AS 4100 Base Plate Design -- Complete Procedure with Worked Examples
A column base plate transfers axial compression, bending moment, and shear from the steel column into the concrete foundation. Under AS 4100 (Steel Structures), the design must satisfy three distinct checks: concrete bearing per AS 3600, plate bending capacity per AS 4100 Section 5, and anchor bolt tension and shear per AS 4100 Section 9. The interaction among these three mechanisms determines the plate dimensions, bolt layout, and weld specification.
This reference covers the full design sequence, from determining design actions through to weld detailing and grout specification. Two worked examples are included: a pure compression base and a combined axial-plus-moment base. The levelling nut (yield line) method and seismic detailing provisions are addressed separately.
Governing Standards and Design Philosophy
Base plate design in Australia draws on two primary standards:
- AS 4100: Steel Structures -- Clauses 5 (members in bending), 9 (connections), and 14 (fabrication)
- AS 3600: Concrete Structures -- Clause 12.6 (bearing), Section 17 (anchoring to concrete)
The design philosophy is limit state design with capacity factors from AS 4100 Table 3.4 and AS 3600 Table 2.2.2. The connection must be designed for the more severe of (a) the actions determined from structural analysis and (b) the minimum notional capacity per the standard.
Capacity Factors
| Limit State | Reference | phi | Application |
|---|---|---|---|
| Plate bending | AS 4100 Tbl 3.4 | 0.90 | Yield of plate in flexure |
| Bolt tension | AS 4100 Tbl 3.4 | 0.80 | Anchor bolt tensile rupture |
| Bolt shear | AS 4100 Tbl 3.4 | 0.80 | Anchor bolt shear rupture |
| Concrete bearing | AS 3600 Tbl 2.2.2 | 0.60 | Crushing of concrete under plate |
| Concrete cone breakout | AS 3600 Sec 17 | 0.60 | Tensile pullout cone in concrete |
| Fillet weld | AS 4100 Tbl 3.4 | 0.80 | Column-to-plate weld |
Step-by-Step Design Procedure
Step 1: Determine Design Actions
The base plate design starts with the factored actions at the column base from the structural analysis. These come from the governing load combinations per AS/NZS 1170.0:
- N* = design axial force (compression positive, tension negative)
- M*_x, M*_y = design bending moments about the major and minor axes
- V*_x, V*_y = design shear forces in the two orthogonal directions
For moment-resisting frames, also determine the plastic moment capacity M_p of the column section -- the connection may need to be capacity-designed for 1.25 to 1.40 times the column plastic capacity.
Step 2: Select Preliminary Plate Geometry
Base plate plan dimensions are established from the column profile plus an edge margin:
- Plate width B_i: column flange width b_f + 2 * a_e (edge distance, minimum 50 mm)
- Plate depth d_i: column depth d + 2 * a_e
- Minimum edge distance to anchor bolt centre: 1.5 * d_b (bolt diameter) or 40 mm, whichever is larger
For a typical UC section, plates are square or nearly square. The initial plate thickness estimate can be based on past experience (20-25 mm for UC200-300 series columns under moderate load) and verified by the bending check in Step 4.
Step 3: Concrete Bearing Check (AS 3600 Clause 12.6)
The bearing capacity of the concrete under the base plate is:
phi Nb = phi_b * 0.85 _ f'_c _ A1 * sqrt(A_2 / A_1)
subject to the upper bound: sqrt(A2 / A_1) <= 2.0, giving a maximum bearing stress of phi_b * 0.85 _ f'_c _ 2.0 = phib * 1.70 * f'_c.
Where:
- A_1 = bearing area of the base plate (B * N)
- A_2 = maximum area of supporting surface geometrically similar to and concentric with A_1
- f'_c = characteristic compressive strength of concrete (typically 25, 32, or 40 MPa)
- phi_b = 0.60
The sqrt(A*2 / A_1) factor accounts for confinement from the surrounding concrete mass. For a base plate on a large footing or slab, A_2 >> A_1 and the full factor of 2.0 can be used, yielding a maximum usable bearing stress of approximately 1.02 * f'_c (phi reduced) or, in pre-factored terms, 1.70 _ f'_c.
Required plate area: A_1 >= N* / (phi N_b / A_1), i.e., N* / f_b where f_b is the factored bearing strength per unit area.
Step 4: Plate Bending Check
The concrete bearing pressure acts upward on the plate underside while the column applies concentrated reactions downward at the flange tips and web. This produces bending in the plate in two regions:
4a. Cantilever bending (plate projection beyond column face):
The plate cantilevers from the column face to the free edge, subject to the uniform bearing pressure q:
M*_cant = q * m^2 / 2 (bending moment per unit width)
Where m = (plate width B - column flange width b_f) / 2 is the cantilever projection.
Required thickness from the plastic section modulus: t*p >= sqrt(4 * M__cant / (phi * f_y))
Where phi = 0.90 for plate bending and f_y is the plate yield stress (typically 250, 300, or 350 MPa).
4b. Two-way bending (plate interior region):
For plates significantly larger than the column footprint, also check the bending of the interior plate region spanning between the column flanges. This is a plate strip loaded by bearing pressure and supported at the flange tips. For a rectangular plate under uniform load:
M*_interior = q * (d_i - 2 * k_s) ^ 2 / 8 (approximately, where k_s approximates the column flange outstand)
The controlling thickness is the larger of the cantilever and interior bending requirements. In most practical cases, cantilever bending governs.
Step 5: Anchor Bolt Tension Design
Anchor bolts resist tension from uplift, overturning moments, and nominal erection loads. For each bolt:
Tensile capacity: phi Ntf = phi * As * f_uf
Where:
- phi = 0.80
- A_s = tensile stress area of the bolt (353 mm^2 for M24, 561 mm^2 for M30)
Nominal tensile strength f_uf by bolt grade per AS 4100 Table 9.3.1:
| Bolt Grade | f_uf (MPa) | f_yf (MPa) | phi N_tf M24 (kN) |
|---|---|---|---|
| 4.6 | 400 | 240 | 113 |
| 8.8 | 830 | 660 | 234 |
For seismic applications, Grade 4.6 bolts are preferred over Grade 8.8 because of their higher elongation capacity (minimum 20% vs 12%), which accommodates inelastic deformation without fracture.
Step 6: Bolt Tension Distribution (Moment-Resisting Bases)
For a base plate subject to combined axial load N* and moment M*:
- Compute the eccentricity e = M* / N*
- If e <= d_i/6: The entire plate is in compression. No bolt tension. The bearing pressure varies linearly from q_max at one edge to q_min at the other.
- If e > d_i/6: A portion of the plate develops tension. The tension force is resolved by the anchor bolts on the tension side.
For a rectangular plate with bolts symmetrically placed at distance d_b (centre-to-centre):
T_total = (M* / d_b) - (N* / 2) [subject to T_total >= 0]
T_per_bolt = T_total / n_bolts_tension_side
Where n_bolts_tension_side is the number of bolts on the tension face (typically 2 for a 4-bolt pattern).
Step 7: Shear Transfer
Shear at the column base transfers through three mechanisms in decreasing order of preference:
- Friction: V*f = mu * N_ where mu = 0.30 for steel on grout (unpainted, clean contact surface). Friction alone is often sufficient for gravity columns.
- Anchor bolts in shear: The bolt shear capacity per AS 4100 Clause 9.3.2.1: phi Vf = phi * 0.62 _ f_uf _ As (threads excluded from shear plane) or phi * 0.80 _ f_uf _ A_core (threads included).
- Shear key: A structural steel lug welded to the underside of the base plate, cast into a recess in the concrete. Required if friction plus bolt shear is insufficient. The key bears against the concrete in a formed pocket and transfers horizontal reaction directly.
Combined tension + shear interaction: Per AS 4100 Clause 9.3.2.3, for bolts with threads in the shear plane:
(T* / phi N_tf)^1.5 + (V* / phi V_f)^1.5 <= 1.0
Step 8: Anchor Bolt Embedment
The bolt must be embedded deeply enough into the concrete to develop its full tensile capacity without pullout or cone breakout. Check three failure modes:
- Concrete cone breakout (AS 3600 Section 17): The pullout cone angle is approximately 35 degrees from the bolt axis. The projected area of the cone at the concrete surface determines capacity. Edge distance and group effects reduce capacity when bolts are closely spaced.
- Bond failure along the bolt shaft (plain bars and headed anchors).
- Steel tensile failure of the bolt itself (the intended failure mode for ductile design).
Minimum practical embedment: 12 _ d_b for hooked anchors, 15 _ d_b for straight headed anchors. For M24 bolts: 288-360 mm. For M30 bolts: 360-450 mm.
Worked Example 1: Axial Compression Only
Problem
Design the base plate for a 250UC72.9 column carrying a design axial compression N* = 1,800 kN. The column bears on a mass concrete pad footing with f'_c = 32 MPa. Use Grade 250 plate and M24 Grade 8.8 anchor bolts. The footing plan dimensions are 2,400 mm x 2,400 mm -- large enough that A_2/A_1 will reach the 2.0 limit.
Column Properties (250UC72.9)
- Depth d = 254 mm
- Flange width b_f = 254 mm
- Flange thickness t_f = 14.2 mm
- Web thickness t_w = 8.6 mm
- Area A_g = 9,310 mm^2
- Yield stress f_y = 300 MPa (Grade 300PLUS)
Step 1: Preliminary Plate Dimensions
Edge distance a_e = 55 mm (comfortably exceeds 1.5 * 24 = 36 mm and the 40 mm minimum).
B = 254 + 2 _ 55 = 364 mm -- round up to 370 mm N = 254 + 2 _ 55 = 364 mm -- round up to 370 mm
A_1 = 370 * 370 = 136,900 mm^2 = 0.1369 m^2
Step 2: Concrete Bearing
With A_2/A_1 taken as 2.0 (large footing relative to plate):
phi N*b = 0.60 * 0.85 _ 32 _ 136,900 _ sqrt(2.0) / 1000 = 0.60 _ 0.85 _ 32 _ 136,900 _ 1.414 / 1000 = 3,162 kN
Bearing capacity check: phi N_b = 3,162 kN > N* = 1,800 kN. OK. Substantial reserve.
Bearing stress under the plate: q = N* / A_1 = 1,800,000 / 136,900 = 13.15 MPa
Step 3: Plate Bending
Cantilever projection: m = (370 - 254) / 2 = 58 mm
Bending moment per unit width: M* = q * m^2 / 2 = 13.15 _ 58^2 / 2 = 13.15 _ 3,364 / 2 = 22,112 Nmm/mm
Required thickness: t*p = sqrt(4 * 22,112 / (0.90 _ 250)) = sqrt(88,448 / 225) = sqrt(393.1) = 19.8 mm
Adopt t_p = 20 mm Grade 250 plate.
Step 4: Anchor Bolt Check
For a pure compression connection, bolts resist nominal erection loads only. Minimum 4 * M24 Grade 8.8.
Tension capacity per bolt: phi N*tf = 0.80 * 353 _ 830 / 1000 = 234.4 kN
Four bolts provide 938 kN total -- more than adequate for any reasonable erection load.
Step 5: Weld Design
For full bearing on grout, the column-to-plate weld transfers only enough load to hold the column in place. Per AS 4100 minimum fillet weld sizes:
Plate thickness 20 mm, column flange thickness 14.2 mm: minimum 6 mm fillet weld.
Specify: 6 mm fillet weld, all around column profile, E48XX electrodes.
Result
| Component | Specification |
|---|---|
| Base plate | 370 x 370 x 20 mm, Grade 250 |
| Anchor bolts | 4 x M24 Grade 8.8, 300 mm embedment |
| Weld | 6 mm fillet all around, E48XX |
| Grout | 40 mm non-shrink cementitious, f_c >= 40 MPa |
Worked Example 2: Combined Axial Compression and Major-Axis Moment
Problem
Design a base plate for a 310UC118 column carrying N* = 900 kN (compression) and M*_x = 220 kNm. The footing is 32 MPa concrete on a 2,800 mm x 2,800 mm pad. Use Grade 300 plate and M30 Grade 8.8 anchor bolts.
Column Properties (310UC118)
- Depth d = 314 mm
- Flange width b_f = 307 mm
- Flange thickness t_f = 18.7 mm
- Web thickness t_w = 11.9 mm
- Plastic section modulus S_x = 1,950 x 10^3 mm^3
- Plastic moment capacity phi M*sx = 0.9 * 300 _ 1,950,000 / 10^6 = 527 kNm
Step 1: Preliminary Plate Dimensions
Because the overturning moment is significant, the plate is extended in the moment direction to engage a wider bolt couple.
B (width, parallel to moment axis): 314 + 2 _ 70 = 454 mm -- round up to 460 mm N (depth, perpendicular): 307 + 2 _ 55 = 417 mm -- round up to 420 mm
A_1 = 460 * 420 = 193,200 mm^2 = 0.1932 m^2
Step 2: Eccentricity Check
e = M* / N* = 220 / 900 = 0.244 m = 244 mm
Plate dimension in the moment direction B = 460 mm. Kern distance = 460/6 = 76.7 mm.
e = 244 mm > 76.7 mm -- significant tension develops on one side. The compression zone is a rectangular stress block at the bearing face.
Step 3: Tension Force in Anchor Bolts
Bolt layout: 4 x M30 bolts at 70 mm edge distance, giving a bolt couple lever arm d_bolt = 460 - 2 * 70 = 320 mm centre-to-centre.
Taking moments about the compression face centroid (assume the resultant compression acts at the centre of the bearing stress block, approximately d_comp = B * 0.42 = 193 mm from the compression edge for full bearing, or at mid-depth of the compression zone):
Using the simplified plastic stress distribution:
C = N* + T_total (equilibrium) C * (B/2 - a/2) = M* + T_total * d_bolt/2 (moment equilibrium about column centroid)
where a = depth of compression stress block. Iterating on a:
Assume a = 180 mm initially. Compression centroid from plate edge ~ a/2 = 90 mm.
Ttotal = (M - N* * (B/2 - a/2)) / d*bolt = (220 - 900 * (0.230 - 0.090)) / 0.320 = (220 - 900 * 0.140) / 0.320 = (220 - 126) / 0.320 = 94 / 0.320 = 293.8 kN
Check compression: C = 900 + 293.8 = 1,193.8 kN at bearing stress q = 1,193.8 _ 1000 / (420 _ 180) = 1,193,800 / 75,600 = 15.8 MPa < 0.60 _ 0.85 _ 32 * 2.0 = 32.6 MPa. OK.
Step 4: Bolt Tension Capacity
M30 Grade 8.8: A_s = 561 mm^2, f_uf = 830 MPa.
phi N*tf = 0.80 * 561 _ 830 / 1000 = 372.5 kN per bolt
Tension per bolt (2 bolts on tension face): T*_bolt = 293.8 / 2 = 146.9 kN
phi N_tf = 372.5 kN > T*_bolt = 146.9 kN. OK. Reserve is more than 2.5x -- adequate for prying allowance.
Step 5: Plate Bending (Cantilever)
Cantilever projection in moment direction: m = 70 - t_f/2 = 70 - 18.7/2 = 60.7 mm
Bearing pressure (worst case, compression side): q = 15.8 MPa
M*_cant = 15.8 * 60.7^2 / 2 = 15.8 * 3,685 / 2 = 29,107 Nmm/mm
Required thickness: t*p = sqrt(4 * 29,107 / (0.90 _ 300)) = sqrt(116,428 / 270) = sqrt(431.2) = 20.8 mm
This is for the compression side cantilever. The tension side also requires checking for prying (plate bending under bolt pull). For tension-side bending, the yield line from the bolt centre to the plate edge governs.
Step 6: Plate Bending (Tension Side -- Prying Check)
Bolt centre to plate edge (at 70 mm edge distance): the plate yields between the bolt and the plate edge under the upward bolt pull.
Using the simplified prying model: the plate must be sufficiently thick to develop the bolt tension without excessive deformation. For M30 bolts, a 25 mm thick plate with 70 mm edge distance provides adequate stiffness to limit prying to below 10% of bolt tension. (Detailed yield-line verification available in the full design guide.)
Adopt t_p = 25 mm Grade 300 plate.
Step 7: Combined Shear + Tension Interaction
For completeness, check the bolt group for the case where lateral shear coexists with tension. If V* = 80 kN (from wind or seismic lateral load):
Shear per bolt: V*_bolt = 80 / 4 = 20 kN
phi V*f (M30, threads in shear plane) = 0.80 * 0.62 _ 830 * 561 / 1000 = 230.6 kN
Interaction: (146.9/372.5)^1.5 + (20/230.6)^1.5 = (0.394)^1.5 + (0.087)^1.5 = 0.247 + 0.026 = 0.273 < 1.0. OK.
Step 8: Embedment
Minimum embedment for M30 hooked anchors: 12 * 30 = 360 mm. Use 400 mm to the hook tangent point plus 150 mm hook tail.
Result
| Component | Specification |
|---|---|
| Base plate | 460 x 420 x 25 mm, Grade 300 |
| Anchor bolts | 4 x M30 Grade 8.8, 400 mm embedment + hook |
| Weld | 8 mm fillet all around column profile, E48XX |
| Grout | 40 mm non-shrink cementitious, f_c >= 40 MPa |
| Shear key | Not required (friction: 0.30 _ 900 = 270 kN > V_ = 80 kN) |
The Levelling Nut Method -- Yield Line Analysis
When the base plate is supported on levelling nuts rather than a continuous grout bed, the load path changes fundamentally: the column load passes through discrete contact points (the nuts) to the anchor bolts and then into the foundation. The plate spans between the nuts and must be designed for the associated bending.
When Levelling Nuts Govern
- Column bases where grouting is delayed (fast-track construction)
- Pre-levelled base plates for precise alignment (crane rails, machinery)
- Temporary column supports before permanent grout placement
- Cases where the grout does not provide full-area bearing (voids, shrinkage)
Yield Line Mechanism
The yield line method models the plate as a plastic mechanism at failure. For a rectangular base plate with bolts at the corners:
- Yield line pattern: Diagonal yield lines radiate from the column corners to the plate edges, subdividing the plate into triangular rigid regions that rotate about the column face.
- External work: The bearing pressure q acting through the virtual displacement of each triangular region.
- Internal work: The plastic moment capacity M_p = f_y * t_p^2 / 4 per unit width, times the rotation angle, integrated along each yield line.
- Equilibrium: The internal work dissipated in the hinges equals the external work done by the bearing pressure:
SUM (Mp * thetai * Lhinge_i) = SUM (q * Aregion_j * delta_j)
Solving for t_p gives the required plate thickness for the yield line pattern. This typically produces a thickness 20-40% higher than the cantilever bending method because the mechanism involves two-way bending with longer effective spans.
Practical Guidance
- Increase design thickness by 25% compared to the grouted cantilever method as a first estimate
- Tighten levelling nuts to snug-tight plus one-quarter turn to pre-compress the plate against the column base
- Use heavy plate washers (minimum 8 mm thick) under both the levelling nut below and the top nut above
- Oversize bolt holes by 2-3 mm for erection tolerance
- For heavily loaded plates (> 2,000 kN), avoid reliance on levelling nuts alone -- specify grouting before applying live load
Weld Design: Column to Base Plate
Minimum Fillet Weld Sizes (AS 4100 Clause 9.7.3.10)
| Thicker part thickness t (mm) | Minimum fillet leg size (mm) |
|---|---|
| t <= 10 | 4 |
| 10 < t <= 20 | 6 |
| 20 < t <= 30 | 8 |
| 30 < t <= 40 | 10 |
| 40 < t <= 50 | 12 |
For a column in full compression bearing on grout, the weld is nominally loaded: it transfers enough force to secure the column during erection but does not need to develop the full column axial capacity. A 6 mm fillet weld around the profile is the standard minimum for UC200-UC310 columns.
For columns with tension or moment (the tension flange in Example 2), the weld on the tension side must develop the full tension flange force. For the 310UC118 example, the tension flange force at the moment M*_x = 220 kNm is approximately M* / (d - t*f) = 220,000 / (0.314 - 0.019) = 220,000 / 0.295 = 746 kN. An 8 mm fillet weld (capacity ~ 1.03 kN/mm for E48XX electrodes on Grade 300 plate, design capacity per AS 4100 Clause 9.7.3.10) around the flange (perimeter ~ 307 * 2 + 18.7 + 307 = 940 mm) provides 940 _ 1.03 = 968 kN > 746 kN. OK.
Weld Electrode Selection
| Electrode | Nominal tensile strength (MPa) | Matching plate steel |
|---|---|---|
| E41XX | 410 | Grade 250 |
| E48XX | 480 | Grade 300, 350 |
| E55XX | 550 | Grade 400, 450 (quenched & tempered) |
Match the electrode to the weaker of the two joined parts. For Grade 250 and 300 plate, E48XX is standard.
Grout Specification
Material Requirements
Structural non-shrink cementitious grout for base plates:
- 28-day compressive strength: minimum 30 MPa (40 MPa recommended for heavy columns)
- Flow: 250-300 mm flow cone (ASTM C939 / AS 1478.2)
- Restrained expansion: 0-0.3% (compensates for drying shrinkage)
- Maximum aggregate size: 5-10 mm for 25-50 mm grout gaps
- Elastic modulus: 25-35 GPa at 28 days
Placement Procedure
- Concrete surface preparation: Roughen to minimum 3 mm amplitude (scabbling or needle-gunning). Clean of laitance, oil, and debris. Saturate surface-dry (SSD) immediately before grouting -- dry concrete absorbs mix water and causes localized shrinkage.
- Form construction: Build a tight form around the plate perimeter with grout inlet holes at one or two sides and vent holes at the opposite edges. The form must extend 10-15 mm above the plate underside to provide hydrostatic head.
- Grout mixing: Use a mechanical paddle mixer. Add the powder to the water (not water to powder). Mix for 3-5 minutes until uniform. Do not over-water -- each 1% excess water reduces compressive strength by approximately 3-5%.
- Placement: Pump or pour from one side only to push air out the opposite vents. Maintain continuous flow until all vents show fresh grout. Do not vibrate the plate during grouting -- vibration can cause segregation and entrapment.
- Curing: Wet cure for minimum 7 days (wet burlap, polyethylene sheet, or curing compound). Ambient temperature must be above 5 deg C and below 35 deg C during curing.
Quality Control
- Cast minimum 3 compression test cubes or cylinders per batch, tested at 7 days and 28 days
- Flow cone test before each placement batch
- Post-cure inspection: tap test for voids (a hollow sound indicates incomplete filling), ultrasonic pulse velocity for critical bases
Seismic Detailing Provisions
For columns in seismic-resisting frames designed to AS 4100 Supplement 1 and AS/NZS 1170.5:
Capacity Design
The base plate connection must be designed for 1.25 to 1.40 times the column plastic moment capacity M_p, not just the actions from linear analysis. This ensures the column yields in flexure before the base plate, welds, or anchor bolts fracture.
Bolt Grade Selection
| Property | Grade 4.6 | Grade 8.8 |
|---|---|---|
| f_uf (MPa) | 400 | 830 |
| f_yf (MPa) | 240 | 660 |
| Elongation (min) | 20% | 12% |
| Seismic use | Preferred | Acceptable if verified |
Grade 4.6 bolts are preferred in seismic applications because of higher elongation. The lower strength is compensated by using more or larger bolts.
Detailing Requirements
| Detail | Non-seismic | Seismic |
|---|---|---|
| Design basis | Factored actions from analysis | 1.25 * column M_p capacity |
| Minimum weld | 6 mm fillet | 8 mm fillet or CJP groove at tension flange |
| Shear transfer | Friction permitted | Shear key required |
| Weld inspection | Visual | Visual + MPI or UT at tension region |
| Bolt embedment | 12 * d_b | 15 * d_b minimum |
| Grout strength | 30 MPa | 40 MPa minimum |
| Base plate stiffeners | Not typically required | Required for M* > 300 kNm |
Comparison with AISC Design Guide 1
For engineers familiar with the AISC method, the AS 4100 approach differs in several details:
| Item | AISC DG 1 (US) | AS 4100 (Australia) |
|---|---|---|
| Bearing stress | phi*c * 0.85 _ f'_c * sqrt(A2/A1) | phi _ 0.85 _ f'_c * sqrt(A2/A1) |
| Bearing phi | 0.65 (concrete) | 0.60 (AS 3600) |
| Plate bending phi | 0.90 | 0.90 |
| Bolt tension phi | 0.75 | 0.80 |
| Cantilever bending model | M* = q * (m - 0.95*d)^2 / 2 | M* = q * m^2 / 2 |
| Yield line method | AISC Manual Part 14, bracket approach | First principles per AS 4100 Commentary |
| Minimum edge distance | 1.5 * d_b (preferred) | 1.5 * d_b or 40 mm (whichever greater) |
| Grout requirements | 1-2 in typical, 4,000 psi min | 25-50 mm typical, 30-40 MPa min |
The AISC DG 1 method accounts for the column profile area when computing the cantilever moment (deducting 0.95 * column depth from the projection), which results in slightly thinner plates for a given geometry. The AS 4100 method (using the full projection m without deduction) is more conservative. When base plate designs from both standards are compared, the AS 4100 plates are typically 2-4 mm thicker for the same column and load combination.
Frequently Asked Questions
What is the governing AS 4100 clause for base plate design? Base plates are governed by AS 4100 Clause 9 (Connections) for the bolted interface and Clause 5 (Members in Bending) for plate flexure. The concrete bearing check is per AS 3600 Clause 12.6. There is no single clause titled "base plate design" -- the design is assembled from the relevant provisions in both standards.
How is the A1/A2 ratio determined for bearing? A_1 is the plan area of the base plate (B * N). A_2 is the maximum area of the supporting concrete surface that is geometrically similar to and concentric with A_1. For a base plate on a footing much larger than the plate, A_2 is limited by the footing plan dimensions. The ratio sqrt(A_2/A_1) is capped at 2.0, representing the maximum beneficial confinement from surrounding concrete.
When do I need to check prying action on anchor bolts? Prying should be checked when the base plate thickness is less than t_min for a given bolt tension -- see the simplified prying model in Step 6 of Example 2. Plates 25 mm and thicker with standard edge distances (>= 1.5 * d_b) typically develop minimal prying for M24 and M30 bolts. For thinner plates or larger bolts, calculate the required thickness or increase the bolt design tension by 20-30%.
What is the minimum base plate thickness per AS 4100? AS 4100 does not specify a numerical minimum for base plates. Engineering practice uses t_p >= max(16 mm, 2 * t_f_column). For most structural columns (UC200-UC310), 20-25 mm is typical. Plates thinner than 16 mm are susceptible to handling damage, welding distortion, and are difficult to grout under without warping.
How do I design for large overturning moments? For M* > 500 kNm on a single column, consider: (a) extending the plate in the moment direction to increase the bolt couple lever arm, (b) using 6 or 8 bolts instead of 4, (c) adding vertical stiffener plates (gussets) between the column flanges and the base plate, (d) specifying a deeper embedment for the tension bolts with supplementary hairpin reinforcement across the pullout cone, and (e) verifying the concrete footing itself for the uplift reaction.
What grout strength is required under a base plate? AS 4100 does not specify grout strength directly, but AS 3600 requires the grout to be at least as strong as the foundation concrete. For typical foundations with f'_c = 32 MPa, specify grout with a minimum 28-day strength of 40 MPa to provide margin. Non-shrink cementitious grout meeting the requirements of AS 1478.2 is the standard product.
Related Pages
- Base Plate Design Guide -- General base plate design principles
- Anchor Bolt Design and Embedment -- Anchor bolt development lengths and tables
- Bolt Grades and Strengths -- Mechanical properties of structural bolts
- Steel Grades Reference -- Australian and international structural steel grades
- Bolted Connection Design -- Bolted connection design per AS 4100
- Base Plate and Anchor Bolt Calculator -- Interactive base plate design tool
Disclaimer
This is a calculation reference, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE), Chartered Professional Engineer (CPEng), or Registered Structural Engineer before use in construction, fabrication, or permit documents. The user bears full responsibility for the accuracy of all inputs and the independent verification of all outputs against the applicable edition of AS 4100, AS 3600, and the project-specific design brief.