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CHAPTER 3. STRUCTURE OF A FELT PROBLEM
*/ will be ignored as a comment no matter where it appears in the file.
3.1.2
3.1.2.1
Expressions
Continuous functions
As a convenience, wherever a numeric value is required for a material characteristic, magnitude of a force, load or displacement boundary condition, or a nodal coordinate, you can
specify an arbitrary mathematical expression, including the operators +, -, *, /, % and
the standard mathematical library functions sin, cos, tan, sqrt, hypot, pow, exp, log, log10,
floor, ceil, fabs and fmod. Note that arguments to the trigonometric functions should be
given in terms of radians just as if you were calling them from a C program using the standard math library. Other than this difference, these functions should be used and should
behave as they are described in the manual pages for your local mathematics library.
In a transient analysis problem the symbol t denotes the time variable in expressions
for force magnitudes or time-varying boundary conditions. These expressions will be dynamically evaulated throughout the course of the simulation. For other parameters (loads,
nodal coordinates, etc.) and for all parameters in a static problems, these expressions will
simply be evaluated as if t=0. For spectral inputs, the symbol w can be substituted for the
independent variable for clarity and to distinguish frequency domain force spectra from
time domain forces.
Expressions can also contain the ternary conditional operator as in the C programming
language: “if a then b else c” is symbolized in a FElt input file as a ? b : c where
a, b, and c are all valid expressions. The logical operators to use in constructing a are
the same as those in C (==, &&, ||, <=, <, >, >=, !=). The conditional construct is
particularly useful in defining things like discontinuous dynamic forces.
3.1.2.2
Discrete functions
Because some forcing functions are easier to express in a discretized (as opposed to continuous) form (e.g., earthquake records), the FElt syntax also includes a mechanism for
specifying a discrete representation of a transient forcing function. The basic specification
consists of a series of time magnitude pairs of the form (t, F) where F is the value of the
function at time t. In evaluating the function, FElt will linearly interpolate between each
adjacent pair for times that fall between two pairs. A single time magnitude pair will be
interpreted as an impulse response function at the given time. Note that the pairs must be
given in order of increasing time.