Design Of Column Base Plates - UB
12/21/2015DESIGN OF COLUMN BASE PLATESAND STEEL ANCHORAGETO CONCRETEOUTLINE1. Introduction2. Base platesMaterialb. Design using AISC Steel Design Guide Concentric axial load Axial load plus moment Axial load plus shear3. Anchor Rodsa. Types and Materialsb. Design using ACI Appendix D Tension Sheara.1
12/21/2015INTRODUCTION Base plates and anchor rods are often the last structuralsteel items to be designed but the first items required onthe jobsite. Therefore the design of column base plate andconnections are part of the critical path. Vast majority of column base plate connections aredesigned for axial compression with little or no uplift.INTRODUCTION(Cont’d) Column base plate connections can also transmit upliftforces and shear forces through: Anchor rods, Friction against the grout pad or concrete, Shear lugs under the base plate or embedding thecolumn base can be used to resist large forces. Column base plate connections can also be used toresist wind and seismic loads: Development of force couple between bearing onconcrete and tension in some or all of the anchorrods.2
12/21/2015INTRODUCTION Anchor rods are needed for all base plates to preventcolumn from overturning during construction and in somecases to resist uplift or large momentsAnchor rods are designed for pullout and breakoutstrength using ACI 318 Appendix DCritical to provide well-defined, adequate load path whentension and shear loading will be transferred throughanchor rodsINTRODUCTION (Cont’d)(Cont’d)Grout is needed to serve as the connection between thesteel base plate and the concrete foundation to transfercompression loads.Grout should have design compressive strength at leasttwice the strength of foundation concrete.When base plates become larger than 24”, it isrecommended that one or two grout holes be provided toallow the grout to flow easier.3
12/21/2015BASE PLATE MATERIALS Base plates should be ASTM A36 material unless othergrade is available. Most base plates are designed as square to match thefoundation shape and can be more accommodating forsquare anchor rod patterns. A thicker base plate is more economical than a thinnerbase plate with additional stiffeners or otherreinforcements.BASE PLATE DESIGN4
12/21/2015DESIGN OF AXIALLY LOADEDBASE PLATES Required plate area is based on uniform allowablebearing stress. For axially loaded base plates, thebearing stress under the base plate is uniformf p max c 0.85 f c A2 1.7 f c A1A2 dimensions of concrete supporting foundationA1 dimensions of base plate Most economical plate occurs when ratio of concrete toplate area is equal to or greater than 4 (Case 1) When the plate dimensions are known it is not possible tocalculate bearing pressure directly and therefore differentprocedure is used (Case 2)DESIGN OF AXIALLY LOADEDBASE PLATES (Cont’d)Case 1: A2 4A11. Determine factored load Pu2. Calculate required plate area A1 based on maximumconcrete bearing stress fp 1.7f c (when A2 4 A2)A1( req ) 3.Pu0.6 1.7 f c Plate dimensions B & N shouldbe determined so m & n areapproximately equal:N A1( req ) B 0.95d 0.8b f2A1( req)N5
12/21/2015DESIGN OF AXIALLY LOADEDBASE PLATES (Cont’d)Case 1: A2 4A14. Calculate required base plate thickness:m N 0.95d2t min l2 Pu0.90 Fy BNn B 0.8b f2where l is maximum of m and n5. Determine pedestal area,A2:A2 4BNDESIGN OF AXIALLY LOADEDBASE PLATES (Cont’d)Case 2: Pedestal dimensions known1. Determine factored load Pu2. The area of the plate should be equal to larger of:1A1 A23.4.2Pu PuA or1 0.6 1.7 f c 0.60 0.85 f c Same as Case 1Same as Case 16
12/21/2015DESIGN OF BASE PLATES WITHMOMENTS Equivalent eccentricity, e, is calculated equal to moment Mdivided by axial force P. Moment and axial force replaced by equivalent axial force at adistance e from center of column. Small eccentricities equivalent axial force resisted bybearing only. Large eccentricities necessary to use an anchor bolt toresist equivalent axial force.DESIGN OF BASE PLATES WITHMOMENTS (Cont’d)If e N/6 compressive bearing stress exist everywheref1, 2 P Mc BNIIf e is between N/6 and N/2 bearing occurs only over aportion of the platef1 2PAB7
12/21/2015DESIGN OF BASE PLATES WITHMOMENTS (Cont’d)1. Calculate factored load (Pu) and moment (Mu)2. Determine maximum bearing pressure, fpf p c 0.85 f c A2 1.7 f c A13. Pick a trial base plate size, B and N4. Determineequivalent eccentricity, e, and maximumbearing stress from load, f1. If f1 fp go to next step, ifnot pick different base plate size.5. Determine plate thickness, tp:tp 4 M plu0.90 Fy Mplu is moment for 1 in wide stripDESIGN OF BASE PLATE WITHSHEARFour principal ways of transferring shear from column baseplate into concrete:1. Friction between base plate and the grout or concretesurface: Vn mPu 0.2 f c AcThe friction coefficient (m) is 0.55 for steel on grout and0.7 for steel on concrete2. Embedding column in foundation.3. Use of shear lugs.4. Shear in the anchor rods.8
12/21/2015DESIGN OF SHEAR LUGS1. Determine the portion of shear which will be resisted byshear lug, Vlgu.2. Determine required bearing area of shear lug:Alg Vlg u0.85 f c 3. Determine shear lug width, W, and height, H.4. Determine factored cantilevered end moment, Mlgu. Vlg u H G M lg u W 2 5. Determine shear lug thickness:tlg 4 M lg u0.90 FyANCHOR RODSTwo categories:a) Post-installed anchors: set after the concrete ishardened.b) Cast-in-place anchors: set before the concrete is placed.9
12/21/2015ANCHOR RODS(Cont’d)Materials: Preferred specification is ASTM F1554:- Grade 36, 55, 105 ksi. ASTM F1554 allows anchor rods to be supplied straight(threaded with nut for anchorage) , bent or headed. Wherever possible use ¾-in diameter ASTM F1554Grade 36:- When more strength required, increase rod diameter to2 in before switching to higher grade. Minimum embedment is 12 times diameter of bolt.CAST-IN-PLACE ANCHOR RODS When rods with threads and nut are used, a morepositive anchorage is formed: Failure mechanism is the pull out of a cone ofconcrete radiating outward from the head of the boltor nut. Use of plate washer does not add any increasedresistance to pull out. Hooked bars have a very limitedpullout strength compared withthat of headed rods or threadedrods with a nut of anchorage.10
12/21/2015ANCHOR ROD PLACEMENT Most common field problem is placement of anchor rods. Important to provide as large as hole as possible toaccommodate setting tolerances. Fewer problems if the structural steel detailer coordinatesall anchor rod details with column base plate assembly.ANCHOR ROD LAYOUT Should use a symmetrical pattern in both directionswherever possible. Should provide sufficient clearance distance for thewasher from the column. Edge distance plays important role for concrete breakoutstrength. Should be coordinated with reinforcing steel to ensurethere are no interferences, more critical in concrete piersand walls.11
12/21/2015DESIGN OF ANCHOR RODS FORTENSION Whenbase plates are subject to uplift force Tu,embedment of anchor rods must be checked for tension. Steel strength of anchor in tension:N s Ase f utAse effective cross sectional area of anchor, AISC Steel ManualTable 7-18fut tensile strength of anchor, not greater than 1.9fy or 125 ksi Concrete breakout strength of single anchor in tension:AN cb N 2 3 NbN b k f c hef1.5A Nohef embedmentk 24 for cast-in place anchors, 17 for post-installed anchors 2, 3 modification factorsDESIGN OF ANCHOR RODS FORTENSION (Cont’d) Ano projected area of the failuresurface of a single anchor remotefrom edges AN approximated as the base ofthe rectilinear geometrical figurethat results from projecting thefailure surface outward 1.5hef fromthe centerlines of the anchor. Example of calculation of AN withedge distance (c1) less than 1.5hefANo 9hef2AN (c1 1.5hef )( 2 1.5hef )12
12/21/2015DESIGN OF ANCHOR RODS FORTENSION (Cont’d) Pullout strength of anchor:N pn 4 Abrg 8 f c Nominal strength in tension Nn min(Ns, Ncb, Npn) Compare uplift from column, Tu to Nn. If Tu less than Nn ok! Nn, must provide tension reinforcingaround anchor rods or increase embedment of anchorrods. If Tu greater thanDESIGN OF ANCHOR RODS FORSHEAR (Cont’d) When base plates are subject to shear force, Vu, andfriction between base plate and concrete is inadequate toresist shear, anchor rods may take shear. Steel Strength of single anchor in shear:Vs Ase f ut Concrete breakout strength of single anchor in shear:AVcb v 6 7VbAvo l Vb 7 do 0.2dof c c11.5 6, 7 modification factorsdol rod diameter, in load bearing length of anchor for shear not to exceed 8do, in13
12/21/2015DESIGN OF ANCHOR RODS FORSHEAR (Cont’d) Avo projected area of the failuresurface of a single anchor remotefrom edges in the directionperpendicular to the shear force Av approximated as the base ofatruncatedhalfpyramidprojected on the side face of themember. Example of calculation of Av withedge distance (c2) less than1.5c1Avo 4.5 c1 2Av 1.5c1 (1.5c1 c2 )DESIGN OF ANCHOR RODS FORSHEAR (Cont’d) Pryout strength of anchor:Vcp kcp N cb Nominal strength in shear Vn min(Vs, Vcb, Vcp) Compare shear from column, Vu to Vn. If Vu less than Vn ok! Vn must provide shear reinforcingaround anchor rods or use shear lugs. If Vu greater than14
12/21/2015COMBINED TENSION AND SHEARAccording to ACI 318 Appendix D, anchor rods must bechecked for interaction of tensile and shear forces:TuV u 1.2 N n VnREFERENCES American Concrete Institute (ACI) 318-02.AISC Steel Design Guide, Column Base Plates, by JohnT. DeWolf, 1990.AISC Steel Design Guide (2nd Edition) Base Plate andAnchor Rod Design.AISC Engineering Journal Anchorage of Steel BuildingComponents to Concrete, by M. Lee Marsh and Edwin G.Burdette, First Quarter 1985.15
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12/21/2015 4 BASE PLATE MATERIALS Base plates should be ASTM A36 material unless other grade is available. Most base plates are designed as square to match the foundation shape and can be more accommodating for square anchor rod patterns. A thicker base plate is more economical than a thinner base plate with additional stiffeners or other
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PRACTICAL DESIGN AND DETAILING OF STEEL COLUMN BASE PLATES 1.0 INTRODUCTION 1.1 Preface Steel column base plates are one of the most ndamental parts of a steel structure, yet the design of base plates is commonly not given the attention that it should by engineers. This results i
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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
Aug 11, 2006 · mechanisms of column base plate details. This investigation is the first phase of a broader study whose aim is to develop improved design guidelines for column base plates. A review of existing design guidelines and published research reveals that shear transfer mechanisms in exposed base pl
Design of Column Base Plates Typical column bases, as shown below, consist of a single plate fillet welded to the end of the column and attached to the foundation with four holding down bolts. The bolts are cas
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