Element Loading

Element loads are surface loads, body loads, inertial loads, and ocean loads. Element loads are always associated with a particular element (even if the input is at the nodes). Certain elements may also have flags.

Flags are not actually loads, but are used to indicate that a certain type of calculation is to be performed.

For example, when the FSI (fluid-structure interaction) flag is active, a specified face of an acoustic element is treated as an interface between a fluid portion and a structural portion of the model. Similarly, MXWF and MVDI are flags used to trigger magnetic force (Maxwell surface) and Jacobian force (virtual displace-ment) calculations, respectively, in certain magnetics elements. Details about flags are discussed under the applicable elements in Element Library (p. 109).

Flags are associated either with a surface (FSI and MXWF) and are applied as surface loads (below), or with an element (MVDI) and are applied as body loads (below). For the FSI and MXWF flags, values have no meaning - these flags are simply turned on by specifying their label on the appropriate command.

For the MVDI flag, its value (which can range from zero to one) is specified, along with the label, on the appropriate command. Flags are always step-applied (i.e., the KBC command does not affect them).

The following topics related to element loading are available:

3.1.8.1. Surface Loads 3.1.8.2. Body Loads 3.1.8.3. Inertial Loads 3.1.8.4. Initial Stresses 3.1.8.5. Ocean Loads

Also see Nodal Loading (p. 49), which refers to loads defined at the nodes and are not directly related to the elements.

3.1.8.1. Surface Loads

Some element types allow surface loads. Surface loads are typically pressures for structural element types, convections or heat fluxes for thermal element types, and so on.

Table 3.1: Surface Loads Available in Each Discipline Label Surface Load

Discipline

PRES [1] [2]

Pressure

Structur-al

CONV Convection

Thermal

HFLUX Heat Flux

INF Infinite Surface

MXWF Maxwell Surface

Magnet-ic Infinite Surface INF

MXWF Maxwell Surface

Electric

CHRGS Surface Charge Density

INF Infinite Surface

TEMP Temperature

FSI Fluid-Structure

Inter-face Fluid

IMPD Impedance

SELV Superelement Load

Vector All

1. Not to be confused with the PRES degree of freedom

2. Buoyancy, wave, current and other ocean loads can be applied to some line and surface element types. For more information, see OCTYPE and related ocean commands.

3.1.8.1.1. Applying Surface Loads

Surface loads (such as pressures for structural elements and convections for thermal element) are typ-ically input via the SF and SFE commands.

Surface loads may also be input in a nodal format. For example, rather than applying surface loading to an element face, it may be convenient to apply the loading the face nodes of an element (which are then processed like face input). For more information, see Nodal Loading (p. 49).

3.1.8.1.2. Multiple Surface Loads

Some elements allow multiple types of surface loads (as shown with the load labels listed under "Surface Loads" in the input table for each element type). Also, some elements allow multiple loads on a single element face (as indicated with the load numbers after the load labels).

Load numbers are shown on the element figures (within circles) and point in the direction of positive load to the face upon which the load acts. A surface load applied on the edge of a shell element is on a per-unit-length basis, not per-unit area.

3.1.8.1.3. Surface Load Labels and Keys

Surface loads are designated by a label and a key. The label indicates the type of surface load and the key indicates where on the element the load acts.

The surface load can be defined on element faces via the SFE command by using a key (LKEY), the load label (Lab), and the load value. The SF command can define surface loads by using nodes to identify element faces. The CONV load label requires two values, the film coefficient and the bulk tem-perature.

3.1.8.1.4. Tapered Surface Load

A tapered surface load, which allows different values to be defined at the nodes of an element, can be entered via the SFE command. Tapered loads are input in the same order that the face nodes are listed.

3.1.8.2. Body Loads

Some element types allow body loads. Body loads are typically temperatures for structural element types, heat-generation rates for thermal element types, and so on.

Table 3.2: Body Loads Available in Each Discipline Label Body Load

Discipline

TEMP [1]

Temperature

Structur-al

FLUE Fluence

HGEN Heat Generation Rate

Thermal

TEMP [1]

Temperature

Magnet-ic

JS Current Density

MVDI Virtual Displacement

VLTG Voltage Drop

TEMP [1]

Temperature Electric

CHRGD Charge Density

HGEN Heat Generation Rate

Fluid

FORC Force Density

1. Not to be confused with the TEMP degree of freedom

3.1.8.2.1. Applying Body loads

Body loads are designated in the "Input Summary" table of each element by a label and a list of load values at various locations within the element.

Body loads are input via the BF or BFE commands. The load values input on the BFE command must correspond to the order indicated in the "Body Load" list.

Body loads can also be applied in a nodal format. For more information, see Nodal Loading (p. 49).

3.1.8.2.2. Temperature Body Loads

For some structural elements, the temperature does not contribute to the element load vector but is only used for material property evaluation. For thermal elements using the diagonalized specified heat matrix option in a transient analyses, a spatially varying heat generation rate is averaged over the element.

Heat-generation rates are input per-unit-volume unless otherwise noted with the element. The element format is usually in terms of the element nodes but may be in terms of fictitious corner points as de-scribed for each element. Corner point numbers are shown on the element figures where applicable.

3.1.8.3. Inertial Loads

Inertial loads (gravity, spinning, etc.), are applicable to all elements with structural degrees of freedom and having mass (that is, elements having mass as an input real constant or having a density (DENS) material property).

Inertia loads are typically entered via the ACEL and OMEGA commands.

3.1.8.4. Initial Stresses

Initial stresses can be set as constant or read in from a file for most current-technology beam, link, plane, solid, and shell element types.

The INISTATE command sets constant initial stress for selected elements and, optionally, only for specified materials. The command also allows you to read in a file specifying the initial stresses. The stresses

specified in the input file can be applied to the element centroids or element integration points, and can be applied to the same points for all selected elements or can be applied differently for each element.

The stresses can also be a written to an external file.

For more information about the initial state capability, see the INISTATE command, and Initial State Loading in the Basic Analysis Guide.

3.1.8.5. Ocean Loads

Ocean loads can be applied to line and surface element types that support such loads. Ocean loading includes the effects of waves, current, drag, and buoyancy. Loading occurs via the ocean family of commands.

For more information, see Applying Ocean Loads in the Basic Analysis Guide.