In order to understand the effect of
bolts number and bolts type on the connection behaviour, 36 multi-bolted
double-lap shear connections are prepared and tested under axial tensile load.
The considered number of bolts in this study are 4, 6, 8, and 10 as shown in
Fig. 3-5.  In addition, for each number,
three types of bolts (i.e., SS, BFRP, and HSFRP) were used. The bolts and hole
diameter are 12 and 12.6 mm respectively as shown in Table 3-3.

3.6.2.1
Load-Displacement

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Fig.
3-15 shows the recorded load-displacement curves for SS, BFRP, HSFRP bolted
specimens with different bolts number. It can be observed from this figure that
for all specimens, in the first stage there was an increase in the displacement
due to the clearance between the bolts and the hole. After that, the slope of
the load-displacement curve increased due to the full contact of bolts with the
holes till reaching the ultimate load where a drop happened due to the failure
of the main plate. SS bolted specimens showed linear behaviour till reaching
its ultimate load, BFRP and hybrid bolted specimens showed approximately linear
behaviour till failure.

As
shown in the load-strain curves in fig. 3-16 the strains ahead of bolts showed
compressive reading due to the contact forces from the bolts which caused
compressive deformation in the region between the bolts as shown in Fig. 3-19.
Strains reading taken before failure because the subsequent results were not
being reliable after failure. Strains front to outer rows showed higher strain
reading than middle rows representing outer rows took higher load than middle
ones, which reached failure first. The variations in load-strain responses may
be happened due to small variation in hole clearance (Lawlor,
McCarthy and Stanley, 2005, Fu and
Mallick, 2001, Persson and
Eriksson, 1999), misalignment of bolts hole, or
tightening torque. Unfortunately, these factors are hardly controlled. As a
result, some bolts might be more tightly than the others or its hole clearance
was less than the others which resulted the bolts to behave differently during
loading.

It
also can be seen from Fig. 3-15 that double lap joints with steel and hybrid
bolts had similar stiffness with changing the bolts number from four to ten,
unlike BFRP bolted joints which its stiffness increased by increasing the
bolts’ number. It worth to mention that after reaching the ultimate load, it
was noticed that specimens with HSFRP bolts did not lose all of their strength
suddenly like specimens with SS and BFRP bolts. However, it showed a ductile
behaviour and gradual degradation which can be attributed to the damage of the
HSFRP bolts which reduce the intensity of the shear out failure. This behaviour
was clearly explained in Chapter two during the shear test of the HSFRP bolts.
From safety and damage control point of view, this behaviour can be used as
safety margin in the case of failure.

3.6.2.2
Comparison of failure load

The
bearing failure started with resin cracks and fibres micro-buckling near the
bolts’ holes at less than 50% of the ultimate load with hearing low sounds of
cracking. This failure appeared as nonlinearity or reduction in the stiffness
before the ultimate failure in the load-displacement curve. This was more
obvious in BFRP bolted joints as seen in Fig. 3-15b.

Table
3-3 shows the failure load of each specimens. The average failure loads of SS,
BFRP and HSFRP bolted joints are compared in Fig. 3-20. It is clear that for
all types of bolted specimens with SS, BFRP or HSFRP bolts, the failure load
increased with increasing the bolt’s number. In terms of specimens with SS
bolts, the failure load increased by 30.5%, 88.9%, and 117.2 % when the number
of bolts increased from four to six, eight, and ten respectively. For specimens
with BFRP bolts, the increase in failure load of specimens with 6, 8, and 10
bolts, as compared to the one with 4 bolts, were 44%, 87.3% and 126.6%
respectively. Ultimately, increasing the HSFRP bolts to 6, 8, and 10 led to
increasing the ultimate load by 35.2%, 60.2% and 95.2% respectively. It is also
evident from Fig 3.15 and 3.20 that the failure load of SS bolted joint and
BFRP bolted joints had approximately the same failure load with the same number
of bolts. On the other hand, specimens with HSFRP bolts had lower failure load
compared with SS and BFRP bolted joints.

Ultimately,
it can be concluded that replacing the SS bolts with BFRP bolts will not affect
the ultimate load capacity and failure mode of the bolted connections. Using
the HSFRP bolts will results in a reduction in the ultimate load, however it
will give a safety margin in the case of failure.

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