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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

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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.

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)

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Anchor rods are needed for all base plates to prevent

column from overturning during construction 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 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.

INTRODUCTION (Cont’d)

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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

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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

A2 = dimensions of concrete supporting foundation

A1 = dimensions of base plate

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)

`

1

2`

max 7.185.0 cccp fA

Aff

Case 1: A2 > 4A1 1. Determine factored load Pu

2. Calculate required plate area A1 based on maximum concrete bearing stress fp=1.7f`c (when A2 = 4 A2)

`)(1

7.16.0 c

ureq

f

PA

)(1 reqAN2

8.095.0 fbd

N

AB

req)(1

3. Plate dimensions B & N should

be determined so m & n are

approximately equal:

DESIGN OF AXIALLY LOADED

BASE PLATES (Cont’d)

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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

DESIGN OF AXIALLY LOADED

BASE PLATES (Cont’d)

Case 1: A2 > 4A1

Case 2: Pedestal dimensions known

2

`

2

185.060.0

1

c

u

f

P

AA

`17.16.0 c

u

f

PA

1. Determine factored load Pu

2. The area of the plate should be equal to larger of:

or

3. Same as Case 1

4. Same as Case 1

DESIGN OF AXIALLY LOADED

BASE PLATES (Cont’d)

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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.

If e < N/6 compressive bearing stress exist everywhere

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

portion of the plate

AB

Pf

21

I

Mc

BN

Pf 2,1

DESIGN OF BASE PLATES WITH

MOMENTS (Cont’d)

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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 maximum

bearing stress from load, f1. If f1 < fp go to next step, if

not pick different base plate size.

5. Determine plate thickness, tp:

y

plu

pF

Mt

90.0

4

`

1

2` 7.185.0 cccp fA

Aff

DESIGN OF BASE PLATES WITH

MOMENTS (Cont’d)

• Mplu is moment for 1 in wide strip

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:

The friction coefficient (m) is 0.55 for steel on grout and

0.7 for steel on concrete

2. Embedding column in foundation.

3. Use of shear lugs.

4. Shear in the anchor rods.

ccun AfPV `2.0m

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DESIGN OF SHEAR LUGS

1. Determine the portion of shear which will be resisted by

shear 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

lg85.0 c

u

f

VA

2

lg

lg

GH

W

VM

u

u

y

u

F

Mt

90.0

4 lg

lg

ANCHOR RODS

Two categories:

a) Post-installed anchors: set after the concrete is

hardened.

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

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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:

- When more strength required, increase rod diameter to

2 in before switching to higher grade.

Minimum embedment is 12 times diameter of bolt.

ANCHOR RODS (Cont’d)

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.

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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.

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DESIGN OF ANCHOR RODS FOR

TENSION

When base plates are subject to uplift force Tu, embedment of anchor rods must be checked for tension.

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 anchors

2, 3 = modification factors

utses fAN

5.1`

efcb hfkN b

No

Ncb N

A

AN 32

Ano = projected area of the failure

surface of a single anchor remote

from edges

AN = approximated as the base of

the rectilinear geometrical figure

that results from projecting the

failure surface outward 1.5hef from

the centerlines of the anchor.

Example of calculation of AN with

edge distance (c1) less than 1.5hef

29 efNo hA

)5.12)(5.1( 1 efefN hhcA

DESIGN OF ANCHOR RODS FOR

TENSION (Cont’d)

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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 tension reinforcing

around anchor rods or increase embedment of anchor

rods.

`

4 8 cbrgpn fAN

DESIGN OF ANCHOR RODS FOR

TENSION (Cont’d)

When base plates are subject to shear force, Vu, 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

do = rod diameter, in

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

b

vo

vcb V

A

AV 76 5.1

1

`

2.0

7 cfdd

lV co

o

b

utses fAV

DESIGN OF ANCHOR RODS FOR

SHEAR (Cont’d)

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Avo = projected area of the failure

surface of a single anchor remote

from edges in the direction

perpendicular to the shear force

Av = approximated as the base of

a truncated half pyramid

projected on the side face of the

member.

Example of calculation of Av with

edge distance (c2) less than

1.5c1

215.4 cAvo

)5.1(5.1 211 cccAv

DESIGN OF ANCHOR RODS FOR

SHEAR (Cont’d)

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 shear reinforcing

around anchor rods or use shear lugs.

cbcpcp NkV

DESIGN OF ANCHOR RODS FOR

SHEAR (Cont’d)

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COMBINED TENSION AND SHEAR

According to ACI 318 Appendix D, anchor rods must be

checked for interaction of tensile and shear forces:

2.1n

u

n

u

V

V

N

T

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 Quarter 1985.

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