ETL 1110-2-365
31 Aug 94
(3) Material property combination 1. This mate-
structure, since the structure is built in lifts (incre-
rial property combination is to be used as a baseline
ments) over a significant period of time. A start time
in the winter could be critical for lift one but could
for comparison with combinations 2 and 3. This
also result in a later lift being placed in the spring.
material property combination is not to be used for
This timing could cause a less critical situation for
design purposes. Comparing combination 1 with 2
the second lift than if it were also placed in the
and 3 provides insight into the effects of the time-
winter. Therefore, the design team should locate
dependent creep and shrinkage. Material property
critical regions (high stress, high strains, potential
combination 1 also provides insight with respect to a
cracking) within the structure and use start dates
traditional heat transfer analysis by including only the
which cause early exposure of these critical areas to
adiabatic temperature rise and the extreme ambient
the coldest and warmest times of the year. The
conditions. This material property combination
design team should be aware that the steeper the
should include the incremental construction.
temperature gradient within the material, the higher
the stress. Therefore, the start date should be chosen
(4) Material property combinations 2 and 3.
to create the highest or steepest internal gradient in
Combinations 2 and 3 are essential in meeting the
those areas cited as critical. The highest gradient in
objectives of a NISA. These material parameter com-
an area would typically occur when the two extremes
binations have been critical combinations in previous
of the internal heat generation and ambient conditions
studies. (Truman, Petruska, and Ferhi 1992; Garner
are 180 deg out-of-phase. Since this situation cannot
et al. 1992)
be explored for every critical area, the design team
must develop a sound strategy for choosing the two
c. Extreme ambient temperature. The extreme
start dates.
ambient temperature data should be obtained from a
weather collection site near the project site. The data
e. Time duration. The time duration used in a
required is the coldest recorded monthly average
NISA should be sufficient to guarantee the inclusion
temperature for each month within any given year
of the maximum or critical response of the structure.
and the hottest recorded monthly average temperature
However, neither the maximum nor critical response
for each month within any given year. An extreme
occurrence time is generally known prior to an analy-
ambient temperature function should be developed as
sis. For this reason, the time duration used in a
a sine wave with a 365-day period which captures the
NISA should last a minimum of one full ambient
coldest and hottest of the extreme monthly average
temperature cycle past the time of service load appli-
temperatures. The extreme ambient temperature is
cation. The minimum time duration may be reduced
used to account for the possibility of seasons
if parametric studies indicate the time at which the
(months) having much higher or lower temperatures
maximum or critical structural response occurs is less
than the average ambient conditions developed using
than the specified minimum.
monthly averages based on multiyear averages. The
weather collection sites and the associated data may
f. Service loads. Service loads shall be applied
be obtained from the Air Force liaison of the National
during the time increment beginning 100 days after
Climatic Data Center, Federal Building, 37 Battery
placement of the last lift in the analytical model.
Park Ave., Asheville, NC, 28801-2733,
Generally, changes in film coefficients and ambient
(704) 271-4218.
conditions due to the application of service loads
need not be considered in the heat transfer analysis.
d. Construction start-date parameters. Combi-
For example, the thermal effect of water need not be
nations 1, 2, and 3 will be used in conjunction with a
considered in the heat transfer analysis, but the
single start date. Then the controlling combination of
weight and pressure due to water must be included in
either material parameter combination 2 or 3 must be
the stress analysis. This simplification, although
used with a second start date. The second start date
theoretically inconsistent, reduces the complexity of
should be selected to provide an opposing ambient
the analysis process. The simplification may be
condition to the original start date. (The first start
inappropriate when the minimum ambient air temper-
date could be in June; the second could be in
ature is much less than the water temperature. If
January.) The selection of start dates is structure and
portions of the structure show high crack potential,
site dependent and should be evaluated by the design
then time-history plots at these locations should be
team. A single start date is inadequate for producing
examined to see the effect of service load application.
the worst conditions at every location within the
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