Post on 13-Apr-2015
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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?