ETL 1110-2-365
31 Aug 94
the heat transfer and stress analyses are not signifi-
then the soil stiffness should be removed after
cantly affected by exceeding the maximum.
30 days to allow the piles to carry the entire load. If
the structure is rock founded, then the structural engi-
neer may choose between modeling the foundation
e.
Stress analysis.
with continuum elements, developing vertical and
(1) Gravity loads. Gravity loads (self weight of
lateral stiffness spring coefficients to model the rock
the structure) will be included in all NISA analyses.
properties, or using super elements to model a major
It is unnecessary to include formwork as a part of the
portion of the foundation. The use of super elements
stress analysis due to the method in which the gravity
(or substructuring) is a technique of modeling where
load of the concrete is applied at early times. In
a super element represents the stiffness of a large
newly placed lifts, the concrete has not aged suffi-
number of regular elements.
ciently to use element body forces to model gravity
loads without causing excessive displacement and
(3) Time increments. The maximum time incre-
cracking. At these times in an analysis, gravity load-
ments to be used in a stress analysis should be the
ing of the newly placed lift shall be applied as an
same as the heat transfer analysis and the increments
equivalent uniform load acting on the top surface of
are given in Table A-2. The small increments at the
the supporting lift. In locations where the newly
early times are necessary to capture the effects of
placed lift spans a void in the supporting lift, the
creep, shrinkage, and the aging modulus of elasticity,
equivalent uniform load shall be applied to the sur-
since it is at the early times that these properties
face at the bottom of the void. Application of the
change the most.
gravity load over a void in this manner is consistent
with normal construction practice where formwork
(4) Smeared crack model. NISA is based on an
supports the new lift by transferring vertical loads to
interactive stress-strain cracking criterion as described
the floor of the void. Equivalent uniform loads
in Annex 2 of Appendix A. The basis for the crite-
should be removed and replaced with element body
rion is data from the slow load test. The aging
forces after the modulus of elasticity in the newly
modulus of elasticity makes the cracking criterion
placed lift has aged to 1,000,000 psi. The time at
age-dependent. The ANACAP-U software checks
which the modulus of elasticity reaches this limit may
calculated stresses and strains against the cracking
be determined from the modulus versus time curve
criterion at each timestep. If the criterion is exceeded
obtained from material testing. The change in gravity
at any integration point (elements used in a NISA
loading can then be made during the nearest subse-
typically have four integration points for each rectan-
quent timestep shown in Table A-2.
gular element), a crack will be introduced perpendicu-
lar to the direction of maximum principal strain. If a
(2) Foundation model. For soil or pile founda-
crack is introduced, the constitutive matrix for the
tions, the foundation material should be included in
element is reformulated and a new stress state is
the model using spring constants. For soil-founded
developed based on zero stress perpendicular to the
structures, the soil can be replaced by springs which
crack. The new constitutive matrix and stresses are
model the stiffness of the soil. For pile-founded
then used for subsequent calculations until the crack
structures, the springs should account for the stiffness
closes. The cracks will close when placed in a com-
of the soil and the piles, including any lateral stiff-
pressive state, and the material will again be able to
ness that the piles provide. Failure to include the
carry compressive loads. With this approach, the
stiffness associated with the soil at nodes between the
entire element matrix is affected if a crack is deter-
nodes where the pile stiffness is modeled will allow
mined at any integration point. This is referred to as
the concrete to develop excessive deformations
a smeared crack model, and it will provide informa-
between the piles at early times. If the results of an
tion which indicates the depth and extent of cracking.
analysis are to be used in evaluating a pile design,
Table A-2
Maximum Time Increments Allowed in a NISA
Days after lift placed
0-2
2-5
5-15
15-35
35-100
100+
Maximum time increment (days)
0.25
0.5
1.0
2.0
5.0
10.0
A-20
Previous Page
