Transcript DESIGN OF COLUMN BASE PLATES AND STEEL ANCHORAGE TO CONCRETE Elena Papadopoulos

DESIGN OF COLUMN BASE
PLATES AND STEEL
ANCHORAGE TO CONCRETE
Elena Papadopoulos
ENCE 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
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
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
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
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
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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
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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
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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
f p max  c  0.85 f c`
A2
 1.7 f c`
A1
A2 = dimensions of concrete supporting foundation
A1 = dimensions of base plate
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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)
Case 1: A2 > 4A1
1.
2.
Determine factored load Pu
Calculate required plate area A1 based on maximum
concrete bearing stress fp=1.7f`c (when A2=4A1)
A1( req) 
3.
Pu
0.6 1.7 f c`
Plate dimensions B & N should
be determined so m & n are
approximately equal
N
A1( req )  
B

A1( req)
N
0.95d  0.8b f
2
Case 1: A2 > 4A1
4.
Calculate required base plate thickness
N  0.95d
m
2
t min
n
B  0.8b f
2
2 Pu
l
0.90 Fy BN
where l is maximum of m and n
5.
Determine pedestal area, A2
A2  4BN
Case 2: Pedestal dimensions known
Determine factored load Pu
2.The area of the plate should be equal to larger of:
1.

Pu
1 
A1 


A2  0.60  0.85 f c` 
3.
4.
2
Same as Case 1
Same as Case 1
Pu
A1 
0.6 1.7 f c`
Design of Base Plates with Moments
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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
f1, 2
P Mc


BN
I
If e is between N/6 and N/2 bearing occurs only over a portion
of the plate
2P
f1 
AB
Design of Base Plate with Small
Eccentricities
1.
2.
Calculate factored load (Pu) and moment (Mu)
Determine maximum bearing pressure, fp
f p  c  0.85 f c`
3.
4.
5.
A2
 1.7 f c`
A1
Pick a trial base plate size, B and N
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
Determine plate thickness, tp
tp 
4 M plu
1.
0.90 Fy
Mplu is moment for 1 in wide strip
Design of Base Plate with Shear
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Four principal ways of transferring shear from column base
plate into concrete
1.
2.
3.
4.
Friction between base plate and the grout or concrete
surface
Vn  mPu  0.2 f c` Ac
The friction coefficient (m) is 0.55 for steel on grout and
0.7 for steel on concrete
Embedding column in foundation
Use of shear lugs
Shear in the anchor rods (revisited later in lecture)
Design of Shear Lugs
1.
2.
Determine the portion of shear which will be resisted by shear
lug, Vlgu
Determine required bearing area of shear lug
Alg 
3.
4.
Vlgu
0.85f c`
Determine shear lug width, W, and height, H
Determine factored cantilevered end moment, Mlgu
 Vlgu  H  G 

M lgu  

 W  2 
5.
Determine shear lug thickness
tlg 
4 M lgu
0.90 Fy
Anchor Rods
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Two categories
 Cast-in place: set before the concrete is placed
 Drilled-in anchors: set after the concrete is hardened
Anchor Rod Materials
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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
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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
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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
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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
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When base plates are subject to uplift force Tu,
embedment of anchor rods must be checked for tension
Steel strength of anchor in tension
N s  Ase f ut
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
AN
` 1.5
 2 3 N b
N

k
f
b
c hef
A No
hef=embedment
k=24 for cast-in place anchors, 17 for post-installed anchors
2, 3 = modification factors
N cb 
Design of Anchor Rods for Tension
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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
ANo  9hef2
AN  (c1  1.5hef )( 2 1.5hef )
Design of Anchor Rods for Tension
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Pullout strength of anchor
N pn   4 Abrg 8 f c`
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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
Design of Anchor Rods for Shear
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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
Vs  Ase f ut

Concrete breakout strength of single anchor in shear
Vcb 
Av
 6 7Vb
Avo
 l 
Vb  7 
 do 
0.2
do
f c` c11.5
6, 7 = modification factors
do = rod diameter, in
l = load bearing length of anchor for shear not to exceed 8do, in
Design of Anchor Rods for Shear
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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
Avo  4.5c1 
2
Example of calculation of Av with edge distance
(c2) less than 1.5c1
Av  1.5c1 (1.5c1  c2 )
Design of Anchor Rods for Shear
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Pryout strength of anchor
Vcp  k cp N cb
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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
Combined Tension and Shear
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According to ACI 318 Appendix D, anchor rods must be
checked for interaction of tensile and shear forces
Tu
Vu

 1.2
N n Vn
References
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American Concrete Institute (ACI) 318-02
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AISC Steel Design Guide, Column Base Plates, by John T. DeWolf, 1990
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AISC Steel Design Guide (2nd Edition) Base Plate and Anchor Rod Design
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AISC Engineering Journal Anchorage of Steel Building Components to
Concrete, by M. Lee Marsh and Edwin G. Burdette, First Quarter 1985
Questions?