Canadian Base Plate Design — Bearing, Anchor Bolts, and Plate Design per CSA S16

Complete reference for column base plate design per CSA S16-19 Clause 13.14. Covers concrete bearing resistance, anchor bolt shear and tension capacity, plate bending design using the cantilever projection method, and a step-by-step worked example for a W310x107 column on 30 MPa concrete.

Quick access: Column design → | Combined loading → | Anchor bolt tension →

CSA S16 Base Plate Design Framework

Per CSA S16-19 Clause 13.14, base plate design covers three limit states:

1. Concrete bearing: Bearing stress beneath the plate must not exceed factored concrete bearing resistance
2. Plate bending: Plate thickness must be adequate to resist bending from bearing pressure
3. Anchor bolts: Tension + shear capacity of anchor rods per Clause 13.12

Material Resistance Factors

Concrete Bearing Resistance

Per CSA S16-19 Clause 13.14.1, based on CSA A23.3:

Br = phi_c × 0.85 × f'c × A1 × sqrt(A2/A1) ≤ 2 × phi_c × 0.85 × f'c × A1

Where:

• phi_c = 0.65
• f'c = specified concrete compressive strength (MPa)
• A1 = base plate area (mm^2)
• A2 = maximum area of supporting concrete that is geometrically similar and concentric with A1
• sqrt(A2/A1) ≤ 2.0

Bearing Resistance for Common Configurations

When the full pedestal area is not available for bearing (A1 = A2), use the first value. When the concrete pedestal is much larger than the plate (A2/A1 ≥ 2), the full 2.0 factor applies, doubling the bearing resistance.

Plate Thickness Design

Per CSA S16 Clause 13.14.3, using the cantilever projection method:

Plate Dimensions

N = B + 2 × m (plate length ≥ column depth + 2m) B = d + 2 × n (plate width ≥ column flange width + 2n)

Where:

• m = projection beyond flange face (parallel to web), measured to column flange face
• n = projection beyond flange, measured to column web centreline

Required Plate Thickness

t_req = sqrt(2 × w × c^2 / (phi × Fy))

Where:

• w = factored bearing pressure = Cf / (N × B)
• c = maximum cantilever distance (max of m, n, lambda_n × n')
• lambda_n = 2 × sqrt(X)/(1+sqrt(1-X)) ≤ 1.0
• X = (4×d×bf/(d+bf)^2) × Cf/(Br_max)
• n' = sqrt(d×bf)/4

Conservative approach: t = sqrt(2 × w × c^2 / (phi × Fy)) where c = max(m, n).

Anchor Bolt Design

Per CSA S16 Clause 13.12, anchor bolts in tension:

Tr = 0.75 × phi_b × Ab × Fu

For combined shear and tension on anchor bolts: (Vf/Vr)^2 + (Tf/Tr)^2 ≤ 1.0

Anchor Bolt Dimensions

Embedment Length (CSA A23.3)

The required embedment length for anchor bolts in tension:

Ld = 0.19 × d_b × Fy / sqrt(f'c)

For Grade 55 (Fy = 380 MPa) anchor bolts in 30 MPa concrete: Ld = 0.19 × 24 × 380 / sqrt(30) = 317 mm for M24 bolt

Minimum practical embedment: 300 mm for M20, 350 mm for M24, 450 mm for M30.

Worked Example — W310x107 Base Plate

Given: W310×107 column, Cf = 2200 kN (factored axial). Concrete pedestal: f'c = 30 MPa, pedestal = 500 × 500 mm. Steel: 350W plate (Fy = 350 MPa). Anchor bolts: 4-M20 F1554 Grade 55.

Step 1 — Plate Area Required: Assume A2/A1 = (500×500)/(N×B), Br ≥ Cf: Try 400×400 plate: A1 = 160,000 mm^2 A2/A1 = 250,000/160,000 = 1.56 sqrt(1.56) = 1.25 Br = 0.65 × 0.85 × 30 × 160,000 × 1.25 / 1000 = 3,315 kN ≥ 2,200 kN. OK.

Step 2 — Bearing Pressure: w = 2200 × 1000 / 160,000 = 13.75 MPa

Step 3 — Cantilever Distances: Column dimensions: d = 310 mm, bf = 310 mm m = (400 - 0.95 × 310)/2 = (400 - 295)/2 = 52.5 mm n = (400 - 0.80 × 310)/2 = (400 - 248)/2 = 76 mm c = max(52.5, 76) = 76 mm

Step 4 — Plate Thickness: t_req = sqrt(2 × 13.75 × 76^2 / (0.90 × 350)) = sqrt(2 × 13.75 × 5776 / 315) t_req = sqrt(158,840/315) = sqrt(504) = 22.5 mm Use 25 mm plate (standard thickness).

Step 5 — Anchor Bolt Check: Assume 60 kN shear per bolt (wind/seismic), 30 kN tension per bolt: Vr = 103.6 kN (per M20 F1554 Gr 55, AX if threads excluded) Tr = 103.6 kN Check: (60/103.6)^2 + (30/103.6)^2 = 0.335 + 0.084 = 0.419 ≤ 1.0. OK.

Result: 400×400×25 mm base plate, 4-M20 F1554 Grade 55 anchor bolts, 300 mm embedment.

Grout Considerations

Per CSA S16 Clause 13.14.2:

1. Grout thickness: 25-50 mm typical between base plate and concrete
2. Grout strength: Minimum f'c_grout ≥ f'c_concrete
3. Bearing through grout: The bearing resistance calculation uses f'c of the concrete, not the grout, provided the grout thickness ≤ 50 mm and grout strength ≥ concrete strength
4. Levelling nuts: Use levelling nuts on anchor bolts to set elevation, then grout, then tighten after grout cures

Frequently Asked Questions

What is the minimum base plate thickness per CSA S16? There is no explicit minimum in CSA S16, but practical minimums are 12 mm (light framing) and 20 mm (typical columns). The cantilever projection method typically governs at 20-35 mm for column base plates. For architectural reasons, 25 mm is the most commonly specified thickness — it provides robust bearing without excessive material cost.

How does the A2/A1 ratio affect concrete bearing resistance? The confinement factor sqrt(A2/A1) accounts for the triaxial stress state beneath the base plate when the supporting concrete pedestal extends beyond the plate. A2 is the maximum geometrically similar area concentric with A1. For a 400×400 plate on a 600×600 pedestal: sqrt(A2/A1) = sqrt(360000/160000) = 1.50, limited to 2.0 maximum. The bearing resistance increases by 50% over the A2/A1 = 1.0 case.

What grade of anchor bolts is typical for Canadian base plates? F1554 Grade 55 (Fy = 380 MPa, Fu = 550 MPa) is the most common specification for anchor bolts in Canadian construction. Grade 36 (Fu = 400 MPa) is used for lightly loaded applications. Grade 105 is used for high-strength applications. ASTM F1554 is the governing standard, accepted by CSA S16 for anchor bolts. For seismic applications, CSA S16 may require ductile anchor bolt materials with minimum elongation.

Do anchor bolts need to be designed for combined shear and tension? Yes, per CSA S16 Clause 13.12.4 using (Vf/Vr)^2 + (Tf/Tr)^2 ≤ 1.0. The factored shear Vf comes from: (a) 100% of lateral load reaction at base (if column participates in lateral system), (b) 25% of axial load as shear (friction assumption — CISC practice), or (c) specified seismic overturning forces. The tension Tf comes from: (a) overturning moment causing uplift, (b) erection/levelling forces, (c) nominal tension from eccentric bearing.

Related Pages

• CSA S16 Column Design
• CSA S16 Combined Loading
• Canadian Bolt Pretension — A325M/A490M
• CSA S16 Beam Design
• CISC Handbook base plate tables
• Column Capacity Calculator
• Base Plate Calculator
• All Canadian References

This page is for educational reference. Base plate design per CSA S16-19 Clause 13.14 and CSA A23.3. Verify concrete bearing and anchor bolt capacities against project-specific material properties. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.

Design Resources

Calculator tools

• Base Plate and Anchor Bolt Design Calculator
• Steel Column Base Design Calculator
• Anchor Bolts Calculator

Design guides

• Base Plate Design Worked Example
• Base Plate Design Checklist
• AS 4100 Base Plate Design Guide
• AS 4100 Base Plate Worked Calculations

Reference pages

• CA Base Plate