Bridges

A bridge carries a road or a railway over something. In yucad3d it is a girder bridge laid along an alignment: a deck slab on girders, standing on abutments at its two ends and piers in between. The road or track on it stays what it already is — an alignment, usually with a corridor — and the bridge is built under it.

Creating one

Bridge › Create Bridge Along Alignment lays a bridge on the selected alignment. Give it a from and a to station — typed the way the drawing shows chainage, 0+120 or 120 — and it spans exactly that stretch. Leave them empty and it runs from the alignment’s start to its end. A station that is not on the alignment is refused, and the message gives the alignment’s range; a word where a station should be is refused too, rather than read as 0+000. Where the alignment carries station equations, the stations you type are its displayed chainage.

The number of spans places the supports evenly between the two ends. The first and last supports are the abutments; every one in between is a pier.

Create Bridge Around Corridor is the other way round: select a corridor and the deck is sized from the sections that corridor actually builds, over the same from / to stretch of its alignment.

Girders

A girder is picked by name from a girder catalogue, and its shape comes from there: the catalogue row gives the dimensions its source prints, and the bridge draws, exports and measures that outline. The shipped catalogue holds the AASHTO Types II–VI and the 63 in and 72 in modified bulb tees, dimensioned from the NCDOT Structure Design Manual (Figures 6-66 and 6-67); a built-in Generic catalogue adds three plain rectangles for a T-beam stem or any bridge with no standard at hand. Choose the girder under Girders in the Properties dock, or name it when you create the bridge.

Every shipped row carries the area its source prints beside the drawing, and a row whose dimensions do not give that area is refused when the catalogue is read — a mistyped dimension is caught the day it is typed. A bridge that names a girder no catalogue has builds no girders and says so in its summary; nothing is drawn in its place.

Catalogues are files in catalogs/bridge/ beside the program, in the same family as the turnout catalogues and the clearance gauges. The girder’s depth is the catalogue section’s own and is shown read-only in Properties.

The Girder Catalog window

Bridge › Girder Catalog lists every catalogue and its girders, with each girder’s dimensions, its source, and its outline drawn beside them together with the area and depth it measures. A shipped or built-in catalogue is read only: Copy to Edit makes your own copy (named …-user) in the catalogue folder, and in that copy you can Add, Duplicate and Delete girders, change a girder’s family, type its dimensions, or give a free polygon point by point. A girder in your copy wins over a shipped one of the same name, so your edits survive the next update of the shipped file.

Every value is checked as you type it: a word where a number belongs is refused and the cell goes back to what it was, and a set of dimensions that does not describe a section says why instead of drawing. Changing a dimension of a row that carried its source’s printed area drops that area and marks the source “edited from” — the row is no longer the one the source printed. Save writes the file, reads it back and registers what it read, and the open bridges redraw with it.

The overhang is the distance from the deck edge to the centre line of an exterior girder. The exterior girders stand that far in from the edges and the rest share the width between them equally.

The deck section

Under Deck and Deck furniture in Properties, the same on both sides:

• Haunch — the concrete between the slab and each girder, as wide as the girder’s top flange and cast with the slab. The girders stand a haunch lower.
• Parapet — the edge beam on the slab at each deck edge (width and height).
• Footway — a raised walk inside each parapet (width and height; width 0 for none).
• Railing — on the parapet, its top a given height above the walking surface beside it (the footway where there is one). Left at −1, the height comes from the project’s bridge standard: the Swiss set reads ASTRA 22001’s 1.10 m. With no standard to give one, no railing is built and the summary says so; Railing height used shows what the bridge will actually build. 0 means no railing.

Each piece stands plumb on the tilted surface below it, so the section’s concrete is width times height, piece by piece: a 12 m deck of 0.25 m slab on five AASHTO Type IV with 50 mm haunches, 0.5 × 0.35 parapets and 1.5 × 0.2 footways is 3.000 + 0.127 + 0.350 + 0.600 = 4.077 m², and that is what the program measures.

Diaphragms (under Girders) stand between each pair of adjacent girders at every support, and as many more inside each span as you ask for. Each fills the bay between the girders’ widest parts, so it never overlaps a girder.

A bridge around a corridor takes its width from the carriageway the corridor builds — its farthest edge of travelled way or paved shoulder (codes ETW / EPS) — plus the margin, not from the whole section: on a high fill the slopes would otherwise make the deck as wide as the embankment. A corridor with neither code is measured whole, and the log says so.

Box girders

Under Superstructure in Properties a bridge is either Girders — a slab on catalogue girders, as above — or a box girder: Bridge Box Girder 1 (a single solid box under the deck) or Bridge Box Girder 2 (an outer box with two hollow cells). These are the same two sections the Composer offers as subassemblies of those names, and not a copy of them: the section is defined once, and the Composer part and the bridge both read that definition. A corridor built from the part and a box girder bridge on the same alignment are the same section — checked to a micron at every station and half way between them, on a grade, across a tangent point and through a spiral.

The deck width and the cross-fall drive the box’s widths and slopes; its own dimensions appear under Box girder in Properties by the part’s parameter names (SoffitDepth, DimensionC, …), each defaulting to the part’s. A box has no catalogue girders, haunches or girder diaphragms. Parapets, footways and the railing stand on it as on a slab (the part’s own barriers are the Composer’s; the bridge draws its own).

The box bears on two lines, each bearing’s outer face on a corner of the soffit; the seat table, the pier cap and the clearance check all read the box’s underside. A box of the default Bridge Box Girder 2 on a 10.68 m deck at −2 % is 4.751 m² of concrete once its two cells are taken out, and that is what the take-off, the IFC and the STEP solid hold. Dimensions that do not make a section — a centre web wider than the box, a cell through its wall — build no box, and the summary says why.

Stations and supports

Everything about where a bridge is, is measured in stations — the host alignment’s own chainage, which is horizontal distance along it. A pier placed at 0+100 is at 0+100 in the plan, in the profile, in the 3D view, in the IFC file, in the STEP solids and in the rebar cage; there is one answer to where it stands, and every output reads it.

The support stations are in the Properties dock under Spans, as a list: 0+000, 0+120, 0+300. Type a new list to move, add or remove supports. The list must have at least two stations and be in increasing order; one that is not is refused with the reason, and nothing changes. From station and To station under Limits move the ends of the bridge the same way.

The supports are sampled as well as the regular step, so every support sits on a point taken straight from the alignment and its profile: the level at a pier inside a vertical curve is the curve’s own level, not an average of two samples either side of it.

Skew and curves

Support skews (°) under Spans gives each support an angle from the normal to the alignment, in the same order as the stations: 30, 0, 0. A skewed support line is the set of points at station s + u·tan θ, offset u: each girder’s bearing sits where the girder crosses that line, and the pier cap, the abutment, the stem and the diaphragms at the support turn with it. The deck ends on the skewed lines of its end supports — between two parallel 30° abutments a 60 m, 12 m deck is a parallelogram of 720 m² — and a girder spans from its own first bearing to its own last, so with one skewed abutment neighbouring girders differ in length by the spacing times tan θ.

Girder layout under Girders says how a girder runs on a curve: Curved follows the alignment (a steel girder is bent to it); Chorded is straight from bearing to bearing, span by span (a precast girder is straight), so it cuts inside the arc by R(1 − cos φ/2) — 0.375 m for a 30 m span on R = 300 m. The summary gives the girders’ lengths.

Staying on the alignment

A bridge records the alignment it is laid along and how it was sampled. Move the alignment, edit its profile, or change the bridge’s limits or its supports, and the bridge re-samples itself onto the host: by default it keeps itself current, as a tunnel does. Switched off for an object, the bridge is marked out of date instead, and Resync Bridges puts every out-of-date bridge back on its host. Either way the rebuild is one undo step and the bridge keeps its identity.

What is re-derived is the centre line only. The supports stay at their stations, and the deck, the girders and the piers stay the size you made them — those are design decisions, and a refresh that re-derived them would redesign the bridge every time the road under it moved. A bridge built by hand, with no host, reads nothing outside itself and is never flagged.

Sampling records the alignment’s own direction at every point as well as its position, and between two points the bridge follows the curve the way the alignment turns — straight, circular or spiral — rather than the chord. So a section anywhere on the bridge, a skewed pier’s bearings at their own stations included, is the alignment’s section there: a corridor built on the same alignment and the bridge agree to a micron, across a tangent point and through a spiral. (Before, the bridge estimated its direction from its sampled points, which turned its section by about a centimetre at the edge of an 11 m deck where a straight met a curve, and it cut the chord inside a curve between samples.) A bridge built by hand, with no alignment to ask, still estimates its direction from its points.

What stands on the ground

Under Substructure in Properties, the same for every pier and for both abutments:

• Pier kind — single column; wall (one wall, the cap’s length, up to the seat); multi-column bent (Columns of them under the cap, the outer ones a column’s width in from its ends); hammerhead (a cap that tapers from the column’s faces to cap end depth at its ends); pile bent (piles run up into the cap, no column).
• The cap is as long as the bearings need: it reaches the outermost bearing’s centre and cap past outer bearing beyond it — with bearings at ±4.5 m and 0.6 m, a 10.2 m cap.
• Abutment kind — seat: a wall up to the seat, a backwall behind the girder ends up to the deck, and a wingwall back from each end; integral: the girders are cast into an end wall that reaches the slab, so there are no bearings there.
• Foundation — none, a spread footing (footing width along the axis, footing margin past the column or wall along the support, footing thickness), or a pile cap on a group of piles (piles across × piles along, their diameter, length and spacing).

Each support sits where the bearings put it: each bearing under its own girder, whose underside follows the deck’s cross-fall, and the seat under the lowest of them. Sections are vertical, as in the Sections tab, so a pier stands plumb however steep the grade.

A foundation’s level is taken where the ground is lowest under it: its top is the soil cover below the lowest of its footprint’s corners, so a footing on a slope is covered on its low side too. The piers and abutments stand on it. With no foundation they stand on the lowest ground under their own footprint.

If there is no ground surface under a support, nothing is built there — no pier, no abutment, no bearing, in any output. A pier of made-up height is a quantity nobody can build, so the bridge does not draw one. The plan still marks the support’s station, in red; the Properties summary lists every support that was not built and why; and the commands that export or reinforce the substructure say which ones they left out. The same happens where the ground is at or above the seat, with nothing to stand on.

In IFC a spread footing is an IfcFooting (PAD_FOOTING), a pile cap an IfcFooting (PILE_CAP), each pile an IfcPile, and the backwall and wingwalls are IfcWalls.

Bearings and levels

A bearing is a Part Builder family: Elastomeric, Pot, Spherical or Guided Sliding, under Bridge / Bearings in the Part Builder, where an office can edit them or make its own. Every one is sized by the same three numbers under Bearings in Properties — width across the support line, length along the axis, height — so any of them fits the same place. A bridge that names a family the Part Builder does not have draws a plain block of that size, and its summary says so. In IFC each bearing is its own IfcBearing with its type (ELASTOMERIC, POT, SPHERICAL, GUIDE). The bearing families never appear among a utility network’s structures.

Each bearing sits under its own girder, whose underside follows the cross-fall, so the seats are at different levels. The cap’s top is flat at the lowest seat and every other bearing stands on a pedestal up to its own: on a 2.5 % cross-fall with girders 3 m apart, pedestals of 0.075 and 0.150 m.

Bridge › Seat & Deck Levels lists every bearing’s top, seat and pedestal, and the deck’s level along each girder line at the tenth points of every span, with the girder’s top beside it. Give it a folder and it also writes both tables as CSV, one pair per bridge. On a 5 % grade with a −2.5 % cross-fall, the seat 3 m out at 0+030 is the profile’s 21.500, less 0.075 of cross-fall, 0.25 of slab, 0.05 of haunch, 1.2 of girder and 0.1 of bearing: 19.825.

Quantities

Bridge › Quantities measures the bridge on the geometry the 3D view and the IFC are built from — never by a second arithmetic of its own:

• per span, cut on the support lines (skewed ones too), so the spans add up to the whole: deck slab, haunches, parapets, footways and girders (m³), girder length (m), and the diaphragms inside the span;
• per support: pier stems, pier cap, abutment wall, backwall, wingwalls, footing, pile cap, pedestals and the support’s diaphragms (m³); the substructure’s formwork (m²: vertical faces, and a cap’s soffit); piles (number and length); bearings, by family;
• for the whole bridge: railing (m), expansion joints at seat abutments (m, along the skewed line; an integral abutment has none), and rebar (kg) from the cages under the bridge, by their bending schedules.

A 20 m span of a 10 m deck on a 0.25 m slab is 50 m³ of slab, and the three spans’ slab and haunches add up to exactly what the IFC exports. A skew shears a span, it does not change a section that is symmetric about the axis. Give the command a folder and it writes one CSV per bridge.

Views

• Plan — the centre line, the deck edges, a line across the deck at each support, and the footprint of what stands there: the same boxes the 3D view builds.
• Profile — in the alignment’s stations: the deck top, the soffit, the required-clearance line, and each support from its seat down to the ground.
• Sections — at a station on the bridge, the deck slab tilted by the cross-fall and the girders under it. Off the bridge, nothing is drawn.
• 3D — the deck, the girders, and each support as its own object.

Reinforcement, checks and output

Generate Deck Rebar places top and bottom mats that follow the cross-fall, as the slab does. Generate Pier Rebar puts corner bars and closed links up every pier stem that was built. Both land as rebar cages under the bridge, with a bending schedule.

Soffit Clearance Check measures how close the bridge comes to what it crosses. It walks every soffit line — each girder’s underside and both deck-edge soffits — along the bridge, so on a cross-fall it is the low-side girder that sets the answer, and it measures against:

• every road alignment that passes under the bridge, over its carriageway: the band between its corridor’s ETW / EPS points and the surface of that corridor’s own section, crown and cross slope included (a road with no corridor is measured along its line only, and the report says so). The requirement is the bridge’s Road clearance; left at −1 it comes from the project’s bridge standard — under the Swiss set, ASTRA 22001’s user clearance, resurfacing and load-movement allowances, 4.50 + 0.10 + 0.30 = 4.90 m, the sum the handbook itself makes for signal portals. A standard that gives none states no requirement; the clearance is still reported. The report also gives each road’s nearest pier and how far it stands from the paved edge, or that it stands in it;
• every railway alignment under it, against a clearance gauge (Rail gauge): the requirement is the gauge’s top plus the electrification allowance you give — not included when 0, and the report says so;
• water, when Over water is set: the lowest soffit above the design water level, against the freeboard;
• the ground: a soffit below it is a breach.

Each finding names the feature, the station, the member and the clearance, with the requirement and what it is made of. The Profile draws the required line under the soffit when there is a requirement to draw.

Export Bridge (STEP) writes the deck, girders and substructure as solids to the file you name.

In IFC 4.3 each bridge is its own IfcBridge, named after it, broken down into IfcBridgeParts the way the standard intends: a Superstructure holding the girders and diaphragms and a Deck part with the slab, footways, parapets and railings; a Substructure holding one Pier or Abutment part per support — column, cap, bearings, pedestals, walls — each with its Foundation part for the footing, pile cap and piles. A box girder is one IfcBeam GIRDER_SEGMENT in the superstructure — deck, webs and soffit in one member, its cells open. Every member is its own product of its own class (IfcSlab, IfcBeam BEAM / PIERCAP / DIAPHRAGM, IfcColumn PIERSTEM, IfcWall PARAPET, IfcRailing, IfcBearing by type, IfcFooting, IfcPile), and the volumes a reader measures are the ones the Quantities command reports.

What a bridge is not, yet

The deck furniture is the same on both sides. Every pier is of one kind and both abutments of another; a pier cannot yet be saved as a template for the next bridge. These are the next steps of the bridge work; this page grows as they arrive.