AS 4100 Base Plate Design — Worked Example

This worked example demonstrates a base plate design for a steel column under axial compression, following AS 4100-2020 and AS 3600-2018 for the concrete bearing check. Educational use only — always verify with a qualified engineer.

Problem Statement

Column: 200UC52.2 (bf = 206 mm, d = 206 mm, tf = 12.5 mm) Applied load: N* = 800 kN (factored compression, strength limit state) Base plate: 350 × 350 × 20 mm, Grade 300 steel (fy = 300 MPa) Concrete: f'c = 32 MPa, 600 × 600 mm pedestal Anchors: 4 × M20 Grade 4.6 bolts (holding-down only, no tension uplift)

Step 1 — Bearing Capacity of Concrete (AS 3600 Cl 12.6)

The bearing strength of the concrete under the base plate:

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

Where:

• phi = 0.65 (bearing)
• A1 = bearing area of plate = 350 × 350 = 122,500 mm²
• A2 = concentric area of pedestal = 600 × 600 = 360,000 mm²
• sqrt(A2/A1) = sqrt(360,000 / 122,500) = 1.71, capped at 2.0

phi_Nc = 0.65 × 0.85 × 32 × 122,500 × 1.71
       = 0.65 × 0.85 × 32 × 122,500 × 1.71
       = 3,713 kN  >> N* = 800 kN  ✓

Bearing check: PASS

Step 2 — Bearing Pressure Under Plate

q = N* / A_plate = 800,000 N / (350 × 350 mm²) = 6.53 MPa

Step 3 — Plate Cantilever Projection

The plate cantilevers beyond the column flange and web. The critical projection lengths:

n_f = (B - bf) / 2 = (350 - 206) / 2 = 72 mm  (beyond flange)
n_w = (D - d) / 2 = (350 - 206) / 2 = 72 mm  (beyond web)

For a 200UC52.2, the controlling cantilever is n = 72 mm.

Step 4 — Plate Bending (AS 4100 Cl 8.3)

The plate acts as a cantilever under the uniform bearing pressure q. Moment per unit width at the column face:

M* = q × n² / 2 = 6.53 × 72² / 2 = 16,919 N.mm/mm = 16.9 kN.mm/mm

Plate design moment capacity (phi × Ms):

phi_Ms = phi × fy × t² / 4   (per unit width, rectangular section)
       = 0.9 × 300 × t² / 4

Required t from M* = phi_Ms:

t_min = sqrt(4 × M* / (phi × fy))
      = sqrt(4 × 16,919 / (0.9 × 300))
      = sqrt(250.7)
      = 15.8 mm

Provided plate thickness = 20 mm > 15.8 mm ✓

Plate bending: PASS

Step 5 — Weld to Column Flange

Assume a 10 mm fillet weld around the column perimeter (all four sides of flange):

Weld length = 2 × (bf + d) = 2 × (206 + 206) = 824 mm

Weld capacity (SP category, E48XX electrode, AS 4100 Cl 9.7.3.10):

phi_vw = phi × 0.6 × fuw × tw
       = 0.8 × 0.6 × 480 × (0.707 × 10)
       = 0.8 × 0.6 × 480 × 7.07
       = 1,630 N/mm

Total weld capacity:

phi_Vw = 1,630 × 824 = 1,343 kN

Average weld demand = N* / weld length = 800,000 / 824 = 971 N/mm

Ratio = 971 / 1,630 = 0.60 ✓

Weld check: PASS

Step 6 — Anchor Bolts (Holding Down, No Uplift)

For axial compression only with no net tension, the M20 Grade 4.6 anchor bolts act as holding-down bolts only. Verify minimum embedment per AS 3600 and confirm no uplift under AS 1170 load combinations.

Summary

Common Pitfalls

1. Selecting a plate larger than needed. A plate that extends too far beyond the column footprint increases the cantilever projection n, which directly increases the required plate thickness. Keep the plate as close to the column size as bearing pressure allows. In this example, a 400 x 400 mm plate would increase n from 72 mm to 97 mm, increasing the required thickness from 15.8 mm to 21.2 mm.

2. Rounding down instead of up. The calculated minimum plate thickness of 15.8 mm must be rounded up to the next standard plate thickness (16 mm or 20 mm), never down to 14 mm. Standard plate thicknesses per AS/NZS 3678: 6, 8, 10, 12, 16, 20, 25, 28, 32, 36, 40 mm.

3. Using fy = 300 MPa for a thin Grade 350 plate. If you specify Grade 350 plate and the thickness is <= 17 mm, the actual fy is 360 MPa, not 350 MPa. Using 300 MPa from a Grade 300 plate when Grade 350 was specified wastes capacity. Conversely, using Grade 350 values for a Grade 300 plate is unconservative.

4. Omitting the weld capacity check. Even when bearing and plate bending clearly govern, the weld must still be checked. A designer who skips this step may discover during fabrication that the specified weld is undersized for the column load.

5. Ignoring load combinations that produce uplift. This example is pure compression, but the same plate and anchors must resist 0.9G + Wu (wind uplift) and 0.9G + Eu (seismic uplift) combinations. The M20 Grade 4.6 holding-down bolts must be adequate for any net tension from these combinations.

6. Not checking the pedestal against punching shear. A small pedestal with a large column load may fail in punching shear at the pedestal-footing interface. This is a reinforced concrete check per AS 3600 Cl. 9.3 and is outside the steel base plate calculation, but it must not be overlooked.

Code Comparison

Frequently Asked Questions

Why does the cantilever projection n govern plate thickness? The plate bends as a cantilever outward from the column face. The larger the projection n (either in the flange direction or web direction), the larger the bending moment at the column face, and therefore the thicker the plate required. A plate that is too large relative to the column wastes material and increases plate thickness rapidly — keep the plate size as close to the column footprint as bearing pressure allows.

Why is 20 mm plate used when 15.8 mm is required? Plates are specified in standard mill thicknesses. After calculating tp,min = 15.8 mm, the next standard plate thickness is 16 mm per AS/NZS 3678 practice. The example rounds to 20 mm to provide additional robustness margin and to use a common stocked thickness. In practice, 16 mm would be acceptable — document the governing thickness and the selected plate thickness separately.

Does the fillet weld need to return around the column flanges? At re-entrant corners (flange-to-web intersections), many engineers provide a weld return of at least 2× the weld size per AS 4100 Section 9.6.3 and good fabrication practice. The worked example uses total weld length = 2 × (bf + d) around the full perimeter. This is conservative — some engineers only weld the flanges. The design must match the actual weld pattern shown on the shop drawing.

What if there is a moment as well as axial load? Combined moment and axial changes the bearing pressure distribution: one side increases and the other decreases or goes into tension. If net tension develops, anchor bolts must be designed for tension breakout per AS 3600 Chapter 17. The plate thickness calculation also changes — you must check the plate at the location of maximum bearing pressure on the compression side, not just at the cantilever centroid.

For this example, what plate thickness would be required if the column load were doubled to 1,600 kN? With N* = 1,600 kN and the same 350 × 350 mm plate, the bearing pressure becomes q = 1,600,000 / (350 × 350) = 13.06 MPa. Using n = 72 mm, the moment per unit width is M* = 13.06 × 72² / 2 = 33,850 N.mm/mm. Required thickness: t_min = sqrt(4 × 33,850 / (0.9 × 300)) = sqrt(501.5) = 22.4 mm. You would select a 25 mm plate (next standard size). The plate area would also need checking against the concrete bearing limit: q = 13.06 MPa vs phi × 0.85 × f'c × sqrt(A2/A1) = 0.65 × 0.85 × 32 × 1.71 = 30.3 MPa — still passes. Alternatively, increase the plate size to reduce q and plate thickness simultaneously.

What weld return length is required at the re-entrant corners between the column flange and web? AS 4100 Section 9.6.3 requires a weld return at re-entrant corners of at least 2 × weld leg size. For a 10 mm fillet weld, the minimum return is 20 mm. In practice, engineers often weld continuously around the full column perimeter (flanges + web), which naturally provides returns at the flange-to-web junction. Some practitioners weld flanges only (not the web) for compression-only bases; in this case, weld returns are still needed at the flange terminations to prevent stress concentrations.

At what eccentricity does the load resultant leave the kern of this 350 × 350 mm plate? The kern of a rectangular plate extends ±D/6 and ±B/6 from the centroid. For a 350 × 350 mm plate, the kern boundary is at ±350/6 = ±58.3 mm from the plate centre. If the moment-to-axial ratio M*/N* exceeds 58.3 mm (eccentricity e > 58.3 mm), the load resultant exits the kern and one side of the plate lifts off. At that point the bearing pressure can no longer be assumed uniform and the anchor bolts on the tension side must be checked for uplift per AS 3600 Chapter 17.

Run This Calculation

→ Base Plate & Anchors Calculator — interactive AS 4100 / AISC 360 / EN 1993 base plate design with full derivation steps.

→ Column Capacity Calculator — axial compression capacity per AS 4100 with slenderness and K-factor inputs.

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

Related pages

• Guides and checklists
• AS 4100 base plate design — procedure
• AS 4100 code guide
• Base plate & anchors calculator
• Column capacity calculator
• Column K-factor reference
• 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
• HSS section properties reference
• How to verify calculator results
• Disclaimer (educational use only)
• Base plate design reference
• Base plate design checklist

Disclaimer (educational use only)

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