How to design Pile Caps & Pad Foundations in MasterSeries [WEBINAR RECORDING]

Posted on April 5th, 2023 in Webinars

Summary

This webinar excerpt from MasterSeries Software demonstrates the Finite Element Analysis (FEA) capabilities within their building design suite, focusing on the modeling and analysis of various structural elements. The speaker guides users through creating and manipulating FEA surfaces, applying diverse loading conditions like patch and line loads, and defining material properties and supports for slabs and walls. Key aspects covered include refining the mesh density to address areas of high stress or error, understanding and interpreting graphical results such as moments and displacements, and the crucial impact of patterned loading on design, especially with high live loads. The demonstration also touches upon advanced concepts like stepped slabs, wall axial loads, raft foundations with subgrade modulus, and the integration of attached beams and openings for comprehensive structural analysis.

This webinar from MasterSeries Software provides a detailed guide on how to model and analyze buildings, particularly slabs, using Finite Element Analysis (FEA). It's an advanced topic building on previous discussions about frame analysis.

Key concepts

• Finite Element Anaylsis
• Slab Design
• Pattern loading
• Mesh Refinement
• Support Conditions

Description

Here's a summary of the key concepts and procedures:

• Software and Capabilities: MasterSeries offers FEA capabilities for surfaces across its product range, including the Building Design Suite, Power Pad, and standalone modules. The core functionality for FEA (frame analysis engine and finite elements) is available in various combinations.

• Creating FE Surfaces (Slabs):

    ◦ An FE surface (slab) can be generated from any four, five, or six co-planar members. This can be done by manually selecting bounding members or using an "auto select" window function.

    ◦ Openings for features like staircases can be defined within the surface by selecting three or four members that form the opening.

    ◦ Supports can be physical columns or continuous supports like walls, which are modeled by applying Y-axis restraints along specific members. Foundation supports for columns are also applicable.

• Applying Loads:

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

    ◦ Various other load types can be added: uniform live loads over the entire surface, patch loads over specific defined areas, line loads along perimeters or internal lines, and varying loads (e.g., triangular loads) by specifying values at three points.

• Material and Thickness: It's crucial to define the material properties (e.g., concrete grade) and thickness of the FE surface for accurate analysis.

• Meshing and Refinement:

    ◦ The software automatically generates a mesh, with nodes at column locations.

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

    ◦ Local mesh intensities can be applied to refine the mesh in areas of high error, often around supports or openings, by reducing element size.

    ◦ 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.

    ◦ Starting with a coarser mesh is often advised to reduce analysis time, and then refining specific areas as needed.

• Interpreting Results and Moments:

    ◦ Results can be viewed graphically, including maximum errors, which highlight areas needing mesh refinement.

    ◦ For bending moments, the webinar recommends using Mrx and Mry (Wood-Armer values) over Mx and My, as they provide more realistic results by considering shear effects at element ends.

    ◦ Tools like peak smoothing (ignoring extreme peaks within a defined diameter) and averaging strips (averaging moments over a defined width, typically 3-5 times the slab thickness) are used to obtain more design-appropriate moment values, rather than designing for small, highly concentrated peaks.

• Patterned Loading:

    ◦ This is a critical technique for high live loads to accurately determine maximum sagging and hogging moments by simulating alternate loaded and unloaded spans.

    ◦ Loads are designated as "alternate," and then "alternate load patterns" and corresponding loading cases are set up, often using auto-generate templates.

• Modeling Specific Scenarios and Tips:

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

    ◦ Stepped Slabs: These are modeled by creating separate FE surfaces for each level of the step and defining connecting members. These surfaces are rigidly connected, allowing moment transfer.

    ◦ Axial Loads in Walls: Axial loads (bearing pressure) in walls can be determined by drawing a line and viewing results in the local Y-axis direction, with local mesh refinement improving accuracy.

    ◦ Raft Foundations: Rafts use "subgrade modulus" (spring supports) to represent soil stiffness. The chosen subgrade modulus significantly impacts deflection and moment distribution, with typical values varying greatly based on soil type and bearing capacity. Soft spots with different subgrade moduli can be modeled by dividing the surface or applying additional reactive loads.

    ◦ Complex structures like spiral staircases can be modeled as a stack of connected FE surfaces, and box culverts can handle triangular or varying loads.

• Slab Design Integration:

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

    ◦ A dedicated slab design module allows for detailed concrete slab design, including automatic sizing of reinforcement, punching shear checks (even at column edges), and cracked section analysis for deflections.

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

    ◦ Attached beams can be defined to integrate them into the slab's analysis and design (e.g., as T-stubs in concrete design).