INTRODUCTION
Before the era of steel and concrete technology, wood was the most common
structural material in residential, public and industrial buildings throughout
Iran, particularly in the Northern provinces which own commercial forest.
Majority of the existing ancient buildings in this country are aged over
one century, which some of them are registered as historical heritage
and are still standing in a good functioning shape. At the time being,
conservation and restoration of these valuable buildings are most concern
of Iran`s office of historical heritage. Inappropriate rehabilitation
of the defected structural elements in historical buildings greatly influence
on their beauty and dignity (Fig. 1).
Over the past 40 years, both Fiber Reinforced Polymer (FRP) and non-FRP
materials have been used to reinforce wooden structural members.
During the last two decades, a special prefabricated Fiber Reinforced
Polymer (FRP) composite was developed to repair and retrofit wooden elements
in field.
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Fig. 1: |
Repair of old stringers with traditional methods |
The advantages of these reinforcing materials lie in their low
weight density, high tensile strength and resistance to corrosion. According
to the recent investigations, FRP materials have been successfully used
to strengthen existing structural members. Svecova and Eden (2004) tested
timber beams reinforced with GFRP dowel bars as shear reinforcement as
well as flexural bar to control the tension failures observed in some
of the specimens. Amy and Sevecova (2004) presented an economical rehabilitation
scheme to strengthen creosote-treated dapped timber beams in both flexural
and shear with glass fiber reinforced polymer bars. The results of study
showed improvement in both flexural strength and shear strength. Borri et al. (2005) presented a method for flexural reinforcement of
old wood beams with CFRP materials. Mechanical tests on the reinforced
wood showed that external bonding of FRP materials may increase flexural
stiffness and bending capacity. Corradi et al. (2006) presented
in-plane shear reinforcement of wood beam floors with FRP. Buell and Saadatmanesh
(2005) presented an investigation on reinforcement timber bridge beams
with a single CFRP. It was reported a 69% increase of the bending strength
if compared with control beam and a compression failure mode. Calderoni et al. (2006) presented flexural and shear behavior of ancient
chestnut beams under both experimental and theoretical evaluation. The
results certified that the simplified methods commonly used to evaluate
the bearing capacity of wooden beams can be safely applied to ancient
structural members. Corradi and Borri (2007) presented a study on Fir
and Chestnut timber beams reinforced with GFRP, which the test results
showed that the reinforced beams produced strong increase in flexural
stiffness and strength. Schober and Rautenstrauch (2007) submitted a study
on reinforcing techniques for restoration and strengthening of existing
timber floors under bending loads. The tests have demonstrated that the
arrangement of the reinforcement and the stiffness of the materials transmitting
the loads, i.e., wood, CFRP and the bonding agent, were of decisive influence
for the overall strength of the specimen.
The purpose of this study is to present a new innovative technology for
rehabilitation of the old wooden members in the field, through testing
small size of wooden members and comparing the experimental results with
numerical calculation in order to be used in historical buildings by means
of Glass Fiber Reinforced Polymer composite (GFRP) sheets.
MATERIALS AND METHODS
Wood materials: This experimental test is based on more than 50
specimens of ancient native Hackberry. These specimens have been obtained
from newly replaced and crushed structural elements of a historical building,
located in town of Babol, near Caspian Sea, built up almost 150 years
ago. Because of the limitation of obtaining enough old wood for tests
and also the major defects present in the main wood, like longitudinal
splitting, cracking, degraded zones and holes due to nailing and insect
attacks, the samples were obtained in size of 25x25x410 mm and tested
as a secondary method under ASTM D-43 for small clear specimens of reinforced
timbers.
 |
Fig. 2: |
Static bending test assembly |
Table 1: |
Mechanical characteristics of the reinforcing materials |
 |
The experimental tests were preformed in mechanical laboratory
of Faculty of Natural Resources of Tehran University in Karage during
May 2006.
GFRP materials: In the recent years FRP has been used as a compatible
reinforcement material for timbers and plywood. The physical/mechanical/chemical
properties of the FRP are very versatile. The FRP may be engineered to
match and complement the orthotropic properties of wood; consequently,
incompatibility problems between the wood and the reinforcing FRP are
minimized. The unidirectional Glass Fiber-Reinforced-Polymer (GFRP) was
used having the physical and mechanical characteristics reported in Table
1.
Resins: The adhesive or glue used in this test was Mac epoxy resin,
which consisted of two parts, resin and hardener mixed with a ratio of
3:1 in volume. The mechanical properties of epoxy resin are reported in
Table 1.
Test set-up: In this experimental investigation, three- point
loading tests were performed as shown in Fig. 2 according
to ASTM D-143 for finding out the flexural strength. The simply supported
test method was selected to simplify the experimental setup. The wood
specimens were supported at the butt and the tip and the load was applied
at the center through a bearing block to the tangential surface nearest
the pith.
Table 2: |
Mean dimensions of mid-span for timber beams reinforced
with GFRP |
 |
The span`s length between the two supports was Ls = 360 mm which the
total length of the specimen was L = 410 mm. These spans were established
in order to maintain a minimum span-to-depth ratio of 14. Both supporting
knife edges were provided with bearing plates and rollers of such thickness
that the distance from the point of support to the central plan is not
greater than the depth of the specimen. The knife edges were adjustable
laterally to permit adjustment for slight twist in the specimen.
During the process of loading, the experimental responses were transferred
to a set of computer that was programmed to register and analyze all data
received through a load cell which was connected to the Instron testing
machine as shown in Fig. 2. The bending tests have been
carried out for a series of control (un-reinforced) beams and for GFRP
reinforced beams with varying area fraction of fiber reinforcement as
shown in Table 2. The mid-span deflection, ultimate
strength and modulus of elasticity have been measured automatically by
means of a load cell attached to the loading devices.
RESULTS AND DISCUSSION
Control specimens: In this flexural investigation, at first, three
different types of more common un-reinforced woods were tested as reference
samples in order to obtain the baseline response. These woods were Hackberry,
Oak and Elm, which their experimental results are shown in Table
3. It clarifies that under the same conditions of testing; the ultimate
strength and failure load of Hackberry is greater than those of Oak or
Elm.
According to the field of investigation, Hackberry wood was the most
common structural elements in a high percentage of ancient buildings;
therefore this wood has been selected for the study. The results of the
test for control specimens are shown in Table 4. Besides,
Fig. 3 exhibits the load-displacement relationship and
the behaviors of linearity up to failure for all control specimens.
In this test, the mid-span deflection, load capacity, modulus of elasticity
in range of the 25 and 75% of elastic region and the mode of failure were
evaluated.
Table 3: |
Experimental results of un-reinforced wood under bending
test |
 |
Table 4: |
Experimental results for control samples |
 |
|
Fig. 3: |
Load-displacement response for five samples of control
specimens |
GFRP-wood hybrids: In this case, the reinforcement of wood, with
glass fiber polymer caused a considerable increase in strength and stiffness
under bending test, as shown in Table 5. The load-displacement
curves for different schemes of reinforcement are shown in Fig.
4a-e.
From Fig. 3, 4a and b, it can be
found that the behavior of load-displacement for all of the control specimens
and low composite products are rather liner up to failure point, but for
those with more than three layers of reinforcement, the linear behavior
changes into nonlinearity as shown in Fig. 4c-e.
The samples under test experienced either a brittle tensile collapse
or a ductile compressive failure. Although both unreinforced and low composite
products failed in tension side, but an abrupt collapse was observed in
unreinforced ones, while all of the low composite products failed rather
ductile as shown in Fig. 5a and a partially ductile
compressive failure followed by peeling or debonding of GFRP in tension
side occurred for the case of three layers of reinforcement (Fig.
5b). In addition, full ductility was exhibited for the case of three
layers of GFRP mounted on tension side and one layer on compression side
as shown in Fig. 5c.
Results have shown that the effect of GFRP composite reinforcement on
the upgrading the mechanical properties of GFRP-wood hybrid is positive.
|
Fig. 4: |
Load-deflection curves for five specimens of different
configurations of GFRP-wood hybrids, (a) one layer reinforcement on
tension side, (b) two layers reinforcement on tension side, (c) two
layers reinforcement on tension side and one layer on compression
side, (d) three layers reinforcement on tension side and (e) three
layers reinforcement on tension side and one layer on compression
side |
In all of the experimental cases, there was a considerable amount of increase
in the strength and stiffness of the reinforced specimens if compared
with control ones. The most effective results in GFRP-wood hybrid were
obtained when the specimens were reinforced with three layers of GFRP
on the tension side and one layer on compression side as shown in Fig.
6.
|
Fig. 5: |
Different types of failure for different wood-hybrid,
(a) brittle tensile failure, (b) partially ductile failure followed
by sudden break in tension side and (c) full ductile failure in compression |
|
Fig. 6: |
Results of different reinforced configurations of GFRP-wood
hybrids |
Table 5: |
The mean value of test results of different reinforced
schemes of wood- GFRP hybrid |
 |
In this case, the maximum increase in load capacity, strength and
stiffness were 57.1, 31.26 and 9.61%, respectively if compared with unreinforced
specimen as shown in Table 5.
Analytical solution of the reinforced wood-GFRP hybrid: According
to the test results, the behavior of stress-strain relation is nearly
linear up to 75% of load capacity and from there it becomes rather nonlinear.
So, if it is assumed that the following relation is accepted.
The stress due to bending moment can be obtained through the equation:
where, Me is the bending moment of the element, S is the section
modules and c is a coefficient that takes into account the timber quality.
This approach can be applied for calculation of bending stresses in the
reinforced element by substituting Se as equivalent section
modules that can be obtained by coefficient η as follows:
Nonlinear approach: The numerical model takes into account the
simultaneous presence of both top and bottom reinforcement of the section,
as shown in Fig. 7. According to the law of Bazan-Buchanan
the stress and strain in compression side is elastic-plastic and in tension
side is linear elastic (Bazan, 1989). Assume the failure not reach to
the composite material; it is possible to reduce the study of the problem
to two failure cases.
• |
Attainment of limit strain in compression region εcwu |
• |
Attainment of limit strain in tension region εtwu
without exceeding limit in the compression region |
 |
Fig. 7: |
Strain and stress distribution on reinforced wood section |
In both case, from the condition of equilibrium it follows that
where, the forces in the compression region are:
and forces in tension side are:
where, AfrpC and Atfrp represent the composite
area in compression zone and ension region.
The preservation of plain sections, confirmed in several experimental
results will be assumed for the congruence equation (Borri et al.,
2005).
εCw/y = εtw/(h-y)
= εCo/y(1-k) = εtfrp/(h+d-y) = εCfrp/(y+d`) |
(6) |
From these equations and for the stress-strain law of material it is
possible for each failure case to find the position of the neutral axis
(y) and the value of the ultimate bending moment capacity of the section.
In particular, the Bazan-Buchanan law can be expressed as follows:
where, m represents the slope of the plastic branch of the Bazan-Buchanan
law:
m = (σCo - σCwu)/(εCou
- εCo) |
(8) |
The Eq. 7 can be modified to take into account the possibility
of having different elastic modulus in tension and compression zone of
the section. In effect, preceding studies on the material indicate that
in an overwhelming number of cases, the difference between the two modules
is almost negligible with respect to other simplifications assumed.
With regard to FRP materials the generic stress-strain relationship is:
σCfrp = ECfrp.
εC frp
σtfrp = Etfrp. εt
frp |
(9) |
which, describe a linear elastic behavior of the two materials. Using
Eq. 2 to 9, it is possible to find the
neutral axis equation for each assumed failure case. If the failure is
reached due to the complete plasticization of the compression region with
ultimate strain εCwu, then the neutral axis equation becomes:
σCoy2(1+α-αδ)-
Ew εCo(h-y)2 +2ρfrpC
EfrpC bhεCwu (y+d`)
-2ρfrpt Efrpt hεCwu(h+d-y)
= 0 |
(10) |
Where:
α = σCwu/σCo,
β = εCo/εCwu |
(11) |
are Bazan-Buchanan parameters and
ρfrpC = AfrpC/bh,
ρfrpt = Afrpt/bh |
(12) |
where, ρ is the area fraction for the two composite materials applied in the
compression and tension areas.
Then if the tension failure occurs, the neutral axis equation is expressed
as follow:
y (2σCo- Φ ((εtwu/(h-y)-εCo)))+y(εCo ((h-y)/εtwy))
H( Φ ((εtwuy/(h-y)-εCo)-σCo)-σtwu (h-y)-2ρfrpt(h+d)εtwu Efrpt
+2ρfrpC h (εtwu)(y+d`)/(h-y)
= 0 |
(13) |
Table 6: |
Comparison between experimental and analytical results
of wood reinforced with GFRP composite |
 |
Table 7: |
Mechanical properties of wood and frp used for analytical
approach. |
 |
Where:
and where, εtwu and σtwu represent, respectively
the limitation of tension strain and stress for wood and εCwu
and σCwu the corresponding limit compression strain and
stress.
After calculation of the y, the strength of section can be expressed
as follows:
Mu= F5(y+d`/2)+F11(y-ky/2)+F12
(y-2/3ky)
+2/3 F2(1-k)y+2/3 F3 (h-y)+F4 (h+d/2-y) |
(15) |
Then ultimate load capacity obtained by following equation will be compared
with test results:
It can be assumed that the yield starts at 75% of ultimate load, so:
where, φ is defined as correction factor. So, as a result of Table
6, the experimental results can be obtained by the following equation:
P | y (test) = φ
P | y (cal.) |
(18) |
The calculation has been done based on following information as given
in Table 7.
CONCLUSIONS
Mechanical tests on the reinforced wood have shown that externally bonded
GFRP composite sheets produces very interesting effects. In general, the
results of the experimental tests have demonstrated an interesting changes
in the strength, stiffness and the behavior of failure in the reinforced
specimens as follows:
• |
Generally, glass fiber polymer reinforcement increased
the load capacity, strength and stiffness for all of the different
configurations. In addition, it has a good advantage in restoration
of structural view since it takes a colorless view after operation.
It`s cheaper price, in comparison with other types of FRP like CFRP
and so on, makes it preferable in the case which very high strength
is not required |
• |
Full length bond between wood and GFRP provided a more ductile failure
if compared to other configuration of reinforcing because it reduced
debonding phenomena and let the specimen to reach to its final capacity
and also higher strength and stiffness. |
• |
Regarding the strength, stiffness and failure mode, samples reinforced
with three layers of the GFRP in tension side and one layer in compression
side were the best performing configuration. |
• |
The results of the experimental tests were almost compatible and
comparable with those obtained from numerical analyses. So, there
would be a reliable method of calculating the strength of old wood
members to obtain the most efficient reinforcement of structural size
|
ACKNOWLEDGMENTS
This study is some parts of a research project of Ph.D Thesis under execution
in University of Tehran and was supported by University of Tehran, Faculty
of Engineering and Natural Resources that is appreciated here, Grant No.
247124.