Canadian Base Plate Design — CSA S16-19 Clause 13.14

Complete reference for column base plate design in accordance with CSA S16-19 Clause 13.14 and CSA A23.3-19 concrete provisions. Covers concrete bearing with confinement factor, plate bending by the cantilever projection method, anchor bolt tension and shear per Clause 13.12, shear lug design, and grouting requirements.

Related pages: CSA S16 Column Design | CSA S16 Bolt Capacity | Canadian Steel Grades | Base Plate Calculator

Design Limit States Under CSA S16

A column base plate assembly must satisfy four limit states:

The base plate bending check (Clause 13.14.3) uses the cantilever projection method, which treats the plate projections beyond the column footprint as cantilever strips under a uniform bearing pressure. This is the same underlying mechanics as the AISC Design Guide 1 approach.

Concrete Bearing Resistance — CSA A23.3-19

The nominal concrete bearing resistance is:

Br = phi_c x 0.85 x f'c x A1 x sqrt(A2/A1) <= 2 x phi_c x 0.85 x f'c x A1

Where:

• phi_c = 0.65 (concrete bearing, same as CSA A23.3 Clause 10.11)
• f'c = specified concrete strength at 28 days (MPa)
• A1 = base plate area (mm^2)
• A2 = maximum area of supporting concrete geometrically similar and concentric with A1
• sqrt(A2/A1) = confinement factor, capped at 2.0

The confinement factor accounts for the triaxial compression state induced in the concrete immediately beneath the plate by the surrounding concrete mass. This is identical in principle to the ACI 318 and AISC 360-22 approach, with the same 2.0 cap.

For a plate on a pedestal: A2 includes the full pedestal area provided it is concentric with the plate. For a plate on a footing: A2 includes the full footing area.

Bearing resistance for typical configurations (f'c = 30 MPa):

*Capped at 2.0 per CSA A23.3.

Plate Bending — Cantilever Projection Method (Clause 13.14.3)

The base plate dimensions N (length, parallel to web) and B (width, parallel to flange) provide cantilever projections m and n:

m = (N - 0.95 x d) / 2 (projection beyond column depth, to column flange face) n = (B - 0.80 x bf) / 2 (projection beyond column width, to web centreline)

The 0.95 and 0.80 factors account for the effective bearing footprint of the column section. The web term uses 0.80 because the effective bearing width of the web is approximately 0.80 x bf for typical W-shapes.

Factored bearing pressure: w = Cf / (N x B) (MPa)

Required plate thickness: tp_req = sqrt(2 x w x c_max^2 / (phi x Fy_plate))

Where c_max = max(m, n, lambda_n_prime x n_prime) and lambda_n_prime and n_prime account for yielding near the column web toes, per the AISC Design Guide 1 refinement also used in Canadian practice:

n_prime = sqrt(d x bf) / 4 X = (4 x d x bf / (d + bf)^2) x Cf/(phi_c x 0.85 x f'c x A1) lambda_n_prime = 2 x sqrt(X) / (1 + sqrt(1 - X)) <= 1.0

For most concentrically loaded column bases, c_max = max(m, n) is sufficient. The n_prime check governs only for very large plates where the bearing pressure is low.

Anchor Bolt Design — CSA S16 Clause 13.12

Tension capacity per bolt: Tr = 0.75 x phi_b x Ab x Fu (Clause 13.12.2)

Where the 0.75 factor accounts for the reduced area at the threaded portion (tensile stress area ~ 0.75 x nominal area). Ab = nominal shank area, Fu = ultimate tensile strength of bolt material.

Shear capacity per bolt: Vr = 0.60 x phi_b x Ab x Fu (threads in shear plane) Vr = 0.70 x phi_b x Ab x Fu (shank in shear plane, threads excluded)

Combined tension and shear (Clause 13.12.4): (Vf/Vr)^2 + (Tf/Tr)^2 <= 1.0

This is an elliptical interaction, more generous than the linear interaction of some codes. It permits a bolt at 80% of its shear capacity to still carry up to 60% of its tension capacity.

Canadian anchor bolt grades (ASTM F1554):

Grade 55 (Fy = 380 MPa, Fu = 550 MPa) is the default for Canadian structural anchor bolts. Grade 36 is used for lightly loaded columns or temporary works. Grade 105 is reserved for high-strength applications (heavy uplift, seismic overturning).

Embedment Length — CSA A23.3

The anchor bolt must be embedded sufficiently into the concrete to develop its tensile capacity without pulling out. Per CSA A23.3-19 Clause 12.3:

Basic development length: Ld = 0.19 x db x Fy / sqrt(f'c) (for headed bolts in tension)

For a M22 (7/8") Grade 55 bolt (Fy = 380 MPa) in 30 MPa concrete: Ld = 0.19 x 22.2 x 380 / sqrt(30) = 1,603 / 5.48 = 293 mm

Practical embedment minimums:

• M20 (3/4"): 250 mm
• M22 (7/8"): 300 mm
• M32 (1-1/4"): 450 mm

For anchor bolts transferring tension, a washer plate or headed anchor at the base of the bolt is required. J-bolts and L-bolts without end anchorage have significantly lower pull-out capacity and should be avoided for primary tension anchors.

Shear Lugs (Shear Keys)

When the factored shear Vf exceeds the shear capacity of the anchor bolts, a shear lug is welded to the underside of the base plate. The lug engages the concrete grout pocket in bearing, transferring shear directly to the foundation without passing through the bolts.

Shear lug bearing on concrete: phi_c x 0.85 x f'c x A_lug_face (bearing area)

Shear lug weld: Full-strength fillet weld around the lug-to-plate interface. The weld must transfer Vf with an allowance for the eccentricity from the plate underside to the bearing face centroid.

Grout pocket: The shear lug sits in a recessed pocket in the concrete that is later filled with non-shrink grout. The pocket must be large enough to allow the lug to bear uniformly. Minimum pocket dimensions: lug width + 25 mm on all sides, depth = lug height + 25 mm for grout flow.

Grout Requirements

Per CISC best practice and CSA A23.3:

• Grout thickness: 25-50 mm between base plate and concrete
• Grout strength: Minimum f'c_grout >= fc'_concrete (specify 35 MPa grout for 30 MPa concrete)
• Dry-pack or flowable grout: For plates up to 600 x 600 mm, dry-pack grout (stiff consistency) is acceptable. For larger plates, flowable grout (prepackaged cementitious grout) is required to ensure full contact.
• Levelling nuts: Required on all anchor bolts. Set plate elevation with nuts below the plate, grout, then torque nuts above the plate after grout curing.
• Grout holes: For plates exceeding 500 x 500 mm, provide 50 mm diameter grout holes at the plate centre. These also serve as air vents during grouting.

Worked Example — W310x158 Column Base Plate

Problem: Design a base plate for a W310x158 column (350W steel). Factored axial compression Cf = 3,200 kN. Factored shear Vf = 180 kN (wind). Concrete pedestal = 600 x 600 mm, f'c = 30 MPa. Plate steel: 300W (Fy = 300 MPa). Anchor bolts: 4-M22 ASTM F1554 Grade 55.

Section properties — W310x158: d = 327 mm, bf = 311 mm, tf = 25.0 mm, tw = 15.9 mm

Step 1 — Plate area: Try 480 x 480 mm plate: A1 = 230,400 mm^2. A2 = 600 x 600 = 360,000 mm^2. A2/A1 = 1.56. sqrt(1.56) = 1.25. Br = 0.65 x 0.85 x 30 x 230,400 x 1.25 / 1000 = 4,776 kN >= 3,200 kN. OK.

Step 2 — Bearing pressure: w = 3,200 x 1000 / 230,400 = 13.89 MPa

Step 3 — Cantilever distances: m = (480 - 0.95 x 327) / 2 = (480 - 311) / 2 = 84.5 mm n = (480 - 0.80 x 311) / 2 = (480 - 249) / 2 = 115.5 mm c_max = max(84.5, 115.5) = 115.5 mm

Step 4 — Plate thickness: tp_req = sqrt(2 x 13.89 x 115.5^2 / (0.90 x 300)) = sqrt(2 x 13.89 x 13340 / 270) tp_req = sqrt(370,529 / 270) = sqrt(1372) = 37.0 mm

Specify 40 mm plate (next standard thickness). At 40 mm, the plate is heavy; consider checking the n_prime/lambda_n refinement to potentially reduce thickness.

n_prime check: n_prime = sqrt(327 x 311) / 4 = sqrt(101,697) / 4 = 319/4 = 79.8 mm X = (4 x 327 x 311 / (327+311)^2) x 3200/(0.65x0.85x30x230400/1000) = skip detailed X — typically <= 0.25.

For X = 0.20: lambda_n_prime = 2 x sqrt(0.20) / (1 + sqrt(0.80)) = 2 x 0.447 / (1 + 0.894) = 0.894 / 1.894 = 0.472. lambda_n x n_prime = 0.472 x 79.8 = 37.7 mm < c_max = 115.5 mm. No reduction.

Final thickness: 40 mm plate, 300W steel.

Step 5 — Anchor bolts: Shear on bolts: Vf = 180 kN. M22 Gr 55: Vr = 128.0 kN per bolt. 4 bolts: Vr_total = 512 kN >> 180 kN. OK. Also check combined tension and shear if uplift is negligible: not governing.

Step 6 — Weld: Column to plate: 10 mm fillet (based on thicker part t_f = 25 mm, min fillet = 8 mm per CSA W59). Flange weld: 2 x 311 x 1.56 = 970 kN >> nominal uplift/moment. Web weld: 2 x 327 x 1.56 = 1,020 kN. OK.

Final specification: 480 x 480 x 40 mm base plate, 300W. 4-M22 ASTM F1554 Gr 55 anchor bolts, 350 mm embedment with headed ends. Levelling nuts and 35 MPa non-shrink grout, 40 mm bed. 10 mm fillet welds.

Uplift and Moment Base Plates

When the column carries significant moment, the base plate must be designed for both the compression block (concrete bearing) and the tension side (anchor bolts). The neutral axis position is iterated until equilibrium is satisfied:

Sum of vertical forces: C (concrete compression) - T (bolt tension) = Cf

Moment equilibrium: Cf x e = C x (N/2 - a/2) + T x (N/2 - e_bolt)

Where a = depth of concrete stress block = C / (0.85 x f'c x B).

For large overturning moments relative to axial load (typical of canopies, sign structures, and tall columns with small gravity load), the tension in the anchors governs, and the plate thickness may be controlled by the tension-side T-stub bending rather than the compression-side cantilever.

Frequently Asked Questions

What is the minimum base plate thickness in Canadian practice? There is no explicit CSA S16 minimum, but practical minimums derived from constructability are 12 mm for lightly loaded columns (Cf < 300 kN) and 20 mm for typical building columns. Plate thicknesses below 12 mm are susceptible to welding distortion and may not provide a flat bearing surface. For architectural columns, 25 mm is the typical minimum to provide a robust appearance at the column base.

How does the A2/A1 ratio affect concrete bearing capacity? The ratio can double the bearing capacity (from 1.0x to 2.0x). For the maximum benefit, the pedestal must be at least 2 times the plate dimension in both directions, and the plate must be centred on the pedestal. If the plate is near the pedestal edge, A2 is limited to the area that is geometrically similar and concentric with A1, which may be less than the full pedestal area.

When should anchor bolts be designed for shear vs using a shear lug? Shear can be resisted by anchor bolts when Vf is less than about 250-300 kN (4 x M22 bolts). Above this, a shear lug becomes more economical. Shear lugs are also preferred when: (a) the column participates in the lateral force resisting system and shear reversal occurs, (b) the anchor bolts are already highly stressed in tension from overturning, or (c) the combined tension-shear interaction exceeds 1.0.

What grout specification is typical for Canadian column base plates? Specify non-shrink cementitious grout (BASF Masterflow, Sika Grout, or Euclid NS Grout) with a 28-day compressive strength of at least 35 MPa. The grout must be placed within 4 hours of mixing at temperatures between 5 and 30 degrees C. For winter construction, heated grout and hoarding are required to maintain the grout temperature above 10 degrees C for 24 hours after placement.

This page is for educational reference. Base plate design per CSA S16-19 Clause 13.14 and CSA A23.3-19. Verify material properties and embedment lengths. All structural designs must be independently verified by a licensed Professional Engineer registered with the relevant provincial engineering association. Results are PRELIMINARY — NOT FOR CONSTRUCTION.