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Equivalent loads

Member loads​

Composite beams intended for export to Compos should be defined as members, even if composed of multiple finite elements. After running a static analysis task, element forces become available along the member length. These forces form the basis for generating the equivalent Compos load definitions.

Compos requires loads to be provided as point loads, uniform loads, linear loads, trilinear loads, patch loads, moments, and constant axial forces. As part of the GSA-Compos integration, the available GSA element forces are interpreted and mapped to these Compos load types.

The mapping approach is based on back‑calculating load intensity from the major‑axis shear forces and end moments of the member. This process is described in more detail in the algorithm section below.

Only major‑axis shear, major‑axis bending, and constant axial forces are considered. Minor‑axis shear, minor‑axis bending, torsion, and varying axial forces are not included in the Compos export.

Gravity loading​

Gravity loading is typically captured as uniform or patch loads when generating Compos files via the GSA integration. Note, as Compos can automatically calculate self-weight of the members if required, to avoid load duplication, Compos files generated via integration will have this option unchecked by default.

Offsets​

Any effects arising from member offsets that may result in minor-axis forces are not mapped into Compos and are therefore omitted.

Algorithm​

The equivalent Compos loading is derived from the analysed major‑axis shear force and end reactions along the composite beam member. The procedure used to identify and classify the loads is outlined below.

Data preparation​

For the selected analysis task, the following quantities are obtained for each composite beam member:

  • Major axis end moments M_yy.
  • Axial force F_x, where only constant axial force along the member is considered.
  • Major axis shear force distribution F_z(x).

The shear force values and corresponding positions are then passed to the load identification routine.

Input validation​

The shear force and position data are checked to ensure that:

  • The vectors contain sufficient points for evaluating changes along the member.
  • The data is consistent and ordered along the member length.

Identification of point loads​

Abrupt changes in the shear force diagram at a single position are interpreted as equivalent point loads. Each such discontinuity is converted into a point load of corresponding magnitude and location.

Identification of distributed loading​

Between shear discontinuities, the shear force data typically varies smoothly. The local behaviour is assessed by fitting curves to small groups of consecutive points. Based on this behaviour:

  • Linear shear variation is interpreted as a uniform distributed load.
  • Curved (parabolic) shear variation is interpreted as a varying (patch) load.

Each continuous region of similar behaviour is treated as a single distributed load segment.

Regional segmentation​

Changes in the fitted shear force behaviour (e.g., linear to curved or vice versa) indicate boundaries of load regions. Each region is processed independently to produce the appropriate Compos load type.

Load consolidation​

Distributed load segments of the same type and with compatible intensities are merged where possible. This produces a concise and practical load representation in the resulting Compos file.

Assembly of the complete load set​

After point and distributed loads have been identified and consolidated, the member’s major axis end moments M_yy and constant axial force F_x, where applicable, are appended to the load set.

Unsupported actions are omitted.

Final adjustments​

The resulting loads are converted to match Compos sign conventions for vertical load, axial load, and bending moment. These loads are then written to the generated Compos file.

Expected differences​

Differences may occur when comparing GSA and Compos force diagrams. These arise from the load mapping process described above and Compos limitations.

Continuity effects​

Compos analyses members as single-span, simply supported beams, Thus, effects from fixities, continuity or adjacent framing which may arise in GSA are not reproduced in Compos. Moment diagrams may therefore differ, particularly for continuous beams.

Simplification of distributed loads​

Loads originating from 2D elements are transferred based on the resulting beam’s shear distribution. Irregular or non-uniform load patterns may therefore appear as simplified patch or uniform loads in Compos.

Shear​

Local fluctuations in shear near supports or point loads may be simplified (or smoothed) when generating equivalent Compos loads, leading to minor differences in shear diagrams.