0.80) requires NDT inspection but gives 33% more capacity than GP (phi = 0.60). For primary base plate connections, SP is standard practice. Using GP without realizing it reduces weld capacity significantly.

6. Not checking all AS 1170 load combinations for uplift. A base plate may be in compression under the gravity combination but in tension under wind uplift (0.9G + Wu). The anchor bolt design must consider all combinations, not just the maximum compression case.

Code Comparison

Frequently Asked Questions

How do I size a base plate under AS 4100? Start with bearing pressure: set plate area A1 = N* / (φ × f'c × sqrt(A2/A1)), then work backward. A practical starting point is A1 ≈ N* / (0.5 × f'c) assuming no confinement benefit. Once plate dimensions B and D are set, check plate thickness from the cantilever bending formula tp ≥ n × sqrt(2 × fp / (φ × fy)), where n is the cantilever projection from the column face and φ = 0.9 for AS 4100.

What is the difference between the AISC and AS 4100 base plate method? Both use the same cantilever bending model. Key differences: AS 4100 uses φ = 0.9 for plate bending, weld φ = 0.8 (SP category) vs AISC φ = 0.75; plate grade AS/NZS 3678 Grade 350 gives fy = 360 MPa (thin plate) vs A36 fy = 250 MPa. The bearing capacity equations reference AS 3600 vs ACI 318 respectively, but both use the confinement factor sqrt(A2/A1).

What weld size connects a base plate to a column flange in AS 4100? A fillet weld to both column flanges is standard. Size from the weld capacity formula φvw = φ × 0.6 × fuw × tw, where fuw = 480 MPa for E48XX electrode and tw = 0.707 × weld leg size. Weld length = perimeter of both flanges. For lightly loaded columns, the minimum weld size per AS 4100 Table 9.7.3.2 governs (typically 6–8 mm for plates 10–25 mm thick).

When do anchor bolts carry tension in a base plate design? Anchor bolts carry tension when the factored axial load becomes net uplift (N* is tensile) or when moment causes net tension on one side of the plate. Under pure compression, anchor bolts only function as holding-down bolts during erection. AS 4100 combined with AS 3600 Chapter 17 governs anchor embedment, breakout capacity, and supplementary reinforcement requirements.

What concrete strength should I use for base plate bearing? Use the specified characteristic compressive strength f'c from the project specification. For pedestals supporting columns in industrial buildings, f'c = 25–32 MPa is common. Higher f'c reduces the required plate area and can reduce plate thickness. Apply the confinement factor sqrt(A2/A1) ≤ 2.0 only when the pedestal is significantly larger than the plate.

What is the minimum fillet weld size for a base plate to column flange connection per AS 4100? Per AS 4100 Table 9.7.3.2, the minimum fillet weld leg size depends on the thickness of the thicker part being joined. For plates 11–17 mm thick, the minimum is 5 mm; for 17–32 mm, the minimum is 6 mm; for over 32 mm, 8 mm. For a 16 mm base plate to a 200UC flange (tf = 12.5 mm), the governing thickness is 16 mm and the minimum weld is 5–6 mm. In practice, 8 mm SP-category welds are common for base plates to ensure adequate heat input for fusion at the thicker plate.

How does the cantilever projection formula differ between the flange and web directions for a UC section? For a UC section, the plate cantilevers beyond both the flange width and the web depth. The effective column footprint used in AS 4100 practice is 0.80 × bf in the flange direction and 0.95 × d_c in the web direction, giving cantilever projections n_f = (B - 0.80 × bf) / 2 and n_w = (D - 0.95 × d_c) / 2. For a 200UC52.2 with bf = d = 206 mm on a 350 × 350 mm plate, n_f = (350 - 164.8) / 2 = 92.6 mm and n_w = (350 - 195.7) / 2 = 77.2 mm — the flange direction governs. For wide shallow sections or plates that extend much further beyond the web, the web cantilever can govern instead.

Run This Calculation

→ Base Plate & Anchors Calculator — full AS 4100 design with bearing, plate bending, weld, and anchor bolt checks. Supports 200UC, 310UC, 250UC, WB, and custom sections.

→ Column Capacity Calculator — axial compression capacity check per AS 4100 with K-factor input.

→ Concrete Footing Calculator — pedestal and spread footing design per AS 3600.

Related pages

• Guides and checklists
• AS 4100 code guide
• AS 4100 base plate worked example
• Base plate & anchors calculator
• Column capacity calculator
• Column K-factor reference
• Column K-factor guide — all 6 end conditions
• Anchor bolt embedment depth reference
• Steel Fy & Fu reference — yield and tensile strength by grade
• Steel grades reference — A36, A572, A992, Grade 300
• Concrete footing design reference
• How to verify calculator results
• Disclaimer (educational use only)
• Base plate design reference
• Base plate design checklist

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.