Baseplate Design for Axial Compression and Bending Using AISC Design Guide 1

Zachary Forbes

Reading time: < 15 minutes

Article

Introducing New Biaxial Bending Provisions in AISC Design Guide 1, 3rd Edition

Baseplate

Software & Services

Structural Connections

Baseplate Design For Axial Compression And Bending Using AISC Design Guide 1

Baseplate design is often considered straightforward for pure axial compression loads but increases in complexity when uniaxial or biaxial bending is included. To address this complexity, engineers need clear, reliable methods and tools to determine the baseplate’s length, width, thickness, and anchor forces. One commonly considered method is the American Institute of Steel Construction’s (AISC) Design Guide 1 (DG1).

AISC DG1, 3rd Edition (June 2024), which is implemented in PROFIS Engineering Premium, provides a widely used, code-consistent method for baseplate design that assumes the baseplate behaves as a rigid element under load with a uniform rectangular bearing stress distribution. The 3rd edition introduces new provisions for baseplate designs with a combined axial compression load and biaxial bending, which addresses a condition not explicitly covered in previous editions.

To support understanding of these new biaxial bending provisions, this article provides a walkthrough of:

Design for Axial Compression Load Only
Design for Combined Axial Compression Load and Uniaxial Bending
Design for Combined Axial Compression Load and Biaxial Bending

Design for Axial Compression Load Only

Introduction

For baseplate designs with a compression load only, AISC DG1 requires two failures modes to be considered:

Concrete Bearing Limit
Baseplate Yielding Limit at Bearing Interface

Concrete Bearing Limit

Baseplate Yielding Limit at Bearing Interface

AISC Design Guide 1 - Table 1 and Equation 2

Summary of Required and Design Strength Equations

The required strength of the failure mode must be less than or equal to the corresponding design strength. See Table 2 below for the required strengths and design strengths of the two failure modes.

Design for Combined Axial Compression Load and Uniaxial Bending

Introduction

For baseplate designs with a combined axial compression load and uniaxial bending, AISC DG1 requires three failures modes to be considered:

Concrete Bearing Limit
Baseplate Yielding Limit at Bearing Interface
Baseplate Yielding Limit at Tension Interface

Table 3: Eccentricity and Critical Eccentricity Equations

Small Moment Case vs. Large Moment Case

Figure 1: AISC DG1 – Fig. 4-7 for Small Moment Case

Figure 2: AISC DG1 – Fig. 4-8 for Large Moment Case

Length of Bearing Area, _Y

Table 4: Length of Bearing Area Equations

Two-way Bending Consideration for Bearing Interface (AISC DG1 Appendix B.4)

Figure 3: Baseplate Design Example with Moment in X-axis

Figure 5: Four Anchor-bolt Pattern Example for Distribution of Tension Loads

Tension in Anchors, T_u

Effective Bending Widths, b_eff,m,and b_eff,n, for Tension Interface

Figure 4: Example Baseplate Design with Moment in X-axis

Summary of Required and Design Strength Equations

The required strength of the failure mode must be less than or equal to the corresponding design strength. See Tables 6 and 7 below for the required strengths and design strengths for small and large moment cases.

Table 6: Axial Compression Load and Uniaxial Bending – Strength Equations for Small Moment Case

Table 7: Axial Compression Load and Uniaxial Bending – Strength Equations for Large Moment Case

Design for Combined Axial Compression Load and Biaxial Bending

Introduction

With uniaxial bending calculations established, these concepts extend naturally to biaxial bending calculations with three unique considerations:

Calculations Required for Both Axes
Bearing Interface Interaction Equation
Tension Load Distribution for Large Moment Cases in Both Axes

Calculations Required for Both Axes

Eccentricity and critical eccentricity are evaluated independently for each axis to classify as a small or large moment case:

Once classified, the failure modes for each axis are calculated accordingly.

Bearing Interface Interaction Equation

Tension Load Distribution for Large Moment Cases in Both Axes

When both axes are classified as large moment cases, the tension forces can be superimposed for a conservative load distribution. For example (Figure 5), if the x-axis calculations result in Anchors 1 and 2 developing tension forces and the y-axis calculations result in Anchors 1 and 3 developing tension forces, superimposition leads to Anchor 1 carrying tension from both axes while Anchor 4 carries no tension. This is expressed as:

Design Implementation and Workflow Efficiency

Using PROFIS Engineering Premium to implement AISC DG1 baseplate designs, engineers can efficiently address the complexity of designs with axial compression loads and uniaxial or biaxial bending.

This same workflow also enables anchoring-to-concrete design in accordance with ACI 318 Chapter 17 allowing baseplate and anchor design to be performed together within a single solution.

Additional loading considerations, including tension and combined tension and bending, can also be evaluated within the same framework extending the workflow beyond compression-controlled designs.

When baseplate behavior exceeds the assumptions of rigid plate analysis, an advanced design approach is available through the Component Based Finite Element Method (CBFEM). This method models the baseplate as a flexible element and captures nonlinear stress distribution, prying effects, and deformation, providing additional insight for more complex loading conditions.

PROFIS Engineering Premium provides an efficient way to implement AISC Design Guide 1 procedures, evaluate advanced CBFEM behavior, and perform anchoring-to-concrete design with a single workflow helping streamline baseplate designs and improve productivity.