Abstract
An experiment, “Connection in Tension”, was conducted in laboratory conditions, and its outcomes are the main topic of the report. Exploring and comparing the expected and actual failure modes of both bottled and welded connections in tension are the primary purposes of the paper. In the process of the experiment, it was revealed that almost all the predictions concerning connection and failure mode matched reality. However, there were some exceptions, for instance, in the contest of high strength Grade 8.8. bolt connection. It did not prove the calculations expecting the failure in a member capacity, and the problem was found in bolt shear. A failure was registered at the moment of the load, which was higher than calculated. The predictions on load ratios were 1.21, 1.4, 1.17, and 1.24 for Grade 4.6 bolts, Grade 8.8 Bolts regarding both single and double pal weld connections.
Discussion
For the experiment, the steel plate was applied, Grade 250 steel (to AS/NZS 3678) was used, and the yield stress was 300 MPa. It is also worthy to note that the weld material of W50X was leveraged (referenced from Table 9.7.3.10(1), AS4100). All the failure modes, which were likely to reveal during the experiment, were taken into consideration. In addition, nominal and design capacity was planned and evaluated too in accordance with AS4100 guidelines. Before the destruction, all examples were preferred, as well as actual and predicted results were monitored and compared. The failure modes, which were calculated, implied section tension yield, section tension fracture (considering all fracture patterns), bolt shear, bolt serviceability, ply in bearing, and weld shear.
Connections
Grade 4.6 and Grade 8.8 bolts are depicted in Figure 1, namely there is a nominal arrangement relating to details and dimensions of the bolted lap joint connection. As for the first one, its minimum tensile strength was 400 MPa, it is also was tensioned by hand. Table 9.3.1 AS4 demonstrates that the latter had a minimum tensile strength of 830 MPa, which was tensioned mechanically to 50kN. To induce this ratio onto the M12 bolt, the calibration should match a torque of 156.20 Nm. A double and single lap welded joint is presented in Figures 2 and 3. The nominal dimensions for the lap lengths and the fillet weld are also shown there. The two steel plates were connected via a W50X weld material.
Discussion of Results – Predicted and observed result differences
The expectations of the experiment in accordance with AS4100 and the actual results, namely the tensile loading of each connection to destruction, are depicted in Table 1. Grade 4.6 bolts were considered to encounter some problems at bolt shear, and Grade 8.8 bolts were planned to encounter an initial bolt slip, as well as a failure at member capacity. The first prediction matched the reality, while the initial bolt slip at 48 kN happened in the moment of failure by bolt shear at 224 kN. However, there was no failure in the connection for the reason of member capacity yielding. It was caused by a higher level of capacity of the Grade 250 steel plate as compared to the figures in Table 2.1 of AS4100. In fact, the yield stress was 300 MPa, and tensile strength was 410 MPa, as is demonstrated in 2.1 of AS4100. It can be explained by the fact that the yield stress and tensile strength values of the plate of steel must be different from the calculated ones.
In addition, there was a deformation between the bolt and the soft steel plate, which is characteristic of ply in bearing. Moreover, the expectations appeared to be correct in the context of failure through weld shear for both single and double lap welds. Apart from it, the calculations did not include the likelihood of eccentricity between the centroid of the section, as well as the load application point. This could lead to the single lap failing at a lower load, which centroid was off center to some extent, as compared to the double lap.
Tightening of bolts in friction grip joint
The necessity to tighten the 8.8 bolts to 50 kN can be explained by the failure load, which was caused by putting bolt shear at maximum. The importance of this action is attributable to placing the bolt into a preload tension. It contributes to creating a “clamping” force, which is helpful for ensuring whether all the forces act uniformly across. It also improves the friction and v for the bolt slip.
The difference in nominal properties, and AS4100 resistance factors
It should be mentioned that some dimensions differed by 1mm, which was included in calculations and did not influence the results. Furthermore, Figure 4 demonstrates differences in nominal and observed material properties. Therefore, the figures of yield stress and tensile strength can be up to 1.35 times in relation to nominal figures explained by the steel manufacturer’s specifications. All the connections, grades, dimensions, and types of steel, bolts and weld were not changed through the experiment. Although their differences in nominal and actual properties and the AS4100 correction factors were involved, an engineer is capable of producing designs for loads, which are close to the calculations. In summary, this design is the safest and most widespread option.