Reinforced Concrete Transfer Slabs – From Modelling to Design [Webinar Recording]

Posted on April 14th, 2025 in Webinars

Summary

This MasterSeries webinar, led by a chartered structural engineer, focuses on the analysis and design of concrete frames, particularly concerning shear checks in transfer slabs using Finite Element (FE) software. It highlights the challenges of designing these complex slab elements, which facilitate column grid changes between floors, by demonstrating how MasterSeries' MasterFrame and FE Concrete Design modules can be used. Key topics include FE surface setup, meshing best practices, interpreting graphical analysis results for shear forces, and performing linear and punching shear checks, with a strong emphasis on the FE Shear Forces method over traditional code methods for improved accuracy in transfer slab design. The session also references important industry guidance and provides a preview of upcoming software updates to assist structural engineers and designers.

Key topics

• Transfer Slab Design
• Finite Element Analysis
• Shear Design
• MasterSeries Software
• Punching Shear Checks

Description

This workshop, led by Patrick McGinley, a chartered structural engineer and technical advisor at MasterSeries, focuses on the analysis and design of concrete frames and slabs, specifically addressing shear design in MasterSeries software. The primary goal is to explore how to conduct linear shear checks for transfer slabs and other critical slab elements, integrating the frame analysis with concrete design.

The session utilises several MasterSeries modules:

• MasterFrame: The general 3D space frame analysis software.

• FE Elements (Finite Elements): An add-on integrated analysis module with MasterFrame.

• FE and Concrete Design Module (for concrete slabs and walls): The primary tool for concrete design, with a focus on slabs in this workshop.

Key Concepts and Features Covered:

1. Transfer Slabs:

    ◦ These are slabs that transfer loads from columns above (planted columns) to different supporting columns below, allowing for column grid changes and architectural freedom without transfer beams.

    ◦ They are structurally inefficient, requiring deeper slabs and more reinforcement, leading to higher embodied carbon.

    ◦ They involve complex load transfers and higher potential for disproportionate collapse, often requiring detailed consideration of accidental loads (e.g., a 35 kN/m² patch load as per IStructE guidance).

    ◦ Finite Element (FE) analysis is crucial for transfer slab design due to their complexity, handling unusual layouts, load paths, openings, and complex shear assessments.

2. FE Surface Setup:

    ◦ Valid FE surfaces must be bounded, indicated by a blue panel in the model space.

    ◦ Openings must be modelled, especially those within six times the effective depth of a column, as they significantly affect shear flow and peak shear stress. Openings near edges also impact meshing.

    ◦ Material properties: Codified materials (e.g., C28/35 concrete) are essential for the FE slab designer to communicate with the FE surfaces.

    ◦ Loading: FE loads can be applied as area, point, line, patch, or perimeter loads. Accidental loads for robustness can be defined as patterned patch loads.

    ◦ Column Placement: For detailed slab design and punching shear checks, columns should be modelled accurately to tie up with the slab edges.

3. Meshing Practices:

    ◦ Global meshing options are set in the analysis menu. A good starting point for element size is not greater than span/10 or 1 metre, whichever is smallest (e.g., 0.5m default).

    ◦ Mesh refinement is vital around areas of significant stress concentration, such as column heads. MasterSeries offers automatic mesh refinement by telling it to mesh columns and walls as stiff regions within slabs.

    ◦ Slab stiff regions account for the cross-sectional sizes of walls and columns, reducing mesh sizes locally.

    ◦ Local mesh intensities can increase elements at selected nodes, further refining mesh around column heads.

    ◦ There must be at least one full mesh element between the column face and the analysis section for shear force assessment, ideally two or three elements between the column and the control perimeter for punching shear.

    ◦ Dummy beams can be used purely to control meshing.

4. Analysis Results Interpretation:

    ◦ Graphical analysis results allow reviewing line elements (axial, bending, shear, deflected shape) and shell elements (forces, moments, displacements, required reinforcements).

    ◦ Contour diagrams (e.g., FV Max) can show the maximum shear values, allowing users to identify critical zones by setting a threshold to the VRdc (shear capacity) value of the slab.

    ◦ Section diagrams on FE surfaces are used for linear shear checks. A line section can be created (e.g., between supporting and planted columns) with an average strip width (e.g., 4D), and shear values can be extracted at a distance of 1D from the column face.

5. Concrete Slab Reinforcement Design (Concrete Slab Design Module):

    ◦ Basic and Peak reinforcement (for column heads) are set up, with larger zones for planted/supporting columns.

    ◦ Shear Unity Ratio Contour outputs identify areas beyond normal punching shear where detailed checks are needed. These exclude zones within the U1 or 2D punching shear perimeter.

    ◦ Punching Shear Checks:

        ▪ Recommendation is to use the FE Shear Forces Method as it provides a more accurate representation by directly extracting shear forces from the perimeters defined in the FE analysis, accounting for slab/column interaction.

        ▪ This is preferred over the Code Method, which relies on simplified assumptions and beta factors that may not be appropriate for transfer slabs.

        ▪ The software allows for an average effective depth multiplier (e.g., 4D recommended by IStructE) to average peak shear stresses.

        ▪ New settings cater to closely spaced planted and supporting columns (e.g., overriding perimeter multiples when spacing is less than 4D face-to-face). If the clear spacing is less than 1.5D, a strut and tie approach is recommended outside the FE analysis scope.

    ◦ Linear Shear Checks: Now provided with strip reinforcement checks. These can be created along specific strips (e.g., between columns) to check shear reinforcement.

        ▪ They are more conservative than punching shear checks because they take shear force values at 1D from the column face (instead of 2D) and specify a maximum spacing of 0.75D for shear links in both directions (as opposed to 1.5D for punching shear).

        ▪ The software calculates the required shear reinforcement (e.g., vertical legs, transverse, and longitudinal spacing). Manual detailing of these shear bars is required.

6. Construction Stage Analysis:

    ◦ Acknowledged as important for transfer slabs due to locked-in stresses during staged building construction.

    ◦ Currently, the general recommendation is to model the transfer slab as a standalone element and apply loads from levels above as support reactions. This feature is on the MasterSeries roadmap for future development.

7. Software Access and Licensing:

    ◦ MasterSeries offers various licensing options, including PowerPad Suite (limited FE elements, 200m² surface, 200-500 members) and Building Design Suite (unlimited members and FE surfaces/sizes).

    ◦ Users can choose quarterly, annual (25% saving), or perpetual (one-off payment) subscriptions.

    ◦ Trials are available via the MasterSeries website.

The workshop also highlighted relevant publications, including those from The Concrete Centre and a free IStructE publication on the design of transfer slabs, along with the MasterSeries Manual and upcoming technical notes.