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Square and octagonal columns are supported by individual piers and pyramid spread footings. The
floor system consists of RC beams in the north-south direction that support the joists and 4 in. thick
one-way slabs in the east-west direction. The beam-column system in the north-south direction
constitutes a poorly detailed moment frame and the lack of beams in the east-west direction implies a
very weak and soft system in that direction. Based on FEMA-356 Life-Safety performance criteria
(FEMA 2000) for a 10% in 50 year event, the lateral-force-resisting system of the building was found
to be inadequate when the current renovation design was undertaken.
Results from linear and non-linear 2-D and 3-D computer analyses of the structure conducted by
Coughlin, Porter and Lundeen, Inc. led to the selection of fluid viscous dampers in conjunction with
hybrid FRC-encased steel truss beams for strengthening the structure in the north-south direction,
while Yielding Steel Braced Frames were selected for the east-west direction. For the addition of the
hybrid beams, appropriate connection details needed to be developed in order to assure adequate
transfer offerees between the new hybrid beams and the existing RC columns. In this paper, a testing
program aimed at evaluating various details for connecting hybrid FRC-encased steel truss beams to
the existing RC columns of the King County Courthouse building structure is described.
EXPERIMENTAL PROGRAM
The experimental program consisted of testing of four beam-column subassemblies under
displacement reversals. Specimens 1 and 2 represented interior connections, while Specimens 3 and 4
represented exterior subassemblies. Fig. I shows a sketch of the test setup used in the experimental
program. The four test specimens were subjected to a displacement pattern similar to that specified in
the ACI ITG/T1.1-99 document (ACI 1999) with story drifts ranging from 0.2% up to 3.5%.
Additional cycles, with an amplitude of approximately 4.5% drift and 5.0% drift, were applied to
Specimens 1 thru 3 and Specimen 4, respectively, after the completion of the cycles to 3.5% drift.
The column and beam cross section dimensions were the same for all four specimens. The column
cross section had an octagonal shape with a side length of 8.25 in. and a 20 in. square beam cross
section was used for all test specimens. These dimensions translated into an approximately 2/3 scale
compared to the existing columns and the hybrid beams planned for the building. Lateral
displacements were applied through a hydraulic actuator connected to the top of the column and to a
strong reaction wall (Fig. 1). A small axial load was applied to the column through hydraulic jacks,
amounting to an axial stress of approximately 150 psi. Three connection details were evaluated for
achieving proper moment connection between the beam and column: 1) use of beam longitudinal
reinforcing bars passing through and epoxy-bonded to the column, 2) use of beam longitudinal
reinforcing bars combined with external steel reinforcement, 3) use of only external steel
reinforcement for full moment transfer. In all cases, shear connection between the embedded steel truss
and the existing column was provided to resist the full shear demand.
Test Specimens
Four beam-column subassemblies were designed and tested to evaluate the behavior of the three
moment connections mentioned above. In the following, a brief description of beam, column and
connection details used in the test specimens is given.