The fatigue testing configuration consisted of the specimens supported at each end on pin-rollers and subjected to two point loads offset from the midspan. The loading configuration and the resulting shear force and bending moment profiles are illustrated in Figure 3–9. The loading offset was intended to create a varying shear profile along the length of the specimens, inducing a relatively early failure in the more heavily loaded shear connectors towards the West end of the specimens. This experimental design was intended to study the impact that a failed through-bolt shear connector had on the neighbouring through- bolts in terms of how the longitudinal shear redistributed and consequently how the overall composite behaviour was affected. Moreover, as shown in Figure 3–9, the loading offset induces approximately three times the shear force in through-bolt Rows 1 and 2 compared to through-bolt Rows 3 - 12, meaning that failure of these through-bolts was expected to occur first. This simplified the instrumentation and monitoring process as it was therefore not necessary to monitor and instrument all twelve through-bolt rows. The loading was applied to the specimens through two pin supports attached to a spreader beam.
Figure 3–9: Static and fatigue testing configuration
3.5.1 Stiffness Testing
The specimen stiffness was evaluated by testing prior to any fatigue testing and at various stages throughout the experimentation. Initial stiffness tests were performed to confirm the load level at which the onset of slip between the concrete deck and the steel girder is apparent as well as to collect data on the behaviour of the specimen before any fatigue damage occurred. The load level for stiffness testing was then adopted as the minimum trial peak load used in the fatigue testing. Subsequent stiffness tests were performed to discover if any global behavioural changes in the specimens or local behavioural changes in the through-bolts had arisen due to the accumulation of fatigue damage.
The stiffness tests were performed in displacement control at a rate of 0.05 mm/s. Displacement control is typically preferable for capturing small variations in the load resisted by the specimens since the actuator continually imposes displacement at a constant rate regardless of the behaviour of the specimen. Conversely, if a crack were to form or if there was a sudden slip and the specimens were loaded in load control, the load cell on the actuator would provide an electronic load feedback to the controller indicating that the required loading increment has not yet been achieved and the controller would respond with instructions to rapidly increase the load until the required loading increment is reached. This may consequent in discrete variations being missed.
Two cycles were performed for each stiffness test so that the results of each cycle could be averaged. Furthermore, at the peak of each stiffness test, the actuator was paused for roughly ten seconds to allow for instrumentation stabilization. The resulting loading-unloading history can be seen in Figure 3–10.
Figure 3–10: Loading-unloading history for stiffness test
The stiffness tests performed subsequent to the cyclic loading stages were performed until the displacement of the composite beam corresponding with the maximum load level in the previous cyclic testing was reached. This displacement is indicated in Figure 3–10 as ΔMax.
The data collected during the stiffness tests included the interfacial slip between the concrete deck and the steel girder, the maximum vertical deflection of the specimen, and the strains in the steel girder flanges and the instrumented through-bolts.
3.5.2 Cyclic Loading
One of the primary focuses of this investigation was to assess the fatigue performance of the through-bolt shear connectors. This required subjecting the through-bolts to cyclic stresses by loading and unloading the specimen repeatedly. As the specimens were loaded, the through-bolts transferred the longitudinal shear forces between the concrete decks and steel girders, increasing the force carried by the through- bolts. When the specimens were subsequently unloaded, the strains in the through-bolts decreased. This oscillation of strains (and corresponding stresses) within the through-bolts was expected to initiate and propagate cracks within the through-bolts until the cracks reached a critical length and the remaining cross-sectional area was less than sufficiently required to resist the forces induced in the through-bolts. At this instance, fatigue failure of the through-bolts was expected to occur and was expected to be brittle.
The specimens were loaded cyclically in load control, using a sinusoidal function. The applied load range was varied from test to test. However, a 10 kN minimum loading was maintained on each specimen to
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Maxprevent impact loading caused by the load apparatus lifting off of the specimen. This is shown as the upper dashed red line in Figure 3–11.
Figure 3–11: Cyclic loading history
The frequency at which the cyclic testing was performed ranged between 0.5 Hz and 2 Hz depending on the desired loading level. Higher load levels generally result in shorter tests since fatigue failure is expected to occur sooner. However, lower frequencies were used at the higher load levels to prevent damage to the actuator and the specimens.