Base Plate and Anchor Rod Design Overview

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.

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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.

Strain compatibility. What theory to use?

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.

Base plate design.

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.

Anchorage design.

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. 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.

Takeaway

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. For our collection of blog posts about base plate and anchorage design please visit Anchor Rods Design.

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