DESIGN OF COLUMN BASE PLATES AND STEEL ANCHORAGE TO CONCRETEElena PapadopoulosENCE 710 Spring 2009

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

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

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

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

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

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 f

A

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)

`)(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

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

1 85.060.0

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 14. 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 EccentricitiesIf 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

y

plup F

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 concrete1. Friction between base plate and the grout or

concrete surface

The friction coefficient () is 0.55 for steel on grout and 0.7 for steel on concrete

2. Embedding column in foundation3. Use of shear lugs4. Shear in the anchor rods (revisited later in

lecture)

ccun AfPV `2.0

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, H4. Determine factored cantilevered end moment,

Mlgu

5. Determine shear lug thickness

`

lglg 85.0 c

u

f

VA

2lg

lg

GH

W

VM u

u

y

u

F

Mt

90.0

4 lglg

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 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 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

Design of Anchor Rods for 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

Design of Anchor Rods for 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

Design of Anchor Rods for 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

bvo

vcb V

A

AV 76 5.1

1`

2.0

7 cfdd

lV co

ob

utses fAV

Design of Anchor Rods for 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

Design of Anchor Rods for 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

Questions?