In also can be seen from fig 3-25a that

In
order to understand the effect of bolts or hole pattern on the connection
behaviour, 36 multi-bolted double-lap shear connections with six different hole
pattern as shown in fig. 3-7 were prepared and tested under axial load. In
addition, for each pattern, two types of bolts (i.e., BFRP, and HSFRP) were
used. The bolts and the hole diameter were 8 and 8.4 mm respectively as shown
in table 3-6.

3.6.5.1
Load- displacement

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Fig. 3-25 shows the recorded
load-displacement curves for BFRP, HSFRP bolted specimens with different bolts
pattern. It can be observed from this figure that for all patterns, in the
first stage the 0.4 mm clearance between the bolts and the holes led to an
increase in the displacement and the load approximately constant. After that,
when the bolts were in a full contact with the holes the slope increased till
reaching the ultimate load with shear-out failure drop. BFRP bolted specimens
showed change in the stiffness at 35-50% of the ultimate load at the
load-displacement curve due to the bearing failure. This failure was not so
clear in the HSFRP load-displacement curve.

It also can be seen from fig 3-25a that
for all pattern with BFRP bolts showed the same behaviour till failure. However,
pattern 5 and 6 showed more ductile behaviour after failure.

In addition, from fig. 3-25b, all HSFRP
bolted specimens under different hole pattern showed the same behaviour till
reaching its ultimate load. As mentioned before HSFRP showed more ductile
behaviour than BFRP bolted specimens after failure, however also pattern 5 and
6 showed more ductile behaviour than other patterns.

3.6.5.2
Comparison of failure load

Fig.
3-25a showed the bearing failure of BFRP bolted specimens at 35-50% of the
ultimate load with hearing low sounds of cracking, and that was more obviously
than HSFRP bolted specimens. Table 3-6 shows the failure load of each specimens.
The average failure loads of BFRP and HSFRP bolted joints with different hole pattern
are compared in Fig. 3-26. In terms of specimens with BFRP bolts, the failure
load increased by 10%, 2.53%, 9.35%, 0.4%, and 10.53% for the second, third,
fourth, fifth, and sixth patterns respectively, comparing with the first
pattern. For specimens with HSFRP bolts, the increase in the failure load of
specimens with the second, third, fourth, fifth, and sixth pattern, as compared
to the first pattern were, 30.55%, 19.5%, 42.8%, 51.24%, 52% respectively.  It is appeared that distribute the bolts in
columns (as in pattern 2) increased the strength more than distribute them in
rows (as in pattern 1 and 3).

In
addition, it is obviously that distributed the bolts in staggered manner as in
pattern 4,5, and 6 increased the strength of the connection than other
patterns. As in the multi-bolted connection the load did not distribution
equally between bolts, first and last rows take larger than middles rows (Feo, Marra
and Mosallam, 2012, Xiaopen and
Ling, 1991), that will lead to
failure faster than if the load was equally distributed. Staggered pattern in
4,5, and 6 will lead to the nonuniformity degree of bolt loading ameliorated as
concluded in (Xiaopen and
Ling, 1991). 

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