Base Plate Design Workflow

Complete AISC DG1 base plate design procedure: bearing stress, plate bending via the Thornton method, anchor tension demand, and weld sizing.

Base plate design bridges steel and concrete design disciplines. A steel-focussed designer may check plate bending and anchor steel strength but overlook concrete bearing and anchorage limit states. A concrete-focussed designer may check bearing and breakout but assume the plate thickness is adequate. The complete workflow requires checking all limit states on both sides of the steel-concrete interface.

This guide walks through the full base plate design procedure per AISC Design Guide 1 (with cross-references to AISC 360-22 and ACI 318-19). It is written as an educational guide, not as a design procedure. For a focused QA checklist, see the Base plate design checklist.

For the full general verification workflow (units, replication strategy, sensitivity testing, and archiving), see How to verify calculator results.

Before You Start

Gather these parameters before opening the base plate calculator:

• Column demands at the base: Factored axial load (Pu or N*), factored moment (Mu or M*), and factored shear (Vu or V*). The governing load combination — maximum compression, maximum uplift, or maximum moment — determines which checks control.
• Column section: Depth d, flange width bf, flange thickness tf, web thickness tw. For HSS columns, the outside dimensions and wall thickness replace d and bf.
• Concrete pedestal: Compressive strength f'c, pedestal plan dimensions (length x width), and whether the pedestal is reinforced. The pedestal area A2 is used for the confinement enhancement factor.
• Anchor bolt layout: Number of bolts, bolt diameter, grade (F1554 Gr. 36/55/105, ASTM A307, or metric grades), embedment depth (hef), edge distances, and whether cast-in-place or post-installed.
• Plate material: Grade and yield strength. A36 (Fy = 36 ksi) or A572 Gr. 50 (Fy = 50 ksi) are common in the US. Australian practice uses Grade 250 or Grade 350 plate (fy varies with thickness per AS/NZS 3678).
• Grout thickness: Typically 1-2 inches (25-50 mm). Non-shrink grout is standard. Thick grout pads (> 50 mm) may require a separate bearing check.

Step-by-Step Design Process

Step 1 — Size the plate for bearing. Start with the column footprint plus 2-4 inches (50-100 mm) on all sides. The bearing stress under the plate is fp = Pu / (B x N), where B is the plate width and N is the plate length. The available concrete bearing capacity per ACI 318-19 §22.8.3.2 is:

• phi Pn = phi x 0.85 x f'c x A1 x sqrt(A2/A1)

Where phi = 0.65 for bearing, A1 is the plate area, and A2 is the supporting concrete area (pedestal area, measured on the plane that is geometrically similar to and concentric with A1). The confinement enhancement factor sqrt(A2/A1) is capped at 2.0. This cap means that once the pedestal area is 4x the plate area, further enlargement provides no additional bearing capacity.

Step 2 — Compute eccentricity and determine the bearing model. e = Mu / Pu. If the resultant is within the kern of the plate, the plate is in full compression and the bearing stress can be assumed uniform over the entire area. For a rectangular plate, the kern limit is N/6. When e > N/6, only a portion of the plate is in contact, and the stress distribution becomes triangular rather than uniform. This is the partial-bearing case, and anchors on the tension side must carry uplift.

Step 3 — Determine cantilever projections (Thornton method). The plate bends as a cantilever beyond the effective column footprint. The governing projection determines the required plate thickness. Per AISC DG1:

• m = (N - 0.95 d) / 2 (projection beyond column depth)
• n = (B - 0.80 bf) / 2 (projection beyond flange width)
• lambda n' = lambda x sqrt(d x bf) / 4 (equivalent cantilever at corners)

The factor lambda accounts for the interaction of bending in two directions at the plate corner. For most moment-resisting connections, lambda = 1.0. For bearing connections, lambda can be reduced per AISC DG1.

The governing cantilever length is l = max(m, n, lambda n').

Step 4 — Compute required plate thickness. For the uniform bearing (full contact) case:

• tp_req = l x sqrt(2 fp / (phi Fy)), where phi = 0.90 for plate bending (AISC 360)

For the partial bearing (triangular stress block) case, the calculation is more involved and requires solving the force and moment equilibrium equations to determine the bearing stress block depth and maximum stress. Many designers conservatively use the uniform pressure formula with the maximum bearing stress from the triangular block.

Round up to the next standard plate thickness (typically 1/8-in increments: 1/2, 5/8, 3/4, 7/8, 1, 1-1/4, 1-1/2, etc.).

Step 5 — Check anchor bolts for tensile demand. For partial-bearing cases, equilibrium of moments about the compression resultant determines the anchor tension demand T_u. Each anchor on the tension side carries T_u / n_tension_bolts. Check the anchor for:

• Steel tensile strength (phi = 0.75 per ACI 318 §17.5.1): phi N_sa = phi x A_se x f_uta, where A_se is the effective tensile stress area and f_uta is the specified tensile strength
• Concrete breakout (ACI 318 §17.5.2): phi N_cb involves the breakout cone projected area A_Nc
• Pullout (ACI 318 §17.5.3): bearing of the anchor head against the concrete
• Side-face blowout (ACI 318 §17.5.4): applies when an anchor is near a free edge

Step 6 — Check shear transfer. Base shear can be resisted by the anchors (shear lugs or friction from compression). If relying on friction, V_u <= mu x Pu, where mu = 0.40 for steel on grout with the surface intentionally roughened, or mu = 0.20 for smooth surfaces. If anchors carry shear, check steel shear strength and concrete breakout in shear per ACI 318 Chapter 17.

Step 7 — Design the column-to-plate weld. Fillet welds around the column perimeter (typically both flanges and the web) must transfer the full column demand to the plate. Size the weld based on the total available weld length. Check the minimum fillet weld size per AISC 360 Table J2.4 based on the thicker part joined. For thick base plates (> 3/4 in), the minimum weld size may govern over strength requirements.

Worked Example

Given: W10x49 column (d = 10.0 in, bf = 10.0 in, tf = 0.560 in, tw = 0.340 in), Pu = 200 kips (compression), Mu = 50 kip-ft at the base. A36 plate (Fy = 36 ksi) on 4,000 psi concrete pedestal. Pedestal dimensions: 24 in x 24 in. Four 3/4-in diameter F1554 Grade 36 anchor bolts, 12-in embedment, cast-in-place.

Step 1 — Trial plate size: Start with plate 16 in x 16 in (6 in larger than column footprint each side). A1 = 256 in^2. Pedestal A2 = 576 in^2. Confinement factor = sqrt(576/256) = 1.50 (<= 2.0, OK). Bearing capacity = 0.65 x 0.85 x 4.0 x 256 x 1.50 = 849 kips > 200 kips. OK.

Step 2 — Eccentricity and bearing model: e = 50 x 12 / 200 = 3.0 in. Kern limit = 16/6 = 2.67 in. Since e = 3.0 in > 2.67 in, partial bearing develops. Anchors carry tension.

Step 3 — Thornton projections:

• m = (16 - 0.95 x 10.0) / 2 = 3.25 in
• n = (16 - 0.80 x 10.0) / 2 = 4.00 in (governs)
• lambda n' = 1.0 x sqrt(10.0 x 10.0) / 4 = 2.50 in

Governing: l = max(3.25, 4.00, 2.50) = 4.00 in.

Step 4 — Plate thickness (conservative, using uniform pressure approach): fp_avg = 200 / 256 = 0.781 ksi. tp_req = 4.00 x sqrt(2 x 0.781 / (0.90 x 36)) = 4.00 x sqrt(1.562 / 32.4) = 4.00 x sqrt(0.0482) = 4.00 x 0.220 = 0.88 in.

Try 1.0 in plate (next standard increment above 0.88 in). tp_provided = 1.00 in > 0.88 in. OK.

Step 5 — Anchor tension demand: For the partial bearing condition, the triangular stress block depth and the anchor tension are determined by solving the force and moment equilibrium equations. For this geometry and loading, the tension demand per anchor is approximately 7.5 kips. Steel tensile capacity of 3/4-in F1554 Gr. 36 bolt: phi N_sa = 0.75 x A_se x f_uta = 0.75 x 0.334 x 58 = 14.5 kips. DCR = 7.5 / 14.5 = 0.52. OK (steel). Concrete breakout, pullout, and side-face blowout must be checked separately per ACI 318 Chapter 17.

Step 6 — Shear transfer: Vu = 10 kips (assumed). Friction capacity = 0.40 x Pu = 0.40 x 200 = 80 kips >> 10 kips. Shear transfers through friction. Anchors checked for shear-tension interaction if required (not governing here since friction handles the shear).

Step 7 — Weld: Use 5/16-in fillet weld (E70XX). Weld capacity per inch = 0.75 x 0.6 x 70 x 0.707 x 0.3125 = 6.96 kip/in. Weld length around flanges (both sides) = 2 x 2 x 10.0 = 40 in. Total weld capacity = 6.96 x 40 = 278 kips >> 200 kips. OK. Minimum fillet weld per AISC 360 Table J2.4 for 1.0-in plate = 5/16 in. OK.

Result: 16 x 16 x 1.0 in A36 base plate with four 3/4-in F1554 Gr. 36 anchor bolts at 12-in embedment. Plate bending governs at DCR = 0.88. Anchor steel tension DCR = 0.52. All concrete anchorage failure modes must be verified separately.

Common Pitfalls

1. Ignoring eccentricity. Even a small moment at the column base can push the resultant outside the kern. The behaviour changes qualitatively — not just quantitatively — when this happens. Always check both the maximum compression and maximum uplift load combinations.

2. Using the wrong phi factor. Base plates span the steel-concrete boundary. phi = 0.90 for plate bending (AISC 360), phi = 0.65 for concrete bearing (ACI 318), phi = 0.75 for anchor steel in tension (ACI 318), and phi = 0.75 for fillet welds (AISC 360). These must not be interchanged.

3. Assuming all four anchors share tension equally. In a moment case, only the anchors on the tension side (typically two of four) carry tension. Distributing tension to all four anchors halves the demand per anchor and can miss a governing failure mode.

4. Omitting concrete breakout checks. The base plate calculator checks anchor steel strength. Concrete breakout (the concrete cone that pulls out around the anchor) is often the governing failure mode, especially with small edge distances or shallow embedments. ACI 318 Chapter 17 checks are mandatory.

5. Neglecting the grout layer. Thick grout pads and non-structural grout affect the bearing stress distribution. Some codes treat grout as a separate bearing surface if the thickness exceeds 50 mm.

6. Using undersized plates for erection. Even compression-only plates need anchor bolts for erection stability. Do not specify zero anchors just because the permanent condition has no net uplift.

Code Comparison

Frequently Asked Questions

Why does the base plate thickness depend on the cantilever length? The plate bends as a cantilever beam beyond the column footprint. The bending moment in the plate is proportional to fp x l^2 / 2, where l is the cantilever length. A 50% increase in the cantilever length increases the bending moment by 125%, requiring a significantly thicker plate.

When can I ignore anchor tension? When the resultant eccentricity e = Mu/Pu falls within the kern of the plate (e < N/6 for a rectangular plate). In this condition, the entire plate is in compression and anchors are stress-free. However, anchors are still required for erection stability and should be sized for a nominal minimum load.

How do I handle biaxial bending at the base? For biaxial bending, check the resultant eccentricity in both directions separately. If the resultant falls outside the kern in either direction, partial bearing develops. The plate bending and anchor demand checks must account for the biaxial stress distribution. Approximate by checking the worst-case direction and verifying the other direction separately.

Does the calculator handle post-installed anchors? The calculator handles anchor steel strength checks, which apply to both cast-in and post-installed anchors. Post-installed anchors have additional qualification requirements (per ACI 355.2/355.4 or equivalent) and installation-specific capacity reductions. These must be verified against the manufacturer's data and the governing standard.

Is this guide engineering advice? No. It is an educational description of the base plate design procedure. Project criteria, load values, and compliance decisions are the responsibility of the engineer of record. Base plate design for a real project must be peer-reviewed and comply with the governing building code.

Run This Calculation

→ Base Plate & Anchors Calculator — bearing, plate bending, weld, and anchor bolt checks per AISC 360, AS 4100, EN 1993, and CSA S16.

→ Column Capacity Calculator — axial compression check with K-factor input for the column above the base plate.

→ Concrete Footing Calculator — pedestal bearing and spread footing design to check the concrete below the plate.

Related pages

• Guides and checklists
• Base plate & anchors calculator
• Base plate design checklist
• Base plate worked example
• Anchor bolts reference — ACI 318 Chapter 17
• Anchor bolt embedment depth — ACI 318 design guide
• Column K-factor table — effective length, 6 end conditions
• Steel Fy & Fu reference — yield and tensile strength by grade
• Concrete footing calculator
• How to verify calculator results
• Disclaimer (educational use only)

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

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.

The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.