Farrokhi, Danesh, and Eshghi 2009
The authors perform and experimental an analytical assessment of reducing the cover plate section and drilling holes in cover plates at beam to column moment connections. Three full scale connections were tested under seismic loading.
System Concept
Reducing the cover plate section and drilling holes in the cover plates are intended to prevent failures at stress concentrations at the welds in connections. It is optimal to achieve plate yielding to weld fracture. The cover plates become a structural fuse as stress concentrations that occur from drilled holes induce plate yielding prior to weld fracture. Yielding cover plates instead of breaking welds serves an additional purpose of being a much easier component to replace.
Experimental Study, Results, and Discussion
Three connections were tested in this experiment: one unchanged connection, and two with cover plates with drilled holes. In the unchanged connection, cracks propagated from the weld roots and there was a significant decrease in stiffness and strength under cyclic loading. The two samples with drilled holes in the cover plates showed consistent hysteretic behavior without as significant a loss in stiffness and strength. It was also observed that connection rotation was greater at the modified connection with drilled holes.
These results indicate that the modified connection is indeed shifting stress concentrations from the welds to the cover plate around the drilled holes, producing a much more ductile connection. This connection can alleviate worries about the quality of the weld roots.
Analytical Study
An analytical study via finite element modeling was performed to examine the effects of certain parameters on the behavior of the connection, namely: cover plate thickness, end plate thickness, the number and diameter of drilled holes, the quality of the weld roots, and other previously studied factors having to do with the geometry of adjacent members.
It was found that the cover plate thickness has a large impact on the stiffness of the connection. The end plate connection has an impact on the stiffness, but does not significantly impact the strength capacity of the connection. It was found that large holes and large numbers of holes both precipitate premature yielding of the connection. The authors recommend that the hole diameter be limited to 0.8 times the cover plate thickness and that the clear distance between the holes be at least 1.25 times the plate thickness.
Reference
Farrokhi, H., Danesh, F., and Eshghi, S. (2009). "A Modified Moment Resisting Connection for Ductile Steel Frames (Numerical and Experimental Investigation)," Journal of Constructional Steel Research, 65.10/11, pp. 2040-2049.