Acknowledgements
Base Plate Design Methods: The authors would like to thank Robert J. Dexter from the University of Minnesota, and Daeyong Lee from the Steel Structure Research Laboratory, Research Institute of Industrial Science & Technology (RIST), Kyeonggi-Do, South Korea, for their writing of Appendix A and the first draft of this Guide. The authors also recognize the contributions of the authors of the first edition of this guide, John DeWolf from the University of Connecticut and David Ricker (retired) from Berlin Steel Construction Company, and thank Christopher Hewitt and Kurt Gustafson of AISC for their careful reading, suggestions, and their writing of Appendix B. Special appreciation is also extended to Carol T. Williams of Computerized Structural Design for typing the manuscript
Base Plate Design Methods | Steel Design Guide
Base plate design is a key part of any structure design because loads are transferred from the superstructure to the foundation via the base plate. The base plate acts as an interface between the superstructure and the foundation; thus, completing the load path into the foundation. Base plates help provide a uniform distribution of superstructure loads to the foundation, and therefore conform to the shape of the foundation, typically a square or a rectangle.
Anchor design software, like PROFIS Anchor, uses the Strain Compatibility method to calculate loads acting on the anchors without detailed consideration of the base plate itself, other than its length and width. This method is based on statics and assumptions from mechanics of materials; for example, that plane sections remain plane and that the steel strain is the same as the concrete strain at all locations. The Strain Compatibility method uses a triangular stress and strain distribution under the compression end of the plate, which results in higher anchor tension forces due to the location of the centroid. Additionally, the plate itself is not considered part of the load path and is therefore assumed to provide no resistance to the load, which results in an overdesign of the anchorage because the calculated tension loads acting on the anchors tend to be higher compared to anchor tension load calculations that include a detailed consideration of the base plate. Since the base plate is not considered in the load transfer between the superstructure and foundation, there is no consideration given to any steel design codes. The only codes and standards that are considered for design of the anchorage are those containing provisions for anchoring-to-concrete.
The new PROFIS Engineering Premium design software offers two methods by which designers can analyze their base plate configuration.
The first method follows the American Institute of Steel Construction (AISC) Design Guide 1: Base Plate and Anchor Rod Design, which is referenced by the AISC Manual of Steel Construction and the International Building Code (IBC). This model code-compliant design method assumes the plate cross-section remains plane under loading and the plate does not undergo any significant deformation, allowing simplified linear-elastic calculations.
The Design Guide 1 assumption may also allow the base plate to be called “rigid.” Thereby the rectangular stress distribution between the interface of the base plate steel and the concrete may be used. The rectangular stress distribution shifts the centroid farther to the tension side of the plate and results in a thinner base plate compared to the thickness calculated using the triangular stress distribution.
Since the rectangular stress distribution considers the base plate to be part of the load path between the superstructure and foundation, the calculated anchor tension forces using the AISC Design Guide 1 methodology are lower than those calculated using the Strain Compatibility method.
Design Guide 1 calculations are algebraic in nature and are based on basic statics principles. The Design Guide 1 assumption is particularly appropriate for column base plate design.
When anchoring applications include attachment of a thin fixture or consist of large moments with eccentricity acting on the fixture, the “rigid” assumption may not be valid. For this type of anchorage, the fixture may need to be designed and analyzed as more flexible. Currently, none of the above referenced codes include methodology to design and analyze more flexible base plates.
However, a designer could utilize analysis tools like Finite Element Modeling (FEM) to better ascertain where the fixture behavior falls on the spectrum between fully flexible and infinitely rigid. FEM analysis involves discretizing the fixture into elements and uses the stiffness of each element to depict more accurate load transfer between them.
The fixture is typically modeled as a shell element and the anchors are modeled as tension-only springs. As a result, the tension load distribution to the anchors is highly influenced by factors such as the applied loads, the profile geometry, the fixture material properties, and the anchor stiffness. PROFIS Engineering Premium provides automatic meshing of the plate elements and spring elements to permit a realistic modeling of the anchorage.
It also permits the user to input their specific preferences of mesh granularity. PROFIS Engineering Premium FEM results can be interpreted by analyzing the percentage of plastic strain in the fixture and the absolute plastic deformation of the fixture. Users can then use their design experience and engineering judgement to determine the permissible plastic strain and fixture deformation for their anchoring application.
One purpose of base plate design is to analyze the transfer of load to the anchors. This analysis is dependent on how rigid the base plate is. Keeping in mind that anchor design per the American Concrete Institute (ACI) publication Building Code Requirements for Structural Concrete (ACI 318) is predicated on the “rigid” fixture assumption, PROFIS Engineering Premium base plate functionality helps permit users to utilize FEM analysis in order to determine whether the results confirm the validity of a “rigid” fixture assumption or require the fixture design to be modified via increasing the fixture thickness, revising the welded connections, or adding stiffeners to achieve a more rigid element.
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