440.7 R 10 Download
The use of unreinforced masonry (URM), particularly for infill walls, is a relatively common practice in building construction throughout the world. Typically, URM walls have very low flexural capacities and possess brittle modes of failure making them highly susceptible to damage when exposed to significant out-of-plane loads. In-plane behavior is also very important to resist lateral loads. Significant research has focused on improving both the out-of-plane and in-plane behavior of URM wall systems by using modern materials such as fiber-reinforced polymers (FRPs) to externally retrofit structures. The use of FRP to retrofit URM wall systems has been proven to be highly effective in improving both the load resistance and the deformability of URM walls subjected to out-of-plane loads and in-plane loads (Velazquez-Concrete Institute (ACI) published a new standard ACI 440.7R-10 entitled " Guide for the Design and Construction of Externally Bonded Fiber-Reinforced Systems for Strengthening Unreinforced Masonry System. " This paper will provide an overview of the ACI design procedures for in-plane and out-of-plane strengthening of URM wall systems. It will also provide two example case studies to further detail the design processes of the new standard in an effort to further disseminate the design standard.
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9th Australasian Masonry Conference
Queenstown, New Zealand
15 – 18 February 2011
STRENGTHENING UNREINFORCED MASONRY STRUCTURES USING
EXTERNALLY BONDED FIBER REINFORCED POLYMER SYSTEMS: AN
OVERVIEW OF THE AMERICAN CONCRETE INSTITUTE 440.7R DESIGN
APPROACH
J.J. MYERS
1 Associate Professor, Missouri University of Science and Technology, Rolla, Missouri, USA
SUMMARY
The use of unreinforced masonry (URM), particularly for infill walls, is a relatively common
practice in building construction throughout the world. Typically, URM walls have very low
flexural capacities and possess brittle modes of failure making them highly susceptible to
damage when exposed to significant out-of-plane loads. In-plane behavior is also very
important to resist lateral loads. Significant research has focused on improving both the out-
of-plane and in-plane behavior of URM wall systems by using modern materials such as
fiber-reinforced polymers (FRPs) to externally retrofit structures. The use of FRP to retrofit
URM wall systems has been proven to be highly effective in improving both the load
resistance and the deformability of URM walls subjected to out-of-plane loads and in-plane
loads (Velazquez-Dimas et al., 2000; Tumialan et al., 2001; Carney, 2003; Hrynyk, 2008;
Tanizawa, 2009).
In 2010, the American Concrete Institute (ACI) published a new standard ACI 440.7R-10
entitled "Guide for the Design and Construction of Externally Bonded Fiber-Reinforced
Systems for Strengthening Unreinforced Masonry System." This paper will provide an
overview of the ACI design procedures for in-plane and out-of-plane strengthening of URM
wall systems. It will also provide two example case studies to further detail the design
processes of the new standard in an effort to further disseminate the design standard.
INTRODUCTION
Masonry is a generic term used to describe a type of construction where clay, or concrete
masonry units, or natural stones are bonded together to form a load-bearing structure or a
component in a structure. Non-load-bearing masonry includes partitions and veneers (ACI
440.7R, 2010). The use of unreinforced masonry (URM) for walls has been and continues to
be common practice in building construction throughout the United States and around the
world.
While unreinforced masonry structures are widely considered a highly sustainable material,
they have shown their vulnerability to major events such as earthquakes, severe wind, blast,
and impact. Furthermore, factors such as change in occupancy, deterioration, or an increase in
lateral-load demand, may generate the need to undertake structural retrofit. The 2008 joint
TMS 402/ACI 530/ASCE 5 Building Code Requirements for Masonry Structures
covers the design and construction of new masonry. Repair, retrofitting, and rehabilitation of
masonry structures are not included in that code. Specifications for masonry structures
are detailed in the joint TMS 405/ACI 530.1/ASCE 6 Specification for Masonry
Structures document.
Within the past two decades, fiber-reinforced polymer (FRP) systems have been developed
for infrastructure applications and used for strengthening and repair of existing structures as
an alternative to traditional strengthening methods, such as steel plate bonding, section
enlargement, and external post-tensioning. The use of FRP to retrofit URM wall systems has
been proven to be highly effective in improving both the load resistance and the deformability
of URM walls subjected to out-of-plane loads and in-plane loads (Velazquez-Dimas, 2000;
Tumialan, 2001; Carney, 2005; Hrynyk, 2008; Tanizawa, 2009). The initial attraction of using
FRP in construction was because it did not experience the common durability problems that
are typically associated with conventional steel reinforcement. Additionally, FRP
reinforcement is lightweight and is available in multiple forms, many of which could easily be
manipulated to match variable structural shapes and geometries (ACI 440.2R, 2002). FRP
materials are readily available in several forms such as laminates, sheets, meshes, and bars
(see Figure 1). They are relatively expensive at the present when compared to concrete and
steel as strengthening materials, but labor and equipment costs to install FRP systems are
often lower. FRP systems can also be used in areas with limited access where traditional
techniques would be difficult to implement such as in tight confined areas where post cured
systems allow for improved flexibility.
Today significant research is still being carried out to assess the use of FRP as an alternative
for steel reinforcement. However, in recent years, the focus of much of the research has
shifted to the use of FRP as a means of retrofitting current infrastructure. As a result, the use
of various externally bonded FRP systems and near surface mounted (NSM) systems
continues to be studied extensively not only in concrete structures, but also in masonry, steel,
and timber structures. The benefits of using FRP as opposed to conventional steel in structural
retrofits are the same as those in new construction and in most cases externally bonded FRP
systems are less intrusive to building occupants. This benefit is primarily because externally
bonded FRP systems and NSM systems are typically easy to install and are less time
consuming than conventional retrofit methods.
There are several different retrofit methods that can be employed to increase the out-of-plane
and in-plane load resistance and improve the behavior of URM wall systems including infill
systems. Conventional masonry retrofitting methods, which typically involve the use of
additional concrete and steel reinforcement, tend to not only add significant mass to a
structure, but in many cases the methods result in a reduction of available space for building
occupants. In addition to the effects on the building, conventional retrofit methods also tend to
be both time consuming and expensive. The use of modern retrofit systems, which involve the
use of fiber reinforced polymers (FRP) or elastomeric coatings, are aimed to address and
improve upon the negative traits associated with conventional techniques of retrofitting
masonry structures.
The two most common retrofit techniques involve externally bonded FRP systems and NSM
systems. Both of these techniques have demonstrated promise for upgrading the flexural and
shear strengthening of masonry systems. Figures 2 and 3 detail both of these systems.
Externally bonded systems have demonstrated more robustness for extreme events, while
NSM allows for the reinforcement to be installed near the surface within the existing bed
joints and therefore impacts the aesthetics of the masonry wall to a lesser degree. More
recently, new coating technologies such as elastomeric polyurea with and without discrete
fibers, as shown in Figure 4, have also shown great promise for hardening of masonry
systems, but the current ACI 440.7R document does not cover this technology at the present
as it is still in the developmental stage.
SCOPE OF ACI 440.7R-10 DOCUMENT
This 440.7R guide offers general information on FRP systems use, a description of their
unique material properties, and recommendations for the design, construction, and inspection
of FRP systems for strengthening URM structures. The guidelines are based on knowledge
gained from a comprehensive review of experimental and analytical investigations and field
applications. At present the guide provides information on the selection and design of FRP
systems limited to externally bonded FRP laminates and near-surface-mounted FRP
bars/strips for increasing the in-plane and out-of-plane strength of existing ungrouted,
grouted, or partially grouted URM walls; infill walls are not included in this guide. The guide
is applicable to URM structures made of clay bricks, concrete masonry units, and natural
stones using conventional types of mortar.
Figure 1. FRP products, CFRP and GFRP rods (left) and CFRP and GFRP laminates (right).
Figure 2. Externally bonded FRP laminate strengthening for masonry.
Figure 3. Near surface mounted (NSM) strengthening for masonry.
Figure 4. Discrete fiber polyurea retrofit for in-plane strengthening (Tanizawa, 2009).
For masonry with significant deterioration, questionable mortar bond, and cracking and/or
element displacement, traditional procedures may be required as well as FRP strengthening.
The evaluation for the need to apply traditional modes of strengthening is not covered in the
guide (ACI 440.7R, 2010).
FRP CONSTITUENT MATERIALS AND PROPERTIES
The behavior of FRP-reinforced masonry structures depends on the physical and material
properties of the existing masonry, as well as the FRP system. The physical and mechanical
properties of the masonry should be investigated and known prior to the retrofit or repair
design. The effects of factors such as loading history and duration, temperature, and moisture
on the properties of FRP systems can influence the design and long-term service performance.
Fiber-reinforced polymer systems are available in a variety of forms such as wet layup,
prepreg, and procured systems. Factors such as fiber volume, type of fiber, type of resin, fiber
orientation, dimensional effects, and quality control during manufacturing, all play a role in
establishing characteristics of the FRP system (ACI 440.7R, 2010). Test methods according to
ASTM standards should be used for material characterization; for test methods not included
in ASTM standards, reference should be made to ACI 440.3R. Preconstruction quality
assurance testing of the FRP strengthening system, hereby called FRP system or FRP
reinforcement, is recommended.
Physical Properties
Fiber-reinforced polymer materials have densities ranging from1.2 to 2.1 g/cm3 (75 to 130
lb/ft3 ), which is four to six times less than that of steel, as indicated in Table 1 (ACI 440.7R).
The material characteristics described in this section are generic and do not apply to all
commercially available products. The coefficients of thermal expansion of unidirectional FRP
materials differ in longitudinal and transverse directions, depending on the fiber type, resin
type, and fiber content. Table 2 (ACI 440.7R) lists the longitudinal and transverse coefficients
of thermal expansion for typical unidirectional FRP materials. Note that a negative coefficient
of thermal expansion indicates the material contracts with increased temperature and expands
with decreased temperature. For reference, concrete has a coefficient of thermal expansion
that varies from 7x10–6 to 11x10–6 /°C (4x10–6 to 6x10–6 /°F) and is usually assumed to be
isotropic (Young et al., 2002). Steel has an isotropic coefficient thermal expansion of
11.7x10–6 /°C (6.5x10–6 /°F).
Mechanical Properties
When loaded in direct tension, FRP materials do not exhibit any plastic behavior, or yielding,
before rupture. The tensile behavior of FRP materials consisting of one type of fiber material
is characterized by a linear elastic stress-strain relationship until failure, which is sudden and
without warning. The tensile strength and stiffness of a FRP material depends on several
factors. Because the fibers in a FRP material are the main load-carrying constituent, fiber
type, fiber orientation, fiber quantity, and method and conditions that the composite is
manufactured affect the tensile properties of the FRP material. Due to the primary role of the
fibers as tensile reinforcement and methods of application, the properties of a FRP system are
sometimes reported based on the net-fiber area (Method 2 of 440.3R). In other examples, such
as in pre-cured laminates, the reported properties are based on the gross-laminate area
(Method 1 of 440.3R). Externally bonded FRP systems should not be used as compression
reinforcement because of insufficient test results validating its use in this type of application
(ACI 440.7R, 2010).
Table 1. Typical densities of materials, g/cm3 (lb/ft3 ).1
Steel GFRP CFRP AFRP
7.9 1.2 to 2.1 1.5 to 1.6 1.2 to 1.5
(490) (75 to 130) (90 to 100) (75 to 90)
1 Table adapted from ACI 440.7R-10.
Table 2. Typical coefficient of thermal expansion for FRP materials.1,2
Direction
Coefficient of thermal expansion
x 10-6 /ºC (x 10-6 /ºF)
GFRP CFRP AFRP
Longitudinal, α L 6 to 10
(3.3 to 5.6)
-1 to 0
(-0.6 to 0)
-6 to -2
(-3.3 to -1.1)
Transverse, α T 19 to 22
(10.4 to 12.6)
22 to 50
(12 to 27)
60 to 80
(33 to 44)
1 Table adapted from ACI 440.7R-10.
2 Typical values for fiber-volume fractions ranging from 0.5 to 0.7.
FRP materials subjected to a constant tensile load over time can suddenly fail after a time
period called the endurance time. As the ratio of the sustained tensile stress to the short-term
strength of the FRP laminate increases, endurance time decreases. The creep rupture time may
also decrease under adverse environmental conditions, such as high temperature, ultraviolet
radiation exposure, high alkalinity, wetting-and-drying cycles, or freezing-and-thawing
cycles. In general, carbon fibers are the least susceptible to creep rupture, aramid fibers are
moderately susceptible, and glass fibers are most susceptible. Therefore, the design guideline
specifies stress limits of ultimate tensile capacity for carbon (55%), glass (20%) and aramid
(30%) to prevent creep rupture of the FRP material while in service.
The fatigue performance of the FRP system is generally considered a non-issue in URM
structures that are typically strengthened with FRP systems because the systems are intended
to resist loads with low cycle counts, such as earthquake, hurricane, and blast loads. ACI
440.2R provides more details regarding the fatigue sensitivity of FRP materials in fatigue
sensitive applications.
It has been widely reported that many FRP systems exhibit reduced mechanical properties
after exposure to certain environmental factors, including high temperature, humidity, and
chemical exposure. The type of FRP strengthening technique may also affect the long-term
durability performance of the strengthened system. The exposure environment, duration of the
exposure, resin type and formulation, fiber type and volume fraction, and resin-curing method
are some of the factors that influence the extent of the reduction in mechanical properties.
GENERAL DESIGN CONSIDERATIONS
The strength design approach taken in the ACI 440.7R document is a limit states approach to
provide acceptable safety levels. All possible failure modes must be investigated to
understand the controlling mechanism of failure. This is particularly critical when
strengthening with FRP for a particular behavior such as flexure. The shear capacity after
retrofit must be investigated to ensure the desirable mode of failure occurs at the desired
strengthening level. The ACI 440.7R document provides guidance on strengthening a
particular masonry element, but ASCE 41-06 is recommended to perform a global analysis of
the overall structure to understand how component strengthening will affect the global
behavior of a structure.
For masonry structures, the ACI 440.7R document does not specify strengthening limits like
the ACI 440.2R document does for concrete structures. The rational for this is that masonry
structures are strengthened for special loading events such as earthquake, wind, hurricane, and
blast loads. These loads are rare and typically not sustained. Therefore, the probability of
simultaneous occurrence of damage to FRP, for example from vandalism or exposure to high
temperatures and high short-term loads, like from earthquake or wind, is low; therefore, a
strengthening limit for these applications is unnecessary (ACI 440.7R, 2010). However, the
document does stipulate that strengthening limits should be considered for two specific cases;
namely, cases that include masonry walls resisting out-of-plane loads due to earth pressure
and walls that are part of the primary lateral load-carrying system resisting in-plane loads
from wind.
Design Material Properties
Because long-term exposure to various types of environments can reduce the tensile
properties and creep rupture / fatigue endurance of FRP laminates, the material properties
used in design equations are reduced based on the environmental exposure condition. Eqs (1)
through (3) give the tensile properties that are used in all design equations. The design tensile
strength is determined using the environmental reduction factor given in Table 3 for the
appropriate fiber type and exposure condition.
ffu = CE ffu * (1)
Similarly, the design rupture strain should also be reduced for environmental-exposure
conditions.
εfu = CE εfu * (2)
Fiber-reinforced polymer materials consisting of one type of fiber oriented predominantly in
one direction are practically linearly elastic until failure. Their modulus of elasticity does not
vary significantly with environmental exposure and loading history and can be computed
according to Eq. [1].
Ef = f fu / ε fu (3)
Constituent materials affect the durability and resistance to environmental exposure of a FRP
system. The environmental-reduction factors given in Table 3 are estimates based on the
relative durability of each fiber type. As more research information is developed and becomes
available, these values may be refined. The relative durability of NSM systems versus surface
applied laminates may be better. Given the lack of data, however, this guide conservatively
recommends use of the same CE factors for both applications.
Table 3. Environmental reduction factors for various FRP systems and exposure systems.1
Exposure conditions Fiber type Environmental reduction
factor, CE
Interior exposure (for example,
partitions)
Carbon 0.95
Glass 0.75
Aramid 0.85
Exterior exposure (including int.
side walls of ext. walls)
Carbon 0.85
Glass 0.65
Aramid 0.75
Aggressive environmental
(basement walls)
Carbon 0.85
Glass 0.50
Aramid 0.70
1 Table adapted from ACI 440.7R-10.
Debonding of the FRP system can occur if the force in the FRP system at the strength limit
state cannot be sustained by the masonry substrate. For a typical FRP system that is linear
elastic until failure, the level of strain in the FRP system will dictate the level of stress
developed in the system. To prevent debonding, a limitation is placed on the strain level
developed in the FRP laminate. The maximum strain and corresponding stress that FRP
systems can attain before debonding from the masonry substrate are defined as effective strain
εfe and effective stress ffe.
Effective Strain and Stress in FRP at the Strength Limit State: flexural-controlled failure
mode
The effective strain εfe and effective stress ffe used for the design of flexural out-of-plane and
in-plane FRP strengthening of masonry walls can be computed according to Eq. (4) and (5),
respectively:
εfe = κ m εfu * ≤ CE εfu * (4)
ffe = Ef ε fe (5)
where κm is a bond reduction coefficient calibrated using available experimental data (ACI
440.7R-10), defined as in Eq. (6). This coefficient is subject to force per unit widths as
detailed in ACI 404.7R-10 since it is based upon current experimental data.
κm = 0.45 for surface mounted FRP systems; = 0.35 for NSM FRP systems (6)
Effective Strain and Stress in FRP at the Strength Limit State: shear-controlled failure mode
The effective strain εfe and effective stress ffe to be used for the design of shear in-plane FRP
strengthening of masonry walls can be computed according to Eq. (7) and (8), respectively:
εfe = κ v εfu * ≤ CE εfu * (7)
ffe = Ef ε fe (8)
The bond reduction coefficient for shear-controlled failure modes κv depends on the FRP
reinforcement index ωf , defined in Eq. (9).
'
1000
1
mn
ff
ffA
EA
for in.-lb units or '
85
1
mn
ff
ffA
EA
for SI units (9)
For shear-controlled failure modes, the bond reduction coefficient is again calibrated based on
experimental data (ACI 440.7R, 2010). The coefficient for shear-controlled failure modes is
equal for both FRP laminates and NSM FRP systems and is given in Eq. (10). Similar to
flexure-controlled failure modes, this coefficient is subject to force per unit widths as detailed
in ACI 404.7R-10.
(10)
It is recognized by ACI 440.7R that in Eq. (4) and (7), the κ values will always control over
the CE values. The κ values, however, were set as a lower bound from experimental data. It is
expected that further experimental data added to the data base may result in higher κ values in
the future. Limitation involving the CE value is presented to establish the design philosophy.
Furthermore, if experimental data for a particular application is available, the designer may
wish to incorporate different CE or κ values. In this case, it is recommended to follow the
limitations given by Eq. (4) and (7).
WALL STRENGTHENING FOR OUT-OF-PLANE LOADS
A number of research projects involving out-of-plane strengthening have been conducted to
study the use of FRP systems for flexural strengthening of masonry walls. The guide provides
a literature overview of several of these studies and may be referenced for greater detail.
Existing Wall Strength
To determine whether FRP strengthening is needed, the existing out-of-plane strength of the
wall should be evaluated first. Unreinforced masonry walls should be analyzed for out-of-
plane seismic forces and wind pressures, or both, and earth pressures as isolated elements
spanning between floor levels and spanning horizontally, or both, between columns or
pilasters (TMS 402/ACI 530/ASCE 5; ASCE 41-06) and the applicable local building code,
or both. This includes analyzing the system for flexural, shear, and axial strengths to
determine what level of retrofit is required.
Nominal Flexural Strength of FRP-Reinforced Masonry Walls Subjected to Out-of-Plane
Loads
The strength design method requires that the flexural strength of the FRP-strengthened wall
(Mn ) times the phi factor exceeds the factored moment (Mu ), as indicated by Eq. (11). ACI
440.7R-10 recommends a phi factor = 0.6, as required in TMS 402/ACI 530/ASCE 5 for
URM walls subject to flexure load, axial load or a combination thereof. Assuming that the
factored axial load Pu acts at t /2 (t = thickness of wall), the nominal flexural strength Mn of
the FRP-strengthened masonry wall can be determined from Eq. (12) using strain
compatibility, internal force equilibrium, and the controlling mode of failure.
un MM
(11)
222
11 c
t
P
c
dfAM uffefn
(12)
where ffe represents the effective stress to be calculated according to Eq. (5) with the strain
level εfe as defined in Eq. (13). The maximum strain level that can be achieved in the FRP
reinforcement is determined by the strain developed in the FRP system at the ultimate limit
state by either crushing of the masonry or FRP system debonding. The maximum strain or
effective strain level in the FRP reinforcement may be calculated as defined in Eq. (13).
**
min ,min fuEfumfe C
c
ct
(13)
The effective stress level in the FRP reinforcement is the maximum level of stress that can be
developed in the FRP reinforcement before reaching the ultimate limit state. This effective
stress can be calculated from the strain level in the FRP system, assuming perfectly elastic
behavior as given in Eq. (5).
The failure mode of that will control the behavior of URM walls strengthened with externally
bonded FRP systems include (a) crushing of the masonry in compression or (b) debonding of
the FRP system from the masonry substrate. Anchorage on the FRP system may delay or
prevent the premature debonding of the external retrofit system. The ACI 440.7R-10
document provides information on details in this regard.
WALL STRENGTHENING FOR IN-PLANE LOADS
A number of research projects involving in-plane strengthening have been conducted to study
the use of FRP systems for shear strengthening of masonry walls. The guide provides a
literature overview of several of these studies and may be referenced for greater detail.
Existing Wall Strength
Similar to flexural strengthening, the existing in-plane strength of the wall should be
evaluated first. The behavior of URM walls under in-plane loads depends on several
parameters related to geometry including height, thickness, slenderness, and bond pattern as
well as mechanical properties of the materials and the loading / support conditions. Three
failure modes may result from in-plane shear including joint sliding (Vjs ), diagonal tension
(Vdt ), and toe-crushing (Vtc ). Joint sliding and diagonal tension are shear-controlled failure
modes while toe-crushing is a flexural-controlled failure mode. Each must be investigated and
existing documents such as TMS 402/ACI 530/ASCE 5, ASCE 41-06, and FEMA 306 may
be referenced. The nominal shear strength of URM walls should be computed as defined in
Eq. (14)
tcdtbjs
URM
nVVVV ,,min (14)
Nominal Shear Strength of FRP-Reinforced Masonry Walls Subjected to In-Plane Loads
The strength design method requires that the shear strength of the FRP-strengthened wall (Vn )
times the phi factor exceeds the factored shear (Vu ), as indicated by Eq. (15). ACI 440.7R
recommends a phi factor = 0.8, as required in TMS 402/ACI 530/ASCE 5 for URM walls
subject to in-plane shear load.
un VV
(15)
Unreinforced masonry walls requiring shear strengthening against in-plane loads are those
walls whose failure mode is due to either stepped joint sliding or diagonal tension. A typical
FRP strengthening scheme performed either with wet layup or NSM systems is indicated in
Fig. 5 (a) and (b), respectively. Other layouts, including fibers placed diagonally, have been
used, but they are not covered in the scope of the ACI 440.7R guide. The design method is
based on the assumption that FRP debonding governs the behavior of the FRP-strengthened
wall. Bonding the FRP into boundary elements such as beams or columns can help reduce the
risk of wall overturning due to out-of-plane excitations. In several situations, however,
extension of FRP may not be practical due to existing field conditions. For example,
intersecting walls, slabs, or columns with dimensions larger than the wall thickness can
preclude extending the FRP beyond the wall. In such cases, other mechanical anchors should
be considered. The in-plane performance of FRP-strengthened walls is highly dependent on
the type of masonry construction and the FRP strengthening layout. Experimental
investigations have shown that FRP systems can significantly increase the shear capacity of
URM walls when the original shear strength of the wall is not large (for example, single-
wythe or ungrouted walls). Contrarily, when the shear strength of the URM wall is large (for
example, multi-wythe or grouted walls), the contribution of FRP has been observed to be
marginal in some situations. Test results also indicate that the FRP layout influences the
wall's structural performance. For instance, in thick walls, FRP placed on the two wall sides
has been shown to be more effective than FRP placed on one side. In the absence of project
specific experimental evidence, the guide recommends FRP strengthening layouts based on
the masonry construction as shown in the guide, where the wall thicknesses are given in
nominal dimensions (ACI 440.7R, 2010).
The nominal shear strength of the FRP-strengthened wall can be computed by adding the FRP
contribution Vf to the nominal strength of the URM wall, computed according to the
provisions discussed in Eq. (14); VnURM in Eq. (16). The nominal in-plane flexural strength of
the FRP-strengthened wall is the minimum of the nominal shear strength given in Eq. (16)
and the nominal lateral strength corresponding to toe-crushing of the URM wall. The FRP
contribution to the shear strength Vf can be determined from Eq. (17a or 17b) based upon the
strengthening system.
a) Horizontal strips b) horizontal bars
Figure5. FRP strengthening of shear-controlled walls (adapted from ACI 440.7R).
f
URM
nsn VVV
, (16)
f
v
ffvf s
d
V
(surface mounted FRP) or
f
v
fvf s
d
V
(NSM FRP systems) (17)
where pfv is computed according to limits specified in ACI 440.7R-10, wf is the width of the
FRP laminates, sf is the center-to-center spacing between each strip, and dv is the effective
masonry depth for shear calculations given by
),(min LHdv (18)
Nominal Flexural Strength of FRP-Reinforced Masonry Walls Subjected to In-Plane Loads
The strength design method requires that the flexural strength of the FRP-strengthened wall
exceeds the factored flexural demand, as indicated by Eq. (19). ACI 440.7R-10 recommends a
phi factor = 0.6, as required in TMS 402/ACI 530/ASCE 5 for URM walls under combination
of axial load and bending. Unreinforced masonry walls requiring flexural strengthening are
those walls whose failure mode is due to toe crushing, as discussed previously. Assuming that
the factored axial load Pu acts at L /2 (L = length of the wall), the nominal moment capacity
Mn of a FRP-strengthened masonry wall subjected to in-plane loading can be calculated from
Eq. (20) according to the assumptions and provisions given in the guide.
un MM
(19)
222
11 cL
P
c
dFM uiin
(20)
where Fi is the force acting on the i -th FRP strip located at a distance di from the extreme
compression fiber. In cases where Pu does not act at L /2, Eq. (20) should account for the
eccentricity of the axial load by inserting an appropriate value instead of L /2.
The nominal lateral strength corresponding to flexural failure of the FRP-strengthened wall
can be obtained as
eff
n
nhk
M
V
(21)
where k is the coefficient that accounts for the boundary condition of the wall, k (k = 0.5 and k
= 1.0 for a fixed-fixed and fixed-free wall, respectively), and heff is the wall height. The
nominal lateral strength of the FRP-strengthened wall is the minimum of the nominal lateral
strength corresponding to flexural failure given in Eq. (21) and the nominal lateral strength
corresponding to shear failure of the URM wall (that is, minimum between joint sliding and
diagonal tension). Similar considerations and identical procedures may be repeated in the case
of NSM FRP systems installed on a wall as an alternative to the wet layup system.
SUMMARY
This paper provides an overview of the ACI 440.7R-10 design procedures for in-plane and
out-of-plane strengthening of URM wall systems. The reader may be referred to that
document for greater details on anchoring of these repair systems, drawings, specifications,
submittals, and a series of design examples for both out-of-plane and in-plane strengthening.
REFERENCES
American Concrete Institute 440.2R-02, "Guide for the Design and Construction of
Externally Bonded FRP Systems for Strengthening Concrete Structures," ACI , 2002.
American Concrete Institute 440.3R Guide Test Methods for Fiber-Reinforced Polymers
(FRPs) for Reinforcing or Strengthening Concrete Structures," ACI 2003.
American Concrete Institute 530/530.1, The Masonry Society 402/405 and ASCE 5/6, Joint
Document, "Building Code Requirements for Masonry Structures," ACI , TMS, and ASCE ,
2008.
American Concrete Institute 440.7R, "Guide for the Design and Construction of Externally
Bonded Fiber-Reinforced Systems for Strengthening Unreinforced Masonry System," ACI ,
2010.
American Society of Civil Engineers 41-06, "Seismic Rehabilitation of Existing Buildings,"
ASCE, 2006.
Carney, P., Myers, J.J., "Static and Blast Resistance of Unreinforced Masonry Wall
Connections Strengthened with Fiber Reinforced Polymers," American Concrete Institute
Special Publication-230, FRPRC-7, November 2005, pp. 229-248.
Hrynyk, T., Myers, J.J., "Out-of-Plane Behavior of URM Arching Walls with Modern Blast
Retrofits: Experimental Results and Analytical Model," American Society of Civil
Engineering–Journal of Structural Engineering, Vol. 134, No. 10, Oct. 2008, pp. 1589-1597.
Young, J.F., Mindness, S., and Darwin, D., "Concrete," Prentice Hall , 2nd Edition, 2002.
Tanizawa, Y., Myers, J.J. Sinclair, R. "In-Plane Response on an Alternative URM Infill Wall
System with and without a Polyurea Retrofit," 9th International Symposium on Reinforced
Polymer Reinforcement for Concrete Structures," Sydney, Australia, 2009, 4p.
Tumialan J. G., 2001, "Strengthening of Masonry Structures with FRP Composites," Doctoral
Dissertation, University of Missouri-Rolla , Rolla, MO, 186 pp.
Velazquez-Dimas, J.; Ehsani, M.; and Saadatmanesh, H., 2000, "Out-of-Plane Behavior of
Brick Masonry Walls Strengthened with Fiber Composites," American Concrete Institute
Structural Journal, V. 97, No. 3, May-June, pp. 377-387.
... W analizie wzmocnień należy uwzględnić również przypadki wandalizmu i wpływu wysokich temperatur. Zalecenia ACI 440.7R-10 [N8] dopuszczają również instalację systemów naprawczych w celu zwiększania nośności dla typowych przypadków obciążeń [45], nie dotyczą jednak naprawy murów już uszkodzonych. Obliczeniową wytrzymałość na rozciąganie ffu laminatu FRP zdefiniowano jako: ...
... The use of fiber-reinforced polymer (FRP) composites of various organic matrices and fiber compositions have proven to be highly effective in enhancing the shear resistance and pseudoductility of masonry structures (Tinazzi 2000;Tumialan et al. 2001;Morbin 2002;Nanni et al. 2003;Grando et al. 2003;Li et al. 2005;Yu et al. 2007;Silva et al. 2008;Myers 2011). Tinazzi (2000) and Morbin (2002) carried out an experimental program applying diagonal compression to clay brick walls retrofitted by embedding glass FRP (GFRP) bars in the mortar joints, known as near-surface mounted (NSM) technique, and adhering external GFRP laminates. ...
Unreinforced masonry (URM) walls have been constructed for the past millennia and are still widely used today. URM walls have proven to have low shear strength and are prone to brittle failure when subjected to in-plane loads caused by earthquake or wind. Retrofitting URM walls is accomplished internally and externally using current techniques, such as placing steel bars in the cavities and grouting, post-tensioning with steel tendons, stitching, and adhering fiber-reinforced polymers (FRP) to increase capacity and enhance pseudoductility. In this study, a fabric-reinforced cementitious matrix (FRCM) system is applied to URM walls to determine its feasibility as an alternative external strengthening technology. The experimental program consists of testing a total of nine clay brick walls under diagonal compression. Two FRCM strengthening reinforcement schemes are applied, namely, one and four reinforcement fabrics. An analytical model is used to calculate the shear capacity of strengthened URM walls and compare its results with the experimental database. The effect of limitations in design approach on shear capacity of strengthened walls is discussed.
Laboratory and in-situ shear tests of walls strengthened with Carbon FRP (Fiber Reinforced Polymer) strips and Glass FRP grid were compared to the results of different calculation models for masonry with FRP. Tests on new and old solid brick specimens showed an increase in shear strength and ultimate displacement. The best results were obtained with horizontally and horizontally-vertically epoxy-bonded strips and modified cement mortar grid configurations, worse with diagonal strips due to peeling failure. ACI and CNR calculation approaches showed the best agreement with experimental results.
Worldwide cultural heritage and especially the heritage buildings in Europe are masonry buildings. Such buildings are generally capable to resist vertical loads but horizontal earthquake actions are often critical. The strengthening upgrade is generally required. One of the most promising methods for the strengthening of masonry walls is application of fibre reinforced polymers (FRP) to the surface of the wall. Three sets of the experimental research were performed recently with new innovative configurations of the fibre reinforcement placement on clay brick masonry. Within the framework of European FP7 research project PERPETUATE two sets of in-situ experiments were performed on 12 walls, belonging to two buildings approximately one hundred years old. Both buildings were made of solid bricks in low strength mortar. Third set of tests was performed in the laboratory on 16 specimens made of contemporary solid bricks in good lime-cement mortar. Tests were performed with different reinforcement configurations. Carbon FRP strips were epoxy-bonded horizontally, diagonally or combined (horizontally and vertically). Glass FRP grids were placed in the modified cement mortar over the entire surface of the wall. Specimens were tested under the constant vertical load and by a displacement controlled horizontal cyclic loading. The carbon fibre reinforced polymer strengthening significantly increased the ultimate displacement capacity in case of the horizontal and combined strengthening. The diagonal strengthening was not so effective because the failure of the diagonally strengthened specimens was governed by the peeling of strips from the masonry. The GFRP grid configuration greatly increased the load bearing capacity but not also the ultimate displacement. The shear strength of the strengthened and un-strengthened specimens was compared to the calculated values of the seismic shear load bearing capacity. For that the Triantafillou (J Compos Constr 2(2):96–104, 1998), Triantafillou and Antonopoulos (J Compos Constr ASCE 4(4):198–205, 2000), Marcari et al. (2011) and Wang et al. (Asian J Civil Eng Build Hous 7(6): 563–580, 2006) calculation models and the design guides ACI 440.7R-10 (2010) and CNR-DT 200/2004 were used.
The ZEMCH2013 International Conference is a follow-up to the 2012 conference that was held in Glasgow UK. It brought together academic and industry experts from Australia, Canada, China, Italy, Japan, United Kingdom, United States, United Arab Emirates, Spain, Philippines, Iran, Saudi Arabia, Qatar among others in interactive sessions that discussed the issues surrounding the analysis, design, production and marketing of low- to zero-energy mass customizable homes and communities from around the world. The conference was open to any stakeholders who are involved in housing research, business, teaching, and policy. The effect of climate change is widely experienced around the world to include rising sea water levels, ambient temperatures that would impact on the indoor comfort of those in buildings. Yet housing is a complex system of energy and environment that demands a delicate balance between the needs and wants of the society. Housing is a major building block of the urban core of cities and a major contributor of carbon emissions and changes are needed in their design to ensure the emissions are lowered. The changes include the reliance on renewable energy, recycling of resources and use of appropriate technology that are customized to the end-user. Housing is a system of energy and environment and required to accommodate wants and needs of individuals and society, which are usually considered to be diverse and dynamic. The 'needs' factor often reflects minimum quality of end-user products (i.e. housing) and may embrace 'adequacy' being prescribed in conventional codes, while the 'wants' may be satisfied only if they are defined clearly by stakeholders (e.g. house-users and builder/developers) at the design decision making stage. 'Mass customisation' is an oxymoron or, perhaps, a paradigm case of a systems approach to identifying the aforementioned wants and needs that should be incorporated into the design of end-user products (or homes). Albeit increasing market demands for achievement of social, economic and environmental sustainability in housing today, conventional homebuilders (and housing manufacturers alike) who are often reluctant to spending extra time, money and effort for information gathering of new products and services are still barely able to adopt recently emerging innovations including mass custom design approaches to the delivery of sustainable affordable homes. Over the years, interest in Zero-energy homes/ buildings is beginning to bear fruit not only worldwide but also in the United States. Not only is the ZEMCH Network working hard in promoting the academic-industry collaborations. The papers presented at the conference explored themes such as sustainability from the social, and economic, design and Construction Management; and environmental perspectives. In addition, it examined how sustainable mass-customization and personalization; value analysis and visualization contributed to reduction in carbon emission. It also took into perspective the effect of user behavior and choice on energy consumption, renewable energy and technology among others. The works presented here in the proceedings are examples from different countries around the globe representing different climates that highlight the hope in a sustainable future. Dr. John Onyango, ZEMCH Network Miami regional director School of Architecture, University of Miami, Coral Gable, FL USA
This chapter briefly discusses the performance and durability of bonded composite systems used for on-site rehabilitation of timber and concrete structures. In spite of some recent developments, the exploitation of their full potential is still often restrained by the lack of structural design guidance, standards for durability assessment and on-site acceptance testing. Therefore, this chapter provides a review of current understanding on the use of hybrid bonded composite systems on the construction site in terms of structural repair, reinforcement, and seismic retrofit. It focuses on the requirements and practical difficulties in the work on-site with regards to the performance and durability of the rehabilitated structure, the characteristics and requirements that must be fulfilled by structural adhesives and advanced polymer composite materials, and the subsequent need for quality control and in-service monitoring. It also highlights the factors affecting performance and durability of bonded joints. Finally, a general overview of the research needs and a bibliography giving references to more detailed information on this topic is given.
Unreinforced masonry (URM) walls are prone to failure when subjected to out-of-plane and in-plane loads. The development of effective and affordable strengthening strategies is a need. In this context fiber reinforced polymer (FRP) materials offer viable solutions to solve or lessen the effects of overloading. This paper describes FRP systems proposed for use in the strengthening of masonry elements as well as the impact of such systems on the building being retrofitted. Also, field applications, some potential and others already implemented applications, are described. Field applications include strengthening for natural hazards (i.e. earthquakes and high wind pressures) as well as man-caused hazards (i.e. blasting). The potential of FRP systems for retrofitting of historic structures is also illustrated. Finally, research needs in this area are discussed.
- Preston Carney
-
![John Joseph Myers]()
Synopsis: Synopsis: Synopsis: Synopsis: Synopsis: Recent world events have illustrated that the sustainability of buildings to blast loads is an ever increasing issue. Many older buildings contain unreinforced masonry (URM) infill walls. Due to their low flexural capacity and their brittle mode of failure, these walls have a low resistance to out-of-plane loads, which includes blast loads. As a result, an effort has been undertaken to examine retrofit methods that are feasible to enhance their out-of-plane resistance. The use of externally bonded and near surface mounted (NSM) Fiber Reinforced Polymer (FRP) laminates and rods have been proven to increase the out-of-plane load capacity. This paper investigates the out-of-plane behavior of URM walls strengthened with FRP subjected to static and blast loading and the capability of developing continuity between the FRP strengthening material and the surrounding reinforced concrete (RC) frame system. There were two phases to this research study. Phase I evaluated strengthened URM walls' out-of-plane performance using static tests. Two strengthening methods were utilized, including the application of glass FRP (GFRP) laminates to the wall's surface and the installation of near surface mounted (NSM) GFRP rods. In both methods, the strengthening material was anchored to boundary members above and below the wall on some of the specimens in the research program. The effects of bond pattern, and the effects of FRP laminate strip width were also investigated in this phase. Phase II involved the field blast testing of two walls to dynamically study the continuity detail for laminates and verify the results obtained in Phase I. The development of continuity between the FRP materials and the surrounding framing system is one approach to improving the blast resistance of URM infill walls.
A series of framed unreinforced masonry URM infill walls were retrofitted with modern materials to evaluate the abilities of these materials to mitigate blast effects. The walls were constructed from traditional and alternative masonry materials to assess the applicability of using a wood-fiber fly ash material for infill construction. The walls were tested in the laboratory under static conditions and were evaluated using several criteria: energy absorption, out-of-plane load resistance, out-of-plane deformability, and the reduction of masonry debris scatter upon collapse. Due to the presence of the surrounding frame structure, all of the walls in this program experienced some form of an arching mechanism. The use of a spray-on polyurea material was found to be highly effective in improving URM energy absorption and reducing masonry fragmentation. Infill walls retrofitted with a combination of fiber-reinforced polymer FRP grids and polyurea material were found to fail prematurely due to a lack of anchorage between the strengthened walls and surrounding structure. A simplified analytical model to estimate the ultimate out-of-plane capacity for FRP strengthened URM arching walls was developed. The analytical model was empirically calibrated using test data from this work as well as previous studies. The model predictions agree well with the experimental results reported in this paper.
The vulnerability of unreinforced masonry buildings (URM) to moderate ground motions is a fact recognized by the earthquake engineering community. In this paper, an innovative retrofitting system for URM buildings using glass fiber reinforced polymer (GFRP) strips is investigated. The experimental results for four retrofitted URM walls subjected to cyclic out-of-plane loading are presented herein. The first three specimens were constructed in single wythe, and the fourth one in double wythe. The height-thickness ratio for all specimens was 28. Depending on the reinforcement ratio, single wythe walls failed in tension, excessive delamination, or a combination of both. Failure modes in the double wythe wall were peeling off of composite strips and splitting of the wythes. From experimental results, it was found that walls were capable of supporting pressures of up to 25 times their weight and deflect up to 1/20 times the wall height. Strength and deformation capacity of the walls were significantly improved by the investigated retrofitting technique.
- Carney
- Preston Wade
Thesis (M.S.)--University of Missouri--Rolla, 2003. Vita. Includes bibliographical references (p. 117-119). "This research study investigated the feasibility of developing continuity between the FRP strengthening material and the surrounding reinforced concrete frame system."--Abstract, p. iii.
Concrete In-Plane Response on an Alternative URM Infill Wall System with and without a Polyurea Retrofit Strengthening of Masonry Structures with FRP Composites Doctoral Dissertation Out-of-Plane Behavior of Brick Masonry Walls Strengthened with Fiber Composites
- J F Young
- S Mindness
- D Darwin
- Y Tanizawa
- J J Myers
- R Sinclair
- J Velazquez-Dimas
- M Ehsani
- H Saadatmanesh
Young, J.F., Mindness, S., and Darwin, D., " Concrete, " Prentice Hall, 2 nd Edition, 2002. Tanizawa, Y., Myers, J.J. Sinclair, R. " In-Plane Response on an Alternative URM Infill Wall System with and without a Polyurea Retrofit, " 9th International Symposium on Reinforced Polymer Reinforcement for Concrete Structures, " Sydney, Australia, 2009, 4p. Tumialan J. G., 2001, " Strengthening of Masonry Structures with FRP Composites, " Doctoral Dissertation, University of Missouri-Rolla, Rolla, MO, 186 pp. Velazquez-Dimas, J.; Ehsani, M.; and Saadatmanesh, H., 2000, " Out-of-Plane Behavior of Brick Masonry Walls Strengthened with Fiber Composites, " American Concrete Institute Structural Journal, V. 97, No. 3, May-June, pp. 377-387.
In-Plane Response on an Alternative URM Infill Wall System with and without a Polyurea Retrofit
- Y Tanizawa
- J J Myers
- R Sinclair
Tanizawa, Y., Myers, J.J. Sinclair, R. "In-Plane Response on an Alternative URM Infill Wall System with and without a Polyurea Retrofit," 9th International Symposium on Reinforced Polymer Reinforcement for Concrete Structures," Sydney, Australia, 2009, 4p.
American Concrete Institute 440.7R Guide for the Design and Construction of Externally Bonded Fiber-Reinforced Systems for Strengthening Unreinforced Masonry System
American Concrete Institute 440.7R, " Guide for the Design and Construction of Externally Bonded Fiber-Reinforced Systems for Strengthening Unreinforced Masonry System, " ACI, 2010.
American Concrete Institute 440.3R Guide Test Methods for Fiber-Reinforced Polymers (FRPs) for Reinforcing or Strengthening Concrete Structures
American Concrete Institute 440.3R Guide Test Methods for Fiber-Reinforced Polymers (FRPs) for Reinforcing or Strengthening Concrete Structures," ACI 2003.
Source: https://www.researchgate.net/publication/272416047_STRENGTHENING_UNREINFORCED_MASONRY_STRUCTURES_USING_EXTERNALLY_BONDED_FIBER_REINFORCED_POLYMER_SYSTEMS_AN_OVERVIEW_OF_THE_AMERICAN_CONCRETE_INSTITUTE_4407R_DESIGN_APPROACH
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