Transverse Beams Under Deck [CSB]

A transverse beam under deck is a steel member placed across the bridge directly under the concrete deck. It runs from deck edge to deck edge and bears on the girders. Each object is one beam. The girders of the deck are lowered so that the beams fit between the deck soffit and the girder top flanges. The beam length, elevation, cross slope and skew all follow the deck, so the beam updates with the deck geometry.

Transverse beams (blue) running from deck edge to deck edge over four steel plate girders, with the deck drawn transparent

Creating many beams at once

To place beams at a regular spacing along a deck, use the generative object Transverse Beams Under Deck (the Transverse Beams Under Deck Generation item of the workflow, or magic icon → Generative: Modeling). You enter a deck, a section, a spacing and a start/end location, and it creates one beam object per position. It can skip positions that fall on floor beams, it always skips floor beam joint zones, and it keeps the first and last beam at least half a flange width inside the deck ends.

How the beam sits between the deck and the girders

• Beam top. The top of the beam sits under the deck soffit at the largest haunch among the girders it crosses.
• Haunch fillets. Where a girder has a smaller haunch, a haunch fillet fills the gap between the beam top and the deck.
• Girder lowering. The girders of the deck are lowered along their length by the haunch plus the depth of the deepest beam on the deck, so every beam bears on the girders. A girder under a deck with transverse beams must therefore belong to that deck only; if it is also listed in another deck, the deck reports an issue on its Girders.
• Shallower sections. A beam with a shallower section than the deepest one gets a filler down to the girder top flange.
• Tub girders. On a tub girder the beam bears on both top flanges, and fillers and fillets are placed at each flange.
• Weights. The steel fillers are applied by Transverse Beam Construction when its Filler Load is set to Include, at the stage the beams are erected. The concrete is applied with the deck pour by the Haunch Load option of the deck construction object. Under each girder the deck soffit is flat at the haunch depth, so the haunch load along the girder includes the concrete over the top flange. Each haunch fillet is applied as a point load at its beam crossing by the one pour that contains the center of the fillet, also at a pour joint; if that pour has Haunch Load off, the fillet is not loaded. A Deck Deconstruction removes these concrete loads together with the deck: the concrete over the top flange and every fillet inside the deconstructed area.
• Shear studs. Shear studs on the girders are not removed or changed where a transverse beam crosses the girder, the same as at a bolted field splice. Under a transverse beam deck the girder sits lower than the deck, so the studs do not reach it. Take this into account when checking the connection.
• Finite element model. The beam is a line element through the deck mesh at the deck mid-plane, offset to the section centroid. It shares nodes with the deck and with the girders at each crossing. With the grid mesh (the deck's default Mesh Algorithm), the row of deck nodes that carries the beam runs across the full deck width, also where the beam itself is inset or trimmed.
• Staged analysis. The beam is included only from the stage at which a Transverse Beam Construction object constructs it.

Seen from below: the beams bear on the top flange of the lowered exterior girder, directly under the deck soffit

A transverse beam across two steel tub girders, bearing on the top flanges of both tubs (deck drawn transparent)

General

Deck: The concrete deck the beam is placed under, selected from the model. A value is required. The object reports an issue in these cases:

• The beam crosses or overlaps another transverse beam on the same deck.
• The deck is a concrete U girder deck or a precast I girder deck with panels, which are not supported.
• A girder of the deck is not a steel plate girder, a steel tub girder or a basic-variation custom shape girder.
• A tub girder of the deck has a top flange spacing that varies along the girder.

Beam Section: Cross section of the beam, selected from the sections defined under Properties. Any section shape can be used; the beam is positioned so that the top of the section sits under the deck. A value is required. The object reports an issue if the beam does not cross any girder, for example when it is placed beyond the girder ends.

Location Method [Deck Relative (0-1)/Station]: How the position of the beam along the deck is entered. Deck Relative (0-1) places the beam by a fraction of the deck length. Station places it at a station on the bridge alignment.

Location (Deck Relative 0-1): Position of the beam as a fraction of the deck length: 0 is the deck start and 1 is the deck end. Only editable when Location Method is Deck Relative. Must be between 0 and 1.

Station: Station of the beam centerline on the bridge alignment, or Start / End for a beam at the deck start or end. Only editable when Location Method is Station. A typed station must be at least half a flange width inside the deck; at a skewed deck end this distance is measured along the station, so it grows to half a flange width divided by the cosine of the end skew.

Resolved Station: The station the beam is actually placed at, after the adjustments near the deck ends (Read-only).

The active location input reports an issue when the beam crosses a girder inside the joint zone of a floor beam, where the girder is not tied to the deck. With the grid mesh it also reports an issue when the beam's finite element row crosses the row of another transverse beam, or a support, pour joint or section change row, inside the deck: the deck mesh cannot follow two rows that cross. Give the beams the same skew, make the beam parallel to that row, or move it so the rows meet outside the deck.

Tub girders

On a tub girder the beam is tied to the girder along its own skewed line. It cannot be tied where that line crosses an existing finite element row of the girder inside the width of the tub, such as a skewed support line, a pour joint, a section change or the line of another transverse beam, or where it runs past the girder end. In these cases the object reports an issue on its location. Place the beam clear of that row, parallel to it, or exactly on it. With the grid mesh the beam's row runs across the full deck width, so this also applies to a tub girder the beam itself does not reach: the row is tied to that tub as well. With another Mesh Algorithm, a beam that does not reach a tub girder is not tied to it.

Skew Method [From Deck/User Defined]: How the skew of the beam is set.

• From Deck takes the skew from the support lines that the girders of the deck sit on. Between two such support lines the skew changes linearly with station, so a beam in a span between a square and a skewed support gets an intermediate skew.
• User Defined uses the Skew Angle below.

Skew Angle: Skew of the beam, measured from the direction perpendicular to the alignment with the same sign convention as a support line skew. Only editable when Skew Method is User Defined.

Beams near the deck ends

The beam is kept on the deck. With Location Method Deck Relative, or with Station set to Start or End, a beam whose flange would extend past a skewed deck end is moved inward by at most one flange width. If it would still protrude by more than about 1 in, it is cut along the deck end line. A typed station is never moved: it reports an issue when it is closer to the deck end than Station allows, and a beam at a typed station that would still protrude at the deck edges is cut along the deck end line in the same way.

Detailing

Inset from Left Deck Edge: Distance by which the left end of the beam is held back from the left edge of the deck, measured across the bridge. With zero the beam runs to the deck edge. Where the deck edge follows a custom edge alignment defined on the Cross-Section, the beam end follows that edge. The object reports an issue if the two insets leave no beam length.

Inset from Right Deck Edge: Distance by which the right end of the beam is held back from the right edge of the deck, measured the same way.

Insets in the finite element model

The beam's finite element line ends at an existing deck mesh node; it does not add a node of its own at the inset end. An inset end that falls between two longitudinal deck grid lines therefore moves to the nearest one, which can be a girder line or the deck edge. On a truss bridge deck, a smaller Trans. Mesh Size brings the element end closer to the inset.

Girder Seating [At Girder Axis/At Low Flange Edge]: Where the beam bears on the girder top flange when the deck has cross slope.

• At Girder Axis: the girders are lowered so that the beam bottom meets the flange at the girder centerline.
• At Low Flange Edge: the girders are lowered further by the cross-slope difference across the top flange, so the beam bears on the lower flange edge. A girder filler fills the gap on the higher side.

All transverse beams on a deck must use the same girder seating; the object reports an issue otherwise. At Low Flange Edge applies to steel plate girders, whose top flange stays level. On tub girders and custom shape girders the top flange follows the cross slope, so both options give the same result.

Quantities

Min Section Area: Smallest cross-sectional area of the beam (Read-only). It is calculated from the 3D geometry and may be slow to recompute.

Max Section Area: Largest cross-sectional area of the beam (Read-only).

Total Surface Area: Total surface area of the beam (Read-only).

Min Cross Section Perimeter: Smallest cross-section perimeter of the beam (Read-only).

Max Cross Section Perimeter: Largest cross-section perimeter of the beam (Read-only).

Volume: Volume of the beam (Read-only).

Min Length Path: Length of the beam measured along its shortest path (Read-only).

Length @COG Path: Length of the beam measured along the path through the section centroid (Read-only).

Max Length Path: Length of the beam measured along its longest path (Read-only).

Weight: Weight of the beam, from its volume and the density of the section material (Read-only).

Material: Material of the beam section (Read-only).

Haunch Fillet Volume: Total volume of the haunch fillets between the beam top and the deck, over girders whose haunch is smaller than the largest one the beam crosses (Read-only).

Haunch Fillet Weight: Weight of the haunch fillets, using the density of the deck concrete (Read-only).

Girder Filler Volume: Total volume of the girder fillers between the beam bottom and the girder top flanges (Read-only).

Girder Filler Weight: Weight of the girder fillers, using the density of the beam material (Read-only).

IFC

Type: The IFC entity type used when the beam is exported to IFC. The default is IfcBeam.

Predefined Type [BEAM/GIRDER_SEGMENT/DIAPHRAGM/DECK/SUPERSTRUCTURE/USERDEFINED/NOTDEFINED]: The predefined type within the selected IFC entity. The default is BEAM.

Parent [BRIDGE/SUPERSTRUCTURE/DECK/SUBSTRUCTURE/FOUNDATION/APPROACH_STRUCTURE/RETAINING_STRUCTURE]: The element the beam is placed under in the IFC spatial hierarchy. The default is DECK.

Properties: Additional custom IFC properties written for this beam on export.