Note: Descriptions are shown in the official language in which they were submitted.
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1.
Anchorage System for Structural Reinforcement of
Fiber Reinforced Plastic Materials and the Like
FIELD OF THE INVENTION
The invention relates to an anchorage system for holding down structural
reinforcing sheet, plate or shell made of fiber reinforced plastic (FRP) or
steel or other
metallic or non-metallic materials which are bonded to the surface of
structural cements
by means of the inherent concentric centering capability in the load transfer
mechanism
of the system.
BACKGROUND FOR THE INVENTION
Structural members, such as walls or columns, in buildings or bridges or other
structural systems are often required to resist uplifting tensile forces and
bending
moments resulting from overturning actions caused by loads imposed on the
structure
due to its occupancy or external environmental actions, especially from the
lateral loads
of strong wind and earthquakes.
There is a large inventory of old structures in Canada and US and around the
world which require repair or strengthening, rehabilitation or retrofit to
restore or
enhance their load carrying capacities to required performance level in order
to ensure
their safe use and operation. Enhancement of the tensile load or bending
moment
resistant capacities of individual structural members, and/or the restoration
of
deteriorated or damaged structural members to their pre-damaged capacities,
are
important part of this process.
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A practical means to enhance or restore the tensile load or bending moment
capacity of a structural member is by adding external surfaced bonded
reinforcing
materials to the structural member. Thin steel plate or sheet has been used
for this
purpose. Recently since the 1990s, fiber reinforced plastic (FRP) sheets have
been
shown to be an attractive alternative to the steel plate. The FRP
alternatives, which
typically are of the types of carbon fiber reinforced plastic (FRP), glass
fiber reinforced
plastic (GFRP), aramid fiber reinforced plastic (AFRP), which is also commonly
known
by the trade name Kevlar, have the advantages of high strength, lightweight
and
excellent corrosion resistance compared to conventional reinforcing steel.
The conventional alternative FRP reinforcing system consists of bonding FRP
sheets to the surface of the structural member by epoxy or other adhesives.
The surface
bonded FRP sheets provide additional tensile load resistance to the structural
member in
the direction parallel to its fiber direction. At the boundaries of the
structural member to
its supporting member or foundation, the load carried by the FRP sheets must
be
transferred to the supporting member or foundation. An anchorage system is
critical for
this load transfer and the effectiveness of the FRP strengthening system.
Previously, the anchorage system has a L-shaped angle anchor with one leg
parallel to the FRP reinforce structural member and another leg parallel to
the surface of
the supporting element. The FRP sheet wrapping around the outer surfaces of
the two
legs of the angle is pressed against the surfaces of the structural member and
the
supporting element by the angle, which is in turn locked down to the
supporting element
by anchor bolts drilled through the surface of one leg of the angle (see
Figure 1).
Because of the eccentricity between the loading direction of the FRP sheet and
the hold
down of the angle to the supporting member, there is significant bending or
prying action
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to the angle shape resulting in large out-of plane distortion of the FRP sheet
from its
loading plane. This leads to a reduced load carrying capacity and resistance
by the FRP
sheet, especially under cyclic load applications when the FRP sheet is
repeatedly
subjected to loading and unloading causing break or cut to the fiber due to
the repeated
cycles of out-of plane deformations and warping. The premature failure of the
FRP
reinforcing system is due to the eccentricity between the load carried by the
FRP sheet
and the lock-down resistance from the angle anchorage system.
Another challenge when using FRP for structural reinforcement is the problem
of
debonding of the FRP sheet from the supporting member or foundation. Nanni et
at. (A.
Nanni, Khalifa, A., T. Alkhrdaji and S. Lansbury, "Anchorage of Surface
Mounted FRP
Reinforcement", Concrete International: Design and Construction, Vol. 21, No.
10, Oct.
1999, pp. 49-54) attempted to employ a U-shaped anchor to prevent such
debonding in
beams reinforced with FRP sheets. In Nanni et al., a U-anchor is embedded at a
bent
portion of the end of the FRP reinforcement sheet into a preformed groove in
the
supporting member or foundation (see Figure 2). The goal is to develop
anchorage force
in the U-anchor by embedment of the FRP sheet. Viscous paste is used to fill
the groove.
Optionally, the end portion of the FRP sheet may wrap around a FRP bar inside
the
groove. However, it is apparent that the FRP bar has no bearing on the
exertion of
anchorage force. Furthermore, the viscous paste may not be strong enough to
hold the
FRP sheet inside the groove.
Furthermore, in Nanni et al., the working principle of the U-anchor system is
that
the load transfer from the FRP sheet to the concrete base is highly dependent
on the
shear and tensile strength of the bond between the FRP sheet and the concrete
on the
inside surface of the groove. The U-anchor arrangement is just a means to
increase the
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length of this bond area available for the transfer of the load, eccentricity
still exists
between the tensile force carried by the FRP sheet on the vertical web of the
beam and
the resultant anchor resistance provided by the bond between the FRP sheet and
the
concrete distributed over the circular inside surface of the groove.
Accordingly, there is a need for an improved anchoring system whereby the
system is able to provide an inherent concentric centering capability in the
load transfer
mechanism and to eliminate the undesirable prying action effect. The present
invention
is for a new self centering anchorage system which eliminates the eccentricity
problem
and allows the FRP material to fully utilize its high strength without
premature failure.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an anchorage system which
devoid the eccentricity problem in transferring the load carried by the
surface reinforcing
agent from the structural member to the supporting member whereby external
hold-down
force against the surface reinforcing agent is provided by an anchor rod or
tube acting
through the anchor rod or tube. According to one aspect of the invention, it
provides an
anchorage system for structural reinforcement comprising: (a) a structural
member and a
supporting member juxtaposing one another, whereby inside comer surfaces of
the
members are bonded with surface reinforcing agent made of structural
reinforcing
material; (b) a cylindrical anchoring means contiguously abutting the inside
corner of the
bonded surfaces of the structural member and supporting member; and (c) a lock-
down
means provided along the longitudinal axis of the anchoring means for
mountably
compressing the anchoring means against the bonded surfaces.
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It is another object of the invention to provide an anchorage mechanism with
easy
installation without the need to employ new or advanced technology to
manufacture or
use. It is a further obj ect of the invention to enable application of the
anchorage
mechanism to a variety of structures of different materials and shapes, such
as reinforced
concrete or masonry structures, structural members with a flat surface, such
as straight
walls and square columns, and structural members with a curved surface, such
as curved
walls and circular columns.
The surface reinforcement agents suitable for the anchorage system of the
present
invention can be selected from FRP sheets, plates and shells and other similar
purpose
metallic or non-metallic materials, including FRP composite materials.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a L-shaped angle anchor in the prior art.
Figure 2 shows an U-shaped anchor embedded within a preforrned groove of a
structural
member in the prior art.
Figure 3A is a cross-sectional side view of the anchorage system of the
present
invention showing a lock-down means holding down an anchor tube together with
sections of the FRP sheet bonded to the reinforced stntctural member and
supporting
member.
Figure 3B is a top view of an anchor tube with the lock-down means mounted
thereon
through a curved sleeve block.
Figure 4 is a cross-sectional view of the anchor tube used in the present
invention.
Figure 5 is a side view of a curved sleeve block.
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Figure 6 is a cross-sectional side view of the anchorage system of the present
invention
showing a lock-down means holding down an anchor rod together with sections of
the
FRP sheet bonded to the reinforced structural member and supporting member.
FYgure 7A is a cross-sectional side view of the anchorage system of the
present
invention showing a continuous anchor strap mounted inside the structural and
supporting members together with sections of the FRP sheet bonded to the
reinforced
structural member and supporting member.
Figure 7B is a cross-sectional side view of the anchorage system of the
present invention
showing a continuous anchor strap with the straps ends projecting through the
supporting
member and threadedly and securely mounted to the supporting member with
washers
and nuts.
Figure 8 is a cross-sectional side view of the anchorage system of the present
invention
showing a lock-down means holding down a half circular anchor tube together
with
sections of the FRP sheet bonded to the reinforced structural member and
supporting
member.
Figure 9 is a cross-sectional side view of the anchorage system of the present
invention
showing a lock-down means holding down an anchor tube with one end of the FRP
sheet
wrapped around thereon and with sections of the FRP sheet bonded to the
reinforced
structural member.
Figure 10 is a cross-sectional side view of the anchorage system of the
present invention
showing a lock-down means holding down an anchor tube against two discontinued
FRP
sheets overlapping at their ends.
Figure 11 is a perspective view of the anchorage system of the present
invention with
FRP sheets bonded to a flat wall.
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Figure 12 is a perspective view of the anchorage system of the present
invention with
FRP sheets bonded to a curved wall.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is applicable to surface reinforcing agent made of
conventional structural reinforcing materials. Preferably, the surface
reinforcing agent is
a surface reinforcing sheet, surface reinforcing plate or surface reinforcing
shell. Also
preferably, the structural reinforcing material is made of non-metal or metal.
More
preferably, structural reinforcing material is made of fiber reinforced
plastic (FRP).
In a typical wall strengthening application, as is the case with the present
invention, it may not be necessary to reinforce the supporting member, such as
the
foundation of a building structure. The surface bonded reinforcement agent,
such as FRP
sheet, may then only be required for the structural member, which is usually
the non-
horizontal structure (in most case, the vertical structure). Accordingly, for
the purpose of
seismic strengthening, continuous FRP sheets are not usually bonded to the
supporting
structures.
The preferred embodiment of the present invention teaches an anchorage system
wherein load transferred from the FRP sheet is applied tangentially to the
circular surface
of the anchor tube or rod, whereas the hold down force exerted by the lock-
down means
of the anchorage system is applied concentrically through the center of the
tube or rod,
thus resulting in always maintaining a pelf centering eccentric arrangement in
the load
transfer mechanism.
Referring now to Figures 3A and 3B, an anchorage system 100 for structural
reinforcement of FRP sheet 200 is constructed by passing FRP sheet 200 around
the
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outer circular surface of an anchor tube 108, thus transferring the load
carried by the FRP
sheet 200 to the anchorage system 100 always in the tangential direction of
the anchor
tube 108. As illustrated in Figure 3A, the structural member and the
supporting member
are perpendicular to one anther, i.e., at 90°. However, as discussed
later in the present
disclosure, application of the present invention is not limited to this
specific structural
orientation.
The FRP sheet 200 is bonded by epoxy 300 or any other conventional bonding
materials to the surface of the strengthening structural member 104 (shown
vertical in
Figure 3A) and to the surface of the supporting member 106 (shown horizontal
in Figure
3A). Typically, structural member 104 is a concrete wall while supporting
member 106
is a concrete foundation. Upon applying tensile load 400 to the FRP sheet 200,
the
resultant action of the applied FRP load and the interface shear force
provided by the
epoxy bond of the FRP sheet 200 to structural member 104 and supporting member
106
is perpendicular to the anchor tube 108 through its center in a direction
equally
subdividing between the structural member surface and the supporting member
surface.
In other words, anchor tube 108 acts as a pulley and the tension stresses
carried by the
FRP sheet 200 attached to the vertical part of structural member 104 equal the
tension in
the horizontal part of the FRP sheet, which is then transferred through the
interface to the
epoxy bonded concrete surface of footing along supporting member 106. The
resultant
action is to pull out the anchor tube 108 in that direction, i.e., away from
the FRP sheet
200, at 45° which is the direction of the resultant of the two FRP
sheet forces on the
structural member and the supporting member.
To resist the tendency of this pull out, a lock-down means 102 is securely
mounted at a 45° angle (i.e. in the direction of the resultant of the
two FRP sheet forces,
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typically bisecting the angle between the surface of the strengthening
structural member
104 and the surface of the supporting member 106) on anchor tube 108 through
pre-
drilled hole 108' to provide an anchoring force through the tube center. The
anchoring
force is applied in a direction exactly opposite to the pull out force. As
shown in Figure
3A, the lock-down means 102 is an anchor bolt 120. Optionally, the anchoring
force is
applied by the lock-down means 102 through a curved sleeve block 114 (see
Figure 5)
onto the anchor tube 108.
In the event that the structural member and the supporting member are not
perpendicular to one another, then the resultant action to pulling out the
anchor tube 108
is at an angle of the resultant of the two FRP sheet forces, typically
bisecting the angle
between the two bonded surfaces of the structural member and supporting
member. As a
corollary, the lock-down means 102 should be mounted on anchor tube 108 at
this
bisecting angle.
ExE ample
The anchor system of the present invention can be illustrated with the
following
strengthening wall example.
The design load of the anchor system is the load that the FRP sheet applies to
the
anchor tube in a wall specimen loaded at the top by a lateral force. The
dimensions of
the anchor tube are selected so that the maximum stress in the anchor tube
under the
design load does not exceed the yield stress of the anchor tube material.
Using deep
beam theory to determine the vertical tensile force distribution at the base
of a flat
rectangular wall panel loaded by a lateral force applied at the top, the
vertical tensile
force is found to be maximum at the end edge of the wall, and it reduces in
magnitude
towards the center of the wall width. This distributed vertical tensile load
(line load) is
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applied on the surface of the anchor tube which is in contact with the FRP
sheets. This
load is only one part of the loads applied to the anchor tube. The second part
is the load
applied from the FRP sheet which extends horizontally on the footing surface
which, due
to the pulley effect, can be considered as equal to the load applied from the
vertical FRP
structural wall sheets. The resultant of these two components is the design
load
mentioned before. The maximum load carrying capacity of the strengthened wall
with
FRP sheets attached on each side of the wall can be determined from mechanics
using
the tensile material strength of the FRP sheets and the strength of the wall
material.
In a concept feasibility and verification study of the anchor system of the
present
invention, a 3-inch external diameter steel mechanical pipe with a 0.5 inch
wall thickness
was chosen for the fabrication of the anchor tube in the anchor system for
strengthening
of a flat reinforced concrete rectangular shear wall of dimensions 100 mm
thick x 1500
mm wide x 1795 mm high loaded by a 500 kN in-plane lateral force at the top. A
curved
sleeve block with its curving surface matching the curvature of anchor tube
was
fabricated from steel plate (3.5" x 25." x 1'~. A hole of 1.5" diameter was
drilled through
the sleeve block and anchor tube for insertion of the lock-down means. The
lock-down
means in this example was an anchoring threaded rod with 1'/<" diameter and
20" in
length with flat washer and nut.
Figure 4 shows anchor tube 108 and the pre-drilled hole 108' for lock-down
means 102 to pass therethrough. Suitable lock-down means 102 include chemical
adhesive anchor, expansion anchor, anchor bolt, anchor strap threaded to
washer and nut
etc.
In the event that there is obstruction for the lock-down means 102 to
penetrate the
FRP bonded structure at an angle, another embodiment of the anchorage system
of the
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present invention provides for an anchor strap 118 to hold down the anchor
tube 108. In
that case, a plurality of anchor strap 118 are projected into the
strengthening structural
member and the supporting member (see Figure 7A), thereby securing the anchor
tube
108 in place. Anchor strap 118 can be made of steel cable or steel rod, or
cable or rod
made of other suitable material.
Sometimes the supporting member may allow the anchor straps to project
through the structure as shown in Figure 7B. In Figure 7B, a plurality of
anchor strap
118 are projected through the supporting member and threadedly and securely
mounted
to the supporting member with washers and nuts 122.
While Figures 7A and 7B show FRP are surface bonded to both sides of the
structural member, it should be noted that the double-sided bonding is
desirable for
strengthening a free standing wall. However, FRP surface double-sided bonding
is
unnecessary in most other cases.
In another embodiment of the anchorage system of the present invention, an
anchor rod 110 is used to hold down or wrap up the FRP sheet instead of an
anchor tube
108 (see Figures 6).
Referring to Figure 8, another embodiment of the anchorage system of the
present invention uses a half circular tube 112 or half circular rod (not
shown). Since
such a system applies the same pulley concept, the resultant action in pulling
out the
anchor tube away from the FRP sheet at 45°, the direction of the
resultant of the two FRP
sheet forces on the structural member and the supporting member which are
perpendicular to one another, is the same. By employing a similar lock-down
means
mounted at a 45° angle onto the half-circular tube or rod, it provides
the necessary
anchoring force through the tube center.
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As discussed earlier, for structural member and supporting member that are not
perpendicular to one another, then the lock-down means 102 should be mounted
on half
circular tube 112 at the angle bisecting the angle between the two bonded
surfaces of the
structural member and supporting member.
Figure 9 teaches another embodiment of the anchor system of the present
invention. It shows a cross-sectional side view of an anchorage system with a
lock-down
means holding down an anchor tube with one end of the FRP sheet wrapped around
thereon and with the remaining sections of the FRP sheet bonded to the
reinforced
structural member. This modified system is particular advantageous when
supporting
member, such as a concrete foundation, does not have sufficient clearance
surface for
continuous FRP bonding,-or the FRP sheet is of limited dimension and the end
of the
sheet ends near the anchor tube.
In another embodiment as shown in Figure 10, the anchor system of the present
invention can be used to enhance the strength and performance of an
overlapping joint of
1 S the free end portions of the two separate FRP sheets. In such a case, the
anchor tube can
accommodate the first free end portion of one FRP sheet bonded to the
structural
member, and the second free end portion of another FRP sheet bonded to the
supporting
member. This results in continuing the FRP sheet bonding of two free end
portions of
FRP sheets.
VYhile the anchorage system of the present invention is applicable for FRP
sheets
bonded to flat surfaces (see Figure 11), due to its unique design, it can be
advantageously
applied to curved surfaces, such as circular columns or curved wall
structures. In the
case of curved walls, such as the one shown in Figure 12, a flexible or bent
anchor tube
is placed along the curvature of the two walls and held down by the lock-down
means at
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suitable spaced apart intervals. Depending on the dimension of the walls and
degree of
the curvature, the anchor tube can be made of materials with flexural strength
to
capacitate necessary bending of the tube.
Application of the anchorage system of the present invention is not limited to
anchorage application of bonded FRP sheet. The structural and/or supporting
surfaces
can be reinforced with bonded or unbonded reinforcing plate or shell made of
FRP or
steel or other metallic or non-metallic materials.
It can be readily observed that the anchorage system of the present invention
is
applicable for rehabilitating existing structures as well as for building new
structures.
It is clear that the inventive concept of this anchorage system is not limited
to
retrofitting or repairing of existing structures, such as seismic upgrade of
structural and
supporting walls. Any new building structures can incorporate the present
inventive
concept and provide for improved structural reinforcements. Thus, the
embodiments
depicted herein are intended to be merely illustrative and not restrictive in
any sense.
It is further understood that the present invention may be carried out in
other
specific way than those herein set forth without departing from the spirit and
essential
characteristics of such invention. The present embodiments are, therefore, to
be
considered in all respects as illustrative and not restrictive, and all
changes coming
within the meaning and equivalency range of the appended claims are intended
to be
embraced therein.
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