Fatigue and cyclic loading of moment-resisting structures connected using gluedin GFRP rods
Mehrab MADHOUSHI
Faculty Member
Gorgan Uni. of Agri. Sci.
Gorgan, 49138-15739, IRAN
[email protected]
- B.S. Degree: 1989-1999,
University of Gorgan, IRAN
- MSc Degree: 1993-1996,
TMU, Tehran, IRAN
- Ph.D. Degree: 1999-2003
Supervisor: Dr. M.P. Ansell
University of Bath, UK
Martin P. ANSELL
Senior Lecturer
Uni. of Bath
Bath, BA2 7AY, UK
[email protected]
BSc, PhD, FIMMM, FIWSc.
President of UK Institute of
Wood Science from 1994 to
1996. Research interests also
include fatigue of timber and
development of natural fibre
composites.
Summary
In this paper the fatigue strengths of beam to column connections based on GFRP glued-in rods are
studied under simulated earthquake loading conditions. Two jointed structures were investigated,
firstly an L-shaped moment-resisting timber (LVL) structure and secondly a U-shaped LVL frame.
The connections consisted of two GFRP pultruded rods for the L-shaped structure and two pairs of
GFRP pultruded rods for the U-shaped structure. The results shows good ductility and capability for
the dissipation of energy under dynamic loading for both jointed structures.
Keywords: Timber connections, epoxy resin, cyclic loading, GFRP glue-in rod,
1. Introduction
Beam to column connections are commonly encountered in timber structures for transferring the
imposed loads from the beam to the column and then to the foundation. Furthermore, in earthquake
zones, timber structures may be subjected to seismic actions and their ability to dissipate energy is
important. Timber connections based on glued-in steel rods [1-5] and glass fibre reinforced plastic
(GFRP) rods [6-8] have been investigated under static loading. These connections are now widely
used in timber connections [9, 10] because of their advantages compared with traditional
connections including the formation of very stiff connection, good fire properties and improved
aesthetics.
Previous studies showed that generally, wooden members show brittle failure under static load in
tension, bending and shear. Hence, wooden members are not very capable of absorbing and
dissipating the energy generated by cyclic loading. On the other hand, joints are capable of absorbing
and dissipating energy, if they are designed appropriately. It has been indicated [11] that the energy
dissipation of joints is mainly based on the ductility and damping of joints. So, understanding the
behaviour of timber joints under cyclic loading is essential. Smith et al [12] believe that the flexibility
of connections enables cyclic forces to be distributed amongst fasteners [12]. Also, ductility in
connections allows a large amount of energy to be absorbed, which is generated as a structure
vibrates under earthquake conditions. The cyclic performance of glued-in rods in timber connections
conducted by Buchanan and Fairweather [13] showed that these joints have good potential for
absorbing and dissipating energy between the members. For this reason, and based on previous
investigations on fatigue of timber connections using glued-in GFRP rods by the authors [14, 15],
two jointed structures are investigated under fatigue and cyclic loading conditions in this paper.
2. Experimental Methods
Two type of joints have been made and studied, based firstly on L-shaped moment-resisting timber
(LVL) structure (Fig. 1a) and secondly a U-shaped LVL frame (Fig. 1b). The connections consisted
of two GFRP pultruded rods for the L-shaped structure and two pairs of GFRP pultruded rods for
the U-shaped structure. The rods were 170 mm long and 16mm in diameter glued in with epoxy
resin.
L-shaped connections were loaded in tension-tension fatigue conditions at R=+0.1 under load
control. Fatigue results are presented in the form of S-N curves and captured hysteresis loops. Ushaped connections were loaded under reversed cyclic loading at R=-1 under displacement control
using a quasi-static loading method. For this purpose, the method described in a CEN standard is
usually used [16] that is based on a quasi-static mode of loading under displacement control.
F1
00
F
0
Grain
Direction
H
90 degrees
90 degrees
0
35
d 170
A
25.5
20
0
35 50
0
D
D
30
0
0
50
C
10
0
0
35
35
0
B
C
Cross-head of test machine
Load Cell
F
F
F
Sample
25.5
50
25
4.5
4.5
25
16
25
25
16
Member A
Steel pins
Metallic
support frame
Bed plate of test machine
(a) Front view
Member B
(a)
(b) Side view
(b)
Fig. 1 Illustration of sample geometry used in (a) L-shaped and (b) U-shaped connections
3. Results
3.1 L-shaped connections
The results show that the fatigue life increases as the load level decreases, although there are some
differences between the number of cycles to failure. The S-logN curve for L-shaped connections is
illustrated in Fig. 2, showing that the joints can withstand more than 106 cycles at the 30 % stress
level.
The pattern of hysteresis loops area versus log number of cycles is shown in Fig. 3. It can be said
that the capacity of L-shaped connections to dissipate energy at higher stress levels (75% and 50%)
increases throughout the cyclic loading, but at the lowest stress level (40%) it is constant.
2
75%
250
y = -21.57x + 221.52
200
R = 0.8484
Hysteresis Loop Area
(Nm.Degree)
Bending Moment (N-m)
2500
300
2
150
100
50
-0.3
0
2000
1500
1000
50%
50%
40%
-1
0
1
2
3
4
5
6
7
0
8
0
Log Number of Cycles
1400
1200
1000
75%
600
75%
400
40%
200
50%
0
0
1
2
3
4
2
3
4
5
6
7
Fig. 3 Hysteresis loop area versus log number
of cycles for L-shaped connections
50%
800
1
Log Number of Cycles
Fig. 2 Bending moment-Log N curve for
L-shaped connections
Dynaic Modulus (Nm/degree)
75%
500
5
6
Log Number of Cycles
7
Dynamic modulus for all samples of L-shaped
connections is plotted in Fig. 4. In all samples
the dynamic modulus reduces over time. At the
75% stress level the dynamic modulus is not
only smaller in magnitude than at the 50% and
40% levels, but it also degrades more rapidly
with log number of cycles. At > 104 cycles it is
clear that the 40% sample retains the highest
dynamic modulus as it suffers the least damage.
Overall, the L-shaped samples suffer almost an
order of magnitude change in dynamic modulus.
Fig. 4 Dynamic modulus versus Log number of cycles for L-shaped connections
30.00
Block 11
25.00
20.00
Sample 2
15.00
Sample 3
Block 10
10.00
5.00
0.00
0
5
10
15
Block Number
20
Viscous Damping Ratio
(%)
Hysteresis Loop Area
(kN. mm)
3.2 U-Shaped Frame
Fig. 5(a) illustrates the area of hysteresis loops on the tension side of samples versus block number.
This figure demonstrates that frame samples are able to dissipate energy in a progressive manner
until failure of the sample, but after that this capability reduces because of damage to the frame.
For calculation of the viscous damping ratio (Qeq), the dissipated energy (Ed) for each cycle in
tension side was related to the available potential energy (EP) for the same cycle on the tension side
as Qeq = Ed/ 2SEp*100 [16] and the trends of viscous damping ratio over the blocks are plotted in
Fig. 5(b).
12.00
10.00
8.00
Sample 2
6.00
Sample 3
4.00
2.00
0.00
0
5
10
15
20
Block Number
Fig. 5 (a) Hysteresis loop area and (b) viscous damping ratio on the tension side of loop versus
block number for frame samples under cyclic loading
In general, Fig. 5(b) shows that the values of viscous damping ratio vary between 4% and 8%, which
are reasonable magnitudes for connections as described by Ceccoti [16]. Finally, it can be concluded
3
that in U-shaped frame the connections are able to withstand reversed cyclic loading conditions
although they are predominantly linear-elastic to failure under static loading.
4. Conclusions
1. For L-shaped structures the fatigue strength increases with decreasing stress levels.
2. The dynamic modulus of L-shaped connections reduces over time at all stress levels.
3. Frame connections can withstand reversed cyclic loading conditions under displacement control
mode.
4. The viscous damping ratio of frame connections varies between 4 and 8% reaching a minimum at
about the point of failure.
5. Bonded-in pultruded GFRP connections are therefore capable of dissipating energy under cyclic
loading and in simulated earthquake conditions.
Acknowledgements
The first author is grateful to the Iranian Government for the PhD studentship, which funded this
research.
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[1]
[2]
[3]
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4
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