ETL 1110-2-365
31 Aug 94
compared to the yield strength of the reinforcing. If
placing temperature or by adding insulation. These
the yield strength of the reinforcing is exceeded, then
are relatively simple changes to make both in the
the structure should be reevaluated with changes to
analysis and in the specifications, although both
other parameters to improve performance.
changes will increase construction costs. Changes in
the analysis will require a change in the initially
(3) Winter protection. The effects of cold fronts
specified temperatures in the heat transfer analysis for
may cause significant cracking within a MCS and
a change in placing temperature, and a change in
should be considered when evaluating the MCS. This
insulation will require the value of the film coeffi-
winter protection evaluation is required mainly to
cients to be revised. An additional item to consider
assess the need, duration, and R-value for possible
for reducing high thermal gradients is to place voids
insulation of the structure. Cold fronts have not been
in the areas of the structure where the concrete may
specifically required in the NISA studies due to their
not be required for structural considerations. This
sporadic and unpredictable occurrences. Yet, they do
will reduce the amount of heat generated which may
occur and are commonly the cause for cracking of
reduce the thermal gradient. In addition, this alterna-
structures while under construction. Their unpredict-
tive will remove concrete and may be a savings
ability in magnitude, occurrence, and duration create
instead of a cost.
significant complexities in computer modeling of their
effects and behavior. A reasonable approximation
(b) Cracking at corners. If cracking occurs at
without specific analysis is to locate all uninsulated
corners of openings and it is desired to limit these
regions of the MCS that have a cracking potential
cracks, but not necessarily eliminate them, then addi-
greater than 80 percent during those periods of con-
tional reinforcing in these areas should be considered.
struction when cold fronts are possible and would be
Typically, reinforcing placed at a 45-deg angle at the
considered detrimental to the structure. A structure
corner of the opening is the most beneficial for con-
with a NISA that exceeds the 80-percent cracking
trolling cracking. This will require adding reinforcing
potential should be insulated or other modifications
elements through the ABAQUS *REBAR option. It
should be made to reduce the risk of cracking during
is also recommended that if reinforcing is to be
a cold front. Insulation should be used whenever the
included in an analysis, the ANAGEN preprocessing
cracking potential exceeds 80 percent during that
software (ANATECH Research Corp. 1992) be used
portion of the year when cold fronts are possible.
in developing the model. Use of ANAGEN will
The evaluation of R-values for insulation purposes
significantly decrease the difficulty of including rein-
may require an additional NISA(s) to be performed.
forcing in the model, particularly for sloping bars.
The design team must use the NISA results coupled
with experience and engineering judgement to
e. Pile reactions. An evaluation of the pile
develop the final requirements for insulation during
reactions should be performed to determine the
construction.
effects of construction procedure, thermal loads, and
aging material properties coupled with service loads
(4) Practical hints. Regions of potential cracking
on the load distribution for the piles. This should be
are project dependent due to the fact that the struc-
a qualitative evaluation where minor pile overload or
ture, the climate, materials, and construction proce-
overstressing should be of little concern.
dures are all typically site specific. If cracking
should occur in a structure, various measures can be
f. Output interpretation. This section is
taken to reduce or eliminate cracking. Presented
intended to give insight into the various methods that
below are some possible areas to consider when
have proven useful in interpretation of analysis results
efforts are being made to improve the structure's
and not to provide a rigid framework of steps to
behavior due to the presence of cracking. Solutions
follow. The structural engineer must sufficiently
for reducing cracking will vary based on the mecha-
process results to comprehend the behavior of the
nism causing the cracking and therefore careful con-
structure and provide the necessary data (plots, dia-
sideration should be given to any items used prior to
grams, tables, etc.) to support conclusions based on
their implementation.
this understanding.
(a) High thermal gradients. If cracking is
(1) Contour plots. Contour plots of temperature,
created due to a high thermal gradient, changes for
stress, net strain, and crack potential are useful in
reducing this gradient may be made by lowering the
selecting zones in the structure for more detailed
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