# Coordinate Systems

Global Coordinates

The global coordinate system is shown in the bottom left corner of the main window. It applies to all elements and cannot be changed.

Global Coordinates are indicated with capital letters (X, Y, Z)

Local coordinates

Each element has its own local coordinate system. These can be changed by the user.

Local coordinates are indicated with lowercase letters (x, y, z).

The quick access button “Local systems” (lower, right) allows displaying the local coordinate systems of elements.

In the menu “Settings/Preferences/Graphic Symbols,” the labels for the local coordinates can be switched on.

Conditions for Local Coordinate Systems

Local coordinate systems are always right-turning, orthogonal coordinate systems.

For planar elements, the local z-coordinate must be perpendicular to the elements’ plane. In consequence, the local z-coordinate can be orientated in two directions only. X-references perpendicular to (or z-references in) the elements’ plane are not allowed.

The local x-direction must be parallel to the elements’ axis for linear elements. The local x-reference cannot be set at will but can be flipped with the appropriate dropdown menu.

Automatic Definition of Local Coordinate Systems

If the user does not give the references to define the local coordinate system, the local systems will be determined automatically according to these principles.

Changing Local Coordinate Systems

The properties of most element types are defined in local coordinates. For some elements (e.g., supports, interface elements), it is possible to choose the coordinate system to be used (global or local).

In the definition dialog of the elements, the direction of the local x and z axis can be set using “References.”

References cannot be selected as local x references for line elements. But it is possible to flip the direction of the local x-axis.

The reference can be defined by selection in the dropdown menu or by the corresponding button and selection of the reference itself (in the main windows, displayed in red).

References that violate the conditions for the definition of local coordinate systems are hidden in the dropdown menu. If possible, the direction of the projection of the selected reference into a ‘valid’ plane is used.

It’s not possible to define local y-references. The local y-direction results from the local x and z directions (perpendicular, right-turning).

Reference vector

A reference vector represents a direction. It doesn’t have a defined length, and its starting point is used exclusively for displaying reasons. It is not possible to define parallel reference vectors. If tried, there will be an appropriate message, including the number of the existing reference vector.

Reference point

When assigning a reference point, the direction from the center of the element to the reference point is used as the direction of the respective local coordinate axis. Reference points can be used to align several elements to a point (e.g., radially placed columns)

Reference angle

For line elements, a reference angle can be defined. Its definition can be given directly in the definition dialog for line elements. A reference angle defines the elements’ rotation around its axis based on the automatically generated local coordinate system.

Local Coordinate System for Special Elements

Ribs inside domains

The local coordinate system for ribs is defined based on the adjacent domains. The local z-axis of the rib is parallel to the local z-axis of the domain.

If multiple domains are connected to the line on which the rib is defined, the bisector is used as a local y-axis of the rib.

The eccentricity of a rib is the distance from the definition axis to the cross-section axis. Its sign is determined based on the local z-direction. For a bottom rib on a horizontal slab, the eccentricity is negative.

Edge hinge

The stiffness of edge hinges is always given based on their respective local coordinate systems. The selected domain defines the local coordinate system of edge hinges.

• x – along the hinge line
• y – perpendicular to the hinge line, in the plane of the selected domain
• z – perpendicular to x and y

The following directions result from the above criteria for the definition of edge hinges in walls (top and bottom).

• x – along the walls’ top and bottom edge
• y – vertical
• z – perpendicular to the walls’ plane