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Use of Shear Lugs for Anchorage to Concrete

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Anchoring structures, systems and components to concrete is a significant activity in the design and construction of a nuclear power plant. Early in this decade the Concrete Capacity Design method (CCD) was adopted by the American Concrete Institute (ACI) for use in the structural design for both commercial and nuclear facilities. This design method and associated qualification tests brings new challenges to designing efficient means for anchoring to concrete structures. Although the CCD method provides guidance on many aspects of concrete anchorage there are a few areas, pertinent to nuclear power plant construction, that are not covered or require significant interpretation of the most recent codes. This paper will focus on the design of shear lugs used to resist significant lateral loads. Results from laboratory tests of shear lugs are presented. These full scale tests considered the interaction of tension and shear loads on the performance of shear lug assemblies. Recommendations for the efficient use of shear lugs are provided.
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FINAL
Proceedings of the ASME 2009 International Conference on Nuclear Engineering
ICONE17
July 12 -16, 2009, Brussels, Belgium
ICONE17-7175
USE OF SHEAR LUGS FOR ANCHORAGE TO CONCRETE
Peter J. Carrato Martin Reifschneider
Principal Civil Engineer Engineering Manager
Bechtel Power Corporation
Frederick, Maryland, 21703, USA
ABSTRACT
Anchoring structures, systems and components
to concrete is a significant activity in the design
and construction of a nuclear power plant. Early
in this decade the Concrete Capacity Design
method (CCD) was adopted by the American
Concrete Institute (ACI) for use in the structural
design for both commercial and nuclear
facilities. This design method and associated
qualification tests brings new challenges to
designing efficient means for anchoring to
concrete structures.
Although the CCD method provides guidance on
many aspects of concrete anchorage there are a
few areas, pertinent to nuclear power plant
construction, that are not covered or require
significant interpretation of the most recent
codes. This paper will focus on the design of
shear lugs used to resist significant lateral loads.
Results from laboratory tests of shear lugs are
presented. These full scale tests considered the
interaction of tension and shear loads on the
performance of shear lug assemblies.
Recommendations for the efficient use of shear
lugs are provided.
1 INTRODUCTION
Shear may be transferred from a structural
component to a concrete substrate thru a
combination of different mechanical interactions;
bearing, of shear lugs attached to the base plate
and the baseplate edge when embedded into the
concrete, and; anchorage, in the form of headed
studs welded to a base plate or anchor bolts
passing through holes in the base plate.
Examples of these mechanical interfaces are
shown in Figures 1a and 1b.
Figure 1a BASE PLATE USING WELDED
STUDS AND SHEAR LUG
Figure 1b BASE PLATE USING ANCHOR
BOLTS AND SHEAR LUG
Bearing resistance is provided when a part of the
baseplate assembly is in direct contact with the
concrete in the direction of the applied load.
Additional shear resistance is provided by a net
confining force. The confining force is the
combination of an applied load and the elastic
tensile strength of engaged anchorage. For the
configuration shown in Figure 1a shear forces
are transmitted from the attachment to the
concrete by; bearing of the edge of the base
plate, dowel action of the welded studs, bearing
on the face of the shear lug, and thru shear
friction as a result of the confining force. The
bearing component of shear resistance is shared
by three components (plate edge, shear lug, and
bearing on the anchors) based on their relative
stiffness. Bearing of the welded studs against
the concrete (dowel action) is generally ignored
when designing the shear resistance for
embedded plates as the anchors typically have a
bending stiffness significantly smaller than the
shear lugs. The load transferred by the embedded
edge of a base plate is equally as effective as a
properly design shear lug and should also be
considered in the design.
Shear transfer mechanisms for the base plate
configuration shown in Figure 1b include dowel
action by the anchor bolt and bearing on the face
of the shear lug. Engagement of the dowel
mechanism is however unreliable due to the
common construction practice of utilizing
oversized holes and grout pads, for column base
plates, as shown in Figure 2.
Figure 2 DETAIL AT CAST-IN-PLACE
ANCHOR BOLT
Oversized holes in baseplates can be as much as
20mm greater in diameter than the cast-in-place
anchor bolt diameter. This practice is common
to accommodate the tighter tolerances required
to properly position the component being
attached (such as a steel frame structure) when
compared to the much greater tolerance allowed
for locating items cast into concrete. The use of
a grout pad is to accommodate floors that are not
placed in a truly flat condition (allowing for
drainage) and the bottom of the plate requiring a
full vertical load bearing condition.
The ability of anchor bolts to engage a lateral
shear is different from that of welded studs.
Welded studs are connected to the base plate
using a full penetration weld. This weld
provides essentially full rotational fixity of the
stud to base plate connection and being integrally
attached to the base plate, all the studs will
engage to resist an applied shear load.
Anchor bolts, especially when they are installed
in oversized holes, do not have the same degree
of fixity as welded studs, and act more like a
pinned connection at the base plate. Depending
on their as-built position in the holes not all
anchor bolts will engage the base plate
simultaneously. Thus the presence of the
oversized hole at the anchor bolt has an effect on
the mechanism for transferring shear from the
base plate. Depending on the actual as-built
location of the bolts in their holes the plate may
have to undergo substantial movement before all
bolts are engaged with the plate. This is
illustrated in Figure 3.
Figure 3 BASE PLATE DISPLACEMENT TO
ENGAGE ANCHOR BOLT
In cases where anchor engagement is needed,
such as when a shear lug is not used, and cast-in-
place anchor bolts are placed in large oversized
holes, a heavy plate washer should be specified
to reduce the gap around the bolt and thus more
efficiently engage the bolts. Plate washers are
fabricated using a standard sized hole, typically
2mm larger in diameter than the bolts, and
should be field welded to the base plate. This
arrangement facilitates appropriate tolerances for
placing the bolts and positioning the attachment
while providing a more reliable means for anchor
engagement. This is shown in Figure 4.
Figure 4 ANCHOR BOLT WITH PLATE
WASHER
2 TYPES OF SHEAR LUGS
A variety of different shear lug configurations
may be used depending on the magnitude and
direction of the shear force that must be resisted.
Shear lugs typically consist of a one or more
pieces of plate welded to the bottom of the
baseplate. When addressing shear loads in two
orthogonal directions, crossed plates can be used
(Figure 5a). Figure 5b is a photograph taken
prior to placing the grout pad and the region of
the concrete substrate that has been blocked out
to accept the shear lug can be seen.
Figure 5a PLATE TYPE SHEAR LUG
Figure 5b PLATE TYPE SHEAR LUG GROUT
POCKET
In many cases, a structural member such as a
wide flange, H shaped or box shaped element is
used as the shear lug. Figure 6 shows a heavy
piece of wide flange being used as the shear
resisting element. In this case the H shaped
member is built up from pieces of plate.
Figure 6 HEAVY H-SHAPED SHEAR LUG
For heavy shear loads a portion of the column
may be extended into a pocket formed in the
concrete. In extreme cases where the shear
capacity of the anchorage is intended to be
greater than that of the column, to ensure ductile
behavior under seismic loads, the embedded lug
will be larger that the structural element being
connected. An example of this type of design is
shown in Figure 7. Figure 7a shows a three
dimensional model of a column and large box
shaped lug that will be embedded in concrete.
The square holes are sleeves in the box that will
allow for cast-in-place anchor bolts. Figure 7b
shows this assembly after fabrication.
Figure 7a MODEL OF LARGE SHEAR LUG
Figure 7b HEAVY SHEAR LUG READY FOR
INSTALLATION
3 FAILURE MECHANISMS
Two distinct modes of behavior are evident for
baseplates with shear lugs and anchors as they
are loaded to failure. Initially the response will
be a bearing mode which will progress into a
steel mode. If there are no bearing components
(shear lugs or plate edges) the baseplate will
exhibit only a steel mode behavior (though some
limited bearing will occur on the anchors).
The bearing mode is controlled by the bearing
capacity of the concrete against the bearing
components, supplemented by a shear friction
from a confining force. Subsequent to the
bearing mode failure, a steel (only) mode
behavior of shear friction from a confining force
will continue till anchor failure. The confining
force is a combination of applied force and
anchor load due to its elongation. Evidence of
both behaviors is predicated on adequate
anchorage and elastic shear lug design and is
limited by the bearing capability of the concrete
substrate.
As with any device used to anchor components
to concrete the two basic failure mechanisms for
shear lugs are either steel or concrete. There are
two potential modes of steel lug failure that will
limit its behavior: bending of the lug, or failure
of the weld between the lug and the base plate.
There are three possible concrete failure
mechanisms that will limit the bearing mode
behavior of the shear lugs. Two of the three are
for failure of a wedge of concrete in front of the
lug (in the direction of the applied shear force).
For lugs that are located away from a free
concrete edge the failure surface to consider is
one that propagates up from the bottom of the
lug to the top of the concrete. This is the most
common type of concrete failure mode and an
example is shown in the photograph in Figure 8.
Figure 8 CONCRETE FAILURE IN FRONT OF
SHEAR LUG
When shear lugs are located close to a free edge
two possible concrete failure mechanisms are
possible dependant on the combination of shear
load and applied axial load (tension or
compression), the bearing area of the plate and
lug, and the distance to the free edge. Figure 9
shows these two possible failure modes near a
free edge. Figure 9a illustrates the failure mode
shown in Figure 10
Figure 9a CONCRETE FAILURE WITH
APPLIED TENSION
Figure 9B CONCRETE FAILURE WITH
APPLIED COMPRESSION
Figure 9b shows the failure surface in which the
tensile capacity of the concrete wedge was less
than the bearing capacity of the concrete in front
of the lug. This failure mode in a laboratory test
is shown in Figure 10.
Figure 10 CONCRETE FAILURE SURFACES
NEAR A FREE EDGE
For plates without bearing surfaces, only a steel
mode of behavior will be exhibited. This mode
of behavior may be limited by a concrete pry-out
mechanism of the anchors. For very rigid
embedded elements loaded in shear it is possible
for them to rotate when loaded and kick out a
concrete failure in the direction opposite to the
applied load. This failure mechanism applies to
both studs and anchor bolts that have insufficient
embedment to ensure tensile failure through the
anchor shaft. Figure 11a exhibits studs that have
failed by pry-out and Figure 11b provides
evidence that the spalling is may be opposite of
the direction of the applied load.
Figure 11a WELDED STUDS WITH PRY-OUT
FAILURE
Figure 11b CONCRETE PRY-OUT FAILURE
SURFACE
The potential for the concrete failure mode to be
either a wedge in front of the lug or a pry-out
failure is a function of the stiffness of the shear
lug, depth of anchor embedment, and the
strength of the concrete. Figure 12 shows two
possible distributions of concrete bearing
pressure on a shear lug. The upper figure shows
the bearing pressure that would be associated
with a lug that is less rigid than that shown in the
lower figure.
Figure 12 POSSIBLE CONCRETE BEARING
PRESSURE ON SHEAR LUGS
4 TESTING AND ANALYSIS
Due to the complex nature of steel to concrete
interaction and the wide variety of anchorage
configuration that incorporate shear lugs, a
classical mechanics approach to design and
analysis is not practical. Empirical design
methods based on extensive testing programs are
available for simple single plate shear lugs, see
References [1], [2] and [3]. These tests focused
on the behavior of base plates that included
welded studs in combination with lugs. None of
the tests investigated the effect of anchor bolts in
oversized holes or the use of grout. Reference
[4] provided design guidance on the effect of
grout on base plate shear resistance.
There are however, a number of conclusions
derived from the available test results that are
pertinent to the design of shear lugs.
The front edge of an embedded baseplate
functions like a shear lug with comparable
shear area.
The shear resistance from bearing on the edge
is compatible with that from bearing on the
properly designed lugs.
The bearing capacity is a direct function of the
bearing area and the ultimate bearing strength
of the concrete remains essentially constant
for tensile or compressive concurrent loads.
The confining force is the algebraic difference
between the between the yield strength of the
tension anchors and the applied axial load.
For both the bearing and steel controlled
modes, the presence of concurrent tension
diminishes shear capacity and concurrent
compression increases shear capacity.
The common practice of resisting all tension
loads by the tension anchors and all shear
loads by the shear lugs (in the bearing mode)
is, a justifiable and conservative procedure.
Tension capacity not utilized in resisting
tension loads can be utilized to resist shear
loads.
5 DESIGN AND CONSTRUCTION
RECOMMENDATIONS
Designing a baseplate to resist shear shall
consider all the connecting components and the
applied loads. The bearing mode design capacity
for baseplates subjected to both concentric axial
and shear loading is as noted in the equation:
Va = CB + SF
where, CB = Concrete Bearing capacity of the
shear lug and plate edge. This should be
in the form of
ΦbKbAbfc
where, Φb = capacity reduction factor
K
b = concrete bearing strength factor
A
b = sum of the bearing areas for the
shear lugs and the embedded plate edge,
only bearing areas below the free
surface of the concrete are effective
f
c = minimum compressive strength of
concrete or grout
and, SF = Shear Friction afforded by the net
confining force. This should be in the
form of
Φsμb(Py – Pa/Φy)
where, Φs = capacity reduction factor
μb = friction factor
P
y = yield strength of tension anchors
P
a = axial load applied concurrent with
shear, positive for tension, negative for
compression
Φy = capacity reduction factor steel
Note the anchor capacity contribution to
confinement force for anchor bolts should be
neglected as it is uncertain whether the anchors
will engage in resisting shear in the bearing
mode.
A number of considerations should be addressed
when detailing and installing baseplates to
ensure design shear capacity requirements are
achieved. Each of these considerations can
directly affect one or both of the behavior modes
noted above.
Affecting the bearing mode: 1) For baseplates
with one or more shear lugs, the baseplate should
be positioned and leveled prior to grouting. A
large shear lug pocket is desirable to ensure that
there is sufficient clearance on all sides, between
the inserted lug and the concrete, allowing for
proper grout installation thus ensuring a fully
effective bearing area.
2) Particularly for large baseplates, a vent hole
should be placed through the baseplate in one or
more locations to facilitate the release of air that
can get trapped under the plate during grout
placement. In some cases it may be easiest to
place the grout through the vent hole and allow it
to flow to the edges. In either case, it is desirable
to place grout from as few locations as possible,
allowing it to flow around the lug as the pocket
fills and prevent the entrapment of air. Proper
grout placement is vital to achieve lug bearing.
3) The grout strength shall be selected to equal or
exceed the concrete compressive strength. If the
grout strength is less than the minimum specified
concrete compressive strength, the grout
compressive strength shall be used to determine
the bearing capacity of the shear lug.
4) Base-plates having bearing components, and
in which the shear load is in the direction of a
free edge, the addition of reinforcement to ensure
a bearing mode failure of the concrete occurs
before a tensile wedge failure is prudent. There is
however little guidance on how much and how to
place such reinforcement to best achieve desired
results.
5) Shear lugs shall be designed to remain elastic
and with small flexural displacements under
design loads. Lugs that deform or are too flexible
will not uniformly resist bearing loads and thus
diminish the bearing capacity of the lugs.
6) Weld design of the shear lug to the base-plate,
either full penetration or fillet, shall ensure
elastic behavior for the combination of shear and
flexural loads resulting from concrete bearing on
the lug. Tension anchor performance will control
the available confinement force in both the
bearing and steel modes.
7) The embedment depth of the anchor must be
sufficient to fully develop the anchor tensile
capacity. If not, the confinement force provided
by the anchor shall be limited to the calculated
tensile capacity of the anchorage controlled by
concrete cone failure.
8) For welded studs, the weld of the anchor to
the baseplate shall be designed to consider the
combination of both tensile loads and bending as
a result of bearing on the anchor shaft.
9) One consideration for installing base-plates
with deep shear lugs is the risk of interference of
the shear lug with reinforcement in the concrete.
Often the shear lug pocket will penetrate into the
concrete surface below the outer layer of
reinforcement. Depending on the purpose of the
reinforcement, it can either be terminated either
side of the pocket, can be trimmed to form the
pocket or as appropriate the lugs designed to fit
between the reinforcing.
Attention to these details will ensure shear lug
performance as expected.
6 REFERENCES
[1] Rotz, J.V., M. Reifschneider, Combined
Axial and Shear Load Capacity of Steel
Embedments in Concrete, Report by
Bechtel Power Corporation 1991.
[2] Michler, H., M. Curbach, Behaviour
and Design of Fastenings of Shear Lugs
in Concrete, International Symposium
on Connections Between Steel and
Concrete, Rilem, Stuttgart, Germany,
September 2001.
[3] Michler, H., Model to Analyse
Fastenings with Shear Lugs, 2nd
International Symposium on
Connections Between Steel and
Concrete, ibidem, Stuttgart, Germany,
September 2007.
[4] ACI 349 “Code Requirements for
Nuclear Safety Related Concrete
Structures,” American Concrete
Institute, Detroit, Michigan, 2006.
[5] Camacho, J., “Seismic Performance of
Exposed Column Base Plates (Phase
I),” Proceeding of the 2007 Earthquake
Engineering Symposium for Young
Researchers, Seattle, Washington,
USA, August 2007.
[6] Hitaka, T., K. Suita, K. Mikiko, “CFT
Column Base Design and Practice in
Japan,” Proceedings of the
International Workshop on Steel and
Concrete Composite Construction
(IWSCCC-2003), Report No. NCREE-
03-026, National Center for Research in
Earthquake Engineering, Taipei,
Taiwan, October 2003.
[7] Grauvilardell, J.E., D. Lee, J.F. Hajjar,
R.J. Dexter, Synthesis of Design,
Testing and Analysis Research on Steel
Column Base Plate Connections in
High-Seismic Zones, Structural
Engineering Report No. ST-04-02,
Department of Civil Engineering,
University of Minnesota, Minneapolis,
Minnesota, October, 2005.
[8] Eligehausen, R., R. Mallée, J.F. Silva,
Anchorage in Concrete Construction,
Ernst & Sohn, 2006.
[9] Dewolf, J.T., D.T. Ricker, Column Base
Plates, AISC, 1990
ResearchGate has not been able to resolve any citations for this publication.
Article
A comprehensive treatment of current fastening technology using inserts (anchor channels, headed stud), anchors (metal expansion anchor, undercut anchor, bonded anchor, concrete screw and plastic anchor) as well as power actuated fasteners in concrete. It describes in detail the fastening elements as well as their effects and load-bearing capacities in cracked and non-cracked concrete. It further focuses on corrosion behaviour, fire resistance and characteristics with earthquakes and shocks. It finishes off with the design of fastenings according to the European Technical Approval Guideline (ETAG 001), the Final Draft of the CEN Technical Specification 'Design of fastenings for use in concrete' and the American Standards ACI 318-05, Appendix D and ACI 349-01, Appendix B. © 2006 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.
Seismic Performance of Exposed Column Base Plates (Phase I)
  • J Camacho
Camacho, J., "Seismic Performance of Exposed Column Base Plates (Phase I)," Proceeding of the 2007 Earthquake Engineering Symposium for Young Researchers, Seattle, Washington, USA, August 2007.
CFT Column Base Design and Practice in Japan
  • T Hitaka
  • K Suita
  • K Mikiko
Hitaka, T., K. Suita, K. Mikiko, "CFT Column Base Design and Practice in Japan," Proceedings of the International Workshop on Steel and Concrete Composite Construction (IWSCCC-2003), Report No. NCREE-03-026, National Center for Research in Earthquake Engineering, Taipei, Taiwan, October 2003.
Combined Axial and Shear Load Capacity of Steel Embedments in Concrete
  • J V Rotz
  • M Reifschneider
Rotz, J.V., M. Reifschneider, Combined Axial and Shear Load Capacity of Steel Embedments in Concrete, Report by Bechtel Power Corporation 1991.
Behaviour and Design of Fastenings of Shear Lugs in Concrete
  • H Michler
  • M Curbach
Michler, H., M. Curbach, Behaviour and Design of Fastenings of Shear Lugs in Concrete, International Symposium on Connections Between Steel and Concrete, Rilem, Stuttgart, Germany, September 2001.
Model to Analyse Fastenings with Shear Lugs
  • H Michler
Michler, H., Model to Analyse Fastenings with Shear Lugs, 2 nd International Symposium on Connections Between Steel and Concrete, ibidem, Stuttgart, Germany, September 2007.
Code Requirements for Nuclear Safety Related Concrete Structures
ACI 349 "Code Requirements for Nuclear Safety Related Concrete Structures," American Concrete Institute, Detroit, Michigan, 2006.
  • J T Dewolf
  • D T Ricker
Dewolf, J.T., D.T. Ricker, Column Base Plates, AISC, 1990