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Elevated Roads and Bridges

In CadnaA bridges, viaducts, and other elevated roadway constructions are modeled by assigning the relative height to the road’s axis at every point or as absolute height and activating the option "self-screening" of the road object. The width of the self-screening bridge plate includes the road width and additional sidewalk etc.

Elevated road with barrier to the left with „Self-Screening“ activated

Entering an elevated Road

  • Select the object "Road" from the toolbox and enter a straight road section from two polygon points.
  • In edit mode - recognized by the road symbol attached to the mouse pointer - the road dialog can be opened by clicking with the right mouse button on the road axis.
  • For CNOSSOS-EU, enter a road width of 5 m, MDTD of 15000 vehicles/day and the road type „national“.
  • Click the button "Geometry" and specify a relative road height 10 m above the ground (default: 0 m).
  • Close the dialogs Geometry and Road by OK.

Vertical Grid

  • Select the object "Vertical Grid" (symbol ) from the toolbox.

  • Draw a line from two polygon points perpendicular to the road axis, approximately in the middle of the road section.

Road with vertical grid (projected into xy-plane)

  • Switch to the edit mode (symbol ).

  • Double-click on the vertical grid to open its dialog.

  • Deactivate the options "global" and specify a receiver spacing of 1 by 1 m and select the representation type "Lines of Equal Sound Level" and activate option „Level Range“.
  • By clicking the button "Calculate" the calculation is engaged and the resulting vertical grid displayed.
  • Click with the mouse into the lower part of the dialog - where the grid is shown - and zoom in the vertical grid using the mouse wheel.

As can be seen the road is radiating to both sides. Thus, the road’s surface is not considered as screening surface by default.

Note

In this example, the two emission lines are located 0.5 m above the height entered for the road itself. This may vary depending on the calculation method selected.

Vertical grid: elevated road not being self-screening

  • Close the dialog Vertical Grid and double-click again on the road axis.
  • Click the button "Geometry".

Activating Self-Screening

The option "Self-Screening" is activated on the road’s Geometry dialog. Additional width and barrier heights can be defined separately for the left and right side. The additional width equals the distance of the outer diffracting edge ("bridge' edge") to the axis of the outer lane.

  • Activate the option "Self-screening" and enter an additional width 4 of meters for the left and the right side.
  • Close the dialogs Geometry and Road by OK.

Instead of the former additional width (per side of the street 1.75 m, see dialog Options|Appearance, „Road“) the additional width is now based on the self-screening definition. Using an auxiliary polygon you can measure that the perpendicular distance from the dotted emission line to the road’s curb is 4 m an either sides. Consider, as the additional width from the self-screening definition determines the radiated pattern it supersedes the additional width defined on the dialog Appearance (Options menu).

  • Double-click on the vertical grid and restart the calculation.

Vertical grid: elevated road with self-screening activated

The resulting grid illustrates the self-screening effect: The bridge's edge is considered as a diffracting edge upon recalculation.

Enter the Barrier’s Height

  • Again, open the dialog Geometry of the road and enter a barrier height on the left of 5 meters.
  • Close both dialogs and start the grid calculation from the dialog Vertical Grid.

Vertical grid: self-screening elevated road with barrier (parapet) on the left side

just the own source is seen

In conjunction with the feature „Self-Screening“ it has to be considered that, both, the additional width and the parapet/s are just seen by the own source (i.e. the road having this option activated). For other sources - besides the road in question - the road’s plate and parapet/s are not seen as a screening surface. This will be illustrated in the following.

  • Open the road’s dialog and change the MDTD to zero vehicles per day.
  • Enter a point source at a height of 10 m aside of the road and in the line of the vertical grid Specify an A-weighted sound power level of 100 dB(A).
  • Open the vertical grid and start the calculation.

The result indicates by the lines of equal sound level propagating unaffected across the road’s plate that the self-screening option activated for the road is not relevant for the sound propagation of the point source.

Vertical grid: self-screening elevated road with no emission and a point source

Dialog Modify Objects

The values required in conjunction with the definition of self-screening property can also be changed via dialog Modify Objects, action Modify Attributes. For this, proceed as follows:

  • Click with the mouse into the white screen area and select the command Modify Objects from the context menu.
  • Select from the list of actions the option „Modify Attribute“ and as object type „Road“.
  • After click on OK the dialog Modify Attribute is displayed.
  • Clicking >>, an attribute for self-screening can be selected to assign a new value. Those attributes start with the prefix SSCR_.

Definition of the additional widths left/right