DESIGN OF COLUMN BASE PLATES AND STEEL ANCHORAGE TO CONCRETE

  DESIGN OF COLUMN BASE PLATES AND STEEL ANCHORAGE TO CONCRETE OUTLINE 1.

  Introduction

2. Base plates a. Material b. Design using AISC Steel Design Guide Concentric axial load

   Axial load plus moment

   Axial load plus shear

   3.

  Anchor Rods a. Types and Materials b. Design using ACI Appendix D Tension

   Shear  INTRODUCTION 

  Base plates and anchor rods are often the last structural steel items to be designed but the first items required on the jobsite.

  

  Therefore the design of column base plate and connections are part of the critical path.

  

  Vast majority of column base plate connections are designed for axial compression with little or no uplift.

  INTRODUCTION (Cont’d)

  

  Column base plate connections can also transmit uplift forces and shear forces through: Anchor rods,

   Friction against the grout pad or concrete,

   Shear lugs under the base plate or embedding the

   column base can be used to resist large forces.

  

  Column base plate connections can also be used to resist wind and seismic loads: Development of force couple between bearing on

   concrete and tension in some or all of the anchor rods. INTRODUCTION (Cont’d)

   column from overturning during construction and in some cases to resist uplift or large moments Anchor rods are designed for pullout and breakout

  Anchor rods are needed for all base plates to prevent

   strength using ACI 318 Appendix D Critical to provide well-defined, adequate load path when

   tension and shear loading will be transferred through anchor rods

  INTRODUCTION (Cont’d)

   Grout is needed to serve as the connection between the steel base plate and the concrete foundation to transfer compression loads. Grout should have design compressive strength at least

   twice the strength of foundation concrete.

   When base plates become larger than 24 ”, it is recommended that one or two grout holes be provided to allow the grout to flow easier.

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 reinforcements.

BASE PLATE DESIGN

  DESIGN OF AXIALLY LOADED BASE PLATES 

  Required plate area is based on uniform allowable bearing stress. For axially loaded base plates, the bearing stress under the base plate is uniform _{`</sub> A <sub>2 `} f    . _{p max c c c} 85 f  1 . 7 f A ₁

  = dimensions of concrete supporting foundation A ₂ A = dimensions of base plate ₁

  

  Most economical plate occurs when ratio of concrete to plate area is equal to or greater than 4 (Case 1)

  

  When the plate dimensions are known it is not possible to calculate bearing pressure directly and therefore different procedure is used (Case 2)

  DESIGN OF AXIALLY LOADED BASE PLATES

  (Cont’d)

  A A Case 1: &gt; 4

  2

  1 1.

  Determine factored load P _u

2. Calculate required plate area A based on maximum

  1

  =1.7f` concrete bearing stress f (when A = 4 A ) <sub>p c 2 2</sub> P <sub>u</sub> A <sub>1 ( req )</sub>  <sub>`</sub> . 6  1 . 7 f _c 3.

  Plate dimensions B &amp; N should be determined so m &amp; n are approximately equal: .

  95 d  . 8 b _f N  A   _{1 req ( )}  

  2 A _{1 ( req )} B  N

  4. Calculate required base plate thickness:

  2 _{` 2 1} 85 .

  DESIGN OF AXIALLY LOADED BASE PLATES

  Same as Case 1

  3. Same as Case 1 4.

  The area of the plate should be equal to larger of: or

  1. Determine factored load P u 2.

   

  f

P

A

  1 6 . _c ^u

  f P A A _{` 1} 7 .

    _c ^u

    

  1   

  60 .

  1 Case 2: Pedestal dimensions known

  where l is maximum of m and n

  2 &gt; 4 A

  Case 1: A

  (Cont’d)

  DESIGN OF AXIALLY LOADED BASE PLATES

  4 ₂ 

  2 _min  BN A

    BN F P l t _y ^u 90 .

  2 8 . _f B b n

  N d m  

  2 95 .

  :

  2

  5. Determine pedestal area, A

  (Cont’d) DESIGN OF BASE PLATES WITH MOMENTS 

  Equivalent eccentricity, e, is calculated equal to moment M divided by axial force P.

   Moment and axial force replaced by equivalent axial force at a distance e from center of column.

   Small eccentricities  equivalent axial force resisted by bearing only.

   Large eccentricities necessary to use an anchor bolt to resist equivalent axial force.

  DESIGN OF BASE PLATES WITH MOMENTS

  (Cont’d) If e &lt; N/6 compressive bearing stress exist everywhere

  P Mc f   _{1 , 2} BN

  I If e is between N/6 and N/2 bearing occurs only over a

  portion of the plate

  2 P

  f  ₁ AB

  DESIGN OF BASE PLATES WITH MOMENTS

  (Cont’d) 1. Calculate factored load (P ) and moment (M )

  u u 2.

  Determine maximum bearing pressure, f

  p _{` `} A ₂ f    . _{p c c c} 85 f  1 . 7 f A ₁ 3.

  Pick a trial base plate size, B and N

  4. Determine equivalent eccentricity, e, and maximum

  bearing stress from load, f . If f &lt; f go to next step, if

  1

  1 p

  5. Determine plate thickness, t : p

  4 M _plu 

• t _p

  M is moment for 1 in wide strip plu .

  90 F _y DESIGN OF BASE PLATE WITH SHEAR

  Four principal ways of transferring shear from column base plate into concrete:

  1. Friction between base plate and the grout or concrete

  surface: _`

   V  m P  . _{n u c c} 2 f A m) is 0.55 for steel on grout and

  The friction coefficient ( 0.7 for steel on concrete 2. Embedding column in foundation.

  3. Use of shear lugs.

  4. Shear in the anchor rods.

DESIGN OF SHEAR LUGS 1.

  Determine the portion of shear which will be resisted by shear lug, V _lgu .

  2. Determine required bearing area of shear lug: 3.

  Post-installed anchors: set after the concrete is hardened.

  a)

  Two categories:

  4 _{lg lg} 

  F M t 90 .

  V M ^u _u y ^u

  G H W

     ^lg _lg

     

     

     

  V A 

  5. Determine shear lug thickness: ^{` lg lg} 85 . _c ^u f

  4. Determine factored cantilevered end moment, M _lgu .

  Determine shear lug width, W, and height, H.

ANCHOR RODS

b) Cast-in-place anchors: set before the concrete is placed.

ANCHOR 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 F1554 Grade 36: 2 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 more positive anchorage is formed:

   Failure mechanism is the pull out of a cone of concrete radiating outward from the head of the bolt or nut.

   Use of plate washer does not add any increased resistance to pull out.

  

  Hooked bars have a very limited pullout strength compared with that of headed rods or threaded rods with a nut of anchorage.

ANCHOR ROD PLACEMENT

   Most common field problem is placement of anchor rods. 

  Important to provide as large as hole as possible to accommodate setting tolerances.

  

  Fewer problems if the structural steel detailer coordinates all anchor rod details with column base plate assembly.

ANCHOR ROD LAYOUT

  

  Should use a symmetrical pattern in both directions wherever possible.

  

  Should provide sufficient clearance distance for the washer from the column.

  

  Edge distance plays important role for concrete breakout strength.

  

  Should be coordinated with reinforcing steel to ensure there are no interferences, more critical in concrete piers and walls. DESIGN OF ANCHOR RODS FOR TENSION 

  When base plates are subject to uplift force T ,

  u embedment of anchor rods must be checked for tension.

  

  Steel strength of anchor in tension:

  N  A f _{s se ut} A _se = effective cross sectional area of anchor, AISC Steel Manual

  Table 7-18 f _ut = tensile strength of anchor, not greater than 1.9f or 125 ksi _y

   Concrete breakout strength of single anchor in tension: _N

_`

_{1 . 5} N   N _cb  N  k f h _{2 3 b b c ef} A _No h _ef = embedment k = 24 for cast-in place anchors, 17 for post-installed anchors  ,  = modification factors _{2 3} DESIGN OF ANCHOR RODS FOR TENSION

  ( Cont’d)

   A

  = projected area of the failure

  no

  surface of a single anchor remote from edges

   A

  = approximated as the base of

  N ₂

  the rectilinear geometrical figure

  A  _{No ef} 9 h

  that results from projecting the failure surface outward 1.5h from

  ef the centerlines of the anchor.

  

  Example of calculation of A with

  N

  edge distance (c ) less than 1.5h

  1 ef A  ( c 

_N

_{1 ef ef} 1 . 5 h )( 2  1 . 5 h )

  

  = rod diameter, in l = load bearing length of anchor for shear not to exceed 8d _o

  ( Cont’d)

  

  When base plates are subject to shear force, V

  u

  , and friction between base plate and concrete is inadequate to resist shear, anchor rods may take shear.

  

  Steel Strength of single anchor in shear:

  

  Concrete breakout strength of single anchor in shear:

   ₆ ,  ₇ = modification factors d _o

  , in b vo v cb

  8 c brg pn

  V A A

  V

  7

  6    ^{5 . 1} ₁ ^{` 2 .}

  7 c f d d

l

  V _{c o}

_o

_b    

     ut se s

  A f

  V 

  DESIGN OF ANCHOR RODS FOR SHEAR

  A f N   DESIGN OF ANCHOR RODS FOR TENSION

  4

  Pullout strength of anchor:

  

  

  Nominal strength in tension N

  n = min(N s

  , N cb

  , N pn

  ) 

  Compare uplift from column, T

  u

  to N

  

n

.

  If T

  `

  u

  less than N

  n

  ok!

  

  If T

  u

  greater than N

  n

  , must provide tension reinforcing around anchor rods or increase embedment of anchor rods.

  ( Cont’d) DESIGN OF ANCHOR RODS FOR SHEAR

  ( Cont’d)

   A

  = projected area of the failure

  vo

  surface of a single anchor remote from edges in the direction perpendicular to the shear force

   A

  = approximated as the base of

  v ₂ A  _{vo  } 4 . 5 c ₁ a truncated half pyramid

  projected on the side face of the member.

  

  Example of calculation of A with

  v

  edge distance (c ) less than

  2 A  _v 1 . 5 c ( ₁ 1 . 5 c  c ) _{1 2}

  1.5c

  1 DESIGN OF ANCHOR RODS FOR SHEAR

  ( Cont’d)

  

  Pryout strength of anchor:

  V  k N _{cp cp cb} 

  Nominal strength in shear V = min(V , V , V )

  

n s cb cp

 Compare shear from column, V to V . u n

   V

  If V less than ok!

  u n  V

  If V greater than must provide shear reinforcing

  u n around anchor rods or use shear lugs.

COMBINED TENSION AND SHEAR

  According to ACI 318 Appendix D, anchor rods must be checked for interaction of tensile and shear forces:

  T _{u u}

  V

    1 .

  2

   N  _{n n}

  V REFERENCES  American Concrete Institute (ACI) 318-02. 

  AISC Steel Design Guide, Column Base Plates, by John T. DeWolf, 1990.

  nd 

  AISC Steel Design Guide (2 Edition) Base Plate and Anchor Rod Design.

  

  AISC Engineering Journal Anchorage of Steel Building Components to Concrete, by M. Lee Marsh and Edwin G. Burdette, First Quarter 1985.

  Terima Kasih dan Semoga Lancar Studinya!