CS454 Auto Lane Generator – MIDAS Support

Intro

Assessment traffic lanes are the part of a CS 454 model that everyone builds by hand and nobody enjoys. The code does not have one set of notional lanes, it has three that coexist in the same assessment: 3.0 m for a single vehicle in each lane, 2.5 m for a convoy, and C/nn for ALL model 2, where nn comes out of Equation 5.18 clamped to a table whose rows below 7.5 m are genuine exceptions to its own rule. Each lane then needs a position measured from something, an eccentricity whose sign depends on which way the element's local axes happen to point, a wheel spacing that MIDAS stores as a third of the value CS 454 actually applies, and a lane item for every element of the line it runs along — a hundred of them on a long deck. Add a special vehicle running with normal traffic and the whole carriageway has to be set out again around it. The CS 454 Auto Lane Generator does the arithmetic and the typing: it reads the model, lets the reference line be set out on the drawing, works out the lanes the loading you pick actually needs, draws them along the deck and across the cross-section with the vehicles standing in them, and only then writes them.

Developed with

MIDAS CIVIL NX 2026 (v2.2) — plugin version 1.0.0

Benefits of this plugin

Three notional lane systems, kept apart — cl 5.8 and Table 5.9a give ALL model 1 a 3.0 m lane for a single vehicle and a 2.5 m lane for a convoy, and cl 5.18 gives ALL model 2 lanes of C/nn. They are generated as separate, separately named sets so each moving load case can pick the one it belongs to, and the cross-section draws them in their own colours one above another, because on a 7.3 m carriageway they give three different answers and none of them is wrong.
Equation 5.18 with its exceptions, not just its rule — Table 5.18's note gives a general rule for carriageways wider than 7.5 m, and the tabulated rows at or below it are not reproduced by that rule. At 4.0 m the rule returns an empty range; at 7.4 m it gives three lanes where the table says two. The table is looked up where it applies and the rule extends it beyond 21.9 m.
The remaining area is conditional, and the condition is stated — cl 5.16 loads the remaining area with 5 kN/m², and cl 5.16.1 permits it to be left unloaded at 7.5 tonnes assessment live loading or below. The lane is then not written at all, and the panel says which clause decided it rather than leaving a missing lane to be noticed.
The special vehicle runs beside the traffic, not through it — the CS 458 combination cases set the vehicle and the associated normal traffic out side by side as one row across the carriageway, because they act at the same moment. Appendix C3 puts normal traffic in all areas where vehicles would not be displaced, so the vehicle takes its width out of the carriageway and the notional lanes fill what is left beside it.
The partial factors that make the combination worth assessing — Table A.1 carries associated normal traffic at 1.30 rather than 1.50 at ULS combination 1 and at 1.00 rather than 1.20 at SLS. Those numbers are reported beside the lanes, because reading them off the ordinary traffic row is the easy mistake and nothing in the lane geometry would reveal it.
The wheel spacing MIDAS stores is not the spacing CS 454 applies — Appendix B puts every axle's two wheel loads 1.8 m apart, and under the BS moving load code the lane's stored value is a third of the real spacing, so 0.6 is written. Everything on the panel and in the drawings is the real 1.8 m and the preview table shows both numbers, so the first lane can be checked against the MIDAS dialog.
Table 5.9a is self-consistent, and the plugin says so when it stops being — lane width minus the Appendix B spacing is exactly the table's own minimum lateral spacing between adjacent vehicles, 3.0 − 1.8 = 1.2 and 2.5 − 1.8 = 0.7. Override the spacing upward and two vehicles centred in adjacent lanes breach cl 5.13; the warning carries the arithmetic rather than just the objection.
Every lane is drawn before it is written — the lanes appear along the deck in plan and across the cross-section at any chainage, with the kerb faces, the lane extents and each lane's width dimensioned on the drawing rather than left to be scaled off it. A dashed outline is the allowable width, the travel MIDAS may give that lane.
The vehicle is drawn standing in its lane — the 0.3 m contact patches of Appendix B are drawn to scale across the section, on the same ruler as the lane they sit in. Lanes carrying a distributed load — the remaining area, the ALL model 2 UDL and KEL, the footways — are drawn empty, because they have no wheels. A special vehicle is drawn as bars rather than patches, because CS 458 holds its axle geometry and this plugin does not.
Footway lanes are refused where the code refuses them — cl 5.30 rules the pedestrian ALL model invalid past 36 m of loaded length with crowds expected and past 400 m outright. Where it applies the footway lane is not generated, with the clause given, instead of a lane being made that should not be loaded.
Accidental vehicle loading follows the barrier, not the verge — cl 5.27 asks for it on members supporting a central reserve, verge or footway that no effective barrier protects, and cl 5.27.1 counts only higher and very high containment systems to CD 377 as effective. The width and the barrier question are asked together, because a verge entered without one is the way a member quietly escapes the check.
The reference line is built, not assumed — every lane eccentricity is measured from a line running along the deck, and a curved or skewed deck often has no single member to use. Pick the nodes the line passes through and the plugin either adopts the beams already running between them or creates a chain of 10 × 10 mm reference beams along a straight or circular alignment, with a node wherever the line crosses the existing structure.
More than one lane set on one deck — a second carriageway, or the same carriageway set out from a different reference line. Running again under the same name rewrites that set in place, which is what you want while settling a layout; a new set takes a name of its own and leaves the first untouched.
Skew where MIDAS accepts it and nowhere else — the skew angles are available only with cross beam distribution, exactly as the MIDAS dialog allows, because a skew is the angle of the lane's ends against the transverse members the load is spread onto. An angle already typed is kept and returns if the distribution is switched back.
A measuring tool on the drawing — click two nodes in plan, or two girder marks on the cross-section, and the carriageway width and kerb offsets can be read straight off the deck instead of being transcribed from the MIDAS window.
Nothing is written until you ask for it — the reference line, the lanes and the effect of every setting are drawn and listed first, and the panel states before you press the button whether Create will add lanes or replace ones you already made.
Removal is by table key, never by name — a name-filtered delete on this API ignores the filter and empties the whole table, so every removal is planned from a fresh read and asks for the exact keys of the lanes concerned.

How to use this plugin?

Open the bridge model in CIVIL NX and start the plugin — the connection details are supplied automatically; the Validation Check card reports the base URI and the MAPI key if either of them fails.
Read the model — on the Model tab press Read model. Nodes, elements, structure groups, sections, the traffic lanes already defined, the units and the moving load code are all read in one pass. CS 454's vehicles live under the BS branch of the Vehicles dialog, so a model with no moving load code is set to BS as part of the lane write; a model already set to another code is never switched without saying what will be lost.
Choose the reference line — on Reference line pick the chain the lane eccentricities will be measured from, either from the suggested lines, by typing an element range, or by clicking the nodes the line runs through. The panel says whether beams already run between the picks or whether new reference beams will be created, reports the minimum radius, and draws the line on the deck before anything is made.
Choose what the structure has to carry — on Lanes tick the ALL model 1 situations, ALL model 2, the single axle load, footways, accidental loading, a CS 458 special vehicle, an HB vehicle, or the special vehicle combinations. The panel asks only for the numbers the ticks you made actually use, so a footway-only check never asks for a carriageway and an ALL model assessment is not asked for verge widths.

Describe the cross-section — enter the carriageway width as CS 454 defines it, between kerbs, raised paving or barriers and including any hard shoulder, then the offset from the reference line to the left-hand kerb. This is not the EN 1991-2 width, and a Eurocode figure carried across is too small, so the hard shoulder is asked for separately and the arithmetic is shown.
Set the assessment live loading level — it decides whether a remaining-area lane exists at all under cl 5.16 and cl 5.16.1, and the panel says which way it has gone before anything is written.
Position the special vehicle where the combinations are used — the vehicle's width comes from CS 458, per vehicle, and is the one number the plugin cannot supply, so it is asked for and flagged every run. Its position in the row and the kerb the lanes pack against are the two controls that change the arrangement, because nothing in a combination slides.
Check the drawings — the cross-section shows the kerb faces, every lane with its width, and the load model standing in each lane with its wheels to scale. Lanes are coloured by set, so the 3.0 m, 2.5 m and C/nn systems can be read against one another and against the girders. The plan shows the same lanes running along the deck, and hovering a row of the table dimensions that lane on the drawing.

Choose how the load reaches the deck — lane element distribution applies the traffic load to the elements the lane runs along; cross beam distribution spreads it through a structure group of transverse members, which is what a grillage wants and what a reference line built from the plugin's own beams needs, since those meet the structure at only a few nodes.
Create the lanes — the whole set is written in one request, the model is re-read, and every lane is checked to have come back, because a silently dropped entry is how this API reports a field it does not accept. Running again under the same prefix rewrites that set; New set takes a fresh prefix and adds another alongside it.

Conclusion

The CS 454 Auto Lane Generator removes the slow, mechanical part of preparing a deck for assessment: working out what Table 5.9a and Equation 5.18 give a particular carriageway, keeping three lane systems apart, setting a special vehicle out beside the traffic it runs with, getting the sign of every eccentricity right, and typing a lane item for every element of the line. What it does not do is decide the engineering. The carriageway width, where the reference line runs, the assessment live loading level, which loading the structure has to carry and how that load reaches the deck are all yours to state — and every one of them is drawn, dimensioned and listed before anything is written to the model. Where CS 454 hands a question to another document, as cl 5.5.1 hands special vehicles to CS 458, the plugin says so rather than answering it quietly.