DESIGN OF COLUMN BASE PLATES AND STEEL ANCHORAGE TO CONCRETE

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

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

 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.

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.

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 c 0.85 c 1.7 c

p f

A A f

f   

Case 1:

A

2 > 4

A

1 1. Determine factored load P_u

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

` ) ( 1 7 . 1 6 . 0 _c u req f P A      A1(req) N 2 8 . 0 95 .

0 d b_f

 

N A B 1(req)

3. Plate dimensions B & N should be determined so m & n are approximately equal:

DESIGN OF AXIALLY LOADED

4. Calculate required base plate thickness:

where

l

is maximum of

m

and

n

5. Determine pedestal area,

A

₂: 2

95 .

0 d

N

m 

2 8 . 0 bf

B

n 

BN F P l t y u 90 . 0 2 min BN A₂ 4

DESIGN OF AXIALLY LOADED

BASE PLATES

_(Cont’d)

Case 1:

A

2 > 4

A

1

Case 2: Pedestal dimensions known

2 ` 2 1 85 . 0 60 . 0 1         c u f P A

A 1 _`

7 . 1 6 . 0 _c u f P A  

1. Determine factored load P_u

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

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 P f₁ 2

I Mc BN

P f1,2  

DESIGN OF BASE PLATES WITH

1. Calculate factored load (P_u) and moment (M_u)

2. Determine maximum bearing pressure, f_p

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, t_p:

y plu p F M t 90 . 0 4  ` 1 2 ` 7 . 1 85 .

0 c c

c

p f

A A f

f   

DESIGN OF BASE PLATES WITH

MOMENTS

_(Cont’d)

• M_plu 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. c c u

n P f A

V m 0.2 `

DESIGN OF SHEAR LUGS

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

2. Determine required bearing area of shear lug:

3. Determine shear lug width, W_{, and height,} H_.

4. Determine factored cantilevered end moment, M_lgu.

5. Determine shear lug thickness: ` lg lg 85 . 0 _c u f V A                 2 lg lg G H W V

M u u

y u F M t 90 . 0 4 _lg lg 

ANCHOR RODS

Two categories:

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

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.

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.

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:

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

f_ut = tensile strength of anchor, not greater than 1.9f_yor 125 ksi

 Concrete breakout strength of single anchor in tension:

h_ef = embedment

k = 24 for cast-in place anchors, 17 for post-installed anchors ₂,₃ = modification factors

ut se s A f

N 

5 . 1 `

ef c b k f h

N 

b No

N

cb N

A A N  23

 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 1.5h_ef from the centerlines of the anchor.  Example of calculation of A_N with

2

9 _ef No h

A 

DESIGN OF ANCHOR RODS FOR

 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 T_u greater than N_n, must provide tension reinforcing around anchor rods or increase embedment of anchor rods.

` 4 brg

8

c

pn

A

f

N





DESIGN OF ANCHOR RODS FOR

TENSION

(Cont’d)

 When base plates are subject to shear force, V_u, 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

d_o = rod diameter, in

l = load bearing length of anchor for shear not to exceed 8d_o, in b vo v cb

V

A

A

V





₆



₇ 1.5

1 ` 2

. 0

7 d f c

d l

V _{o c}

o b        ut se

s

A

f

V



DESIGN OF ANCHOR RODS FOR

 A_vo = projected area of the failure surface of a single anchor remote from edges in the direction perpendicular to the shear force  A_v = 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

 

2 1 5 .

4 c

A_vo 

) 5 . 1 ( 5 .

1 c1 c1 c2

Av  

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 V_u greater than V_n must provide shear reinforcing around anchor rods or use shear lugs.

cb cp cp k N

V 

DESIGN OF ANCHOR RODS FOR

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

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.

Terima Kasih

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