ON THE EFFECTS OF CIRCULAR BOLT PATTERNS ON THE ...

forces in a given bolt row. Borgsmiller [13] introduced a simplified version of the Kennedy et al. [5]
method with only two stages of plate behavior; thick plate with no prying force, and thin plate behavior
with maximum prying force. The threshold between thin and thick plate behavior was established as the
point in which prying force was negligible. Therefore, the intermediate stage introduced by Kennedy et
al. [5] was ignored. Consequently, this design philosophy was adopted by the AISC Design Guide-16 [8]
with the assumption that the column and end-plate are relatively stiff and remain elastic, and bolts are
not subjected to noticeable prying forces. These models have been used for many years as a design
philosophy to design a steel moment resisting bolted connections.
On January 17, 1994, the Northridge earthquake became a turning point for the design philosophy of
the steel connections. This earthquake caused many unexpected failures on numerous fully-welded
beam-to-column connections. More than 150 welded steel moment frame structures sustained brittle
fractures in their welded moment connections, which demonstrated in no uncertain terms the
vulnerability of welded connections to seismic loading. This earthquake event initiated a series of new
research studies to investigate the true behavior of steel moment resisting connections subjected to
cyclic loading. The premature brittle failure of welded connections was also noticed in the 1995 Hyogo-
ken Nanbu (Kobe) earthquake. To address this vital issue, the U.S. Federal Emergency Management
Agency (FEMA) initiated a 5-year program to develop and verify reliable and cost-effective methods for
the inspection, evaluation, repair, rehabilitation, design, and construction of steel moment frame
structures. This program was managed and administrated by the joint venture between Structural
Engineers Association of California, the Applied Technology Council, and the California Universities for
Research in Earthquake Engineering (SAC). This research is commonly known as the SAC Steel
Project. SAC was divided into two phases. The initial phase focused on determining the cause of the
fully welded connection failures, and the second phase of the research focused on finding alternative
connections which can sustain seismic loading. This investigation included experimental testing and
numerical modeling to determine suitable welded connection or bolted connection with an end-plate for
use in seismic force resisting moment frame. Among the important tasks identified by FEMA/SAC Phase
II, the connection performance and system (frame) performance under earthquake loads were most
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