MasterFrame Finite Elements Hints & Tricks [WEBINAR RECORDING 2021]

Posted on October 21st, 2021 in Webinars

This webinar by MasterSeries Software focuses on modeling and analyzing buildings using Finite Element Analysis (FEA), building upon previous discussions on frame analysis.

Here's an easy-to-follow summary of the key concepts and procedures:

• Software Overview: The webinar highlights the MasterSeries product range, including the Building Design Suite, Power Pad, and standalone modules, all of which incorporate FEA capabilities for surfaces.

• Creating FE Surfaces (Slabs):

    ◦ An FE surface (slab) can be created from any four, five, or six members as long as they are co-planar.

    ◦ The process involves selecting bounding members or using an "auto select" window function to define the surface.

• Defining Openings and Supports:

    ◦ Openings: Openings (e.g., for staircases) can be defined within the FE surface by selecting four or three members that delineate the opening.

    ◦ Supports: The FE surface can be supported by physical columns, or continuous supports (like walls) can be defined by applying Y-axis restraints along specific members. Foundation supports are also applicable for columns.

• Applying Loads:

    ◦ Self-weight: The self-weight of the slab is automatically applied based on material thickness.

    ◦ Live Loads: Simple uniform live loads can be applied over the entire surface.

    ◦ Patch Loads: These are loads applied over a specific, defined area of the surface.

    ◦ Line Loads: Continuous line loads can be applied along a perimeter or any line within the structure.

    ◦ Varying Loads: Triangular or other varying loads can be defined by specifying load values at three points on the surface.

• Material and Thickness: Material properties (e.g., concrete grade) and the thickness of the FE surface are defined to inform the analysis.

• Meshing and Refinement:

    ◦ Automatic Mesh Generation: The software automatically creates a mesh, with nodes at column locations.

    ◦ Column Placement: It's generally not recommended to pull columns in slightly from the edges (e.g., 100-200mm) as this can create obtuse, nearly triangular elements in the mesh, leading to errors.

    ◦ Local Mesh Intensities: To counteract areas with high errors (often around supports or openings), the mesh can be locally refined by reducing the element size in problematic zones.

    ◦ Meshing to Internal Members: Enabling "mesh inside to members inside FE surfaces" or "mesh to internal columns and walls" can improve mesh accuracy by aligning elements with internal structural components.

    ◦ Course Mesh Preference: Starting with a coarser mesh is often recommended to reduce analysis time, and then refining specific areas as needed.

• Analyzing Results and Interpreting Moments:

    ◦ Error Visualization: Graphical results can show areas of high "maximum errors," which indicate where mesh refinement might be beneficial.

    ◦ Moment Values (Mx, My, Mrx, Mry): The webinar emphasizes using Mrx and Mry (Wood-Armer values) for more realistic bending moments, as Mx and My can show very high peaks over small areas.

    ◦ Peak Smoothing: This tool helps to minimize exaggerated peak values by ignoring everything within a defined diameter around a peak, providing a better indication of moments.

    ◦ Averaging Strips: Moments can be averaged over a defined width (e.g., 3-5 times the slab thickness) to get a more design-appropriate value, rather than designing for tiny, high peak values.

• Patterned Loading:

    ◦ Significance: Patterned loading is crucial for accurately assessing moments, especially with high live loads, as it considers alternate loaded and unloaded spans.

    ◦ Implementation: This is done by designating live loads as "alternate" and defining "alternate load patterns" and corresponding loading cases (e.g., L1 only, L2 only). Templates can be used for auto-generation of these cases.

• Modeling Tips for Specific Scenarios:

    ◦ Column/Surface Intersection: When two surfaces cross (e.g., a column passing through a slab), a member must be defined to ensure a node point exists at the intersection, allowing for proper support.

    ◦ Stepped Slabs: Stepped slabs can be modeled by creating separate FE surfaces for each level of the step and defining connecting members. The surfaces are rigidly connected unless otherwise specified, allowing moment transfer.

    ◦ Axial Loads in Walls: To determine axial loads (bearing pressure) in walls, a line can be drawn, and results viewed in the local Y-axis direction, with local mesh refinement aiding accuracy.

    ◦ Raft Foundations: Rafts utilize "subgrade modulus" (spring supports) to represent soil stiffness, which significantly impacts deflection and moment distribution. Typical values vary greatly based on soil type and bearing capacity.

    ◦ Soft Spots: Areas with different subgrade modulus (soft spots) can be modeled by cutting the FE surface into subsurfaces and assigning different properties, or by applying additional reactive loads.

    ◦ Spiral Staircases: These are modeled as a stack of connected FE surfaces.

• Slab Design Integration:

    ◦ The software can calculate required areas of reinforcement (e.g., "area steel top" or "area steel bottom") using strips.

    ◦ A dedicated slab design module allows for detailed concrete slab design, including automatic sizing of reinforcement and cracked section analysis for deflections.

    ◦ Punching Shear: The program accounts for punching shear, even at column edges.

    ◦ Openings Near Edges: For openings close to the slab edge, it's recommended to refine the mesh significantly around the opening and potentially introduce drop beams to strengthen the area.

    ◦ Attached Beams: Beams can be defined as "attached beams" to the slab, allowing them to be considered in the analysis and design (e.g., as T-stubs in concrete design).