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 f

AAff ≤×=φ

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

ureq f

PA×

=

∆+= )(1 reqAN2

8.095.0 fbd −=∆

NA

B 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

295.0 dNm −

= 28.0 fbB

n−

=

BNFPlt

y

u

90.02

min =

BNA 42 =

Case 2: Pedestal dimensions known

2

`2

1 85.060.01

×

=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 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

ABPf 2

1 =

IMc

BNPf ±=2,1

Design of Base Plate with Small Eccentricities1. Calculate factored load (Pu) and moment (Mu)2. Determine maximum bearing pressure, fp

3. Pick a trial base plate size, B and N4. 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

plup F

Mt

90.04

=

`

1

2` 7.185.0 cccp fAAff ≤×=φ

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 (µ) 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≤=φµφ

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

`lg

lg 85.0 c

u

fV

Aφ

=

+

=

2lg

lgGH

WV

M uu

y

u

FM

t90.0

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 ofheaded 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 7-18fut= tensile strength of anchor, not greater than 1.9fy or 125 ksi

Concrete breakout strength of single anchor in tension

hef=embedmentk=24 for cast-in place anchors, 17 for post-installed anchorsψ2, ψ3 = modification factors

utses fAN =

5.1`efcb hfkN =b

No

Ncb N

AAN 32ψψ=

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 φNn ok If Tu greater than φNn must provide tension

reinforcing around anchor rods or increase embedment of anchor rods

`4 8 cbrgpn fAN ψ=

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

ψ6, ψ7 = modification factors

do = rod diameter, in

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

bvo

vcb V

AAV 76ψψ= 5.1

1`

2.0

7 cfddlV coo

b

=

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 φVn ok If Vu greater than φVn 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

2.1≤+n

u

n

u

VV

NT

φφ

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