1 Steel Design Guide Base Plate and Anchor Rod Design Second Edition 1 Steel Design Guide Base Plate and Anchor Rod Design Second Edition JAMES M. FISHER, Ph.D., P.E. Computerized Structural Design, S.C. Milwaukee, Wisconsin and LAWRENCE A. KLOIBER, P.E. LeJuene Steel Company Minneapolis, Minnesota AMERICAN INSTITUTE OF STEEL CONSTRUCTION, INC. Copyright © 2006 by American Institute of Steel Construction, Inc. All rights reserved. This book or any part thereof must not be reproduced in any form without the written permission of the publisher. The information presented in this publication has been prepared in accordance with recognized engineering principles and is for general information only. While it is believed to be accurate, this information should not be used or relied upon for any specific application without compe- tent professional examination and verification of its accuracy, suitability, and applicability by a licensed professional engineer, designer, or architect. The publication of the material contained herein is not intended as a representation or warranty on the part of the American Institute of Steel Construction or of any other person named herein, that this information is suitable for any general or particular use or of freedom from infringement of any patent or patents. Anyone making use of this information assumes all liability arising from such use. Caution must be exercised when relying upon other specifications and codes developed by other bodies and incorporated by reference herein since such material may be modified or amended from time to time subsequent to the printing of this edition. The Institute bears no responsi- bility for such material other than to refer to it and incorporate it by reference at the time of the initial publication of this edition. Printed in the United States of America First Printing: May 2006 Acknowledgements The authors would like to thank Robert J. Dexter from AISC would also like to thank the following individuals the University of Minnesota, and Daeyong Lee from the who assisted in reviewing the drafts of this Design Guide for Steel Structure Research Laboratory, Research Institute of their insightful comments and suggestions. Industrial Science & Technology (RIST), Kyeonggi-Do, Victoria Arbitrio Donald Johnson South Korea, for their writing of Appendix A and the first Reidar Bjorhovde Geoffrey L. Kulak draft of this Guide. The authors also recognize the contribu- Crystal Blanton Bill R. Lindley II tions of the authors of the first edition of this guide, John Charles J. Carter David McKenzie DeWolf from the University of Connecticut and David Brad Davis Richard Orr Ricker (retired) from Berlin Steel Construction Company, Robert O. Disque Davis G. Parsons II and thank Christopher Hewitt and Kurt Gustafson of AISC James Doyle William T. Segui for their careful reading, suggestions, and their writing of Richard M. Drake David F. Sharp Appendix B. Special appreciation is also extended to Carol Samuel S. Eskildsen Victor Shneur T. Williams of Computerized Structural Design for typing Daniel M. Falconer Bozidar Stojadinovic the manuscript. Marshall T. Ferrell Raymond Tide Roger D. Hamilton Gary C. Violette John Harris Floyd J. Vissat Allen J. Harrold v vi Table of Contents 1.0 INTRODUCTION .....................................................1 3.3.3 Base Plate Flexural Yielding at Tension Interface ...............................25 2.0 MATERIAL, FABRICATION, 3.3.4 General Design Procedure ....................25 INSTALLATION, AND REPAIRS ..........................2 3.4 Design of Column Base Plates with Large Moments ................................................25 2.1 Material Specifications ......................................2 3.4.1 Concrete Bearing and 2.2 Base Plate Material Selection ............................2 Anchor Rod Forces ...............................25 2.3 Base Plate Fabrication and Finishing ................3 3.4.2 Base Plate Yielding Limit 2.4 Base Plate Welding ............................................4 at Bearing Interface ..............................26 2.5 Anchor Rod Material .........................................5 3.4.3 Base Plate Yielding Limit 2.6 Anchor Rod Holes and Washers ........................6 at Tension Interface ...............................27 2.7 Anchor Rod Sizing and Layout .........................7 3.4.4 General Design Procedure ....................27 2.8 Anchor Rod Placement and Tolerances ............7 3.5 Design for Shear ..............................................27 2.9 Column Erection Procedures .............................8 3.5.1 Friction ..................................................27 2.9.1 Setting Nut and Washer Method .............8 3.5.2 Bearing ..................................................27 2.9.2 Setting Plate Method ..............................9 3.5.3 Shear in Anchor Rods ...........................29 2.9.3 Shim Stack Method ................................9 3.5.4 Interaction of Tension and 2.9.4 Setting Large Base Plates .......................9 Shear in the Concrete ...........................30 2.10 Grouting Requirements .....................................9 3.5.5 Hairpins and Tie Rods ..........................30 2.11 Anchor Rod Repairs ........................................10 4.0 DESIGN EXAMPLES ............................................31 2.11.1 Anchor Rods in the Wrong Position ....10 4.1 Example: Base Plate for Concentric Axial 2.11.2 Anchor Rods Bent or Not Vertical .......10 Compressive Load 2.11.3 Anchor Rod Projection Too Long (No concrete confinement) ..............................31 or Too Short ..........................................10 4.2 Example: Base Plate for Concentrix Axial 2.11.4 Anchor Rod Pattern Rotated 90° ..........12 Compressive Load 2.12 Details for Seismic Design D ..........................12 (Using concrete confinement) .........................32 4.3 Example: Available Tensile Strength of a 3.0 DESIGN OF COLUMN BASE w-in. Anchor Rod ............................................34 PLATE CONNECTIONS .......................................13 4.4 Example: Concerete Embedment Strength .....34 3.1 Concentric Compressive Axial Loads .............14 4.5 Example: Column Anchorage for 3.1.1 Concrete Bearing Limit ........................14 Tensile Loads ...................................................34 3.1.2 Base Plate Yielding Limit 4.6 Example: Small Moment Base Plate Design ..37 (W-Shapes) ...........................................15 4.7 Example: Large Moment Base Plate Design ..38 3.1.3 Base Plate Yielding Limit 4.8 Example: Shear Transfer Using Bearing .........40 (HSS and Pipe) ...................................16 4.9 Example: Shear Lug Design ............................40 3.1.4 General Design Procedure ....................16 4.10 Example: Edge Disttance for Shear ................42 3.2 Tensile Axial Loads .........................................18 4.11 Example: Anchor Rod Resisting Combined 3.2.1 Anchore Rod Tension ...........................19 Tension and Shear ...........................................42 3.2.2 Concrete Anchorage for REFERENCES ...............................................................45 Tensile Forces .......................................19 3.3 Design of Column Base Plates with APPENDIX A .................................................................47 Small Moments ................................................23 3.3.1 Concrete Bearing Stress .......................24 APPENDIX B .................................................................55 3.3.2 Base Plate Flexural Yielding Limit at Bearing Interface ....................24 vii viii 1.0 INTRODUCTION Column base plate connections are the critical interface the equations shown herein are independent of the load ap- between the steel structure and the foundation. These con- proach and thus are applicable to either design methodology. nections are used in buildings to support gravity loads and These are shown in singular format. Other derived equations function as part of lateral-load-resisting systems. In addition, are based on the particular load approach and are presented they are used for mounting of equipment and in outdoor sup- in a side-by-side format of comparable equations for LRFD port structures, where they may be affected by vibration and or ASD application. fatigue due to wind loads. The typical components of a column base are shown in Base plates and anchor rods are often the last structural Figure 1.1. steel items to be designed but are the first items required Material selection and design details of base plates can on the jobsite. The schedule demands along with the prob- significantly affect the cost of fabrication and erection of lems that can occur at the interface of structural steel and steel structures, as well as the performance under load. reinforced concrete make it essential that the design details Relevant aspects of each of these subjects are discussed take into account not only structural requirements, but also briefly in the next section. Not only is it important to design include consideration of constructability issues, especially the column-base-plate connection for strength requirements, anchor rod setting procedures and tolerances. The impor- it is also important to recognize that these connections tance of the accurate placement of anchor rods cannot be affect the behavior of the structure. Assumptions are over-emphasized. This is the one of the key components to made in structural analysis about the boundary conditions safely erecting and accurately plumbing the building. represented by the connections. Models comprising beam or The material in this Guide is intended to provide guidelines truss elements typically idealize the column base connection for engineers and fabricators to design, detail, and specify as either a pinned or fixed boundary condition. Improper column-base-plate and anchor rod connections in a manner characterization can lead to error in the computed drifts, that avoids common fabrication and erection problems. This leading to unrecognized second-order moments if the Guide is based on the 2005 AISC Specification for Structur- stiffness is overestimated, or excessive first-floor column al Steel Buildings (AISC, 2005), and includes guidance for sizes if the stiffness is underestimated. If more accurate designs made in accordance with load and resistance factor analyses are desired, it may be necessary to input the stiffness design (LRFD) or allowable stress design (ASD). of the column-base-plate connection in the elastic and plastic This Guide follows the format of the 2005 AISC Specifi- ranges, and for seismic loading, possibly even the cyclic cation, developing strength parameters for foundation sys- force-deformation relations. The forces and deformations tem design in generic terms that facilitate either load and from the structural analyses used to design the column-base- resistance factor design (LRFD) or allowable strength de- plate connection are dependent on the choice of the column- sign (ASD). Column bases and portions of the anchorage base-plate connection details. 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 Appendix D, which is exclu- sively based on the strength approach (LRFD) for the design of such embedment. Other steel elements of the foundation system, including the column base plate and the sizing of anchor diameters are equally proficient to evaluation using LRFD or ASD load methods. In cases such as anchors sub- jected to neither tension nor shear, the anchorage develop- ment requirement may be a relatively insignificant factor. The generic approach in development of foundation de- sign parameters taken in this Guide permits the user a choice to develop the loads based on either the LRFD or ASD ap- proach. The derivations of foundation design parameters, as presented herein, are then either multiplied by the resistance factor, φ, or divided by a safety factor, Ω, based on the ap- propriate load system utilized in the analysis; consistent with the approach used in the 2005 Specification. Many of Figure 1.1. Column base connection components. DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 1 Table 2.1. Base Plate Materials Thickness (t ) Plate Availability p t ≤ 4 in. ASTM A36 [a] p ASTM A572 Gr 42 or 50 ASTM A588 Gr 42 or 50 4 in. < t ≤ 6 in. ASTM A36 [a] p ASTM A572 Gr 42 ASTM A588 Gr 42 t > 6 in. ASTM A36 p [a] Preferred material specification The vast majority of building columns are designed for the stability required during erection with an ironworker on axial compression only with little or no uplift. For such col- the column. This regulation has essentially eliminated the umns, the simple column-base-plate connection detail shown typical detail with two anchor rods except for small post- in Figure 1.1 is sufficient. The design of column-base-plate type structures that weigh less than 300 lb (e.g., doorway connections for axial compression only is presented in Sec- portal frames). tion 3. The design is simple and need not be encumbered This Guide supersedes the original AISC Design Guide 1, with many of the more complex issues discussed in Appen- Column Base Plates. In addition to the OSHA regulations, dix A, which pertains to special structures. Anchor rods for there has been significant research and improved design gravity columns are often not required for the permanent guidelines issued subsequent to the publication of Design structure and need only be sized to provide for column sta- Guide 1 in 1990. The ACI Building Code Requirements for bility during erection. Structural Concrete (ACI, 2002) has improved provisions Column base plate connections are also capable of trans- for the pullout and breakout strength of anchor rods and mitting uplift forces and can transmit shear through the an- other embedded anchors. Design guidance for anchor rods chor rods if required. If the base plate remains in compres- based on the ACI recommendations is included, along with sion, shear can be transmitted through friction against the practical suggestions for detailing and installing anchor rod grout pad or concrete; thus, the anchor rods are not required assemblies. These guidelines deal principally with cast-in- to be designed for shear. Large shear forces can be resisted place anchors and with their design, installation, inspection, by bearing against concrete, either by embedding the col- and repair in column-base-plate connections. umn base or by adding a shear lug under the base plate. The AISC Design Guide 7, 2nd edition, Industrial Build- Column base plate moment connections can be used to ings: Roofs to Column Anchorage (Fisher, 2004), contains resist wind and seismic loads on the building frame. Moment additional examples and discussion relative to the design of at the column base can be resisted by development of a force anchor rods. couple between bearing on the concrete and tension in some or all of the anchor rods. 2.0 MATERIALS, FABRICATION, This guide will enable the designer to design and specify INSTALLATION, AND REPAIRS economical column base plate details that perform adequate- ly for the specified demand. The objective of the design pro- 2.1 Material Specifications cess in this Guide is that under service loading and under ex- treme loading in excess of the design loads, the behavior of The AISC Specification lists a number of plate and threaded column base plates should be close to that predicted by the rod materials that are structurally suitable for use in base approximate mathematical equations in this Design Guide. plate and anchor rod designs. Based on cost and availability, Historically, two anchor rods have been used in the area the materials shown in Tables 2.1 and 2.2 are recommended bounded by column flanges and web. Recent regulations of for typical building design. the U.S. Occupational Safety and Health Administration (OSHA) Safety Standards for Steel Erection (OSHA, 2001) 2.2 Base Plate Material Selection (Subpart R of 29 CFR Part 1926) require four anchor rods in Base plates should be designed using ASTM A36 material almost all column-base-plate connections and require all col- unless the availability of an alternative grade is confirmed umns to be designed for a specific bending moment to reflect 2 / DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN Table 2.2. Anchor Rod Materials Tensile Nominal Tensile Nominal Shear Nominal Shear Stress Maximum Material Strength, Stress,[a] Stress (X type),[a, b] (N type),[a, c] Diameter, ASTM F (ksi) F = 0.75F (ksi) F = 0.50F (ksi) F = 0.40F (ksi) in. u nt u nv u nv u Gr 36 [d] 58 43.5 29.0 23.2 4 4 5 5 Gr 55 75 56.3 37.5 30.0 4 1 F Gr 105 125 93.8 62.5 50.0 3 120 90.0 60.0 48.0 1 A449 105 78.8 57.5 42.0 1� 90 67.5 45.0 36.0 3 A36 58 43.5 29.0 23.2 4 A307 58 43.5 29.0 23.2 4 A354 150 112 75.0 60.0 2� Gr BD 140 105 70.0 56.0 4 [a] Nominal stress on unthreaded body for cut threads (based on major thread diameter for rolled threads) [b] Threads excluded from shear plane [c] Threads included in the shear plane [d] Preferred material specification prior to specification. Since ASTM A36 plate is readily avail- 2.3 Base Plate Fabrication and Finishing able, the plates can often be cut from stock material. There Typically, base plates are thermally cut to size. Anchor rod is seldom a reason to use high-strength material, since in- and grout holes may be either drilled or thermally cut. Sec- creasing the thickness will provide increased strength where tion M2.2 of the AISC Specification lists requirements for needed. Plates are available in 8-in. increments up to 14 in. thermal cutting as follows: thickness and in 4-in. increments above this. The base plate sizes specified should be standardized during design to fa- “…thermally cut free edges that will be subject to calculated cilitate purchasing and cutting of the material. static tensile stress shall be free of round-bottom gouges When designing base plate connections, it is important to greater than x in. deep … and sharp V-shaped notches. consider that material is generally less expensive than labor Gouges deeper than x in. … and notches shall be removed and, where possible, economy may be gained by using thick- by grinding and repaired by welding.” er plates rather than detailing stiffeners or other reinforce- Because free edges of the base plate are not subject to tensile ment to achieve the same strength with a thinner base plate. stress, these requirements are not mandatory for the perimeter A possible exception to this rule is the case of moment-type edges; however, they provide a workmanship guide that can bases that resist large moments. For example, in the design be used as acceptance criteria. Anchor rod holes, which may of a crane building, the use of a seat or stool at the column be subject to tensile stress, should meet the requirements of base may be more economical, if it eliminates the need for Section M2.2. Generally, round-bottom grooves within the large complete-joint-penetration (CJP) groove welds to limits specified are acceptable, but sharp notches must be heavy plates that require special material specifications. repaired. Anchor rod hole sizes and grouting are covered in Most column base plates are designed as square to match Sections 2.6 and 2.10 of this design guide. the foundation shape and more readily accommodate square Finishing requirements for column bases on steel plates anchor rod patterns. Exceptions to this include moment- are covered in Section M2.8 of the AISC Specification as resisting bases and columns that are adjacent to walls. follows: Many structural engineers have established minimum thicknesses for typical gravity columns. For posts and light “Steel bearing plates 2 in. … or less in thickness are permit- HSS columns, the minimum plate thickness is typically 2 in., ted without milling, provided a satisfactory contact bearing and for other structural columns a plate thickness of w in. is is obtained. Steel bearing plates over 2 in. … but not over 4 commonly accepted as the minimum thickness specified. in. … in thickness are permitted to be straightened by press- DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 3