Non-Tangential Alignments [TN]


  
 Applies To 
  
 Product(s):Bentley MX 2004 Edition
 Version(s):2004 Edition
 Environment: N/A
 Area: N/A
 Subarea: N/A
 Original Author:Bentley Technical Support Group
  

 

 

 

 

 

 

 

 

Purpose

The purpose of this TechNote is to explain the reasons for the inclusion of non-tangential alignments into Bentley MX 2004 Edition, the benefits they will bring, their limitations and the impact they could have on historical MX projects.

Introduction

Until MX Version 2.5 MX had always required continuity of bearing in alignment geometry, then the Straight Line Design options were implemented in Design and Interface. These new options helped users design strings with bearing discontinuities, however, they were not available in Alignment.

Now, in Bentley MX 2004 Edition, non-tangential alignments have been introduced. As this TechNote outlines there are many implications of using non-tangential alignments on the way in which you use MX and the effect it could have on projects created in earlier versions of MX.

Note: The Straight Line Design options have been removed from the menu structure (although they are still available from Linemode). It is not advisable to combine the use of non-tangential Alignments with the straight line Design/Interface options.

Getting started

Non-tangential alignments can only be created in new projects. This is signified by the tick box towards the bottom of the MX start up panel shown in Figure 1.

 

If you wish to change the default setting on the start up panel, so that the non-tangential alignment box is ticked on or off by default for a new project, please edit the file ..\windows\mosswin.ini. Towards the bottom of this file is the setting:

[DISCO]
DiscontinuousAlignments=FALSE

If you want the non-tangential alignment tick box to be always on for a new project, ensure the setting is TRUE; if you want it to be off for all new projects, then ensure the setting is FALSE, as in the example above.

It is important to note that even with the non-tangential setting turned on, tangential alignment design will create the same information as it did prior to Bentley MX 2004 Edition.

Using a 3D string as a reference string

The most important change in MX to cater for non-tangential alignments is with 3D strings.

Until now, when designing from a 3D reference string in MX, a curve fitting algorithm was used to calculate the position of the new string. In Bentley MX 2004 Edition, a new improved algorithm has been introduced which inspects the geometry of the string in the background so that when designing from a 3D string the specified offset is maintained around corners. No geometry strings are created or stored in this process.

Note: The new non-tangential design algorithms do not apply when using DESIGN 103/105/105, Intersection of two slopes or DESIGN 107/108, Subgrade design. In these cases, the original curve fitting routine is used because of conflicts encountered if the subsidiary string lies on the opposite side of the reference string.

Designing the same strings with non-tangential design gives the results shown in Figure 3. You can see that extra points have been generated from the two normals emanating from the reference string kink point. These extra points generated to improve corner conditions can have implications for historical data. The prime example is when an input file is used from an old project. If the input file uses a 3D string as a reference string, the resultant string will have more points in it than it would if it were designed with the non-tangential alignment setting turned off.

Any reference to the point number of the new 3D string will be incorrect if the input file was developed in a project that did not have non-tangential alignment setting on. This will result in points not being found or the wrong position being specified.

If a 3D string has been created in a non-tangential project, the string report will contain a 40000 indicator in the header information (see Figure 4). Note that the report also shows any points of non-tangency in the string with a bearing discontinuity label.

Within major option DESIGN, when you are specifying start and end points with non-tangential alignments or 3D strings you cannot use XY SPRD as the options will only snap to the nearest point on the reference string.

If a corner condition is less than 90 degrees on its inside angle, MX may not be able to calculate a suitable solution for the corners of designed strings or earthworks. This is dependent on the offset and slope from the corner.

When designing from a string with points of non-tangency it is not possible to specify the start and end points in the reverse direction

 

Editing non-tangential 3d strings

Some differences in the way the Modify Strings and Modify String Points options work (major option EDIT) are to be expected on this new type of 3d string. Options such as Convert to a 6d Master String use curve fitting to determine new points for interpolated chainages on the new string. A summary of effects is listed below, EDIT option by option - where the option is not listed there is no change or effect:

  1. Edit 012 Join two strings. If you join two strings using this option the second string specified governs the string direction of the new string. In the same way the second string will govern whether the new string has a 40000 indicator or not. The 40000 indicator will also be honored through the join strings wizard.
  2. Edit 026 Insert a point, interpolated point will always honor the point of non tangency.
  3. Edit 027 Convert to 6D master string - use of field 2 "REGU" to create regular chainages will almost certainly remove the "kink" points in the string, resulting in a very different looking master string to the 3d string.
  4. Edit 028 Create a string with extra points, will honor the point of non tangency.

Alignment Creation

Interactive alignment design

Non-tangential alignments can be generated in both Alignment and Quick Alignment. Major options HALGN, VALGN and VERAT also support non-tangential alignments. HALGN supports fix to fix scenarios for straights and curves, while VALGN and VERAT support grade to grade.

To connect elements in Alignment (to generate a non-tangential connection) go to Special Geometry and select Fix to Fix. Ensure that the check box for non-tangential connection is set.

In vertical alignment (element method) it is not possible to connect a fix grade to a fixed curve as a point of non-tangency.

Quick Alignment design

Using Quick Alignment, you will have to change the parameter settings so that the default horizontal radius is set to 0 (zero), and the parameters for both the hog and sag K / vertical radius values are set to 0 (zero). Alternatively, you can set the radius for an individual curve to zero. Note that this does not give you the ability to connect curves in a non tangential manner.

Points of non-tangency in Alignment and Quick Alignment are annotated with a special symbol (macrosymbol - "IADSSKEW" diamond shaped dumbbells).

Care should be taken when exchanging alignments with other design systems as they may not support non-tangential alignments.

Create from 3D String

Using the Modify String > Generate 6D master string option (EDIT 027) to generate an alignment is discussed separately later.

Import from CAD
Non-tangential alignments can also be imported from CAD packages. Warnings are given when a non-tangential alignment is created. A SmartLine in MicroStation or a Polyline in AutoCAD will be converted to non-tangential master and geometry strings if they contain points of non-tangency.

Other Import methods
Import of non-tangential alignments from other MX projects via GENIO, LandXML and Softdesk / Land Desktop are all now supported.
Note: Tangential alignments can still be created in non-tangential projects in the same way as previous versions of MX.

 

The values of Common Radius and Transition Constant Ratio should be left blank to generate a non-tangential connection

The following steps diagrammatically highlight the relationship between the "Fix to Fix" functionality and the "Re-specify Link" functionality.

Link 1; Fixed Straight - 2 Points. Link 2; Fixed Curve - 2 Points and Radius Linked through the Non Tangential Connection functionality (note the dumbbell), the Re-specify Link option cannot be used to adjust either link 1 or 2.

1

 

Link 3; Float a Curve - Point and Radius
Re-specify Link option cannot be used on Links 1 &2 however it can be used on Link 3

2

 

Link 4; Float a Curve - Point and Radius
Re-specify Link option cannot be used on Links 1 & 2 however it can be used on Link 3 & 4

3

 

Link 5; Fixed Curve - 2 Points and Radius, linked into the alignment through the Non Tangential Connection functionality.
Re-specify Link option cannot be used on Links 1 or 2, nor link 4, or 5 because of the new non- tangential connection however it can still be used on Link 3.

 4

Link 4; replaced with a Free Arc - Radius. A leading Transition and a Trailing Transition were also added. This is now a tangential connection.

5 

The re-specify link option can not be used on Links 1 & 2 however it can be used on Links 3, 4, & 5.

If the Re-specify Link option is attempted on a link with a non-tangential connection at either end then the following message is displayed. 

To summarize - you cannot re-specify a link if it has a non-tangential connection at either end. Instead you have to delete the link and recreate the connection.

Swap Link

Horizontal Alignment > Element Alignment > Edit Alignment > Link > Swap Link

The implementation of Non Tangential Alignment has also had an impact on the Swap Link option. Remember that Swap link re-analyses the element to be swapped and another element either side of it.

Taking the same alignment as an example the Swap Link functionality can only be used on Link 5 as all other Links would be affected by the non-tangential connection between links 1 & 2.

 

In essence Swap Link can not be applied to the two adjacent links either side of the Non Tangential Connection (Tangent Point). A more detailed explanation of the Swap Link functionality can be referenced from the Help.

Designing the 3D strings

When designing from a non tangential reference string a "kink" will be introduced into the designed 3D string. The position of this "kink" is calculated in plan by using the bearing of the points before and after the "kink" to extend the string links. Note here that the "kink" may involve a curved element, but the extension to the "kink" position will be on a constant bearing.

This affects the position of the calculated corner in the following design minor options.

100: Add String: Constant horizontal offset / Constant Crossfall
101: Add string: Linear Horizontal offset / Constant Crossfall
102: Add string: Reverse Horizontal offset / Constant Crossfall
110: Add String: Constant Horizontal offset / Constant Vertical
111: Add String: Linear Horizontal offset / Constant Vertical offset
112: Add string: Reverse Horizontal / Constant Vertical Offset

Elevations at corners

On the outside of a non-tangential corner created using DESIGN options, the level remains the same around the corner. The level at this corner may need consideration as the flat spot could lead to ponding as shown in Figure 6 below. The design contours are displayed in white.

Interface/Earthworks

When generating Earthworks from a non-tangential alignment the interface is only calculated to a triangulation model. If you have not already created a triangulation model, the Earthworks Wizard will give an option to generate one automatically.

Additional sections are cut through the triangulation on the outside of the corner at the points of non-tangency, in an attempt to improve the earthworks string. The normal slope condition will be retained from the level datum, but in the direction defined by these sections. This may give apparent slopes greater than required at the corners.

The Earthworks GUI allows 4 angular increments ( ie 5 sections with a bisector in the middle) However if the angular deflection at the kink causes an increment greater than 10 degrees, then more sections are added to reduce the angular increment below the 10 degree maximum increment value. An odd number will always be created to ensure that the bisector is formed.

In LINEMODE the user has extra flexibility here, allowing a circular corner condition on the outside of the corner and control over the number of incremental sections used. This means that the 259 option has been extended to allow fields 2, 4 and 5 as in the 359 option.

Internal corners have a section cut at the angle bisector position. Retention of slope conditions in this direction may give rise to an unusual position for the earthworks string point at the corner.

Any internal corner loops will automatically be removed from the earthworks string, In the background when you design on the inside a loop is generated and before MX removes the loop. MX takes an average elevation of the points making up the loop, and applies it to the point on the inside corner.

Cross sections

At a point of non-tangency, a skewed section (also known as a bisector or secant section) is created when generating cross sections. See Figure 7  

Because the section is not created normal to the reference string, it is flagged as a skewed section in MX reports, see Figure 8.

Drawing cross-sections

If the skewed section is drawn in section using MXDRAW, the SECTDRAW command macro, or using working drawings, the section will be annotated with the word SKEW and a angle value in degrees minutes and seconds. See Figure 9. This angle is the deflection angle of the section from the normal direction to the string at this point.

It is important that these sections are recognized in this way in case they are being used for setting out purposes.

 

Volumes

On a project with non-tangential alignments enabled accurate volumes should be calculated using the prismoidal volumetric method. It is not recommended that you generate volumes from stored sections referenced from a non-tangential alignment as the average end area of the sections is used to calculate the volume. The skewed sections could affect the results if there is significant cut or fill on one side of the section.

Profiles/Long Sections

When sectioning about strings with 40000 indicators, Secondary Interpolation (SINT) must be on to get the correct representation of the string. If SINT is not used then the representation will be only partially correct because you will only get primary cuts. (For details on primary cuts please refer to the on line documentation heading "Analysis by Section: Secondary interpolation"). However if you are sectioning a triangulation, where SINT isn't used, then the result will be correct.

Areas of the program where this will apply are:

  1. Analysis > Section > long / sections /profiles on the define section panel.
  2. Add-ins > longsection / profile along a string.
  3. Add-ins longsection / profile between two points.
  4. Design > Quick Alignment, vertical profile, adding collinear profiles.
  5. Draw > working drawings, create new drawing, page set up (longsection / profile) Create section string.
  6. Section 171, Long section along a string.

MX Applications

MXRENEW, MXURBAN, and MXROAD Pavement and Subgrade Design have been enhanced to accommodate non-tangential alignments. If the reference string selected has a 40000 header, then these packages identify areas where sections created normal to the reference string might not cut the design or existing edges. This is done by inspecting the maximum offsets to the design or existing edges at each skewed section. Normals are raised to the reference string from these points, and the chainages found mark the limits of the area affected by the bearing discontinuity, as shown in Figure 10 below:

 

This figure shows the area of angular discontinuity hatched in red. The relevant options in MXRENEW, MXURBAN, and MXROAD Pavement and Subgrade Design will display this hatching to make it easy to identify these areas.

If you try to select a point or chainage that is inside one of these areas, for example in the MXURBAN Street Furniture or Thresholds options, you will get a message telling you that it cannot be used because it is inside this area. This also applies to Parametric Fitting, where you will not be allowed to modify widths, crossfalls etc in this area.

When you click OK in the MXRENEW and MXURBAN Parametric Fitting, and Pavement and Layer Design, any cross sections that are inside the area of angular discontinuity will be deleted, apart from the skewed section, which is always retained.

In practice, horizontal bearing discontinuities in highway design are likely to be very slight (less than two degrees) so the area of angular discontinuity will be very small. It is therefore unlikely that it will result in cross sections being deleted.

MXRAIL also supports horizontal bearing discontinuities.

MXSITE uses the Straight Line Design options to create turning heads. This method of operation has not been changed.

Summary

Non-tangential alignments are useful for a host of situations including site layout, quarry design, and straight-line design. Projects where existing road alignments have slight deflections can now be designed. MX can now import non-tangential alignments from CAD packages. The new improved algorithm for 3D strings enables proper geometric offsetting of strings giving greater design flexibility.

Remember there are some new limitations that you may not expect to encounter because of the introduction of this functionality:

See Also

Product TechNotes and FAQs

MX TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

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