In order to understand the effect ofbolts number and bolts type on the connection behaviour, 36 multi-bolteddouble-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 inFig. 3-5. In addition, for each number,three types of bolts (i.e., SS, BFRP, and HSFRP) were used. The bolts and holediameter are 12 and 12.
6 mm respectively as shown in Table 3-3. 18.104.22.168Load-DisplacementFig.
3-15 shows the recorded load-displacement curves for SS, BFRP, HSFRP boltedspecimens with different bolts number. It can be observed from this figure thatfor all specimens, in the first stage there was an increase in the displacementdue to the clearance between the bolts and the hole. After that, the slope ofthe load-displacement curve increased due to the full contact of bolts with theholes till reaching the ultimate load where a drop happened due to the failureof the main plate. SS bolted specimens showed linear behaviour till reachingits ultimate load, BFRP and hybrid bolted specimens showed approximately linearbehaviour till failure.
Asshown in the load-strain curves in fig. 3-16 the strains ahead of bolts showedcompressive reading due to the contact forces from the bolts which causedcompressive deformation in the region between the bolts as shown in Fig. 3-19.Strains reading taken before failure because the subsequent results were notbeing reliable after failure. Strains front to outer rows showed higher strainreading than middle rows representing outer rows took higher load than middleones, which reached failure first.
The variations in load-strain responses maybe happened due to small variation in hole clearance (Lawlor,McCarthy and Stanley, 2005, Fu andMallick, 2001, Persson andEriksson, 1999), misalignment of bolts hole, ortightening torque. Unfortunately, these factors are hardly controlled. As aresult, some bolts might be more tightly than the others or its hole clearancewas less than the others which resulted the bolts to behave differently duringloading.Italso can be seen from Fig.
3-15 that double lap joints with steel and hybridbolts had similar stiffness with changing the bolts number from four to ten,unlike BFRP bolted joints which its stiffness increased by increasing thebolts’ number. It worth to mention that after reaching the ultimate load, itwas noticed that specimens with HSFRP bolts did not lose all of their strengthsuddenly like specimens with SS and BFRP bolts. However, it showed a ductilebehaviour and gradual degradation which can be attributed to the damage of theHSFRP bolts which reduce the intensity of the shear out failure. This behaviourwas 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 assafety margin in the case of failure.3.6.2.
2Comparison of failure loadThebearing failure started with resin cracks and fibres micro-buckling near thebolts’ holes at less than 50% of the ultimate load with hearing low sounds ofcracking. This failure appeared as nonlinearity or reduction in the stiffnessbefore the ultimate failure in the load-displacement curve. This was moreobvious in BFRP bolted joints as seen in Fig. 3-15b.Table3-3 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 forall types of bolted specimens with SS, BFRP or HSFRP bolts, the failure loadincreased with increasing the bolt’s number. In terms of specimens with SSbolts, the failure load increased by 30.5%, 88.9%, and 117.
2 % when the numberof bolts increased from four to six, eight, and ten respectively. For specimenswith BFRP bolts, the increase in failure load of specimens with 6, 8, and 10bolts, 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 toincreasing the ultimate load by 35.2%, 60.
2% and 95.2% respectively. It is alsoevident from Fig 3.15 and 3.20 that the failure load of SS bolted joint andBFRP bolted joints had approximately the same failure load with the same numberof bolts.
On the other hand, specimens with HSFRP bolts had lower failure loadcompared with SS and BFRP bolted joints. Ultimately,it can be concluded that replacing the SS bolts with BFRP bolts will not affectthe ultimate load capacity and failure mode of the bolted connections. Usingthe HSFRP bolts will results in a reduction in the ultimate load, however itwill give a safety margin in the case of failure.