Design of column base plates anchor bolt

DESIGN OF COLUMN BASE

PLATES AND STEEL

ANCHORAGE TO

CONCRETE

Khaled Eid

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

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

Introduction

Anchors to appear in concrete drawings with

location of each anchor in x and y direction

Pedestal should be designed to suit the

supporting column and anchors

Usually allow for enough edge distance of 6d

bolt

Usually use to nuts to avoid slip

Introduction

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

Bearing end plate

Shear lugs under the base plate or embedding the column base to transfer the shear force.

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

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

In seismic zones the pedestal should carry 2.5 the

factored design load

Introduction

Grout is needed to adjust the level

Grout to transfer the load from steel plate to

foundation

Grout should have design compressive strength at

least twice the strength of foundation concrete

When base plates become larger than 600mm, 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 to match the

pedestal shape

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

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=4A1)

`)(17.16.0 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

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

Case 2: Pedestal dimensions

known

2

`

2

185.060.0

1

c

u

f

P

AA `1

7.16.0 c

u

f

PA

1.Determine factored load Pu

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

of:

3. Same as Case 1

4. Same as Case 1

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 Plate with Small

Eccentricities

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 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 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

1. Mplu is moment for 1 in wide stripy

plu

pF

Mt

90.0

4

`

1

2` 7.185.0 cccp fA

Aff

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 (revisited later in lecture)

ccun AfPV `2.0m

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

Cast-in place: set before the concrete is placed

Drilled-in anchors: set after the concrete is hardened

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 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

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 issued

anchor bolt layout for placing the anchors form his 3d

model

Anchor Rod Layout

Should use a symmetrical pattern in both

directions 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 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 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


Tension

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


Tension

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


Shear

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


Shear 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


Shear

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

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

Common mistakes

Careful when considering the location of

anchors to concrete walls

Bolts miss alignment or clash with gusset

plate