ASDIP STEEL includes column base plate design as well as the anchorage system to the concrete support, based on the AISC Design Guides # 1, the Blodgett's "Design of Welded Structures" textbook, and the ACI 318 code. This article provides an engineering background on the current design philosophy of column base plates and anchor rods.
Base plates are structural elements at the bottom of columns to spread the concentrated load over a larger supporting area, so that the bearing stresses are under the acceptable limits. Depending on the type of frame the column belongs to, the base plate may be subject to compression, tension, bending and shear. In practice, most columns are W-sections subjected to compression, shear, and bending about the strong axis, but HSS columns under biaxial bending are also common.
There are two main theories for base plate design: one is based on the AISC Design Guides # 1 and it assumes that the base plate is flexible, therefore the strain compatibility is ignored. This theory provides equations for two cases: when the Eccentricity e < L/6, and when e > L/6. The transition between these two cases is not smooth, and the results may be quite different for slightly different eccentricities in the border line of the two cases.
The second theory is based on the Blodgett's textbook, and it assumes that the plate is rigid and therefore the plane sections remain plane after bending, so the strain compatibility is enforced. This theory provides consistent equations for any eccentricity values, as shown in the graphs below. Note the abrupt changes in both the rod tension and the plate thickness per the AISC approach for eccentricities nearby the kern. It seems that the equations of the AISC approach need to be further refined to produce consistent results for any eccentricity. ASDIP STEEL performs the calculations per either theory.
For compression columns subjected to small moments, there's no tension in the anchor rods, so the goal is to keep the concrete bearing pressure under the acceptable limits, as shown in the left image below. In this case the maximum plate moment will be produced by the bearing pressure acting upwards on the cantilever portion of the plate. For plates with small cantilevers the maximum plate moment will occur between the flanges of the W-column.
As the applied moment increases, only a portion of the plate is under compression and the anchor rods provide the required tension to maintain the static equilibrium, as shown in the right image below. In this case the maximum plate moment will be produced by the worst case of a) the bearing pressure acting upwards on the cantilever portion of the plate, or b) the tension force acting on an effective area of 45 degrees to the column face.
Once the design moment is known, the plate thickness is calculated accordingly. Plate thickness increments of 1/4" (6 mm), and plate size increments of 1" (25 mm) are common practice. A grout pad is normally specified to level up the plate on the concrete support.
Anchor rods are normally used to connect the column base plate to the concrete support. The ACI 318 methodology consists in the design of anchor rods for tension and for shear separately, and then check the interaction effects.
The design of anchor rods for tension implies checking the limit states of steel strength, concrete breakout, pullout, and side-face blowout. The calculation of the breakout is particularly important since a concrete failure would be non-ductile, and therefore should be avoided. Anchor reinforcement may be provided in order to avoid a breakout failure, and in this case the tension is taken completely by the rebars. ASDIP STEEL performs all these Code checks and generates a graphic view of the tension breakout area.
For small to moderate shear loads, the anchor rods may be used to transfer the shear to the foundation. In this case the limit states to be checked include the steel strength, concrete breakout and concrete pryout. Since the holes in the plate are oversize, it's unlikely that all the rods will bear against the plate to resist the shear. Unless the washers are welded to the plate, only the front rods are effective for calculation purposes. Anchor reinforcement may be provided to prevent a brittle failure. ASDIP STEEL performs all these Code checks and generates a graphic view of the shear breakout area.
For a more detailed overview of anchorage design please see the blog post Anchor Bolt Design - The Complex ACI Provisions (Part 1). Below is a typical anchorage design report in ASDIP STEEL. Note that the interaction of tension and shear usually controls the design of the anchor rods.
Base plate design is very important, particularly for structural columns subjected to compression, shear, and bending. The design of anchor rods implies multiple limit state checks for tension and shear. ASDIP STEEL includes the design of base plates and anchor rods, with multiple options to optimize the design easily.
Detailed information is available about this structural engineering software by visiting ASDIP STEEL. For a design example see the post Base Plate and Anchor Rods Design Example Using ASDIP STEEL.
You are invited to download the Free 15-day Software Trial, or go ahead and Place your Order. This post originally appeared at https://www.asdipsoft.com/base-plate-and-anchor-rod-design/.
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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.
Figure 1: Example of triangular stress distribution used in Strain Compatibility method.
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.
Figure 2: Design of base plate with axial compressive load, per AISC Design Guide 1.
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.
Figure 3: Example of strain distribution of a plate demonstrating non-rigid behavior.
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.