Base Plates Design

Base plates are elements required at the end of columns to distribute the concentrated load of the column over a much larger area of the material that supports it. The design of column base plates involves two main considerations: One, spread the load so as to maintain the bearing pressures under the allowable values, and the second is with the connection, or anchorage, of the base plate and column to the concrete foundation.

For frame analysis it may be important to consider the degree of fixity of the entire anchorage, so the design of the base plate and the anchor bolts should consider the interaction of axial load and bending moment.

The program performs the elastic design of a column steel base plate resting on a concrete pier and subjected to any service load combination of axial load and bending moment, including uplift loading. The moment is assumed acting about the strong axis of a steel column welded to the plate. In addition, this program computes and checks the maximum bearing stress on the pier, as well as the tension and shear forces per rod, necessary to design the anchorage system.

For axially loaded base plates, such as those in frames assumed to be pinned at the base, the program is based on either the cantilever model or the H-shape model of the Murray/Stockwell method covered in the AISC Manual.

For base plates with moment, two design theories are considered:
a) For plates assumed rigid, the strain compatibility is enforced in accordance with the Blodgett method ("Design of Welded Structures")..
b) For plates assumed flexible, the strain compatibility is ignored in accordance with the DeWolf method ("AISC.Design Guides")

For plates subjected to axial tension or uplift, the Murray method is used. The anchorage design is performed following the procedure presented by Marsh and Burdette, as per the allowable stresses given by the AISC.

Input Data

The input data required includes the plate, column and pier dimensions, the distance from rods to center of column, the materials properties and the acting service loads, as shown below:

Example

As an example, consider a W8x24 steel column designed to resist a bending moment of 40 k-ft and an axial load of compression of 60 k, welded to the plate shown below. Design the plate thickness and check the bearing stresses on a pier with an area of 900 in2. In addition, find out if the tension force can be taken by (2) 1-1/2" diam. A-307 rods and compute the rod length.

Output

The figure to the right above shows the template of the Base Plate Design program.

The 14 x 14" plate size is adequate since the maximum bearing stress is 68% of the allowable bearing stress for that pier. The plate thickness required is 1-5/8" and the rod length is 18". The combined tension-shear stress for the anchor rods is 35% of the allowable value, therefore the design is correct.

ASDIP generates a praphical view of the designed base plate and the resulting bearing pressures and anchor rod forces, as shown.