ETL 1110-2-365
31 Aug 94
will be used to develop the final design parameters to
conditions, and film coefficients. The material prop-
be used in evaluating the various monoliths.
erties must then be defined and should include the
conductivity, density, and specific heat of any materi-
(3) Final NISA. A final NISA should be com-
als used in the analysis. This will require properties
pleted late in the design process, using the design
for both the concrete and foundation and possibly air.
parameters selected in the initial NISA to verify the
The initial temperature of the concrete must be
selected designs. The analysis should be based on
defined and is typically assumed to be the placing
the final design layout and the parametric combina-
temperature. A definition of the air temperatures
tion which produced the worst condition in the initial
should be made as described in paragraph A-2c. Fin-
NISA studies. Should actual conditions during con-
ally, a definition of the time history must be made as
struction deviate from those assumed for the final
shown in steps 6 and 7 of Figure A-1. This includes
analysis, it may be necessary to perform another
defining the length of each step and its increment,
NISA using the actual field conditions.
changing the model as necessary, applying, removing,
and changing film coefficients as required, applying
c. NISA process. The NISA process is basically
the heat generation (paragraph A-5a(2)), and defining
composed of a heat transfer analysis and a stress
any required output. It is critical that a temperature
analysis. The heat transfer analysis is performed to
output file be defined properly to ensure that tempera-
determine how the temperatures within the structure
tures needed for the stress analysis are computed
change with time. The stress analysis is performed to
properly.
determine the stress and strain state of the structure
based on these changing temperatures, gravity loads,
(3) Stress analysis. A flow chart defining the
changing material properties, and the boundary condi-
steps in a stress analysis is presented in Figure A-2.
tions. A description of these two types of analyses is
The node and element data defined in the heat trans-
provided in the following paragraphs. Parametric
fer analysis for the concrete are typically used in the
studies are an important part of performing a NISA
stress analysis and these data can then be used to
and are used to assist the engineer in making the
identify the needed node and element sets. Input
proper decisions for design and construction parame-
parameters for the user material subroutine must be
ters. Use of parametric studies is discussed in para-
determined by calibrating the model for the concrete
graph A-2g. Once analyses are completed, it is
mixture being analyzed with the test results for that
necessary to evaluate the results as described in para-
mixture. If springs are used in place of continuum
graph A-6 to determine the effects of various
elements to model the foundation, then spring con-
parameters. Finally, the results, conclusions, recom-
stants must be determined and used as a definition of
mendations, and any cost savings should be reported
the spring properties. A definition of the initial
as described in paragraph A-7.
boundary conditions must be specified prior to begin-
ning a time-history analysis. As in the heat transfer
(1) General. To date, NISA's have been per-
analysis, the final process in the stress analysis is to
formed using the finite element (FE) code ABAQUS
define the time-history analysis to take place as
(Hibbitt, Karlsson, and Sorensen 1989). Since experi-
shown in steps 6 and 7 of Figure A-2. This includes
ence has been gained by using ABAQUS and its
defining the time of the steps and their increments,
associated user supplied subroutines (UMAT,
defining changes in the model, application of
DFLUX, and HETVAL), discussion will be based on
mechanical loads, accessing the temperature data
the methods used by ABAQUS for performing a
from the heat transfer analysis to define thermal
NISA as well as these user supplied subroutines used
loads, and definition of the output desired.
by ABAQUS.
d. Coordination. A design team consisting of
(2) Heat transfer analysis. A flow chart defining
structural, materials, geotechnical, cost, and construc-
the steps in a heat transfer analysis is presented in
tion engineers should be established prior to perform-
Figure A-1. The first step is the basic step necessary
ing a NISA study. Interdisciplinary coordination is
for any FE analysis in which the structure and foun-
essential to ensure that the complex structural analysis
dation are discretized into a group of elements
is based on reliable concrete and foundation proper-
defined by nodes. Once the nodes and elements have
ties and realistic construction techniques. The struc-
been defined, it is necessary to define node and ele-
tural, materials, and construction engineers should
ment sets for items such as material properties, initial
predict an appropriate set of construction conditions
A-2
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