AS 4100 Base Plate Design — Complete Procedure & Worked Examples

Base plates transfer column axial loads, bending moments, and shear forces to concrete foundations. In Australia, AS 4100 (Steel Structures) combined with AS 3600 (Concrete Structures) governs the design of steel column base plate connections.

This page covers the full design procedure including bearing checks, plate bending, anchor bolt design, the levelling nut (yield line) method, and worked examples.

Design Standards

• AS 4100: Steel Structures — governs the steel base plate design
• AS 3600: Concrete Structures — governs the concrete bearing strength
• AS 4100 Clause 9.1: Connections general requirements
• AS 4100 Clause 9.3: Bearing-type connections
• AS 3600 Clause 12.6: Bearing stress on concrete

The design philosophy is limit state design (LSD) with capacity factors per AS 4100 Table 3.4.

Base Plate Design Procedure — Step by Step

Step 1: Determine Design Actions

Obtain the design actions at the base of the column from the structural analysis:

• N* (design axial force): compression or tension
• M*x and My (design bending moments)
• V*x and Vy (design shear forces)

These come from the load combinations in AS/NZS 1170.0 (or the project-specific combination factors).

Step 2: Determine Base Plate Geometry

Select preliminary base plate dimensions:

• Plate width (B): typically column flange width + 2 × edge distance (min 50 mm each side)
• Plate depth (N): typically column depth + 2 × edge distance
• Plate thickness (tp): initially estimate based on bearing requirements

The plate must extend beyond the column profile to distribute the load to the foundation. Minimum edge distance to anchor bolts is typically 1.5 × bolt diameter or 40 mm, whichever is greater.

Step 3: Concrete Bearing Check

Check the concrete bearing strength per AS 3600 Clause 12.6:

f_b = φ × 0.85 × f'c × √(A2/A1) ≤ 2 × f'c

where:

• φ = 0.6 (AS 3600 capacity factor for bearing)
• f'c = characteristic compressive strength of concrete (typically 25, 32, or 40 MPa)
• A1 = base plate bearing area (B × N)
• A2 = maximum area of the supporting surface that is geometrically similar to and concentric with A1

The √(A2/A1) factor accounts for beneficial confinement from the surrounding concrete. It is limited to 2.0, giving a maximum bearing stress of 1.7 × f'c.

Required plate area:

A1 ≥ N* / f_b

Step 4: Plate Bending Check

The base plate must resist bending caused by the spread of load from the column to the plate edges. Two critical bending planes:

4a. Cantilever bending (plate extending beyond column flange):

The plate bends as a cantilever from the column flange face to the plate edge. The bending moment per unit width:

M* = w × m² / 2

where m is the cantilever distance from the column face to the plate edge.

Required plate thickness:

tp ≥ √(4 × M* / (φ × fy))

where φ = 0.9 for plate bending.

4b. Bending between column flanges (for light axial loads on large plates):

For plates that are significantly larger than the column, check bending of the plate strip between the column flanges. This strip is loaded by the bearing pressure and spans between the column flanges.

Step 5: Anchor Bolt Design

Anchor bolts resist tension from overturning moments and uplift forces.

5a. Tension capacity per bolt:

φNt = φ × As × fy_bolt

where:

• φ = 0.8 (AS 4100 Table 3.4 for bolts in tension)
• As = tensile stress area of the bolt
• fy_bolt = yield stress of bolt material (typically 640 MPa for Grade 8.8)

5b. Required number of bolts for tension:

n ≥ N*t / φNt

where N*t is the total tension from the overturning moment.

5c. Embedment length:

The anchor bolt embedment into concrete must develop the full bolt capacity. Per AS 3600:

Ld = (fy_bolt × As) / (0.5 × √f'c × π × db)

Minimum embedment is typically 12 × bolt diameter for hooked anchors and 15 × db for straight anchors.

Step 6: Shear Transfer

Shear forces are transferred from the column to the foundation through:

1. Friction between base plate and grout: Vf = μ × N where μ ≈ 0.3 for steel on grout
2. Anchor bolts in shear: if friction is insufficient, bolts resist the remainder
3. Shear keys: for large shear forces, a shear key (lug) welded to the underside of the base plate

For anchor bolts in shear:

φVf = φ × 0.62 × fy_bolt × As

Check combined tension and shear interaction per AS 4100 Clause 9.3.

The Levelling Nut Method (Yield Line Analysis)

Background

When base plates are supported on levelling nuts (rather than being grouted with full bearing), the load transfer mechanism changes. The plate is effectively supported at discrete points (the nuts) rather than on a continuous bearing surface.

This is common in:

• Pre-levelled column bases
• Adjustable base plates for precise alignment
• Temporary bracing connections
• Industrial structures where grouting is delayed

Yield Line Method

The yield line theory models the plate as developing plastic hinges at failure. For a rectangular base plate with anchor bolts at the corners:

1. Identify the yield line pattern. The most common pattern is diagonal yield lines from column corners to plate edges, dividing the plate into triangular regions.

2. Equilibrium of each region. Each triangular region of the plate rotates about its support line (the column face) under the applied bearing pressure.

3. Work equation. The internal work done by the plastic hinges equals the external work done by the bearing pressure:

Σ(Mp × θ × L_hinge) = Σ(q × A_region × δ)

where:

• Mp = fy × tp² / 4 = plastic moment capacity per unit width
• θ = rotation angle of each yield line
• L_hinge = length of each yield line
• q = bearing pressure (uniform)
• A_region = area of each triangular region
• δ = average displacement of each region

4. Solve for required thickness. The yield line analysis typically gives a thicker plate than the simple cantilever method because the plate must span between the levelling nuts rather than bearing directly on the grout.

Practical Design Guidance

For base plates with levelling nuts:

• Increase the design thickness by 20-30% compared to a fully grouted plate
• Ensure nuts are tightened to snug-tight to prevent plate flexure under service loads
• Check the plate for prying action if bolts are in tension
• Consider oversized holes (2-3 mm clearance) for erection tolerance
• Washers must be used under both the levelling nut and the top nut

AS 4100 Specific Provisions

AS 4100 does not provide explicit provisions for the levelling nut method. The design must follow:

• General connection provisions (Clause 9.1)
• Bolt design rules (Clause 9.3)
• Plate element rules (Clause 6.2) for plate slenderness
• First principles yield line analysis (as described above)

The capacity factor for plate bending is φ = 0.9.

Worked Example: UC Base Plate Under Axial Load

Problem

Design a base plate for a 250UC72.9 column carrying a design axial load of N* = 1800 kN. The foundation is 32 MPa concrete. Use Grade 250 steel plate and M24 Grade 8.8 anchor bolts.

Given Data

Column properties (250UC72.9):

• d = 254 mm (total depth)
• bf = 254 mm (flange width)
• tf = 14.2 mm (flange thickness)

Materials:

• Plate: fy = 250 MPa
• Bolts: fy = 640 MPa (Grade 8.8)
• Concrete: f'c = 32 MPa

Step 1: Plate Dimensions

Plate width: B = 254 + 2 × 55 = 364 mm → use 370 mm Plate depth: N = 254 + 2 × 55 = 364 mm → use 370 mm

Plate area: A1 = 370 × 370 = 136,900 mm²

Step 2: Concrete Bearing Check

Assuming A2/A1 = 2.0 (conservative for a pad footing):

f_b = 0.6 × 0.85 × 32 × √2.0 = 23.1 MPa (limited to 2 × 32 = 64 MPa, OK)

Bearing capacity: φRb = 23.1 × 136,900 / 1000 = 3,162 kN > N* = 1,800 kN ✓

Step 3: Plate Bending

Cantilever distance from column flange to plate edge:

m = (370 - 254) / 2 = 58 mm

Bearing pressure under full plate area:

q = N* / A1 = 1,800,000 / 136,900 = 13.1 MPa

Bending moment per unit width:

M* = q × m² / 2 = 13.1 × 58² / 2 = 22,034 N·mm/mm

Required plate thickness:

tp = √(4 × M* / (φ × fy)) = √(4 × 22,034 / (0.9 × 250)) = √391.7 = 19.8 mm

Use tp = 20 mm Grade 250 plate.

Step 4: Anchor Bolts

For pure axial compression, anchor bolts resist erection loads and nominal tension. Minimum 4 bolts:

Bolt tension capacity: φNt = 0.8 × 353 × 640 / 1000 = 180.7 kN per bolt (As = 353 mm² for M24)

4 × φNt = 723 kN — more than adequate for erection loads.

Embedment: Ld = 12 × 24 = 288 mm minimum. Use 300 mm embedment with standard hook.

Result

• Base plate: 370 × 370 × 20 mm Grade 250
• Anchor bolts: 4 × M24 Grade 8.8 with 300 mm embedment
• Grout: 50 mm non-shrink grout under full plate area

Worked Example: Base Plate with Overturning Moment

Problem

The same 250UC72.9 column base has N* = 600 kN (compression) and M*x = 120 kN·m. Check the base plate and anchor bolts.

Eccentricity

e = M / N = 120,000 / 600 = 200 mm**

Since e = 200 mm > N/6 = 370/6 = 61.7 mm, tension develops on one side.

Anchor Bolt Tension

Assuming 4 bolts at 310 mm × 310 mm spacing (55 mm from plate edge):

Bolt group moment of inertia (per bolt): I_bolt = 2 × (310/2)² = 96,100 mm² (two bolts per side)

Tension per bolt:

T = M × 155 / (2 × 155²) = 120,000 / (2 × 155) = 387 N/mm...**

Simplifying with the couple approach:

T_bolt = (M - N* × 0) / (2 × 155) = 120,000,000 / 310,000 = 387 kN**

Wait — let me redo this properly.

Net tension on the tension side:

T_total = M / d_bolt - N/2 = 120,000 / 0.31 - 600/2 = 387.1 - 300 = 87.1 kN**

No — the correct approach for the bolt group:

T_per_bolt = (M / n × d_bolt_spacing) / n_bolts_per_side**

Using the simplified rectangular stress block approach:

T_total = (N × e) / (N_plate/2) - N*/2 = (600 × 200) / 185 - 600/2 = 648.6 - 300 = 348.6 kN**

T*_per_bolt = 348.6 / 2 = 174.3 kN per bolt

φNt = 180.7 kN > T* = 174.3 kN ✓ (barely, increase to M30 if more margin desired)

Plate Bending Under Tension

The plate must also be checked for bending due to the bolt tension pulling the plate edge up. This prying action increases the bolt tension and creates additional bending in the plate.

Use a thicker plate (25 mm) for this case to handle the combined bearing and prying.

Raised Foundation Bolt Connection

Detailing

For base plates on raised piers or pedestals:

1. Anchor bolt sleeves: Use 75 mm × 75 mm pockets around each bolt for grouting after leveling
2. Plate washers: Minimum 50 mm × 50 mm × 8 mm under nuts
3. Grout: Non-shrink, minimum 30 MPa, pour after alignment
4. Edge distance: Minimum 100 mm from bolt center to concrete edge
5. Bolt projection: Minimum 2 bolt diameters above top of plate for nut + washer

Bending of Anchor Bolts in Raised Position

When the base plate is levelled on nuts and there is a gap between plate underside and concrete, the anchor bolt segment between the concrete and the plate is in free span. If lateral loads are applied before grouting:

Bending moment in bolt:

M_bolt = V × L_gap**

where L_gap is the free length of bolt between concrete surface and plate underside.

Check combined tension + bending in the bolt using the interaction formula:

(N/φNt)² + (M/φM) ≤ 1.0**

Base Plate Design Checklist

• Design actions determined (N*, M*, V*) from structural analysis
• Load combinations per AS/NZS 1170.0 applied
• Concrete bearing stress ≤ φ × 0.85 × f'c × √(A2/A1)
• Plate bending checked for cantilever action (m distance)
• Plate thickness ≥ required from bending calculation
• Anchor bolt tension capacity ≥ bolt tension demand
• Anchor bolt embedment develops full bolt capacity
• Shear transfer mechanism identified (friction, bolts, or shear key)
• Combined tension + shear interaction checked per AS 4100 Clause 9.3
• Plate fabrication tolerances specified
• Grout specification (non-shrink, min 30 MPa)
• Edge distances to concrete face ≥ 100 mm
• Base plate detail drawn with bolt layout, dimensions, and welds

Prying Action on Anchor Bolts

When anchor bolts are in tension (from overturning moment or uplift), the base plate bends as the bolt pulls upward, creating a prying force at the plate edge. This prying action increases the bolt tension beyond the applied load.

Prying Force Mechanism

The prying force Q develops because the base plate deforms between the anchor bolt and the edge of the plate. The bolt stretches, the plate bends, and the edge of the plate bears against the grout surface. Three equilibrium states exist:

1. No prying: The plate is rigid enough that the bolt can stretch without plate bending — the bolt force equals the applied tension T.
2. Partial prying: The plate bends, creating contact at the plate edge. The edge reaction Q adds to the bolt force: T_bolt = T + Q.
3. Full prying: The plate yields at the bolt line, and a plastic hinge forms, limiting further increase in prying force.

Simplified Prying Analysis (per AS 4100 Commentary and AISC Manual Part 9)

The AS 4100 Commentary suggests using the T-stub analogy adapted from AISC Manual Part 9:

Check for prying:

1. Determine the tributary width p for each bolt. For end anchor bolts: p is typically half the bolt spacing plus edge distance, limited to the flange width or plate geometry.
2. Calculate the required plate thickness to avoid prying:

t_min = sqrt(4 x T x b' / (p x Fy x (1 + delta x alpha'))

where:

• T = tension per bolt (service or factored as applicable)
• b' = distance from bolt centerline to the edge of the plate face minus half the bolt head/nut diameter
• p = tributary width per bolt
• Fy = plate yield stress
• delta = ratio of net area to gross area at the bolt line
• alpha' = coefficient depending on connection geometry (typically 1.0 for preliminary design)

3. If the actual plate thickness t >= t_min: Prying is negligible — use T_bolt = T.
4. If t < t_min: Prying develops. The bolt force increases to T_bolt = T x (1 + Q/T).

Practical Guidance for Prying

• Plates 25 mm and thicker typically experience minimal prying for M24 bolts at standard edge distances.
• For thinner plates (16-20 mm) with tension anchors, increase the bolt capacity by 20-30% to account for prying, or verify by calculation.
• Increasing edge distance reduces prying (more plate length generates more bending resistance).
• Adding stiffener plates (vertical gussets) between the plate and column eliminates the prying effect for bolts near the stiffeners.
• Prying is most significant when bolts are near the plate edge (edge distance < 1.5 x bolt diameter).

Weld Design for Base Plates

Fillet Welds Between Column and Base Plate

The column-to-base-plate weld must transfer the design axial force, moment, and shear from the column into the plate. For columns with base plates in full compression (no tension), nominal welds around the column profile are sufficient for load transfer.

Minimum fillet weld size per AS 4100 Clause 9.7.3.10:

Weld design for axial load:

For a column subject to axial compression N*:

• Full bearing: The base plate bears directly on the grout/concrete. The weld transfers only enough load to hold the column in place during construction. For this case, a 6 mm fillet weld around the column profile is usually adequate (AS 4100 recommends minimum nominal welds for bearing connections).
• Partial bearing or tension: The weld must develop the full tension capacity of the column cross-section or the design tension, whichever is smaller.

Weld design for combined axial + moment:

When the base plate is subject to axial compression plus moment, the weld is designed for the maximum tensile force on the tension side:

Tension force per unit length = (N/A_column + M/Zy) x effective weld length**

For a UC column (doubly symmetric), the weld demand varies around the profile:

• The compression flange transmits the maximum compressive stress from the column
• The tension flange is critical for weld design when the overturning moment creates tension
• The web transmits the vertical shear from the column

Typical weld schedule for column base plates:

Base Plate to Concrete Connection

The base plate transfers its load into the concrete through:

1. Bearing (compression): Direct contact between steel plate and grout/concrete
2. Anchor bolts (tension): Bolts embedded in concrete
3. Friction (shear): Between plate and grout
4. Shear key (large shear): A structural steel lug welded to the plate underside

Grout Pad Design and Specification

Grout Material Properties

Structural grade non-shrink cementitious grout:

• Compressive strength: Minimum 30 MPa at 28 days (typically 40-60 MPa)
• Flow: 250-300 mm flow cone (for 25-50 mm gap)
• Maximum aggregate size: 5-10 mm (for 25-50 mm gap)
• Expansion: 0-0.3% restrained expansion (non-shrink)
• Elastic modulus: 25-35 GPa (depends on compressive strength)

Grout Pad Geometry

• Thickness: 25-50 mm typical. Minimum 25 mm for adequate flow under the plate. Maximum 50 mm unless designed for lateral loads (thin grout beds can crush under edge compression).
• Extent: Full area under the base plate. Partial grouting creates stress concentrations and should be avoided.
• Bearing surface: The concrete foundation should be roughened (minimum 3 mm amplitude) to ensure composite action between grout and concrete.

Grout Placement Procedure

1. Formwork: Construct a tight form around the base plate perimeter. Leave grout holes at the top and vent holes at the edges.
2. Pre-wetting: The concrete surface should be saturated surface dry (SSD) before grouting. Dry concrete absorbs water from the grout, reducing strength and creating shrinkage cracks.
3. Grout mixing: Follow manufacturer instructions precisely. Over-watering reduces strength by 30-50%.
4. Placement: Pump or pour grout from one side only to avoid air entrapment. Maintain continuous flow until all vents show fresh grout.
5. Curing: Wet cure for 7 days minimum. Apply wet burlap or curing compound.

Grout Quality Assurance

• Compression testing: Minimum 3 cubes/cylinders per batch, tested at 7 and 28 days
• Flow testing: Check flow cone value before placement
• Inspection: Visual inspection for voids after grout hardens. Tap test or ultrasonic testing for critical applications.
• Bond testing: Pull-off test for grout-to-concrete bond (minimum 1.0 MPa for acceptable bond)

Seismic Detailing for Base Plates

Ductility Requirements

For columns in seismic frames, base plates must be designed for ductile behavior per AS 4100 Supplement 1 and NZS 1170.5:

• Capacity design: The base plate connection should be designed for the overstrength capacity of the column, not just the design actions from analysis. Typically 1.25 to 1.4 × the design actions.
• Yield mechanism: The preferred yield mechanism is yield in the column section above the base plate, not in the base plate itself, the welds, or the anchor bolts. This requires the base plate and anchor bolts to be designed for 1.25 to 1.5 × the column plastic moment capacity.
• Anchor bolt ductility: Bolts must have sufficient ductility (minimum 12% elongation) to accommodate inelastic deformations. Grade 4.6 bolts are preferred over Grade 8.8 in seismic applications because Grade 4.6 has higher elongation.
• Bolt thread length: Limit threaded embedment to prevent bolt fracture at the grout surface. Use a longer unthreaded shank length or debond the threaded portion to distribute strain.

Seismic Detailing Requirements

Tie-Down and Uplift Design

For columns subject to net uplift (wind or seismic overturning):

1. Bolts in tension: All tension forces are carried by the anchor bolts. The base plate must be stiff enough to distribute tension from the column to the bolts without excessive prying.
2. Combined tension + shear interaction: Check per AS 4100 Clause 9.3. The interaction formula: (T*/phiNt)^1.5 + (V*/phiVf)^1.5 ≤ 1.0 (for threads in the shear plane).
3. Pullout cone failure: Check concrete cone breakout per AS 3600. The concrete cone angle is typically 35 degrees from the anchor point. For shallow embedment, use supplementary reinforcement (tie bars across the potential cone plane).
4. Edge distance check: For anchors near slab edges, verify that the concrete edge does not spall under tension. Minimum edge distance = 1.5 × embedment depth for critical tension anchors.
5. Fatigue: For cranes, vibrating equipment, or wind-loaded columns, check anchor bolt fatigue per AS 4100 Appendix K or AS 3600.

Frequently Asked Questions

What capacity factor do I use for base plate design in AS 4100? Use φ = 0.9 for plate bending and φ = 0.8 for bolt tension per AS 4100 Table 3.4. For concrete bearing, use φ = 0.6 per AS 3600.

What is the levelling nut method for base plates? The column base plate is supported on anchor bolt nuts for leveling, rather than on a grout bed. The plate must be designed for bending between the discrete support points, typically using yield line analysis. This results in a thicker plate than a fully grouted connection.

How do I size anchor bolts for a steel column base? Determine the bolt tension from the overturning moment (M*/d_bolt_spacing). Select bolt diameter so that φNt ≥ T*_bolt. For M24 Grade 8.8 bolts, the tension capacity is about 181 kN. Ensure embedment develops full bolt capacity into the concrete.

What concrete grade is needed under a base plate? Minimum f'c = 25 MPa for typical column bases. For heavy columns (N* > 2000 kN), use f'c = 32 or 40 MPa. Higher concrete strength reduces the required plate size.

Do I need shear keys on base plates? Only if the shear force exceeds the friction capacity (μ × N* where μ ≈ 0.3). For most gravity columns, friction is sufficient. For braced frame or moment frame bases with significant shear, a shear key (plate lug welded under the base plate) is required.

What is the minimum base plate thickness? AS 4100 does not specify a minimum, but good practice is tp ≥ max(20 mm, 2 × tf_column). For most columns, 20-30 mm is typical. Very lightly loaded columns may use 16 mm minimum.

How do I handle base plates with large moments? For large overturning moments, the plate becomes thick and bolts become large. Consider:

• Extending the plate beyond the column in the moment direction
• Using more bolts (6 or 8 instead of 4)
• Stiffening the plate with vertical fin plates or gussets
• Increasing the bolt grade or diameter

Related Pages

• Base Plate Design — General base plate design principles
• Anchor Bolt Embedment — Anchor bolt development length tables
• Bolt Grades — Mechanical properties of structural bolts
• Steel Grades — Fy and Fu values for structural steel
• Column Design Guide — Full column design procedure
• Foundation Types — Concrete foundation systems
• Bolted Connections — Bolted connection design per AISC/AS 4100

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE), Chartered Professional Engineer (CPEng), or Structural Engineer before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.