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OCR for page 49
47
Partial Diaphragm and Slab Casting Effects
In the first full-size specimen, a partial diaphragm was used.
Initially, the bottom one-third of the height of the diaphragm
was cast. After 28 days, the remaining part of the diaphragm
was cast along with the slab. The idea was that the weight of
the slab would cause the ends of the girder to rotate into the
partial pour, compressing the diaphragm and the girder ends.
This would then prevent positive moment cracking due to
creep shrinkage. It is not clear how well this actually worked.
During the process of casting the slab, about 50 microstrain
of compression was seen at the diaphragm or at the girder
ends. This would translate to a stress of 200 psi. However,
the system was already in tension from positive moments,
which had occurred since the partial diaphragm was cast.
Even with the compressive stresses generated when the slab
Figure 70. Positive moment connection with 180o hooks. was cast, the system was still in a net state of tension.
After a few hours, the slab concrete began to heat up from
hydration. The top of the specimen expanded, the system
stirrups arrest form in the embedded specimens, but are less
cambered up, the center reaction decreased, and the end
pronounced or nonexistent in the nonembedded specimens.
reactions increased. This caused a positive moment on the
This type of detail may be useful in providing additional duc-
connection, which relieved all compression caused by cast-
tility in seismic zones.
ing the slab. Later, the slab cooled and contracted. This
The use of horizontal bars through the web enhanced the
caused the system to deflect downward, increasing the cen-
performance of the connection. This type of connection was
ter reactions and decreasing the end reactions. It appears that
stiffer and more resistant to fatigue. However, when the con-
a net negative moment on the order of 500 k-ft was created
nection failed, there was considerable cracking in the beam.
and that the girder ends were compressed into the diaphragm.
This may not be desirable. The decision to test a connection
with horizontal web bars was made after the girder sections A net negative strain of 100 microstrain was observed in the
were cast. Holes for bars were drilled into the webs between girder ends and in the diaphragm. This would correspond
the shear stirrups. There was no additional reinforcement in to a stress of approximately 400 psi. Thus, it appears that
the web around the holes. Had the holes been cast into webs the partial diaphragm worked, but not by the mechanism
and additional reinforcing added, the cracking in the girders assumed.
might have been less severe. A tension tie at the top of the girders should improve per-
The test results showed that all of the connection details formance because it seems that much higher compressive
performed adequately, and each had advantages and dis- stresses were generated at the bottom of the diaphragm once
advantages. There was no detail that performed markedly continuity was established. This tension tie either can be a
better or worse than any other. Thus, the selection of a spe- mechanical tie between the girder tops or it can be made by
cific detail should be left to the preference of the engineer, pouring the entire diaphragm and a portion of the slab in the
state DOT, or both. negative moment region.
BRIDGE BEHAVIOR
Effects of Creep and Shrinkage
Temperature Effects
The first full-size specimen was constructed and then mon-
The most striking result in the full-size tests was the influ- itored for a period of 4 months after the deck was cast. Accord-
ence of temperature in the system. During a single 24-h ing to the model of the system, the girders should camber
period, the end reactions often changed as much as ±5 kips, upward due to creep and shrinkage. Before the deck was cast,
20% of the average value of 25 kips. This change generated a partial diaphragm was cast. A total of 28 days elapsed
diaphragm moments of ± 250 k-ft, approximately 60% of the between the time the partial diaphragm was cast and the time
positive cracking moment for the section or 2.5 times the live- the deck was added. During this time, the girders cambered
load positive moment. The AASHTO LRFD Specifications upward. This increased the end reactions, causing positive
require that temperature effects be considered (12), but these moment at the connection. Tensile strains were seen in the
effects are often ignored in design. The experimental results girder end and in the diaphragm.
show that temperature effects can be significant. Alabama When the deck is added, the models predict that the deck
attributes cracking in bridges to temperature effects (20, 21). will shrink and the differential shrinkage between the slab and
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48
girders will cause the formation of negative moment at the Continuity
connection. This was not observed. In fact, over the monitor-
ing period, the end reactions increased and the center reaction In the first full-size specimen, cracks formed at interface
decreased, indicating the formation of positive moment. of the girders and the diaphragm. These cracks varied in
Similar results were seen in three projects using high- width (due to the asymmetrical bar pattern), but some were
performance concrete (26). The girders in these projects as wide as 0.02 in. The crack did not extend into the slab.
ranged from 60 to 300 days old. The data did not show Continuity was assessed by loading the specimen such that
the expected downward camber or formation of negative the moment at the diaphragm was either the positive or neg-
moment. The data presented by Ramirez et al. (14), show ative live-load moment and observing changes in reactions
that the effect of differential shrinkage is overestimated by and strains in the cross section. This specimen showed con-
the models. This is also consistent with anecdotal evidence tinuity under all loads. This is consistent with observation
as there have been no reports of severe negative moment dis- from Alabama (20, 21).
tress occurring in continuous-for-live-load bridges. The second full-size specimen had cracks between the
The problem does not appear to be the structural model. diaphragm and the girders of 0.07 in. These large cracks were
As noted in the previous section, there was a clear differen- induced by increasing the positive moment by raising the end
tial contraction of the deck due to temperature after casting, supports. This specimen maintained continuity until the con-
nection appeared near failure. This determination that the con-
and the behavior was consistent with the model predictions
nection was near failure was based on the observations that
for a contraction of the deck. The girders deflected down-
the interface crack had propagated into the slab, the bottom of
ward, the end reactions increased, the bottom of the connec-
the diaphragm was cracked, and a diagonal crack had formed
tion showed compressive strain, and there were tensile strains
on the face of the diaphragm. The tests of the stub specimens
at the top of the connection. The problem appears to be that
indicated that these cracks were signs of the impending fail-
values used for deck shrinkage are not correct. In most mod-
ure of the connection. At the point were the connection seemed
els, the values for deck shrinkage are based on unrestrained
near failure, the system still maintained approximately 70%
shrinkage values. These values either are obtained experi-
continuity.
mentally or are from empirical equations based on the results
It therefore appears the cracking at the diaphragm does not
of experimental studies. These values of shrinkage do not
affect continuity unless the cracking is severe. One reason for
seem to duplicate the actual field conditions. Many models
this is probably the condition of the center support. In most
do not correctly account for the restraining effects of the rein- models, the system is modeled as having a single center sup-
forcing bar or the girder, actual field relative humidity, and port. In reality, the center support consists of two supports,
rewetting of the deck by rain or snow. one under each girder end. The girder ends and the diaphragm
This difficulty in assessing the effects of differential shrink- then form a "member" between the two center supports. The
age has an impact on the assumed stresses in the connection. presence of a crack on either side of the diaphragm does not
The models generally show that negative moment caused by appear to alter the stiffness of this intermediate member
differential shrinkage will mitigate positive moments cause by enough to cause a complete loss of continuity. However, as
creep and shrinkage of the girders. However, if these negative the diaphragm cracks and the cracks propagate into the slab,
moments do not form, the actual positive moments on the con- this center section softens enough to cause some loss of con-
nection will be much greater. This issue needs further study. tinuity. From the results of the stub specimen tests, it appears
Some states attempt to solve the creep and shrinkage that continuity will only be completely lost when the con-
issues by specifying maximum ages, minimum ages, or both nection fails and a hinge forms.
for the girders at the time continuity is established. The max- The models predict some loss of continuity as soon as the
imum age limit is to prevent the formation of the large neg- section cracks; however, this is a limitation of the models. In
ative moments caused by differential shrinkage of the deck theory, as soon as the section cracks, the crack propagates
slab. Since the data suggest the models overestimate this into the slab (see Figure 12). In reality, the crack starts at the
effect, there does not appear to be a reason to limit the age of bottom of the joint and propagates upward to the deck slab.
the girder at the time continuity is created. However, a min- Since the joint between the girder and the diaphragm is a cold
imum age does seem advisable. If continuity is formed when (or construction) joint, the crack can follow this joint easily,
the girders are young, creep and shrinkage in the girders but seems to be arrested when it hits the deck slab. Only
cause large positive moments to form. Since the data suggest when the connection is about to fail does the crack propagate
that these positive moments are not mitigated by the deck into the slab. At this point, the cracked section at the joint
slab shrinkage as much as the models predict, the actual pos- matches that predicted by the model, and only then does the
itive moments that develop may be worse than predicted. If predicted loss of continuity occur.
the girders are allowed to age, much of the creep and shrink- This experimental program tested a single system using I
age will occur before continuity is established. This will lower girders. This certainly does not cover the entire range of
the magnitude of the positive moments that form. bridge types, so only limited recommendations can be made.
