COLUMN ERECTION METHODS

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The behavior of the base connection may be affected by the type of construction method used to erect the column to the foundation. These methods may result in obstructions between the base plate and concrete/grout foundation interface that are not typically considered during design but may influence connection behavior. Three common procedures are used to erect columns on the foundation: leveling nuts, setting plates and shim stacks . Local practice and the weight of the column generally determine which of these methods are used.

o   Leveling Nut Method - Leveling nuts and washers are installed to the cast-in anchor rods, onto which the base plate is placed atop, and the column is set to proper elevation by turning the leveling nuts. The use of leveling nuts may be not be suitable for erecting heavy columns, as the anchor rods would be loaded in compression prior to grouting. The compression strength of the rods, as well as the possibility of rods pushing out through the bottom of the footing, should be checked. The use of leveling nuts may affect the behavior of grouted base connections loaded in shear, flexure and axial compression. For example, the leveling nuts would constrain the anchor rods in bending which may increase the resistance of the anchor rod bearing mechanism in shear. In addition, the leveling nut detail provides a compressive load path through bearing from the base plate to the anchor rods, which may affect flexural and axial behavior, especially if the grout pad has deteriorated due to crushing/spalling damage.

o   Setting Plate Method - Setting plates (sometimes called leveling plates) can be used to erect columns but are generally more costly than leveling nuts. After the anchor rods have been set, a pre-fabricated steel setting plate (usually 1/4 inches thick) is installed over the rods. Grout is spread over the concrete foundation under the setting plate, and the setting plate is tapped down to the desired elevation and proper level. After the grout cures, the column base plate is set atop the setting plate. Note that this method results in an unbonded steel-to-steel interface between the base plate and the leveling plate, which may affect connection behavior. For example, base plate slip under shear loading may occur between the setting plate steel, rather than the grout or concrete, which may affect the frictional response.

o   Shim Stack Method - The use of steel shim stacks is a traditional method for erecting columns and is advantageous since all compression is transferred from the base plate to the foundation without involving the anchor rods. Steel shim stacks are typically set at the four edges of the base plate as the column is lowered. Similarity as with the leveling nuts, shim stacks may affect base connection behavior by proving a stiff compressive load path between the base plate and the concrete/grout foundation.