In Phase I of the precast concrete
beam-to-column connection study at
NIST, six specimens were tested. Two
of the specimens were monolithic concrete
connections designed to UBC
( 1985) Seismic Zone 2 criteria. The
other four specimens were designed to
UBC (1985) Seismic Zone 4 criteria.
Two of the Zone 4 specimens were
monolithic concrete specimens while
the remaining two were precast concrete
with post-tensioned beam-to-column
connections. The precast concrete
elements were connected by two posttensioning
bars. The construction joint
between the beam and column was
filled with a fiber reinforced grout and
the post-tensioning ducts were grouted
after tensioning.
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Results from the monolithic concrete
tests are used as a benchmark
reference for both present and future
precast concrete tests. The objective of
the test program was to develop an
economical moment-resistant precast
beam-to-column connection for seismically
active regions. The following
paragraphs present the conclusions
drawn from the results of the Phase I
tests.
1. Failure of the monolithic Zone 2
specimens occurred in the joint region
due to a combination of high joint
stresses and inadequate confinement.
The monolithic Zone 4 specimens
failed as a result of beam hinging and
deterioration. Failure of the precast
concrete specimens was characterized
by plastic elongation of the post-tensioning
bars and crushing and spalling
of the concrete cover in the beams.
Joint shear stresses for the Zone 4
specimens were below the recommended
value of 20 {7; ( t: in psi)
while the shear stresses in the monolithic
Zone 2 specimens were above
the recommended value.
2. The ultimate displacement ductilities
for the monolithic Zone 2 specimens
were 6. These ductilities corresponded
to story drifts of 4.1 and 4.3
percent. The ultimate displacement
ductility of I 0 for the precast concrete
specimens was higher than 6 obtained
for their companion monolithic specimens.
However, since the precast concrete
specimens were stiffer than the
monolithic specimens, the story drifts
at failure for the Zone 4 post-tensioned
and monolithic specimens were
almost identical. The story drifts at
failure for the Zone 4 specimens
ranged from 3.0 to 3.4 percent. The
post-tensioned specimens were approximately
twice as stiff as the
monolithic Zone 4 specimens and five
times as stiff as the monolithic Zone 2
specimens.
3. The flexural strength of the precast
concrete Zone 4 connections was
slightly greater than flexural strength
of the monolithic Zone 4 specimens.
The measured maximum beam moments
were on the average 18 and 13
percent greater than the calculated moments
(as defined previously) for the
precast concrete specimens and the
monolithic Zone 4 specimens, respectively.
The monolithic Zone 2 specimens
achieved measured maximum
beam moments that were on the average
8 percent greater than the calculated
moment.
4. When comparing the energy dissipated
per cycle, the behavior of the
precast concrete specimens was closer
to that of the monolithic Zone 2 specimens
than to the monolithic Zone 4
specimens. On a per cycle basis, the
post-tensioned specimens dissipated
about 30 percent of the energy dissipated
by the monolithic Zone 4 specimens.
However, since the precast concrete
specimens achieved higher displacement
ductilities than the monolithic
specimens, the average of the
cumulative energy dissipated up to
failure by the post-tensioned specimens
was approximately 80 percent of
that for the monolithic specimens. The
average of the cumulative energy dissipated
up to failure by the monolithic
Zone 2 specimens was about 33 percent
of that dissipated by the monolithic
Zone 4 specimens.