ETL 1110-2-536
31 Dec 94
and A-20 to A-25. ABAQUS calculated a maximum
since the orientation will be in the traditional
principle stress of 715 psi which occurs during the
upstream-downstream direction. In addition, since the
cooling period when maximum temperature differen-
dam is almost continuously dry, and channel flows
tials occur. Earlier testing of the RCC mix indicated
carry a phenomenal silt load, this cracking of the
a 28-day tensile strength capacity of 200 psi. The
structure is not of great concern. No additional
715-psi principle stress calculated is far in excess of
expense was warranted in lessening the cracking
the tested direct tensile capacity. This may be attrib-
potential by reducing the placing temperatures or by
uted to the fact that the analyses were performed
installing transverse joints.
using the original version of UMAT. That version
contained inconsistencies which resulted in the crack-
(2) Postconstruction inspections revealed one
ing threshold to be computed in an unconservative
crack in the structure located high on the left abut-
manner. This inconsistency has been corrected in the
ment as a result of a slope change in the foundation.
current version of UMAT which was not available
No other cracks are apparent. Less cracking has been
when this study began. However, newer versions of
observed because the restraint provided by the foun-
software available at WES have the capability to
dation is probably lower than full restraint assumed
predict and plot direction and magnitude of cracks.
by the simple analysis.
Observation of the stress contours for the longitudinal
model indicate higher stresses occur at the foundation
(3) Examination of the temperature history plots
interface and in the upper reaches of the abutment
indicates that the longitudinal model cools at a much
and at the spillway. This can be attributed to the
slower rate. The benefits of performing a longitudi-
temperature differential occurring between the
nal model are significant when time and costs are of
exterior and interior elements and the rigidity of the
concern for smaller projects, or projects of this type
foundation. At the time of year for postconstruction
where certain cracking will not adversely effect the
cooling, the average ambient temperature is decreas-
performance of the structure. However, in this case,
ing causing a larger temperature differential from
the thermal gradients of the longitudinal model should
surface elements to interior elements. Time history
be calibrated with the more accurate transverse model
stress plots of various points of high stresses
to produce nearly the same rate of cooling. This may
observed in the contour plots are presented in Appen-
become a significant factor in the analysis because:
dix A, Figures A-26 to A-32 and A-33 to A-40. The
stress time histories presented in Figures A10.4 and
While cooling is at a slower rate than might
A10.6 indicate that some cracking may have
be expected, this will cause volume changes
occurred. Jumps in stress, as indicated in these plots,
and hence the maximum stresses to occur at
typically do not occur unless a crack has formed.
a later age in the model. Since the higher
Discussion of these results are presented below.
stresses occur later in time, the aging modu-
Likewise, the transverse model predicted stresses that
lus will be higher. Since the criteria for
are higher than the limited cracking stress. The high-
cracking is partially based on stress, the
est stress occurred at element 217 at the foundation/
potential for cracking will be unconservative
RCC interface. In addition, a region of high stress
for this case.
occurs along the downstream exposed face of the
dam.
The thermal stresses are being applied at a
later age; therefore, the effect of creep will
play a less significant role in stress relax-
d. Discussion.
ation. This may be conservative; however,
as long as a large amount of effort has been
(1) A simple thermal analysis was done for the
expended to accomplish a material investiga-
project during the design phase of the project. This
tion, it would be prudent to spend the same
analysis indicated that during the normal summer
amount of effort to calibrate the longitudinal
weather conditions at the site, the structure may crack
model. This will ensure that the analysis will
at three locations. Two-crack locations were
incorporate the more accurate creep data in
estimated to be located where the foundation changes
the earlier time steps when cooling would be
from a horizontal surface at elevation 635 to the
expected to cause higher stresses. Hence, the
sloped abutments. The third crack was speculated to
effects of stress relaxation due to creep will
be in the center of the spillway. None of these
be incorporated into the analysis.
cracks pose a threat to the stability of the structure
A-5
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