📄 Understanding FE Surface Attached Beams
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Understanding FE Surface Attached Beams

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See 📄 Attached Beams

An attached beam is engineered to function compositely with an FE surface, such as a slab or wall. Its primary role is to modify the stiffness of the FE surface by accounting for the beam's position relative to the slab's centroid. A key assumption for attached beams is the shear transfer between the beam and the slab, implying that the beam cannot act as an independent vertical support. The influence of an attached beam is solely on the stiffness within the FE surface, not on its vertical support behaviour.

Modelling and Defining Attached Beams

  • Boundary Members: A single line element can serve as a shared boundary for multiple FE surfaces, allowing for the subdivision of slabs. However, overlapping FE surfaces must be avoided, as the software cannot detect them, which would lead to incorrect meshing. If a boundary element is not specifically designated as an attached beam within the FE menus, MasterFrame will automatically treat it as a Dummy FE Member.
  • Beam Section: When modelling concrete elements, since the FE surface represents the slab, the attached line element only needs to model the additional concrete element below or above the slab. For instance, in a downstand beam, the depth of the concrete line element would be set as the overall beam depth minus the slab thickness. For an attached steel beam, the line element should be sized as the steel section itself.

  • Positioning/Offset: You can define the position of the attached beam relative to the FE surface's local z-axis, which controls how the beam interacts with the slab. Options include:

    • Centre: The centroid of the attached beam aligns with the FE surface's centreline.
    • Bottom: The top of the beam coincides with the bottom of the FE surface.
    • Top: The underside of the beam coincides with the top surface of the FE surface. An additional z-direction offset can be specified to further refine the beam's position. Multiple groupings of attached beams can be created to apply different offsets.
  • Loads and Analysis: When member loads are applied to an FE surface where the member is an attached beam, it will act compositely with the FE surface. If the beam is not attached, it behaves like a MasterFrame Dummy member, transferring reactions to end nodes but ignoring beam bending in the analysis.
  • Combined Beam-Plate Results: The Attached Beam menu provides options to define the concrete flange width for combining results and to calculate the bending of the beam element itself for automated design. The "Use Combined Beam-Plate Results" option uses the axial force in the FE surface and the beam element's centroid offset to calculate bending moment in the beam.

Process for Attaching a Beam to an FE Surface

  1. Navigate to Properties > FE Surface Attached Beams from the top menu, or access the "Attached Beams" tab within the FE menu.
  2. Click the selection tool icon to enter select mode.
  3. Hover your mouse over the desired line element (beam) in the graphical window and click the left mouse button to select it. The selected element will highlight in red, and its member number will appear in the right-hand pane.
  4. Specify the Beam-Plate Offset (Centre, Bottom, Top) and any additional z-direction offset.
  5. If needed, create new attached beam groupings to apply different offsets to other beams. You can also amend the line element beta to align members with the FE surface orientation, especially for surfaces not aligned with global axes.

Considerations for FE Meshing with Attached Beams

The presence of attached beams can influence the FE mesh. When a beam is attached to an FE surface, or the mesh is set to align with members, the mesh size may be reduced locally to ensure elements fit within the gap between the member and the boundary. This local mesh refinement can affect an area around the member to maintain compatibility between finite element edges and nodes. If the member is very close to the FE surface, it can force an extremely small mesh, potentially making it difficult to form a valid mesh. Reviewing the alignment of structures near FE surface edges and considering simplification where possible can help mitigate meshing issues.