How is concrete bearing capacity checked for an AS 4100 base plate?

Per AS 3600 Clause 12.6: φ × 0.85 × f'c × A1 × √(A2/A1) with φ = 0.60 and √(A2/A1) capped at 2.0. For a 310UC137 column on a 540x540 plate over a 900x900 pedestal with 32 MPa concrete, the bearing capacity is 7,940 kN — well above the example's 2,400 kN demand.

How is base plate thickness calculated per AS 4100?

Base plate thickness is derived from cantilever bending: M* = w × aₘ² / 2 where aₘ is the larger cantilever distance from column face to plate edge. Required thickness: tₚ ≥ √(4M* / φfᵧ) with φ = 0.90 per AS 4100 Table 3.4. For 310UC137 with 540x540 plate under 2,400 kN: aₘ = 115.5 mm, w = 8.23 MPa, t_req = 31.2 mm — use 32 mm plate.

How are anchor bolts designed per AS 4100 Clause 9.3?

Anchor bolt tension capacity per AS 4100 Clause 9.3.1: φNₜf = φ × Aₛ × fᵤf where φ = 0.80. For M24 Grade 4.6 (Aₛ = 324 mm², fᵤf = 400 MPa): φNₜf = 103.7 kN per bolt. Four anchors provide 415 kN total. Shear transfer is handled by a 20 mm × 100 mm shear key cast into the pedestal.

What grout thickness and strength are required?

Standard grout thickness is 25-50 mm. For heavy columns like 310UC137, use 40 mm non-shrink cementitious grout at 40 MPa minimum strength. The grout spreads bearing stress at 45 degrees, increasing effective bearing area by roughly 2 × t_grout on each side — for 40 mm grout, the effective plate dimension increases from 540 mm to 620 mm.

Why does a 310UC137 need a 32 mm thick base plate?

The 310UC137 is a heavy column (137 kg/m, 300 mm+ flanges) with high axial capacity. The large cantilever from column face to plate edge (115.5 mm for a 540 mm plate) generates significant bending moment. The 32 mm thickness follows directly from the bending demand. Lighter columns (200UC, 250UC) typically use 20-25 mm plates.

AS 4100 Base Plate Complete Worked Example — 310UC137, M24 Anchors

A column base plate transfers axial compression from the steel column into the concrete foundation. Getting the plate dimensions, thickness, and anchor bolt arrangement right is fundamental to steel design. This page walks through a complete AS 4100 base plate design for a 310UC137 column carrying 2,400 kN, with four M24 Grade 4.6 anchor bolts on a 540x540x32 mm plate. Every number is shown. Every clause is referenced.

PRELIMINARY — NOT FOR CONSTRUCTION. This is an educational worked example. All designs must be independently verified by a licensed Professional Engineer before use in any project.

Design Brief

Parameter	Value
Column	310UC137 (Grade 300PLUS)
Column dimensions	d = 321 mm, bf = 309 mm, tf = 21.7 mm, tw = 13.8 mm
Axial load (factored)	N* = 2,400 kN (compression only)
Concrete strength	f'c = 32 MPa
Pedestal size	900 x 900 mm
Base plate steel	Grade 250 (fyp = 250 MPa, fup = 410 MPa)
Anchor bolts	4 x M24 Grade 4.6 (fuf = 400 MPa)
Bolt layout	150 mm from column face, 440 mm gauge
Grout	40 MPa non-shrink cementitious, 40 mm thickness

Step 1 — Concrete Bearing Check (AS 3600 Clause 12.6)

The base plate area A1 must be large enough that the concrete bearing pressure does not exceed the capacity.

Try a 540 x 540 mm plate:

A1 = 540 x 540 = 291,600 mm2
Bearing pressure: fbp = N* / A1 = 2,400,000 / 291,600 = 8.23 MPa

A2 (pedestal area, 900 x 900) = 810,000 mm2
sqrt(A2/A1) = sqrt(810,000 / 291,600) = sqrt(2.778) = 1.667 (capped at 2.0, OK)

Concrete bearing capacity per AS 3600 Cl 12.6:
Design bearing = 0.60 x 0.85 x 32 x 291,600 x 1.667 = 7,940 kN

Check: 2,400 kN << 7,940 kN --> Bearing OK by wide margin

The bearing check passes comfortably. The A2/A1 confinement factor of 1.667 provides substantial additional capacity. If the pedestal were smaller (600 x 600), sqrt(A2/A1) drops to 1.111, reducing capacity to approximately 5,300 kN — still adequate, but with less margin.

Step 2 — Base Plate Thickness (Cantilever Bending Method)

The plate thickness is governed by bending of the plate projection beyond the column face. The bearing pressure acts upward, and the plate cantilevers from the column profile.

Cantilever distances:

a1 = (540 - 321) / 2 = 109.5 mm  (parallel to column depth)
a2 = (540 - 309) / 2 = 115.5 mm  (parallel to flange width)
Governing cantilever: am = max(109.5, 115.5) = 115.5 mm

Bending moment per unit width (1 mm strip):

M* = w x am^2 / 2 = 8.23 x 115.5^2 / 2 = 54,890 Nmm/mm = 54.9 kNm/m

Required plate thickness (AS 4100 Table 3.4, bending capacity factor = 0.90):

Ms = 0.90 x fyp x Z_plastic  where Z_plastic = t^2 / 4 (per mm width)
Set Ms >= M*:
0.90 x 250 x (t^2 / 4) >= 54.89  (in kNmm per mm width)
56.25 x t^2 >= 54,890
t^2 >= 976
t >= 31.2 mm

Use 32 mm plate.

This is a thick plate, but normal for a 310UC137 under high axial load. The large cantilever from the column flange face (115.5 mm) drives the thickness requirement.

Alternative check: a 500 x 500 plate gives A1 = 250,000 mm2, bearing pressure = 9.60 MPa, am = 95.5 mm, t >= 26.8 mm (use 28 mm). The 28 mm plate saves material but increases bearing pressure by 17%. The original 540 x 540 x 32 design is preferred for simplicity.

Step 3 — Anchor Bolt Design (AS 4100 Clause 9.3)

For a compression-only column, anchors mainly serve for erection stability and accidental tension during construction. Four M24 Grade 4.6 bolts are specified.

Bolt tension capacity per AS 4100 Cl 9.3.1:

M24 Grade 4.6: fuf = 400 MPa
Tensile stress area: As = pi x (20.32)^2 / 4 = 324 mm2
Design tension capacity per bolt = 0.80 x 324 x 400 = 103.7 kN
Four bolts: 4 x 103.7 = 414.8 kN total

Bolt shear capacity (if shear key not provided, threads in shear plane):

Design shear capacity per bolt = 0.80 x 0.62 x 400 x 324 = 64.4 kN
Four bolts: 4 x 64.4 = 257.6 kN total

Recommendation: Provide a 20 x 100 mm shear key fillet-welded to the plate underside, cast into a 150 x 150 x 100 mm pocket. This eliminates reliance on anchor bolts for shear transfer — standard Australian practice for heavily loaded columns.

Step 4 — Weld Design: Column to Base Plate

The column is welded to the base plate with fillet welds around the column profile. For compression-only with full bearing, welds are nominally sized.

Minimum fillet weld per AS 4100 Table 9.6.3.2:

For plate t = 32 mm: minimum leg = 8 mm
For column flange tf = 21.7 mm: minimum leg = 6 mm
Specify 8 mm continuous fillet weld all around, Grade E48XX electrodes.

Weld shear capacity per mm: 0.80 x 0.6 x 480 x (0.707 x 8) = 1.30 kN/mm
Total weld length (310UC137 perimeter): approximately 1,260 mm
Total capacity: 1,260 x 1.30 = 1,638 kN (adequate for positioning)

Step 5 — Grout Pad Specification

Per Australian industry practice and AS 3600 guidance:

Grout type:      Non-shrink cementitious (AS 1478.2 Type B)
Thickness:       40 mm (provides tolerance for concrete surface)
Strength:        40 MPa at 28 days
Placement:       Flowable, through two 50 mm grout holes in plate
Curing:          Wet cure 72 hours minimum

The 40 mm grout thickness increases effective bearing area via 45-degree load spread:

B_eff = 540 + 2 x 40 = 620 mm
N_eff = 540 + 2 x 40 = 620 mm
A1_eff = 620 x 620 = 384,400 mm2
Effective bearing pressure: 2,400,000 / 384,400 = 6.24 MPa (24% reduction)

Complete Specification Summary

Item	Specification
Base plate	540 x 540 x 32 mm, Grade 250
Anchor bolts	4 x M24 Grade 4.6, 600 mm embedment, 100 mm projection
Bolt gauge	440 mm centres, 150 mm from column flange face
Bolt holes	26 mm diameter (2 mm oversize)
Grout	40 mm non-shrink cementitious, 40 MPa minimum
Welds	8 mm CFW all around column profile, Grade E48XX
Shear key	20 x 100 mm flat bar x 100 mm projection, 8 mm FW
Concrete	32 MPa, 900 x 900 mm pedestal, reinforced per AS 3600

Design Checks Summary

Check	Demand	Capacity	Ratio	Status
Concrete bearing	8.23 MPa	27.2 MPa effective	0.30	OK
Plate bending	32 mm provided	31.2 mm required	0.98	OK
Anchor tension	0 kN	103.7 kN per bolt	0.00	OK
Anchor shear	0 kN (shear key)	64.4 kN per bolt	—	OK
Weld capacity	Nominal	1,638 kN total	—	OK

Frequently Asked Questions

Why does a compression-only column need anchor bolts at all?

Anchor bolts serve three purposes even for pure compression columns: (1) erection stability — holding the column plumb before the frame is complete, (2) accidental tension — wind uplift during construction or unbalanced loading, and (3) robustness — providing nominal tension tie for disproportionate collapse resistance per AS 1170.0. The bolts also prevent the column from shifting off the base plate during erection.

Could I use a smaller plate with a thicker grout pad?

Reducing the plate to 500 x 500 reduces cantilever distance (am = 95.5 mm) and allows a thinner plate (28 mm vs 32 mm). However, bearing pressure increases from 8.23 to 9.60 MPa, leaving less margin. A 28 mm plate is an off-standard thickness (standard increments: 25, 32, 36, 40 mm), so a 32 mm plate may actually be cheaper and more readily available from stock.

What changes if the column has moment in addition to axial load?

A moment-resisting base plate requires checking combined axial-plus-moment. Bearing pressure becomes non-uniform (triangular distribution), and anchors on the tension side must carry the uplift component. Plate thickness also increases because tension-side anchors impose prying action. For a 310UC137 with 2,400 kN axial plus 150 kNm moment, tension-side anchors carry approximately 170 kN each — beyond M24 Grade 4.6 capacity. M30 Grade 8.8 anchors and a 40 mm plate would be indicated.

Is a shear key always required?

For small columns (150UC, 200UC) with low shear, anchors alone may suffice. For a 310UC137, shear demand is typically high enough that a shear key is recommended. Anchors in shear rely on bearing against concrete through the grout layer, and grout can crush locally under repeated loading, loosening the connection. A shear key provides a positive, direct load path that does not degrade over time.

How do I verify base plate thickness by yield line analysis?

The cantilever method is a simplified elastic approach. Yield line analysis provides a more refined result by considering the plastic collapse mechanism of the plate. For a rectangular plate with central H-shaped column footprint, yield lines form between the column perimeter and plate edges. The yield line capacity is typically 10-15% higher than the cantilever method prediction, meaning a 32 mm plate has some reserve capacity. For critical base plates (moment connections, seismic), yield line analysis per AISC Design Guide 1 should be performed.

This worked example is for educational purposes only. Verify all assumptions, material grades, and code clauses against the current editions of AS 4100, AS 3600, and AS/NZS 3679.1 before use in any project. All designs must be independently checked and certified by a licensed Professional Engineer.