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Barrier with special crowning

Barriers having a special crowning can be edited and calculated in CadnaA, depending on the calculation standard selected. In this case, select the desired crowning from the list box „Crowning“ on dialog Barrier.

Option „Crowning“

  • normal: models a vertical screen with no crowning
  • Cantilever to the left/right: The designations „left“ or „right“ refer to a viewing direction from first to last polygon point of the barrier. Enter the horizontal and vertical extent of the cantilever (in m). The location of the cantilevered part is displayed in the graphics by a dashed line.

Note

At the cantilevered surface is no reflection is calculated. So, the cantilever is assumed to be fully absorbing. With a single barrier, the level calculated with sources below and outside the cantilevered part is valid.

  • T-shaped (only available with option MITHRA or NMPB08): Enter the dimensions (in meters) of the T part perpendicular to the surface (permissible value range 0.85 to 1.50 m).
  • Cylindric (only available option MITHRA or NMPB08): Enter the diameter (in m) of the cylinder (permissible value range 0.60 to 1.00 m).

Note

In CadnaA, the Mithra barrier crownings are considered only:- with NMPB 08 (Industry, Road, Fer), whether Mithra option is on or off (see General Tab)- with Mithra option activated (and hence Angle Scanning, see General Tab): for every standard being selected.

  • Roof Edge (3D): When barriers are modeled inside buildings this option caused the building polygon to be generated by triangulation using the barrier’s and the building’s height. In this case, just the roof ridge modeled by the barrier causes screening, but not the triangulated roof edges (see Defining a Roof Ridge).
  • Curved cantilever to the left/right (for 3D): The designations „left“ or „right“ refer - as with the cantilevered barrier - to a viewing direction from first to last polygon point of the barrier. The calculation is analogous to the cantilevered barrier, except that in the 3D special view, a cylindrical curved cantilever with the entered curvature diameter (in m) is displayed. The edge position of the cantilever is shown in the graphics as a dashed line. The same restrictions apply as for a cantilevered barrier.

Button "Geometry"

Via the button „Geometry“ the barrier’s height is specified (see Object’s Geometry).

Additional Information

Crowning "Normal"

The type „(normal)“ is applicable to all kinds of calculation methods. The polygon describing the barrier is considered in the calculation of the screening effect. The diffraction algorithm according to calculation method selected is applied.

Crowning with Cantilever

Whether the option „Cantilever top the left/right“ is applicable depends on the calculation method selected. The following chapters contain related information:

Guidelines for Input

Enter a value for the horizontal and vertical part of the cantilever. The height of the barrier displayed on the dialog Geometry is the height of the diffracting edge at vertical, plane barriers.

By selecting "left" or "right" the cantilever directs into this direction when looking from the first to the last polygon point. The cantilevered part is displayed using a dashed line.

As an example, the barrier height is 6 m (on dialog Geometry), and the cantilever horizontal and vertical 1 m results in the following figure.

Barrier with cantilever displayed in 3D-View „General Parallel“

Barrier with cantilever in 3D-Special view

Crowning „T-shaped“

Barriers with crowning attribute „T-shaped“ are valid for all calculation methods with "Mithra-compatibility" activated. The value for the width has to be between 0.85 and 1.5 m. The T-shaped crowning is created symmetrically on top of the barrier, with half the width on either side (is shown in 3D-Special).

Crowning „Cylindric“

Barriers with crowing „Cylindric“ are valid for all calculation methods with "Mithra-compatibility" checked. After entering a value for the diameter between 0.60 and 1.00 m a cylindric crowning is created with the upper centre line of the barrier (is shown in 3D-Special).

Crowning „Roof Edge (3D)"

The triangulation of the roof surface inside a building’s polygon occurs just for barriers having the option „Roof Edge (3D)“ selected. This enables to model real roof structures, also for display in the 3D-Special view providing a screening effect (see Defining a Roof Ridge).

When modeling a roof ridge, however, this option does not generate gable walls raising up to the roof ridge. In order to enter a gable wall up to ridge height, additional points making up the building polygon are required (at the intersections between roof and exterior wall). In this case, use the option „Height at every Point“ from dialog Geometry of the building (see Defining a Roof Ridge).

Application Notes for Cantilever

With regard to the practical application of barriers with cantilever the following situations apply:

  • source below the cantilever,
  • source outside of the cantilever.

Source below the cantilever

In order to calculate these situations correctly, the shortest ray path must be found starting from the source around the cantilever’s edge to the receiver point. The method applied will retrieve the shortest path difference for a single barrier in the entire space. Regardless of the receiver point’s location, one or two diffracting edges are taken into account. As mentioned already, the sloping surface of the cantilever is considered to be full absorbing. This feature enables to model porches or galleries alongside of roads with their lower surfaces being absorbing.

Example: point source and barrier with cantilever

Vertical Grid: point source directly below the cantilever’s edge

Vertical Grid: point source below the cantilever (Dz without limit)

Source outside the cantilever

The same procedure is applied with sources below the cantilevered part. The levels are valid for single barriers within the entire space.

Vertical Grid: point source outside of the cantilever