FE Composites [FEA]

An FE Composite merges the forces from several finite elements into a single set of section forces at the section's centre of gravity. A steel I-girder modelled with, say, two shell elements for the deck, one for the web and two beam elements for the flanges has its actions spread across five elements — but design is carried out on the section, so those actions must be resolved back to one point.

Without this, the individual element forces cannot be checked against a section capacity at all.

FE Composites are a reporting object. They change no analysis result; they resolve existing element forces onto a common reference.

Why this matters for live load. In influence-surface analysis, OpenBrIM finds the single critical vehicle position that maximizes the force effect at the section's centre of gravity, and reports the whole section under that one position. The alternative — taking each element's own critical vehicle position and summing the results — pairs actions that never occur together, giving a result that is conservative but not real. One position for the whole section is both correct and less wasteful.

Data

Path: The line along which section forces are extracted. It defines the start and end of the extraction and is divided into segments; at each station the program constructs a plane perpendicular to the path and collects the elements it cuts.

Engineering influence. The path is the spine of the member being designed — it should follow the girder line, not the model's global axes. Because the cutting planes are perpendicular to it, a path that does not follow the member cuts obliquely through the section, collecting a distorted set of elements and resolving their forces about the wrong axis. On a curved or skewed girder this is the difference between correct section forces and quietly wrong ones.

Segments: How many stations the path is divided into.

Engineering influence. This sets the longitudinal resolution of the design forces. Too few segments and the extraction misses the critical section — peak moment near midspan, peak shear near supports — reporting values at stations either side of the true maximum and under-predicting. More segments cost extraction time and produce more design points. Place enough that the critical sections of the member are captured.

Alignment: The alignment the composite is related to, for station-based positioning.

Spacing: The transverse spacing used when the composite represents a repeated member, such as a line of girders.

Beta Angle: The rotation of the extraction section about the path.

Engineering influence. This orients the section's local axes, and therefore what "major axis moment" and "minor axis moment" mean in the reported forces. An incorrect angle reports the right total actions resolved about the wrong axes, so a girder's strong-axis moment appears partly as weak-axis moment — which will not be caught by a total-force check but will produce a wrong design result.

Pos / PosRel: The position of the extraction reference, absolute and relative.

Engineering influence. These locate the point the forces are resolved to. Because moment is force times lever arm, the reference position directly determines the reported moment: resolving to the centroid of the composite section gives the section moment a design check expects, while resolving to some other point mixes axial force into the moment through the lever arm. This is the parameter to check when the reported axial force looks right but the moment does not.

Group: The FE Group whose elements are eligible to be collected.

Engineering influence. The connectivity parameter. Only elements in this group are gathered by the cutting planes, so an incomplete group omits part of the section — dropping the deck shells from a composite girder, for instance, which removes their contribution to the section forces and understates the design actions. A group that is too broad collects elements from an adjacent member and overstates them.

Verification

• Sum the axial forces of the individual elements at a station by hand and compare against the composite's reported axial force. They must agree; a difference means the group or the cutting plane is missing elements.
• Confirm the number of elements collected at each station equals the number the section is built from.
• Check the reported moment against Σ(element force × lever arm to the reference point) at one station.
• Display the extraction stations and confirm the cutting planes are perpendicular to the member, not skewed.
• Confirm the critical sections — midspan, supports — coincide with extraction stations, and increase the segment count if not.
• Check that reported major and minor axis moments are the way round you expect, which validates the beta angle.
• For live load, confirm a single vehicle position is being reported for the whole section rather than per-element maxima.

Common mistakes

• An incomplete group, silently omitting part of the section — most often the deck shells — and understating design actions.
• A path that does not follow the member, so the cutting planes slice the section obliquely.
• Too few segments, missing the critical section and under-predicting peak moment or shear.
• Wrong beta angle, resolving strong-axis actions partly onto the weak axis.
• A reference position that is not the section centroid, mixing axial force into the reported moment through the lever arm.
• A group that reaches into an adjacent girder, overstating the section forces.
• Combining per-element live-load maxima manually instead of using the composite, producing conservative but unreal results.