Design of Column Base Plates

8/3/2019 Design of Column Base Plates

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DESIGN OF COLUMNBASE PLATES AND

STEEL ANCHORAGE TOCONCRETEElena Papadopoulos

ENCE 710 Spring 2009


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Outline

Introduction

Base plates

Material

Design using AISC Steel Design Guide Concentric axial load

Axial load plus moment

Axial load plus shear

Anchor Rods

Types and Materials

Design using ACI Appendix D Tension

Shear


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Introduction

Base plates and anchor rods are often the laststructural steel items to be designed but the firstitems required on the jobsite

Therefore the design of column base plate andconnections are part of the critical path


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Introduction

Vast majority of column base plate connectionsare designed for axial compression with little or nouplift

Column base plate connections can also transmituplift forces 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 usedto resist wind and seismic loads

Development of force couple between bearing onconcrete and tension in some or all of the anchor rods


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Introduction

Anchor rods are needed for all base plates toprevent column from overturning duringconstruction and in some cases to resist uplift or

large moments Anchor rods are designed for pullout and breakout

strength using ACI 318 Appendix D

Critical to provide well-defined, adequate load

path when tension and shear loading will betransferred through anchor rods


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Introduction

Grout is needed to serve as the connectionbetween the steel base plate and the concretefoundation to transfer compression loads

Grout should have design compressive strength atleast 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


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Base plate Materials

Base plates should be ASTM A36 material unlessother grade is available

Most base plates are designed as square to

match the foundation shape and can be moreaccommodating for square anchor rod patterns

A thicker base plate is more economical than athinner base plate with additional stiffeners or

other reinforcements


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Base Plate Design

Base plate design in this lecture is using AISC Steel Design GuideColumn Base Plates (First Edition) by John T. DeWolf. A Second Editionwas published in 2006.


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Design of Axially Loaded BasePlates

Required plate area is based on uniform allowablebearing stress. For axially loaded base plates, thebearing stress under the base plate is uniform

A2 = dimensions of concrete supporting foundation

A1 = dimensions of base plate

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

When the plate dimensions are known it is notpossible to calculate bearing pressure directly and

therefore different procedure is used (Case 2)

`

1

2`max 7.185.0 cccp f

AAff


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Case 1: A2 > 4A1

1. Determine factored load Pu2. Calculate required plate area A1 based on maximum

concrete bearing stress fp=1.7f`c (when A2=4A1)

`)(1 7.16.0 c

u

req f

P

A

)(1 reqAN2

8.095.0 fbd

N

AB

req)(1

3. Plate dimensions B & Nshould be determined so m& n are approximatelyequal


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Case 1: A2 > 4A1

4. Calculate required base plate thickness

where l is maximum of m and n

5. Determine pedestal area, A2

2

95.0 dNm

2

8.0 fbBn

BNF

Plt

y

u

90.0

2min

BNA 42


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Case 2: Pedestal dimensionsknown

2

`

2

185.060.0

1

c

u

fP

AA `1 7.16.0 c

u

fPA

1.Determine factored load Pu2.The area of the plate should be equal to largerof:

3. Same as Case 1

4. Same as Case 1


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Design of Base Plates withMoments Equivalent eccentricity, e, is calculated equal to moment

M divided by axial force P

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

Small eccentricities equivalent axial force resisted bybearing only

Large eccentricitiesnecessary to use an anchor boltto resist equivalent axial force


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Design of Base Plate with Small

Eccentricities

If e


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Design of Base Plate with Small

Eccentricities

1. Calculate factored load (Pu) and moment (Mu)

2. Determine maximum bearing pressure, fp

3. Pick a trial base plate size, B and N

4. Determine equivalent eccentricity, e, and maximumbearing stress from load, f1. If f1 < fp go to next step,

if not pick different base plate size

5. Determine plate thickness, tp

1. Mplu is moment for 1 in wide strip

y

plu

pF

Mt

90.0

4

`

1

2` 7.185.0 cccp fA

Aff


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Design of Base Plate withShear

Four principal ways of transferring shear from columnbase plate into concrete

1. Friction between base plate and the grout orconcrete surface

The friction coefficient (m) is 0.55 for steel on groutand 0.7 for steel on concrete

2. Embedding column in foundation

3. Use of shear lugs

4. Shear in the anchor rods (revisited later in lecture)

ccun AfPV `2.0m


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Design of Shear Lugs

1. Determine the portion of shear which will be resisted byshear lug, Vlgu

2. Determine required bearing area of shear lug

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

4. Determine factored cantilevered end moment, Mlgu

5. Determine shear lug thickness

`

lg

lg 85.0 c

u

f

V

A

2lg

lg GHWVM uu

y

u

F

Mt

90.0

4 lglg


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Anchor Rods

Two categories

Cast-in place: set before the concrete is placed

Drilled-in anchors: set after the concrete is hardened


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Anchor Rod 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 orheaded

Wherever possible use -in diameter ASTM F1554Grade 36

When more strength required, increase roddiameter to 2 in before switching to higher grade

Minimum embedment is 12 times diameter of bolt


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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 with that of

headed rods or threaded rods with

a nut of anchorage


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Anchor Rod Placement

Most common field problem is placement of anchorrods

Important to provide as large as hole as possible to

accommodate setting tolerances

Fewer problems if the structural steel detailercoordinates all anchor rod details with column base


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Anchor Rod Layout

Should use a symmetrical pattern in bothdirections wherever possible

Should provide ample clearance distance for

the washer from the column Edge distance plays important role for

concrete breakout strength

Should be coordinated with reinforcing steel toensure there are no interferences, more criticalin concrete piers and walls


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Design of Anchor Rods forTension When base plates are subject to uplift force Tu,

embedment of anchor rods must be checked fortension

Steel strength of anchor in tension

Ase =effective cross sectional area of anchor, AISC Steel Manual Table 7-18

fut= tensile strength of anchor, not greater than 1.9fy or 125 ksi

Concrete breakout strength of single anchor in

tension

hef=embedment

k=24 for cast-in place anchors, 17 for post-installed anchors2, 3 = modification factors

utses fAN

5.1`

efcb hfkN b

No

N

cbN

A

AN

32


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Design of Anchor Rods forTension ANo=Projected area of the

failure surface of a singleanchor remote from edges

AN=Approximated as the baseof the rectilinear geometricalfigure that results from

projecting the failure surfaceoutward 1.5hef from thecenterlines of the anchor

Example of calculation of AN with edgedistance (c1) less than 1.5hef

29 efNo hA

)5.12)(5.1( 1 efefN hhcA

D i f A h R d f


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Design of Anchor Rods forTension

Pullout strength of anchor

Nominal strength in tension Nn = min(Ns, Ncb,

Npn)

Compare uplift from column, Tu, to Nn If Tu less than Nn ok

If Tu greater than Nn must provide tensionreinforcing around anchor rods or increaseembedment of anchor rods

`

4 8 cbrgpn fAN

D i f A h R d f


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Design of Anchor Rods forShear When base plates are subject to shear force, Vu, and

friction between base plate and concrete is inadequateto 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

do = rod diameter, in

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

b

vo

vcb V

A

AV 76

5.1

1

`

2.0

7 cfdd

lV co

o

b

utses fAV

D i f A h R d f


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Design of Anchor Rods forShear Avo=Projected area of the failure

surface of a single anchor remotefrom edges in the directionperpendicular to the shear force

Av=Approximated as the base of atruncated half pyramid projected onthe side face of the member

Example of calculation of Av with edgedistance

(c2) less than 1.5c1

215.4 cAvo

)5.1(5.1 211 cccAv

D i f A h R d f


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Design of Anchor Rods forShear

Pryout strength of anchor

Nominal strength in shear Vn = min(Vs, Vcb,

Vcp)

Compare shear from column, Vu, to Vn If Vu less than Vn ok

If Vu greater than Vn must provide shearreinforcing around anchor rods or use shearlugs

cbcpcp NkV


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Combined Tension and Shear

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

2.1n

u

n

u

V

V

N

T


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References

American Concrete Institute (ACI) 318-02

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

AISC Steel Design Guide (2nd 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 Quarter1985


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Questions?

Design of Column Base Plates

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