Signs of internal forces

Table of Contents

The direction of the signs are always determined based on the active coordinate system.

Internal Forces #

The following description of sign convention is valid for linear and surface elements. It is designed in a way to allow the determination of the acting force at any time without doubt, specially for elements with arbitrary orientation in three-dimensional space.

Normal force #

  • Positive: Tension
  • Negative: Compression

Bending moment #

  • Positive: Tension stress at the surface with positive local z-coordinates
  • Negative: Tension stress at the surface with negative local z-coordinates

Using this convention, bending moments get the inverse sign compared to the classical sign convention used in planar slab calculations. As a consequence, the field moments in slabs are negative, the support moments (above columns and walls) are positive.

Shear force #

  • Positive: Shear force acts in positive local coordinate direction
  • Negative: Shear force acts in negative local coordinate direction

Torsion moment (line elements) #

  • Positive: Right-turning, around an axis in positive local x-direction
  • Negative: Right-turning, around an axis in negative local x-direction

Coordinate System for Results #

In the main window, the deformations (displacements, rotations) are shown in global coordinates (capital letters X, Y, Z). But in the result overview (clicking on the element), local coordinates are used (small letters x, y, z).

For the deformations (displacements, rotations), global coordinates (capital letters X, Y, Z) are used in the main view. In the summary (click on the beam), the deformations are shown in local coordinates (lower case letters x, y, z). All other results refer to local coordinates (lower case letters x, y, z).

Reinforcement #

The “reinforcement values” and the “actual reinforcement” refer to the element’s local coordinate system.

  • Top: Surface with positive local z-coordinates.
  • Bottom: Surface with negative local z-coordinates.

For slabs the names are intuitive. ‘Top’ and ‘bottom’ refer to the surfaces as expected, as long as the local coordinate system (local z-reference) is unchanged by the user.

For the walls the expressions ‘top’ and ‘bottom’ refer to the two surfaces of the wall. If different 

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reinforcements should be used on the two surfaces (e.g. walls in the ground) the surfaces can be identified by the local coordinate system (the surface to which the green z-axis points is the ‘top’). 

The x- and y-direction is also given by the local coordinate system. Without manual changes, the local y-axis is vertical, the x-axis is horizontal in the walls’ plane.

Source: Ingware AG