Alignments

This chapter shows how to draw a road centerline in Terrace, with its curves and spirals, stations, offset edges and curb returns, and how Terrace works out the superelevation and checks the design against the standard. When you finish it you'll be able to lay out an alignment, change it, read its problems and print its geometry, stakeout and superelevation reports.

Alignments, PIs, curves and spirals

An alignment is the line a road follows on the plan, usually its centerline. Everything else in the road hangs on it: the ground and design profiles, the corridor, the sample lines, the plan-and-profile sheets.

You draw an alignment the way a highway engineer lays one out on a map: by its PIs (points of intersection), the corners where two straight tangents meet. Terrace puts a curve of a given radius at every inside PI, so the road turns smoothly instead of at a corner.

A spiral (clothoid, or transition curve) is a curve whose radius changes steadily from straight to the circular radius. It sits between the tangent and the circular curve on each side, so a driver turns the wheel gradually and the road has room to tilt into its superelevation. Terrace labels the points along a curve the usual way: TS (tangent to spiral), SC (spiral to curve), CS (curve to spiral) and ST (spiral to tangent), or PC and PT for curves without spirals.

Along the alignment, distances are stations, written kilometres + metres: 35+200.00 is 35.2 km from the start of the stationing. The sample project's Teresa Bypass Road starts at Km 35+200.

The Roads tab

Every alignment tool is in the Alignment group of the Roads ribbon tab. When an alignment is selected, a contextual Alignment tab appears as well, with the editing tools, the next steps (Ground Profile, Profile View, Corridor, Show Profile) and the Properties, Zoom To and Report buttons.

The full list of Roads commands is in the command reference.

Drawing the centerline by clicking PIs

This is the usual way to start a road: you click the PIs on the plan, over your surface or a background map, and Terrace fits the curves for you as you go.

1. Type CREATEALIGNMENT (CALIGN) and press Enter. Or click Roads ▸ Alignment (or Draw the road centerline in the Next step panel): the New Alignment dialog opens; click Click PIs… at its bottom left and it closes and starts the same command.
2. Read the first line Terrace writes: it tells you the defaults it picked (see below). Change any of them now with the options on the prompt, or keep them.
3. Click the start point, then click each PI in turn. A live preview follows the cursor: the tangents as a light line and the solved alignment with its curves and spirals.
4. Press Enter after the last point to finish. You need at least two points.

While you place PIs, the prompt offers:

Each PI keeps the radius and spiral that were set when you clicked it. To give one PI a sharper curve, choose Radius, type it, click that PI, then set the radius back.

When you finish, Terrace adds the alignment, checks it against the standard and writes a summary, for example CL-1: 0+000.00 to 1+224.10 (1,224.095 m), 2 curves. Any problems are listed under it (see Design checks). The alignment appears in the Project browser under Alignments, with stations, curve data and PI labels drawn on the plan.

The defaults Terrace picks

You don't need to look anything up before drawing. CREATEALIGNMENT takes its design values from the project's standard (DPWH unless you chose AASHTO):

• Road class: the class of the last alignment you drew, or National Secondary in a DPWH project (Rural Collector in AASHTO).
• Terrain: rolling.
• Design speed: the class's speed for the terrain. National Secondary is 80 km/h on level ground, 60 km/h rolling and 50 km/h mountainous, so a new DPWH road starts at 60 km/h.
• Maximum superelevation (e max): the highest rate in the standard's tables, 10 % for DPWH.
• Curve radius: 1.5 times the minimum radius for the speed at that e max, rounded up to 10 m (50 ft in US projects). At 60 km/h that is 160 m.
• Spiral length: the standard's spiral length for the speed, or the superelevation runoff length when that is longer, rounded up to 5 m (10 ft). At 60 km/h with R = 160 m that is 50 m.
• Name: CL-1, CL-2 and so on.

To change them, choose an option at the first prompt, before you click the start point:

The new alignment gets superelevation turned on, with the e max above and the normal crown and lane width of its road class (1.5 % and 3.35 m for National Secondary). In a US-feet project its stations are labelled every 100 ft with ticks every 50 ft. You can change all of these later in Properties.

Making an alignment from a polyline

If the route is already drawn, for example a centerline from an old plan or a DXF, turn it into an alignment instead of clicking it again. You can do it with the New Alignment dialog or with the command.

The New Alignment dialog

1. Select the polyline: click it in the drawing. (If you select several, the dialog uses the first.)
2. Click Roads ▸ Alignment, or Draw the road centerline in the Next step panel. The New Alignment dialog opens with every field filled in, and says From the selected polyline.
3. Change what you need and click Create.

Each polyline vertex becomes a PI. Arcs in the polyline stay as curves; every other corner gets a simple circular curve of the radius above (the dialog adds no spirals). Terrace checks the alignment and writes, for example, Alignment CL-1 made: 0+000.00 to 1+226.07., with the number of design problems if there are any.

If no polyline is selected, the dialog says so at the top. Select one and open it again, or click Click PIs… to draw the alignment by clicking (it closes the dialog and starts CREATEALIGNMENT, as in Drawing the centerline by clicking PIs).

With the command

1. Start CREATEALIGNMENT and set the name, class, speed or terrain if you want.
2. Choose Polyline (type P, Enter).
3. Click the polyline (a line works too).

Each polyline vertex becomes a PI. Arcs in the polyline stay as curves; every corner without an arc gets the command's default curve and spirals, and Terrace says how many it added (Curves of R = 160 m added at 1 corner). A closed polyline gives a loop that ends where it starts. The polyline itself stays in the drawing; erase it if you no longer need it.

Alignment Properties

Select an alignment (click it on the plan, or click its name in the Project browser) and the Properties panel shows it. At the top are its status (Up to date or Has problems), the buttons Zoom to, Rebuild and Report…, and the list of problems, if any. Every change you make here is one undo step, and everything built on the alignment follows at once.

The radius and spirals of each curve aren't in Properties: change them with the curve's grip (next section).

Editing with grips, ADDPI and DELETEPI

Grips

A selected alignment shows two kinds of grips: one at each PI, and one at the middle of each curve, just inside the PI. Dragging them is the quickest way to try an idea, because the alignment, and everything built on it, is solved again as you move.

1. Select the alignment.
2. Click a grip. The command line says which one (** PI 2 ** or ** CURVE RADIUS AT PI 2 **).
3. Move the cursor. The alignment is redrawn live, with its new curve data.
4. Click to put the grip down. The change is one undo step (Grip Edit).

The PI grip moves the PI. The radius grip sets the radius from how far you pull the curve's middle from the PI: towards the PI makes it sharper, away makes it flatter. Instead of clicking, you can type the radius and press Enter.

Adding and deleting PIs

1. Click Add PI (ADDPI) or Delete PI (DELETEPI).
2. Pick the alignment: click it, type its name, or press Enter for the one shown in angle brackets (the selected or last one).
3. For Add PI, click where the new PI goes: the preview shows the alignment with the PI inserted between its nearest neighbours. For Delete PI, click near the PI to remove.
4. Keep clicking to add or delete more. Press Enter to stop.

Each click is its own undo step, and Terrace writes how many PIs the alignment has now. An alignment always keeps its two end points. Offset alignments and curb returns don't have PIs of their own: they follow their parent roads.

Stations and station equations

An alignment's stations start at its Start station (Properties), 0+000.00 by default. A road that continues an existing one usually starts at the kilometre post where you join it, like the sample's 35+200.

A station equation is a break in the stationing: from a point on, the stations jump to another value. You need one when a realigned road must keep the stations of the old road beyond the change, or when it meets a kilometre post.

1. Click Station Equation (STATIONEQUATION, STAEQ) and pick the alignment.
2. Give the back station: type it (0+500) or click the point on the alignment.
3. Type the ahead station, the value the stations take from there (1+000).
4. Choose whether stations ahead are Increasing (the default) or Decreasing.

Labels, reports, profiles and sections all use the equated stations.

Offset alignments

Offset alignments are lines a fixed distance from a centerline: edges of pavement, shoulders, right-of-way lines. They follow their parent: move a PI of the centerline and the edges move with it.

1. Click Offset Alignment (OFFSETALIGNMENT, OALIGN) and pick the alignment.
2. Type a distance: positive for the right side, negative for the left. Or choose Both and type one distance for both sides. Pressing Enter takes the default: the road class's lane width on both sides (3.35 m for National Secondary).

The new alignments are named after the parent, side and distance (CL-1 L 3.35 and CL-1 R 3.35), drawn on layer C-ROAD-EDGE, and take the parent's speed, class and stations. To change the distance later, select the offset and edit Offset in Properties.

Curb returns

Where two roads meet, the edges are joined by a curb return, a circular arc in the corner. Terrace builds it as an alignment of its own, tied to both roads.

1. Click Curb Return (CURBRETURN, CRETURN).
2. Pick the first road: a centerline or, usually, an edge. Then pick the second road. They must cross; if they don't, Terrace says so and stops.
3. Type the radius, or press Enter for the default (10 m, or the last radius you used).
4. Move the cursor into the corner you want: the return is previewed in whichever of the four corners the cursor is in. Click to make it.

Returns are named CR-1, CR-2… and drawn on C-ROAD-EDGE. To build a whole intersection, make the edges of both roads and add a return in each corner.

Superelevation

Superelevation is the tilt of the road across a curve, so that part of the sideways force is carried by the slope. On a tangent the road has its normal crown (falling both ways from the centerline); approaching a curve it rotates until it slopes fully towards the inside of the curve, then rotates back after.

How Terrace computes it

Superelevation is worked out for every curve of an alignment that has it turned on (new alignments do), with nothing for you to enter:

• The rate comes from the standard's superelevation table for the alignment's e max and design speed, at the curve's radius.
• The runoff (from level crown to full super) is long enough to keep the edge within the standard's maximum relative gradient for the speed, for the lanes rotated and lane width. The runout (from normal crown to level crown) is at the same rate of rotation.
• On curves with spirals, the runoff is the whole spiral: level crown at the TS, full super at the SC.
• On simple curves, two thirds of the runoff is on the tangent and one third on the curve.
• The section turns as a plane about the Pivot: the outside lane goes from −crown to level, to +crown (reverse crown), to the full rate; the inside lane stays at −crown until reverse crown and then follows.
• Between two curves of the same direction (compound curves) the rate changes gradually, not in a step.
• The pavement widening on each curve comes from the standard (DPWH DGCS Table 3-22, at least 0.60 m).

Where to see and change it

Change the inputs in the alignment's Properties, group Superelevation: turn it on or off, choose the Maximum rate, and set the Normal crown, Lanes rotated, Lane width and Pivot. The corridor's lanes tilt by these values, so cross sections and volumes follow at once. The design speed also matters: a faster road needs more superelevation and longer runoff.

To see the stations, open the Superelevation report. Spiral lengths shorter than the runoff, and tangents too short for the transitions between two curves, are reported as problems.

Design checks and the problems list

Every time an alignment changes, Terrace checks it against the standard at its design speed and e max. The checks are:

• a curve radius below the minimum for the speed and e max;
• a spiral shorter than the standard's minimum, or shorter than the superelevation runoff;
• a tangent between two curves too short for their superelevation transitions;
• a curve that doesn't fit between its neighbours (the tangents overlap): it's drawn as an angle point;
• spirals that turn more than the PI deflects (spirals too long for a small turn): the PI gets no curve.

Problems show up in four places:

• the command line, right after the command that made or changed the alignment;
• the Project browser, with a red dot and a ⚠ 2 count beside the alignment;
• the top of the alignment's Properties, as plain sentences with the station and a Zoom to link that zooms the plan to the place;
• the Next step panel, as Fix 2 problems: it selects the first object with problems and zooms to the first one.

Problems never stop you from working: the alignment is still built and everything follows it. Fix them by changing the radius, the spirals or the design speed, or by moving PIs.

Alignment reports

Four reports are about alignments. Open them with Report on the contextual Alignment tab, Report… in Properties, or the object's menu in the Project browser. The Reports dialog opens on Alignment geometry for the selected alignment; pick another report in the list on the left and another alignment at the top.

Every report can be saved with Save CSV… (for a spreadsheet) or Save PDF… (to print), or copied with Copy. The Reports chapter covers the dialog in full.

What's next

With the centerline drawn, the next step is its profile: sample the ground along it and design the grade line. See Profiles.