CSA S16 Base Plate Design — W250x73 Worked Example

Base plate design sits at the critical steel-to-concrete interface and must satisfy two standards simultaneously: CSA S16:2019 for the steel side and CSA A23.3 for concrete bearing and anchor bolt provisions. This guide walks through a complete worked example with a W250x73 column, 400x400x25 mm base plate, and 4xM24 anchor bolts — with numbers you can verify in our free base plate calculator.

What you will learn

• How to set up a base plate design to CSA S16:2019
• Concrete bearing capacity with and without confinement
• Base plate bending check to determine required plate thickness
• Anchor bolt tension and shear capacity
• A full worked example with real numbers
• How CSA S16 compares to AISC 360 for base plate design

Copyright and standards notice

This site does not reproduce copyrighted code clauses or proprietary tables verbatim. Discussion of CSA S16 and CSA A23.3 here is high-level and intended to help you understand verification workflows. Always consult the official published standards for authoritative requirements.

Step 1 — Define the problem

The connection must transfer 1200 kN compression with 30 kN-m moment through the plate into a concrete pedestal.

Step 2 — Identify all limit states

Base plate connections span the steel-concrete interface, so limit states come from both sides:

Steel side (CSA S16:2019):

1. Base plate bending (Cl. 25.3) — plate must be thick enough to resist bending from bearing pressure
2. Weld capacity — column-to-plate weld (not covered in this example)

Concrete side (CSA A23.3): 3. Concrete bearing (Cl. 10.8) — bearing pressure must not exceed concrete capacity 4. Anchor bolt tension (Annex D) — individual anchor tensile capacity 5. Concrete breakout in tension (Annex D) — cone-shaped failure surface 6. Anchor bolt shear — steel shear capacity of anchor 7. Concrete breakout in shear — edge failure under shear 8. Combined tension-shear interaction — when anchors resist both simultaneously

Step 3 — Resistance factors

CSA uses the following resistance factors:

Step 4 — Worked example

Check 1: Concrete bearing capacity

Without confinement (A2 = A1): Bearing resistance = phi_c x 0.85 x f'c x A1 = 0.65 x 0.85 x 30 x (400 x 400) / 1000 = 2652 kN

With confinement (pedestal larger than plate): The confinement factor = sqrt(A2/A1), limited to 2.0: A1 = 400 x 400 = 160,000 mm² (plate area) A2 = 500 x 500 = 250,000 mm² (pedestal area, geometrically similar and concentric) sqrt(A2/A1) = sqrt(250000/160000) = 1.25

Confined bearing resistance = phi_c x 0.85 x f'c x A1 x sqrt(A2/A1) = 0.65 x 0.85 x 30 x 160000 x 1.25 / 1000 = 3315 kN

Utilization (confined): 1200 / 3315 = 0.36 (OK — bearing is not critical for this loading)

Check 2: Eccentricity and bearing stress distribution

Check the load eccentricity: e = Mf / Cf = 30 / 1200 = 0.025 m = 25 mm

Kern distance for a 400 mm plate: e_kern = N/6 = 400/6 = 66.7 mm

Since e = 25 mm < e_kern = 66.7 mm, the bearing pressure is fully compressive (no tension in the anchors from this load combination). The stress distribution is trapezoidal:

Maximum bearing stress: f_max = Cf/A1 x (1 + 6e/N) = (1200 x 1000) / 160000 x (1 + 6 x 25/400) = 7.50 x 1.375 = 10.31 MPa

Minimum bearing stress: f_min = Cf/A1 x (1 - 6e/N) = 7.50 x (1 - 0.375) = 4.69 MPa

Allowable bearing stress (confined): phi_c x 0.85 x f'c x sqrt(A2/A1) = 0.65 x 0.85 x 30 x 1.25 = 20.72 MPa

Utilization: 10.31 / 20.72 = 0.50 (OK)

Check 3: Base plate bending

The plate must resist bending from the bearing pressure. For a W250x73 column, the critical section for plate bending is at the face of the column flange or web.

Cantilever projection (critical dimension): m = (N - 0.95 x d) / 2 = (400 - 0.95 x 253) / 2 = 79.8 mm n = (B - 0.80 x bf) / 2 = (400 - 0.80 x 254) / 2 = 98.4 mm

The larger cantilever (n = 98.4 mm) governs plate bending.

Bending moment at the critical section (uniform pressure approximation): Using the maximum bearing stress for conservative design: M_plate = f_max x n² / 2 = 10.31 x 98.4² / 2 / 1000 = 49.9 kN-mm per mm width

Wait — let's convert properly: M_plate = 10.31 x (98.4)² / 2 = 49,918 N-mm per mm width = 49.9 N-mm/mm

Required plate thickness: t_req = sqrt(4 x M_plate / (phi_s x Fy)) = sqrt(4 x 49.9 / (0.90 x 300)) = sqrt(0.665) = 0.82 mm ... this seems too thin. Let me recalculate with proper units.

Actually, the bearing pressure is in MPa (N/mm²), and cantilever is in mm: M_plate = f_max x n² / 2 = 10.31 N/mm² x (98.4 mm)² / 2 = 49,917 N-mm per mm of plate width

t_req = sqrt(4 x M_plate / (phi_s x Fy)) = sqrt(4 x 49,917 / (0.90 x 300)) = sqrt(739.5) = 27.2 mm

The required plate thickness is 27.2 mm. Our 25 mm plate is slightly under — utilization = (27.2/25)² = 1.18 (FAILS).

Design decision: Increase plate thickness to 30 mm or increase plate dimensions to reduce cantilever projection.

With a 30 mm plate: phi x Mp = phi_s x Fy x t² / 4 = 0.90 x 300 x 30² / 4 = 60,750 N-mm/mm Utilization = 49,917 / 60,750 = 0.82 (OK)

This is why base plate bending often governs — it sizes the plate thickness.

Check 4: Anchor bolt tension

For this load combination, the eccentricity is within the kern, so anchors are not required for tension. However, anchors must be checked for the uplift load case.

Consider a wind uplift combination where:

• Factored tension (Tf) = 150 kN (net uplift on column base)
• Tension per anchor = 150 / 4 = 37.5 kN

M24 Grade 8.8 anchor tensile capacity:

• Tensile stress area: At = 353 mm²
• Ultimate tensile strength: fu = 830 MPa

phi_s x At x fu = 0.85 x 353 x 830 / 1000 = 249.1 kN per anchor

Utilization: 37.5 / 249.1 = 0.15 (OK — steel tensile capacity is rarely the governing anchor check)

Check 5: Concrete breakout in tension

Concrete breakout typically governs over anchor steel capacity. For a single M24 anchor with 300 mm embedment:

Basic breakout strength (Annex D, simplified): Nb = k x sqrt(f'c) x hef^1.5 where k = 10 (cast-in-place anchors), hef = 300 mm

Nb = 10 x sqrt(30) x 300^1.5 / 1000 = 10 x 5.477 x 5196 = 284.6 kN per anchor (unfactored)

Factored breakout: phi_c x Nb = 0.65 x 284.6 = 185.0 kN per anchor

For a group of 4 anchors, the projected failure cones overlap, reducing the group capacity. The group factor depends on anchor spacing relative to 3 x hef = 900 mm. With anchors at approximately 250 mm spacing, there is significant overlap.

Utilization (single anchor under uplift): 37.5 / 185.0 = 0.20 (OK)

Results summary (30 mm plate)

Controlling limit state: Plate bending at 82% utilization with 30 mm plate.

Verify these numbers: Base Plate Calculator -- enter the W250x73 with 400x400x30 plate and compare.

CSA S16 vs AISC 360 — base plate design comparison

Engineers who work across both Canadian and American codes should be aware of the key differences:

The bearing capacity formulas are essentially identical between CSA and AISC. The main differences arise in load combinations (NBCC vs ASCE 7 produces different factored demands) and material grades (G40.21 vs ASTM grades with slightly different Fy/Fu values).

Step 5 — Sensitivity analysis

Common mistakes in CSA base plate design

1. Using the wrong bearing area. The effective bearing area A1 depends on the plate geometry, not the column footprint. Using column flange area instead of plate area underestimates bearing capacity.

2. Ignoring plate bending. Adequate bearing pressure means nothing if the plate is too thin to transfer the load. Plate bending almost always governs plate thickness.

3. Mixing CSA and ACI provisions. While the bearing formulas are similar, the breakout models, psi-factors, and edge distance provisions differ in detail. Do not interpolate between CSA A23.3 Annex D and ACI 318 Chapter 17.

4. Forgetting load reversals. Wind and seismic combinations can produce net uplift. Anchors must be checked for tension, not just assumed to resist compression.

5. Using incorrect edge distances for breakout. Concrete breakout capacity is extremely sensitive to edge distance. Moving an anchor 50 mm closer to an edge can reduce breakout capacity by 40%.

6. Neglecting the shear transfer mechanism. Base plate shear can be transferred by friction, anchor bolt shear, shear lugs, or bearing against the pedestal. The assumed mechanism must match the detailing.

Frequently Asked Questions

Why does base plate design need two standards? Because the connection spans the steel-concrete interface. CSA S16 governs plate bending and steel components. CSA A23.3 governs bearing, anchor embedment, and concrete breakout. Both must be satisfied simultaneously.

When is a shear lug needed? When the shear force exceeds what friction can reliably transfer. The friction coefficient depends on the grout condition. For factored shear above approximately 0.20 x Cf, a shear lug or alternative mechanism should be considered.

How do I choose between more anchors and a thicker plate? More anchors help with tension and breakout but do not help with plate bending. A thicker plate helps with bending but adds cost and weight. Usually, increasing plate dimensions (reducing cantilever) is more effective than increasing thickness.

Can I use Grade 350W plate instead of 300W? Yes. The higher yield strength (350 vs 300 MPa) reduces the required plate thickness. For this example, t_req drops from 27.2 mm to 25.2 mm — the 25 mm plate would just work.

What grout thickness is typical? 25-50 mm of non-shrink grout is standard practice. Thicker grout pads require additional consideration for load path and may affect the bearing area calculation.

Key Takeaways

• Plate bending usually governs plate thickness. Bearing is rarely the controlling check for typical column loads.
• Check eccentricity first: if e < N/6, the full plate is in compression and anchors carry no tension from that load case.
• Concrete breakout often governs over anchor steel for tension. Always check breakout per CSA A23.3 Annex D.
• CSA and AISC bearing formulas are nearly identical, but load combinations and material grades differ.
• Edge distance is the most sensitive parameter for anchor breakout capacity.
• Always check uplift load cases even for primarily compression columns — wind and seismic combinations can reverse the loading.

Run This Calculation

Base Plate & Anchors Calculator — full base plate design per CSA S16:2019, AISC 360, AS 4100, and EN 1993 with bearing, bending, weld, and anchor checks.

Bolted Connection Calculator — for checking bolt groups in the column splice above the base plate.

Further Reading

• Base plate design checklist — step-by-step verification guide
• Anchor bolt embedment depth — design guide
• Anchor bolt reference — development length and edge distance
• Rebar reference — bar sizes and areas for supplementary reinforcement
• Steel Fy and Fu reference — yield and tensile strength by grade
• CSA S16 code notes
• EN 1993-1-8 steel connections guide
• How to verify calculator results
• Base plate design reference
• Steel grades reference
• Why spreadsheets fail for steel design

Disclaimer (educational use only)

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All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

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