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reinforcement was used over the remaining part of the beam and beam longitudinal reinforcement was
kept the same as for Specimen 2.
Specimen 4
Test Specimen 4 represented an exterior subassembly similar to Specimen 3. However, no reinforcing
bars were used in the hybrid beam, and thus the steel reinforcement consisted only of a steel truss
embedded in the FRC beam. Moment transfer in this specimen was achieved through the use of
external steel rods. The use of a moment connection through only external reinforcement eliminates
the need for drilling holes in the RC column, and thus the risk of cutting longitudinal and transverse
reinforcement, which required wrapping of the column. A sketch of the connection details used to
transfer the forces from the steel truss to the external rods is shown in Fig. 4.
Material Properties
Ready-mix concrete from a local supplier was used in all four specimens. For the FRC used in the
beams, 1.2 in. long hooked steel fibers with a diameter of 0.02 in. were added at a volumetric fraction
of 1%. Concrete compressive strength for the columns ranged between 4100-5600 psi, while that of the
FRC ranged between 3500-4100 psi. Grade 60 steel was used for all reinforcing bars in the columns
and beams. A3 6 steel was used for the steel truss members and Grade B7 bolts were used for the
through-rods in Specimens 2 and 3, and the external rods in Specimen 4.
EXPERIMENTAL RESULTS
The behavior of the test specimens was evaluated in terms of their load vs. displacement hysteretic
response, cracking pattern, beam rotations, and energy dissipation capacity.
Load vs. Displacement Behavior and Cracking Pattern
The load vs. displacement response for Specimens 1, 3 and 4 is shown in Fig. 5. Specimen 1, with
moment connection relying on epoxy injection of the longitudinal beam bars passing through the
column, experienced significant pinching due to slippage of the longitudinal beam bars. Yielding of
these bars began at 1.0% drift and for story drifts up to 1.4%, pinching in the hysteretic loops was
moderate. However, at larger drift levels, the amount of pinching increased severely due to an almost
total loss of bond between the longitudinal beam bars and the column concrete, which led to the
opening of a large gap at the beam-column interface. Flexural cracking in the FRC beams was
observed at early stages of the test. However, concentrated rotations at the beam ends due to the
opening of the gap dominated beam rotations, and thus beam flexural cracks did not open significantly
during the cycles performed at larger drifts. In terms of lateral strength, Specimen 1 maintained its
strength up to 4.5% story drift with little loss of stiffness during repeated cycles at the same drift level
Specimen 3 5 which had the moment connection with combined external steel plates and epoxy
injection of the longitudinal beam bars passing through the column, exhibited a stable hysteretic
response with good stiffness retention and energy dissipation capacity, as shown in Fig. 5b. In this
specimen, the use of external steel plates with tight holes connected to the beam through-bolts