Column base plates are structural elements designed to spread the load in such a way that the bearing pressures at the support are within the allowable limits. Under the presence of biaxial moments, the base plates may be in partial bearing, leading to a complex design. This document is a step-by-step biaxial base plate example using ASDIP STEEL software.
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As an example, consider the column base plate shown below, which supports a steel column in a multistory building. The column size is W10x49, and it's exposed to dead, live, and seismic loads, whose reactions are shown below. The concrete support is 36" x 36" in plan, but the column is located eccentrically at 12" from one corner in both directions. Design the base plate and the anchor rods for the ASCE 7-16 load combinations.
Base Plate Design Steps
Enter the information given in the statement of the example, which are the loads and the materials. In this example the analysis of the structure provided the reactions per load case. Since the column is 10" deep, in the Geometry tab let's specify a base plate 17" x 17" to allow space for the anchor rods with an edge distance of 2".
Use the Materials tab to enter the material properties, such as the concrete and reinforcing steel strengths. Specify the number and size of anchor rods. In this case let's try (10)-1" diameter anchor rods, distributed as 3 end rods and 2 side rods. Use A36 steel for the base plate. Assume that the project is in a seismic zone, with Sds = 0.60, therefore the seismic provisions will be enforced.
The At-a-Glance tab shows a summary of the design for a quick overview of your work as you go. The green check marks indicate that the checks passed. Note that the program calculates a minimum plate thickness of 1.1". Let's use a 1-1/4" plate.
Anchor Rods in Tension
Design the anchor rods for tension. Since the base plate is placed eccentrically on the support, the tension breakout capacity will be very limited, therefore in the Anchorage > Tension Analysis tab specify anchor reinforcement to transfer the tension load to the foundation. Let's try 1 #5 rebar for each anchor rod to avoid the breakout failure. The anchor reinforcement must be detailed to develop the required strength.
The Condensed tab shows a more detailed set of calculations grouped by topic. Note that the pullout strength controls the design in tension, with a design ratio of 0.50 as shown below.
Anchor Rods in Shear
In the Anchorage > Shear Analysis tab specify the way the shear load will be resisted. In this case the shear load will be resisted by anchor rods only. Specify the washers to be welded to the base plate in order to force all anchor rods to be effective in shear.
Similarly to the tension analysis, since the base plate is close to the borders of the support, the shear breakout capacity will be very limited. Therefore, it's necessary to specify anchor reinforcement to avoid the shear breakout failure. In this case let's use #4 hairpins per rod, detailed to develop the required strength. The controlling shear limit state is concrete pryout with a design ratio of 0.58. The combined stress ratio is 0.82.
ASDIP STEEL generates a plan view of the biaxial base plate, showing the extent of the bearing pressure diagram and the tension load per anchor rod. Note that the maximum bearing pressure at the corners is 2.1 ksi, and the maximum tension per rod is 12.7 kips for the load combination shown below.
The image below at the left shows graphically the calculation of the tension breakout area. Note that ASDIP STEEL shows in yellow the anchor rods in tension. In addition, the effective anchor embedment is used in the calculation. The image at the right shows the calculation shear breakout area. Note that in this case all anchor rods are effective in shear, and this is reflected in the calculations.
ASDIP STEEL includes the design of biaxial base plates and anchor rods, with multiple options to optimize the design easily. The design of biaxial base plates may be cumbersome and error-prone, with multiple limit states checks. This biaxial base plate example shows that the design can be completed and optimized with ASDIP STEEL in minutes.
For engineering background, please read the blog post Base Plate and Anchor Rod Design Overview. For user interface, see How to Design Biaxial Base Plates Using ASDIP STEEL. For our collection of blog posts about base plate and anchorage design please visit Anchor Rods Design.
Javier Encinas, PE
ASDIP Structural Software