📄 Consideration of Damp Proof Courses (DPCs) for Edge Conditions in MasterSeries Masonry Design
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Consideration of Damp Proof Courses (DPCs) for Edge Conditions in MasterSeries Masonry Design

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When dealing with a damp-proof course (DPC) or damp-proof membrane (DPM) at the base of a masonry wall, determining the correct edge fixity is a common question for structural engineers, as the membrane creates a physical discontinuity in the mortar joint.

Here is how the MasterSeries software and structural codes handle fixity conditions at DPC levels:

1. The Standard Assumption: Pinned (Simply Supported)

The most common and "safest" engineering assumption is to model the base of the wall at the DPM/DPC location as a pinned (simply supported) edge.

  • Structural Behaviour: Standard DPC materials typically provide adequate shear resistance to stop the wall sliding sideways, but they lack the ability to transfer moment (bending) forces because they break the tensile bond between the masonry courses.
  • Code Guidance: Clause 20.1 of the British Standards states that flexural tension should only be relied upon at a damp-proof course if the material has been explicitly proven by testing to permit the joint to take tension. Without this proof, it cannot be assumed to be fixed.

2. When Can a Base Be Modeled as "Fixed"?

If you need a fixed base to make a wall panel pass its design checks (which provides significantly more moment capacity), there are three main scenarios where a fixed condition is justifiable:

  • Engineering Bricks: A very popular and structurally robust solution is to omit the plastic DPC membrane entirely and instead use one or two courses of high-strength, low-water-absorption engineering bricks (e.g., Class A, 20-30 N/mm²). These bricks naturally stop moisture ingress while allowing standard mortar to bond them, thus maintaining full structural continuity and allowing you to model the base as fully fixed.
  • Proprietary DPCs: There are specific proprietary "force-transmitting" DPC materials on the market that have been tested and certified to maintain continuity and transmit flexural forces across the joint.
  • Upper Floors: If you are designing a wall panel on a second or third floor where a DPC is not required, and the masonry is continuous past the floor slab, the base can naturally be taken as fixed.

3. Partial Fixity

In some cases, the DPC joint can be considered partially fixed rather than fully pinned. Guidance (such as from the Concrete Masonry Designer's Handbook) suggests that you can sometimes rely on the vertical gravity load pressing down on the wall to enhance the moment resistance at the DPC, giving it a degree of partial fixity. The software allows you to manually input a partial fixity percentage to represent this.

4. The "Free" Edge (Warning)

At the absolute worst, if a DPC material provided zero shear resistance, the base would have to be modeled as a free edge. However, this is highly unusual in practice, as almost all standard DPCs provide enough friction/shear resistance to be considered simply supported.

Summary for Software Input: In the MasterSeries Edge Fixity tab, you will generally set the base fixity to Pinned when a standard DPC is present. It is ultimately your responsibility as the engineer to verify that if you choose to set it to Fixed, the material specified on site (like engineering bricks or a tested proprietary DPC) can physically support that assumption.