Base Plate And Anchor Rod Design - Texas A&M University
1Steel Design GuideBase Plate andAnchor Rod DesignSecond EditionJAMES M. FISHER, Ph.D., P.E.Computerized Structural Design, S.C.Milwaukee, WisconsinandLAWRENCE A. KLOIBER, P.E.LeJuene Steel CompanyMinneapolis, MinnesotaAMERICAN INSTITUTE O F S T E E L C O N S T RU C T I O N , I N C.
1.0 INTRODUCTIONColumn base plate connections are the critical interfacebetween the steel structure and the foundation. These connections are used in buildings to support gravity loads andfunction as part of lateral-load-resisting systems. In addition,they are used for mounting of equipment and in outdoor support structures, where they may be affected by vibration andfatigue due to wind loads.Base plates and anchor rods are often the last structuralsteel items to be designed but are the first items requiredon the jobsite. The schedule demands along with the problems that can occur at the interface of structural steel andreinforced concrete make it essential that the design detailstake into account not only structural requirements, but alsoinclude consideration of constructability issues, especiallyanchor rod setting procedures and tolerances. The importance of the accurate placement of anchor rods cannot beover-emphasized. This is the one of the key components tosafely erecting and accurately plumbing the building.The material in this Guide is intended to provide guidelinesfor engineers and fabricators to design, detail, and specifycolumn-base-plate and anchor rod connections in a mannerthat avoids common fabrication and erection problems. ThisGuide is based on the 2005 AISC Specification for Structural Steel Buildings (AISC, 2005), and includes guidance fordesigns made in accordance with load and resistance factordesign (LRFD) or allowable stress design (ASD).This Guide follows the format of the 2005 AISC Specification, developing strength parameters for foundation system design in generic terms that facilitate either load andresistance factor design (LRFD) or allowable strength design (ASD). Column bases and portions of the anchoragedesign generally can be designed in a direct approach basedon either LRFD or ASD load combinations. The one areaof anchorage design that is not easily designed by ASD isthe embedment of anchor rods into concrete. This is due tothe common use of ACI 318 Appendix D, which is exclusively based on the strength approach (LRFD) for the designof such embedment. Other steel elements of the foundationsystem, including the column base plate and the sizing ofanchor diameters are equally proficient to evaluation usingLRFD or ASD load methods. 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 choiceto develop the loads based on either the LRFD or ASD approach. The derivations of foundation design parameters, aspresented herein, are then either multiplied by the resistancefactor, φ, or divided by a safety factor, Ω, based on the appropriate load system utilized in the analysis; consistentwith the approach used in the 2005 Specification. Many ofthe 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 equationsare based on the particular load approach and are presentedin a side-by-side format of comparable equations for LRFDor ASD application.The typical components of a column base are shown inFigure 1.1.Material selection and design details of base plates cansignificantly affect the cost of fabrication and erection ofsteel structures, as well as the performance under load.Relevant aspects of each of these subjects are discussedbriefly in the next section. Not only is it important to designthe column-base-plate connection for strength requirements,it is also important to recognize that these connectionsaffect the behavior of the structure. Assumptions aremade in structural analysis about the boundary conditionsrepresented by the connections. Models comprising beam ortruss elements typically idealize the column base connectionas either a pinned or fixed boundary condition. Impropercharacterization can lead to error in the computed drifts,leading to unrecognized second-order moments if thestiffness is overestimated, or excessive first-floor columnsizes if the stiffness is underestimated. If more accurateanalyses are desired, it may be necessary to input the stiffnessof the column-base-plate connection in the elastic and plasticranges, and for seismic loading, possibly even the cyclicforce-deformation relations. The forces and deformationsfrom the structural analyses used to design the column-baseplate connection are dependent on the choice of the columnbase-plate connection details.Figure 1.1. Column base connection components.DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 1
mentary to the AISC Seismic Provisions notes some significant differences:1. Long anchor rods embedded in concrete will strain muchmore than high-strength bolts or welds in beam-to-columnconnections.2. Column base plates are bearing on grout and concrete,which is more compressible than the column flanges ofthe beam-to-column connections.3. Column base connections have significantly more longitudinal load in the plane of the flanges and less transverseload when compared to beam-to-column connections.4. The shear mechanism between the column base and thegrout or concrete is different from the shear mechanismbetween the beam end plate and the column flange.5. AISC standard hole diameters for column base anchorrods are different than AISC standard holes for highstrength bolts.6. Foundation rocking and rotation may be an issue, especially on isolated column footings.As the Commentary to the AISC Seismic Provisions suggests, research is lacking regarding the performance and design of base details for high seismic loading. However, theCommentary also acknowledges that these details are veryimportant to the overall performance of the SLRS. Therefore, careful consideration must be given to the design ofthese details.Figure 2.6. Typical moment base detail.3.0 DESIGN OF COLUMN BASE PLATECONNECTIONSThis section of the Design Guide provides the design requirements for typical column base plate connections inbuildings, such as the one shown in Figure 1.1.Five different design load cases in column base plate connections are discussed: Section 3.1 Concentric Compressive Axial Loads Section 3.2 Tensile Axial Loads Section 3.3 Base Plates with Small Moments Section 3.4 Base Plates Large Moments Section 3.5 Design for ShearIn column base connections, the design for shear and thedesign for moment are often performed independently. Thisassumes there is no significant interaction between them.Several design examples are provided in the following sections for each loading case.The general behavior and distribution of forces for a column base plate connection with anchor rods will be elasticuntil either a plastic hinge forms in the column, a plasticmechanism forms in the base plate, the concrete in bearingcrushes, the anchor rods yield in tension, or the concretepullout strength of the anchor rod group is reached. If theconcrete pullout strength of the anchor rod group is largerthan the lowest of the other aforementioned limit states, thebehavior generally will be ductile. However, it is not alwaysnecessary or even possible to design a foundation that prevents concrete failure.Figure 2.7. Embedded moment base detail.DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 13
For example, in statically loaded structures, if the strengthis much larger than the demand, the ductility is not necessaryand it is acceptable to design with the limit state of tensile orshear strength of the anchor rod group governing the design.However, frames designed for seismic lateral load resistanceare expected to behave in a ductile manner and, in this case,it may be necessary to design the foundation and the column-base-plate connection so that the concrete limit statesof tensile or shear strength of the anchor rod group do notgovern the design. See ACI Appendix D, Section D3.3.4.OSHA RequirementsThe regulations of the Occupational Safety and Health Administration (OSHA) Safety Standards for Steel Erection(OSHA, 2001) require a minimum of four anchor rods incolumn-base-plate connections. The requirements excludepost-type columns that weigh less than 300 lb. Columns,base plates, and their foundations must have sufficient moment strength to resist a minimum eccentric gravity loadof 300 lb located 18 in. from the extreme outer face of thecolumn in each direction.The OSHA criteria can be met with even the smallest ofanchor rods on a 4-in. 4-in. pattern. If one considers onlythe moments from the eccentric loads (since including thegravity loads results in no tensile force in the anchor rods),and the resisting force couple is taken as the design forceof the two bolts times a 4-in. lever arm, the design momentstrength for w-in. anchor rods equals (2)(19.1 kips)(4 in.) 306 kip-in. For a 14-in.-deep column, the OSHA requiredmoment strength is only (1.6)(0.300)(18 7) 12.0 kip-in.3.1. Concentric Compressive Axial LoadsWhen a column base resists only compressive column axialloads, the base plate must be large enough to resist the bearing forces transferred from the base plate (concrete bearinglimit), and the base plate must be of sufficient thickness(base plate yielding limit).Equation J8-2: A Pp (0.85 f c′A1 ) 2 1.7 f c′A1 A1 These equations are multiplied by the resistance factor, φ, forLRFD or divided by the safety factor, Ω, for ASD. SectionJ8 stipulates the φ and Ω factors (in the absence of CodeRegulations) for bearing on concrete as follows:φ 0.60 (LRFD)Alternatively, ACI 318-02 stipulates a φ factor of 0.65 forbearing on concrete. This apparent conflict exists due to anoversight in the AISC Specification development process.The authors recommend the use of the ACI-specified φ factor in designing column base plates.The nominal bearing strength can be converted to a stressformat by dividing out the area term Pp equations such that,On the full area of a concrete support:fp(max) 0.85 fc′When the concrete base is larger than the loaded area onall four sides: A f p(max) (0.85 f c′) 2 1.7 f c′ A1 The conversion of the generic nominal pressure to anLRFD or ASD available bearing stress isfpu(max) φ fp(max) (LRFD)f pa(max) The 2005 AISC Specification, Section J8, provides thenominal bearing strength, Pp, as follows:Equation J8-1:Pp 0.85fc′A1 on the full area of a concrete support.f p(max)Ω(ASD)The concrete bearing strength is a function of the concretecompressive strength, and the ratio of geometrically similarconcrete area to base plate area, as indicated in Section 10.17of ACI 318 (ACI, 2002), as follows:(3.1.1 Concrete Bearing LimitThe design bearing strength on concrete is defined inACI 318-02, Section 10.17, as φ(0.85fc′A1) when the supporting surface is not larger than the base plate. When thesupporting surface is wider on all sides than the loaded area,the design bearing strength above is permitted to be multiplied by A2 A1 2.Ω 2.50 (ASD)f p(max) φ 0.85 fc′)A2A1A2 2A1wherefp(max) maximum concrete bearing stress, ksiφ strength reduction factor for bearing, 0.65 perSection 9.3, ACI 318-02fc′ specified compressive strength of concrete, ksi14 / DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN
A1 area of the base plate, in.2A2 maximum area of the portion of the supportingsurface that is geometrically similar to and concentric with the loaded area, in.2The increase of the concrete bearing capacity associatedwith the term A2 A1 accounts for the beneficial effects ofthe concrete confinement. Note that A2 is the largest areathat is geometrically similar to (having the same aspect ratioas) the base plate and can be inscribed on the horizontal topsurface of the concrete footing, pier, or beam without goingbeyond the edges of the concrete.There is a limit to the beneficial effects of confinement,which is reflected by the limit on A2 (to a maximum of fourtimes A1) or by the inequality limit. Thus, for a column baseplate bearing on a footing far from edges or openings, A2 A1 2. 2.The bearing stress on the concrete must not be greaterthan fp(max):Many column base plates bear directly on a layer of grout.Because, the grout compressive strength is always specifiedhigher than the concrete strength—the authors recommendthat the grout strength be specified as two times the concretestrength—it is conservative to use the concrete compressivestrength for fc′ in the above equations.The important dimensions of the column-base plate connection are shown in Figure 3.1.1.3.1.2 Base Plate Yielding Limit (W-Shapes)For axially loaded base plates, the bearing stress under thebase plate is assumed uniformly distributed and can be expressed asf pu Pu(LRFD)BNf pa Pa(ASD)BNThis bearing pressure causes bending in the base plate atthe assumed critical sections shown in Figure 3.1.1(b). ThisPu f pu(max) (LRFD)A1Pa f pa(max) (ASD)A1Thus,PuA1( req ) f pu(max)A1( req ) (LRFD)Pa(ASD)f pa(max)When A2 A1, the required minimum base plate area canbe determined asA1( req ) Pu(LRFD)φ0.85 f c′A1( req ) ΩPa(ASD)0.85 f c′When A2 4A1, the required minimum base plate area canbe determined as1 Pu (LRFD)A1( req ) 2 φ0.85 f c′ 1 ΩPa (ASD)A1( req ) 2 0.85 f c′ Figure 3.1.1. Design of base plate with axial compressive load.DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 15
bearing pressure also causes bending in the base plate in thearea between the column flanges (Thornton, 1990; Drakeand Elkin, 1999). The following procedure allows a singleprocedure to determine the base plate thickness for both situations.The required strength of the base plate can be determinedas l 2 M pl f pu (LRFD) 2 l 2 M pl f pa (ASD) 2 It is conservative to take λ as 1.0.For the yielding limit state, the required minimum thickness of the base plate can be calculated as follows (Thornton,1990) (AISC, 2005):tmin ltmin l2 Pu(LRFD)φFy BN2ΩPa(ASD)Fy BNwhereφ resistance factor for flexure, 0.90Where the critical base plate cantilever dimension, l, is thelarger of m, n, and λn′,m n Fy specified minimum yield stress of base plate, ksiN 0.95d2Since l is the maximum value of m, n, and λn′, the thinnest base plate can be found by minimizing m, n, and λ. Thisis usually accomplished by proportioning the base plate dimensions so that m and n are approximately equal.B 0.8b fλn ′ λ2db f3.1.3 Base Plate Yielding Limit (HSS and Pipe)4N base plate length, in.B base plate width, in.bf column flange width, in.d overall column depth, in.n′ yield-line theory cantilever distance from column web or column flange, in.2 Xλ 11 1 X 4db P uf(LRFD)X (d b f ) 2 φc Pp 4db f Ω Pc a(ASD)X (d b f ) 2 Pp wherePu the required axial compressive load (LRFD), kipsPa the required axial compressive load (ASD), kipsPp 0.85 f c′A1Ω factor of safety for ASD, 1.67A2A1For HSS columns, adjustments for m and n must be made(DeWolf and Ricker, 1990). For rectangular HSS, both mand n are calculated using yield lines at 0.95 times the depthand width of the HSS. For round HSS and Pipe, both m andn are calculated using yield lines at 0.8 times the diameter.The λ term is not used for HSS and Pipe.3.1.4 General Design ProcedureThree general cases exist for the design of base plates subject to axial compressive loads only:Case I:Case II:Case III:A2 A1A2 4A1A1 A2 4A1The most direct approach is to conservatively set A2 equalto A1 (Case I); however, this generally results in the largestbase plate plan dimensions. The smallest base plate plan dimensions occur when the ratio of the concrete to base platearea is larger than or equal to 4, i.e., A2 4A1 (Case II). Baseplates resting on piers often meet the case that A2 is largerthan A1 but less than 4A1, which leads to Case III.When a base plate bears on a concrete pedestal larger thanthe base plate dimension, the required minimum base platearea cannot be directly determined. This is because both A1and A2 are unknown.As mentioned before, the most economical base platesusually occur when m and n, shown in Figure 3.1.1(b), are16 / DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN
equal. This situation occurs when the difference between Band N is equal to the difference between 0.95d and 0.8bf.In selecting the base plate size from a strength viewpoint,the designer must consider the location of the anchor rodswithin the plate and the clearances required to tighten thebolts on the anchor rods.Steps for obtaining base plates sizes for these cases aresuggested below. Anchor rod design is covered in Section3.2.Case I: A2 A1N base plate length, in.B base plate width, in.bf column flange width, in.d overall column depth, in.n′ yield-line theory cantilever distance from column web or column flange, in.λ The largest base plate is obtained when A2 A1.A1( req ) ΩPa(ASD)0.85 f c′whereφPP φ0.85 f c′ A1 (LRFD)0.85 f c′ A1PP(ASD) ΩΩ3. Optimize the base plate dimensions, N and B.N A1( req ) where 1 4db f ΩPa(ASD)X (d b f ) 2 Pp 2. Calculate the required base plate area.Pu(LRFD)φ0.85 f c′1 1 X 4db f Pu(LRFD)X (d b f ) 2 φPp 1. Calculate the required axial compressive strength, Pu(LRFD) or Pa (ASD).A1( req ) 2 XFind l max (m, n, λn′)0.95d 0.8b ftmin l2thenB tmin lA1( req )NNote that the base plate holes are not deducted from thebase plate area when determining the required base platearea. As mentioned earlier in the Guide, from a practicalview point set N equal to B.4. Calculate the required base plate thickness.N 0.95dm 2n B 0.8b fλn ′ λ22 Pu(LRFD)φFy BN2 Pa Ω(ASD)Fy BN5. Determine the anchor rod size and the location of the anchor rods. Anchor rods for gravity columns are generallynot required for the permanent structure and need only tobe sized for OSHA requirements and practical considerations.Case II: A2 4A1The smallest base plate is obtained when A2 4A1 for thiscase.1. Calculate the factored axial compressive load, Pu (LRFD)or Pa (ASD).db f2. Calculate the required base plate area.4A1( req ) Pu(LRFD)2φ0.85 f c′DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 17
A1( req ) ΩPa(ASD)2 (0.85 f c′)3. Optimize the base plate dimensions, N and B.Use the same procedure as in Step 3 from Case I.4. Check if sufficient area, A2 exists for Case II applicability(A2 4A1).Based on the pier or footing size, it will often be obviousif the condition is satisfied. If it is not obvious, calculateA2 geometrically similar to A1. With new dimensions N2and B2, A2 then equals (N2)(B2). If A2 4A1, calculate therequired thickness using the procedure shown in Step 4 ofCase I, except thatφPp φf c′2 A1 (LRFD)PpΩ f c′2 A1(ASD)Ω5. Determine the anchor rod size and location.Case III: A1 A2 4A11. Calculate the factored axial compressive load, Pu (LRFD)or Pa (ASD).2. Calculate the approximate base plate area based on theassumption of Case III.A1( req ) Pu(LRFD)2φ0.85 f c′A1( req ) ΩPa(ASD)2 (0.85 f c′)3. Optimize the base plate dimensions, N and B.Use the same procedure as in Step 3 from Case I.4. Calculate A2, geometrically similar to A1.5. Determine whetherPu φPp φ0.85 f c′A1Pa A2(LRFD)A1 0.85 f c′A1 A2 (ASD) A1Ω ΩPpIf the condition is not satisfied, revise N and B, and retryuntil criterion is satisfied.6. Determine the base plate thickness using Step 4, as shownin Case I.7. Determine the anchor rod size, and their locations.3.2 Tensile Axial LoadsThe design of anchor rods for tension consists of four
Five different design load cases in column base plate con-nections are discussed: Section 3.1 Concentric Compressive Axial Loads Section 3.2 Tensile Axial Loads Section 3.3 Base Plates with Small Moments Section 3.4 Base Plates Large Moments Section 3.5 Design for Shear In column base connections, the design for shear and the
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