Using the Pipe Matching and Matchline Offset Design Constraints

Product(s): SewerCAD, StormCAD, SewerGEMS, CivilStorm
Version(s): CONNECT Edition, V8i
Area: Modeling

Problem

How do the "Pipe Matching" and "Matchline Offset" design constraints work, when performing a constraint-based design with the StormCAD or SewerCAD solver?

How do I determine if pipes should match inverts or crowns?

Solution

The pipe matching design constraints help the design solver choose the vertical alignment between multiple pipes connected to a node (such as a manhole). They are found in the Node tab of the Design alternative or Design Constraints.

Pipe Matching

The "Pipe Matching" dropdown enables you to specify whether the connection elevation of the incoming and outgoing pipes with the structure should be matched relative to the pipes' Inverts or Crowns. Please refer to the following illustration to understand what this means:

Traditionally, most engineers will consider matching the crown elevations of incoming and outgoing pipes. The elevation drop can help to offset energy losses through the structure, and the likelihood of having pressurized flow in a smaller upstream pipe is reduced.

When inverts are matched, the bottom of the structure can be formed to convey some flows with less energy loss.

In a practical sense, there is likely a negligible hydraulic benefit between these pipe matching strategies, and you may need to use your own engineering judgement along with local standards to decide which to use.

From a least-cost perspective, if a minimum cover constraint is driving the elevations of pipe inverts, then matching pipe crowns will typically result in the least amount of excavation. However, if the ground surface is fairly flat, or if the storm sewer slopes in the direction opposite that of the ground slope, then minimum pipe slope requirements will typically drive the invert elevations. In this situation, matching pipe inverts will minimize excavation. In the absence of regulations to the contrary or extenuating circumstances, the engineer may choose to match inverts to minimize construction costs in this case.

See "Vertical Alignment" in chapter 11.2 of our book "Stormwater Conveyance Modeling and Design"

Matchline Offset

The Matchline Offset setting enables the design solver to offset a manhole or catchbasin's outgoing pipe invert from the incoming pipe invert by a specified amount. The matchline offset is measured as the relative vertical distance between either the pipes' Inverts or Crowns depending on what you select.

This setting can be set globally in the Node tab of the Default Design constraints or design alternative, or on a per-manhole basis by setting local design constraints.

Note that there is an order of priority with the design constraints so it is possible that in some situations the matchline offset cannot be applied if doing so would violate a higher priority constraint.

Advanced Matchline Offset

In some situations, such as the Michigan County, Oakland County and Livingston County Storm Sewer Standards, you may be required to impose a more advanced matchline such as 0.8 times the change in pipe diameter (see more details here). At this time the matchline offset is a fixed value globally or on a per-pipe basis for design constraints. A dynamic matchline offset will be considered for a future release (reference # 1010223)

See Also

Troubleshooting Constraint Based Design Results

Tips for constraint-based design

Order of constrained based design priorities for StormCAD (GVF-Rational) and SewerCAD (GVF-Convex)

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