Based on the observations and results
during fabrication, testing, and
data analysis of the two large-scale
precast concrete beam-to-column connections,
the following conclusions
can be made:
1. Precast concrete frames using the
two tested beam-to-column connections
can be built easily and quickly.
2. The specimens exhibited ductile
behavior. The lateral load carrying capacity
was maintained nearly constant
up to drifts of 3.5 percent, which are
larger than the maximum drift values
allowed in most design codes around
the world.
3. The specimens were designed to
develop plastic hinges in the beams
next to the columns and to impose
joint shear demands close to the nominal
joint shear strength for monolithic
construction. As expected, the behavior
was controlled by the joint at large
drifts. Joint degradation and stiffness
decay were recorded after the beam
top steel reinforcement of the beam
had yielded in tension.
4. In both structures, beam rotation
took place inside and outside the joint.
Joint mechanisms of resistance were
impaired by the development of tensile
strains due to beam rotation inside
the joint. Beam rotation inside the
joint does not usually occur in monolithic
construction.
5. In Specimen J1, where hoops
were used to achieve continuity, premature
bending flexibility of hoop
legs transverse to the loading direction,
as well as pullout of beam bottom
bars, contributed to initial joint
damage.
6. Specimen J2 (in which continuity
of bottom longitudinal reinforcement
of the beams is provided by a steel bar
inserted through overlapping Ushaped
prestressing strands) performed
better than Specimen J1. Specimen
J2 exhibited a more uniform
distribution of beam cracking and
yielding under negative bending.
7. Continuity reinforcement in the
form of hoops or U-shaped strands
should be sufficiently strong and stiff
to avoid plastic behavior under maximum
demands calculated from a capacity
design approach.
8. Joint shear strengths of Specimens
J1 and J2 were 80 and 90 percent,
respectively, of those expected
for monolithic construction. Moreover,
the initial shear cracking occurred
at lower levels of nominal
shear stress than in monolithic construction.
This phenomenon was attributed
to premature beam rotation inside
the joint.
9. The test results showed that the
structural response of the precast
frame was satisfactory. Although the
connections tested did not fully emulate
monolithic construction, they can
be used in precast concrete frame systems
or in hybrid systems, provided
that their strength and stiffness are
taken into account.
10. To improve the cyclic behavior
of the connections, beam rotations inside
the joint should be minimized.
One approach to accomplishing this
objective is to force the concentration
of beam flexural rotations away from
the column faces, i.e., relocate the
beam plastic hinges. An alternative to
reducing beam rotations inside the
joint is to place unbonded post-tensioning
tendons through the joint.