(e.g., time between lifts, lift heights, type of form-

accounted for by performing parametric studies using

work, formwork removal, construction start date,

combinations of the upper and lower bounds

insulation requirements, etc.) which will approximate

described in paragraph A-2*b*.

actual field conditions and which can be adequately

modeled. The materials engineer and structural engi-

(b) Start times. Variations in behavior will

neer should develop a set of time-dependent curves to

occur due to construction of a monolith beginning at

be used in the analytical model for the aging

different times of the year and these variations should

modulus, adiabatic temperature rise, creep, and

be accounted for. Assuming construction starts at

shrinkage based on the results of laboratory testing.

different times of the year may identify additional

These curves will be banded to reflect the typical

critical areas of the structure. Minimum requirements

variations specified in paragraph A-2*b*(2) and the

for analyses with different start times are presented in

confidence level the materials engineer has with the

paragraph A-2*d*.

local site conditions. Other concrete properties (e.g.,

tensile strain capacity, the coefficient of thermal

(c) General. Variations in other parameters may

expansion, thermal conductivity, specific heat, dens-

also be accounted for by varying the parameter of

ity, and Poisson's ratio) should be provided for the

interest while other parameters remain constant.

proposed concrete mixtures by the materials engineer

General guidance on performing a parametric study is

through results of test data. The geotechnical engi-

given in paragraph A-2*g*. This approach can be used

neer and structural engineer should develop appropri-

to identify and confirm cost-saving construction tech-

ate values for the thermal conductivity, coefficient of

niques and to increase the structural designer's confi-

thermal expansion, specific heat, density, and

dence in the results that are being produced. For

Poisson's ratio for the foundation material, and the

cases when cracking appears to be imminent for a

pile-subgrade reaction moduli. The structural engi-

given set of conditions and the variation of some

neer should obtain the monthly average ambient air

other parameter could induce crack initiation, a para-

temperatures as described in paragraph A-2*c*. It will

metric study may be valuable in assessing the design-

be the structural engineer's responsibility to ensure

er's confidence in the satisfactory behavior of the

that the specified parameters are properly modeled for

structure.

the numerical analysis. When modeling assumptions

must be made, the structural engineer should consult

(2) Methods of analysis. Two-dimensional

with other design team members as necessary, but the

NISA's of entire monolith cross sections are currently

final decision on how to implement the various

practical. Three-dimensional (3-D) NISA's are typi-

parameters will be made by the structural engineer.

cally performed on isolated portions of structures, or

sometimes entire monoliths may be modeled, pri-

marily to determine the 3-D behavior of the structure

This guidance is based on proven methods of FE

in all directions. Three-dimensional NISA's may also

analysis, on NISA's performed on Corps of Engineers

be used to confirm 2-D results. Most structures

projects, and from independent parametric studies.

should be modeled using a plane-stress approach.

The past experience has highlighted the following

Plane-strain modeling may also be considered, but

points.

studies have shown that differences between the plane

stress and plane strain approach are minimal for the

(1) Variations in input data. The analysis

results in the plane being evaluated (Truman,

requires reliable, but not exact, input data for mean-

Petruska, and Ferhi 1992 and Garner et al. 1992).

ingful results. Since exact data are not available,

The selection of an appropriate approach is a matter

parametric studies are valuable in predicting the

of engineering judgment with consideration given to

trends in behavior that can be expected in a given

factors such as the volumetric change which may

structure.

occur in the out-of-plane direction, the ability of the

ends of the monolith to move with respect to each

(a) Material properties. Variations in material

other, and the length of the monolith. For example,

properties due to scatter of test data, differences in

if volume changes are small, the selection between

behavior of the material between actual and that

plane stress and plane strain will create little differ-

predicted by the numerical model, and expected dif-

ence in the results, but if these volumetric changes

ferences between the laboratory mixture and the

are large the length of the monolith may be the deter-

actual mixture used during construction can be

mining factor for which model to use. The longer a

A-7