AISC DESIGN GUIDE 1
Base Plate and Anchor Rod Design
|Publication Date:||1 May 2006|
Column base plate connections are the critical interface between
the steel structure and the foundation. These connections are used
in buildings to support gravity loads and function as part of
Base plates and anchor rods are often the last structural steel items to be designed but are the first items required on the jobsite. The schedule demands along with the problems that can occur at the interface of structural steel and reinforced concrete make it essential that the design details take into account not only structural requirements, but also include consideration of constructability issues, especially anchor rod setting procedures and tolerances. The importance of the accurate placement of anchor rods cannot be overemphasized. This is one of the key components to safely erecting and accurately plumbing the building.
The material in this Guide is intended to provide guidelines for engineers and fabricators to design, detail and specify column-base-plate and anchor rod connections in a manner that avoids common fabrication and erection problems. This Guide is based on the 2005 AISC Specification for Structural Steel Buildings (AISC, 2005a), hereafter referred to as the AISC Specification, and includes guidance for designs made in accordance with Load and Resistance Factor Design (LRFD) or Allowable Strength Design (ASD).
This Guide follows the format of the AISC Specification, developing strength parameters for foundation system design in generic terms that facilitate either LRFD or ASD. Column bases and portions of the anchorage design generally can be designed in a direct approach based on either LRFD or ASD load combinations. The one area of anchorage design that is not easily designed by ASD is the embedment of anchor rods into concrete. This is due to the common use of ACI 318-08, Appendix D, which is exclusively based on the strength approach (LRFD), for the design of such embedments. ASD and LRFD methods are equally proficient at evaluating other steel elements of the foundation system, including the column base plate and the sizing of anchor diameters. In cases such as anchors subjected to neither tension nor shear, the anchorage development requirement may be a relatively insignificant factor.
The generic approach in development of foundation design parameters taken in this Guide permits the user a choice to develop the loads based on either the LRFD or ASD approach. The derivations of foundation design parameters, as presented herein, are then either multiplied by a Resistance Factor, φ, or divided by a Safety Factor, Ω, based on the appropriate load system utilized in the analysis; consistent with the approach used in the AISC Specification. Many of the equations shown herein are independent of the load approach, and thus are applicable to either design methodology. These are shown in singular format. Other derived equations are based on the particular load approach and are presented in a side-by-side format of comparable equations for LRFD or ASD application.
The typical components of a column base are shown in Figure 1.1. This figure shows anchor rods that are threaded and nutted at the embedded end. Anchor rods also may be headed or have hooked ends.
Material selection and design details of base plates can significantly affect the cost of fabrication and erection of steel structures, as well as the performance under load. Relevant aspects of each of these subjects are discussed briefly in the next section. Not only is it important to design the column-base-plate connection for strength requirements, it is also important to recognize these connections affect the behavior of the structure. Assumptions are made in structural analysis about the boundary conditions represented by the connections. Models comprising beam or truss elements typically idealize the column base connection as either a pinned or fixed boundary condition. Improper characterization can lead to error in the computed drifts, leading to unrecognized second-order moments if the stiffness is overestimated, or excessive first-floor column sizes if the stiffness is underestimated. If more accurate analyses are desired, it may be necessary to input the stiffness of the column-baseplate connection in the elastic and plastic ranges, and for seismic loading, possibly even the cyclic force-deformation relations. The forces and deformations from the structural analyses used to design the column-base-plate connection are dependent on the choice of the column-base-plate connection details.
The vast majority of building columns are designed for axial compression only with little or no uplift. For such columns, a simple column-base-plate connection detail like that shown in Figure 1.1 is sufficient. The design of column-baseplate connections for axial compression only is presented in Section 3. The design is simple and need not be encumbered with many of the more complex issues discussed in Appendix A, which pertains to special structures. Anchor rods for gravity columns are often not required for the permanent structure and need only be sized to provide for column stability during erection.
Column-base-plate connections are also capable of transmitting uplift forces and can transmit shear, including through the anchor rods if required. If the base plate remains in compression, shear can be transmitted through friction against the grout pad or concrete, thus the anchor rods are not required to be designed for shear. Large shear forces can be resisted by bearing against concrete, either by embedding the column base, or by adding a shear lug under the base plate.
Column-base-plate moment connections can be used to resist wind and seismic loads on the building frame. Moment at the column base can be resisted by development of a force couple between bearing on the concrete and tension in some or all of the anchor rods.
This Guide will enable the designer to design and specify economical column base plate details that perform adequately for the specified demand. The objective of the design process in this Guide is that under service loading, and under extreme loading in excess of the design loads, the behavior of column base plates should be close to that predicted by the approximate mathematical equations in this Design Guide.
Historically, two anchor rods have been used in the area bounded by column flanges and web. Recent regulations of the U.S. Occupational Safety and Health Administration (OSHA)-Safety Standards for Steel Erection (OSHA, 2001) (Subpart R of 29 CFR Part 1926)-require four anchor rods in almost all column-base-plate connections, and require all columns to be designed for a specific bending moment to reflect the stability required during erection with an ironworker on the column. This regulation has essentially eliminated the typical detail with two anchor rods except for small post-type structures that weigh less than 300 pounds (e.g., doorway portal frames).
This Guide supersedes the original AISC Design Guide 1 -Column Base Plates. In addition to the OSHA regulations, there has been significant research and improved design guidelines issued subsequent to the publication of Design Guide 1 in 1990. The ACI Building Code Requirements for Structural Concrete, ACI 318-08 (ACI, 2008), has improved provisions for the pullout and breakout strength of anchor rods and other embedded anchors. Design guidance for anchor rods based on the ACI recommendations is included, galong with practical suggestions for detailing and installing anchor rod assemblies. These guidelines deal principally with cast-in-place anchors and with their design, installation, inspection and repair in column-base-plate connections.
AISC Design Guide 7, Industrial Buildings: Roofs to Column Anchorage (Fisher, 2004), hereafter referred to as AISC Design Guide 7, contains additional examples and discussion relative to the design of anchor rods.