FELT(4fe)

NAME
felt - finite element problem description file format

DESCRIPTION
The felt(4fe) file format is used by the programs of the finite element package, felt(1fe), velvet(1fe), and burlap(1fe) to describe a finite element problem. The file is human readable and consists of a friendly, intuitive syntax rather than a table of numbers. Syntactic and semantic errors are detected and reported assuring that only valid problem descriptions are used. In general white space is unimportant, arbitrary numeric expressions may be used, and case of keywords is unimportant. As per standard convention, boldface items represent keywords, italicized items represent the syntax of the grammar, and items in brackets are optional. The file syntax is shown below.

    [ problem-description ]
    [ analysis-parameters ]
    [ object-definitions ]
    [ appearance-description ]
    end

  Problem description
The problem-description section specifies the problem title and the number of nodes and elements in the problem. If this section is not specified then the problem will be unnamed and is assumed to contain zero nodes and zero elements. This section may be absent for example in defaults files which define objects but do not specify an actual problem instance. The format for a problem-description is given below.

    problem description
    [ title = string ]
    [ nodes = integer ]
    [ elements = integer ]
    [ analysis = static | transient | modal | static-thermal| transient-thermal| spectral ]

If the title is missing then the problem is unnamed. If nodes or elements is missing then none of that type of object are expected. The assignments can occur in any order and can even be repeated with the last assignment being used.

  Analysis parameters
The analysis-parameters section defines any parameters for a specific type of analysis. Currently, this section is used only if the analysis mode is transient. The format of the analysis-parameters section is given below.

    analysis parameters
    [ alpha = expression ]
    [ beta = expression ]
    [ gamma = expression ]
    [ step = expression ]
    [ start = expression ]
    [ stop = expression ]
    [ Rm = expression ]
    [ Rk = expression ]
    [ nodes = [ node-list ] ]
    [ dofs = [ dof-list ] ]
    [ mass-mode = lumped | consistent ]

The alpha, beta, andgamma parameters are used in numerical integration schemes (transient and transient-thermal analysis). start, stop, and step define the range of time or frequency interest for transient or spectral analyses. In transient analyses, start is meaningless and duration and dt can be used as aliases for stop and step, respectively. Rk and Rm are global Rayleigh (stiffness and mass) damping proportionality constants. The node-list is a comma or white space separated list of node numbers that are of interest in the analysis. Similarly, the dof-list is a list of the degrees of freedom (Tx, Ty, Tz, Rx, Ry, and Rz) that are of interest.

An object-definition section defines objects of a specified type. Objects include nodes, elements, materials, constraints, forces, and distributed loads. Each of these types of objects is discussed below. Multiple object-definition sections are allowed and the sections may occur in any order.

  Nodes
Nodes are points in cartesian space to which elements are attached. A node must have a constraint and may have an optional force. A node is identified by a natural number. The syntax is as follows:

    nodes
    node-definitions

where a node-definition takes the following form:

    node-number
    [ x = expression ]
    [ y = expression ]
    [ z = expression ]
    [ constraint = constraint-name ]
    [ force = force-name ]
    [ mass = expression ]

The node-number starts the definition. Each node must have a unique number. If a cartesian coordinate is not given then the coordinate of the previous node is used. Similarly, if no constraint is given then the constraint applied to the previous node is used. As above, the assignments can appear in any order and any number of times. As indicated above, some objects are identified by their name and some by their number. Elements and nodes have numbers while materials, forces, loads, and constraints have names.

  Elements
Elements are linear, planar, or solid objects which are attached to nodes. Each element must have a material and may have optional loads. Furthermore, each element has a type, or definition. Like nodes, elements are identified by a unique natural number. Elements of specific type are defined with the following syntax:

    element-type elements
    element-definition

where an element-type is one of the following:

    spring
    truss
    beam
    beam3d
    CSTPlaneStrain
    CSTPlaneStress
    iso2d_PlaneStrain
    iso2d_PlaneStress
    quad_PlaneStrain
    quad_PlaneStress
    timoshenko
    htk
    brick
    ctg
    rod

and an element-definition has the following form:

    element-number
    nodes = [ node-list ]
    [ material = material-name ]
    [ load = load-name-list ]

The element-number starts the definition. Each element must have a unique number. If no material is given then the material applied to the previous element is used. The load-name-list is a list of up to three loads to apply to the element. The node-list is a comma or white space separated list of node numbers. Each type of element requires a certain of nodes and in some cases a special "null node" which is numbered zero may be used to indicate a gap or filler in the list.

  Materials
Elements are made of a type of material. Each material has a name and certain physical properties not all of which may be used by any one element. The syntax for defining materials is as follows:

    material properties
    material-definitions

where material-definition has the following form:

    material-name
    [ color = string ]
        # color for velvet
    [ E = expression ]
        # Young's modulus
    [ Ix = expression ]
        # moment of inertia about x-x axis
    [ Iy = expression ]
        # moment of inertia about y-y axis
    [ Iz = expression ]
        # moment of inertia about z-z axis
    [ A = expression ]
        # cross-sectional area
    [ J = expression ]
        # polar moment of inertia
    [ G = expression ]
        # bulk (shear) modulus
    [ t = expression ]
        # thickness
    [ rho = expression ]
        # density
    [ nu = expression ]
        # Poisson's ratio
    [ kappa = expression ]
        # shear force correction
    [ Rk = expression ]
        # Rayleigh damping coefficient (K)
    [ Rm = expression ]
        # Rayleigh damping coefficient (M)
    [ Kx = expression ]
        # thermal conductivity in the x-direction
    [ Ky = expression ]
        # thermal conductivity in the y-direction
    [ Ky = expression ]
        # thermal conductivity in the z-direction
    [ c = expression ]
        # heat capacitance

The material-name starts the definition. If an attribute of a material is not specified then that attribute is zero. The assignments may occur in any order. The color specifies the color to use in drawing the material within velvet, and is ignored by other applications.

  Constraints
Constraints are applied to nodes to indicate about which axes a node can move. The syntax for defining a constraint is as follows:

    constraints
    constraint-definitions

where constraint-definition has the following form:

    constraint-name
    [ color = string ]
        # color for velvet
    [ tx = c | u | expression ]
        # boundary translation along x axis
    [ ty = c | u | expression ]
        # boundary translation along y axis
    [ tz = c | u | expression ]
        # boundary translation along z axis
    [ rx = c | u | expression | h ]
        # boundary rotation about x axis
    [ ry = c | u | expression | h ]
        # boundary rotation about y axis
    [ rz = c | u | expression | h ]
        # boundary rotation about z axis
    [ itx = expression ]
        # initial displacement along x axis
    [ ity = expression ]
        # initial displacement along y axis
    [ itz = expression ]
        # initial displacement along z axis
    [ irx = expression ]
        # initial rotation about x axis
    [ iry = expression ]
        # initial rotation about y axis
    [ irz = expression ]
        # initial rotation about z axis
    [ vx = expression ]
        # initial velocity along x axis
    [ vy = expression ]
        # initial velocity along y axis
    [ vz = expression ]
        # initial velocity along z axis
    [ ax = expression ]
        # initial accel. along x axis
    [ ay = expression ]
        # initial accel. along y axis
    [ az = expression ]
        # initial accel. along z axis

The constraint-name starts the definition. A value of c for a boundary condition indicates that the axis is constrained; a value of u indicates that the axis is unconstrained. An expression indicates a displacement (non-zero) boundary condition and may contain the t variable for time varying boundary conditions in transient analysis problems. The initial dislacement, velocity and acceleration specifications are only used in transient problems. A value of h for a rotational boundary condition indicates a hinge. By default, all axes are unconstrained. The color specifies the color to use in drawing the constraint within velvet, and is ignored by other applications.

  Forces
Forces, or point loads, may be applied to nodes. The syntax for a force definition is as follows:

    forces
    force-definitions

where a force-definition has the following form:

    force-name
    [ color = string ]
        # color for velvet
    [ Fx = expression ]
        # force along x axis
    [ Fy = expression ]
        # force along y axis
    [ Fz = expression ]
        # force along z axis
    [ Mx = expression ]
        # moment about x axis
    [ My = expression ]
        # moment about y axis
    [ Mz = expression ]
        # moment about z axis
    [ Sfx = expression ]
        # frequency-domain spectra of force along x axis
    [ Sfy = expression ]
        # frequency-domain spectra of force along y axis
    [ Sfz = expression ]
        # frequency-domain spectra of force along z axis
    [ Smx = expression ]
        # frequency-domain spectra of moment about x axis
    [ Smy = expression ]
        # frequency-domain spectra of moment about y axis
    [ Smz = expression ]
        # frequency-domain spectra of moment about z axis

The force-name starts the definition. If the force or moment is not specified then it is assumed to be zero. The expressions for forces may be time-varying. Time-varying expressions include the single variable t to represent the current time in the solution of a dynamic problem or consist of a list of discrete (time, value) pairs. Frequency varying expressions for spectra can also use w to represent the independent variable (radial frequency). The color specifies the color to use in drawing the force within velvet, and is ignored by other applications.

  Loads
Distributed loads, or loads for short, are applied to elements. The syntax for a defining a distributed load is as follows:

    distributed loads
    load-definitions

where a load-definition has the following form:

    load-name
    [ color = string ]
        # color for velvet
    [ direction = dir ]
        # direction
    [ values = pair-list ]
        # local nodes and magnitudes

The load-name starts the definition. The dir is one of LocalX, LocalY, LocalZ (local coordinate system), GlobalX, GlobalY, GlobalZ (global coordinate system), parallel, or perpendicular. The pair-list is a sequence of pairs. A pair is a node number and an expression enclosed in parentheses. The node number refers to the position within the element rather than referring to an actual node. The color specifies the color to use in drawing the load within velvet, and is ignored by other applications.

  Appearance Description
The appearance-description section is used by velvet to describe the appearance of a problem. This section is currently not used by felt. The appearance includes the state of the drawing area and any tool figures. This section consists of two subsections, the canvas-configuration section and the figure-list section. The canvas-configuration section has the following syntax.

    canvas configuration
    canvas-parameters

where a canvas-parameter has the following form:

    [ node-numbers = boolean ]
        # node numbering
    [ element-numbers = boolean ]
        # element numbering
    [ snap = boolean ]
        # snap grid status
    [ grid = boolean ]
        # visible grid status
    [ snap-size = expression ]
        # snap grid size
    [ grid-size = expression ]
        # visible grid size
    [ node-color = color-name ]
        # node color
    [ element-color = color-name ]
        # element color
    [ label-font = font-name ]
        # labeling font
    [ tool-color = color-name ]
        # tool figure color
    [ tool-font = font-name ]
        # text figure font
    [ x-min = expression ]
        # x-axis minimum
    [ x-max = expression ]
        # x-axis maximum
    [ y-min = expression ]
        # y-axis minimum
    [ y-max = expression ]
        # y-axis maximum
    [ x-pos = expression ]
        # x position of drawing area
    [ y-pos = expression ]
        # y position of drawing area
    [ width = expression ]
        # width of viewport window
    [ height = expression ]
        # height of viewport window
    [ scale = expression ]
        # scale of drawing area

A boolean is either true or false. A color-name is the name of a valid X11 color. Similarly, a font-name is the name of a valid X11 font. The last five parameters are probably not very meaningful to the user. The figure-list section has the following syntax.

    figure list
    figure-definitions

where a figure-definition has the following form:

    figure-type
    [ x = expression ]
        # x coordinate
    [ y = expression ]
        # y coordinate
    [ width = expression ]
        # width
    [ height = expression ]
        # height
    [ start = expression ]
        # starting angle
    [ length = expression ]
        # arc length
    [ text = name ]
        # text string
    [ color = name ]
        # color
    [ font = name ]
        # text font
    [ points = [ point-list ] ]
        # line points

The figure-type starts the definition and is one of rectangle, polyline, text, or arc. Note that not all properties have meaning for all figures. Any unneeded property is ignored. If a color or font property is not given then the previous property is used. The point-list is a list of (x-coordinate, y-coordinate) pairs.

  Expressions
An expression can be either constant or time-varying. As discussed above, time-varying expressions contain the variable t or consist of a list of discrete (time, value) pairs. If a time-varying expression is given where a constant expression is expected, the expression is evaluated at time zero. An expression has one of the following forms, where all operators have the precedences and associativities given to them in the C programming language.

    expression ? expression : expression
        # in-line conditional
    expression || expression
        # logical or
    expression && expression
        # logical and
    expression | expression
        # integer inclusive or
    expression ^ expression
        # integer exclusive or
    expression & expression
        # integer and
    expression == expression
        # equality
    expression != expression
        # inequality
    expression < expression
        # less than
    expression > expression
        # greater than
    expression <= expression
        # less than or equal
    expression >= expression
        # greater than or equal
    expression << expression
        # integer shift left
    expression >> expression
        # integer shift right
    expression + expression
        # addition
    expression - expression
        # subtraction
    expression * expression
        # multiplication
    expression / expression
        # division
    expression % expression
        # integer remainder
    - expression
        # arithmetic negation
    ! expression
        # logical negation
    ~ expression
        # integer bitwise negation
    ( expression )
        # enforce precedence
    sin ( expression )
        # sine
    cos ( expression )
        # cosine
    tan ( expression )
        # tangent
    pow ( expression , expression )
        # power (exponentiation)
    exp ( expression )
        # exponential
    log ( expression )
        # natural logarithm
    log10 ( expression )
        # base-10 logarithm
    sqrt ( expression )
        # square root
    hypot ( expression , expression )
        # Euclidean distance
    floor ( expression )
        # floor
    ceil ( expression )
        # ceiling
    fmod ( expression , expression )
        # floating point remainder
    fabs ( expression )
        # absolute value
    number
        # literal value
    t   # current time

Finally, a discretely valued expression has the following syntax, where the optional + indicates that the list represents one cycle of an infinite waveform.

    ( expression ',' expression ) ... [ + ]

AUTHOR
The felt file format was developed by Jason I. Gobat (jgobat@mit.edu) and Darren C. Atkinson (atkinson@ucsd.edu).

SEE ALSO
corduroy(1fe), felt(1fe), velvet(1fe), xfelt(1fe), corduroy(4fe).