Beam to Plate – MIDAS Support

Intro

A beam model shows what a girder does as a line. It cannot show shear lag across a wide box, the distortion of a cross-section, or the local effects around a support, and seeing those means a plate model. Rebuilding a bridge deck as plates by hand is days of work: every section drawn out as walls of the right thickness, a mesh laid along every member, nodes merged where members meet, the supports and neighbouring elements reconnected, and every beam load turned into nodal loads without changing its resultant. The Beam to Plate plugin does that from the model you already have. Select the beams in CIVIL NX, check how each section has been read, and the plugin writes the plate mesh with its loads, links, virtual beams and structure groups in one confirmed step, keeps the construction stages working, and can undo it again.

Developed with

MIDAS CIVIL NX 2026 (v1.0.0)

Benefits of this plugin

Any section CIVIL NX gives geometry for — rolled and fabricated steel, plate, box and tub girders with their stiffeners, PSC-I, PSC-TEE, PSC-MID and PSC-Half girders, box girders of one, two or many cells, composite box, tub and general composite sections, combined sections, value sections, catalogue sections and tapered members, each turned into walls with their own thicknesses.
Sections read the way CIVIL NX builds them — the dimension conventions were measured by writing probe sections to a live CIVIL NX and reading back the properties it computed, so nothing here is assumed from a drawing. Box girder joints, the haunches of a multi-cell deck, R-octagons, the radii of rolled angles, the corner radii of hollow sections, and the reference-line distances that lean a plate girder’s webs are all reproduced exactly.
Stiffeners become plates too — longitudinal stiffeners on the webs and flanges of steel and composite girders, as flats, tees or U-ribs, placed and spaced as CIVIL NX places them and counted only where CIVIL NX counts them in the section properties, with an option to include the ones it leaves out. Riveted angles, which the API does not export at all, are recovered from the published properties and checked against a second moment the fit never used.
Transverse stiffeners you define yourself — a CIVIL NX section has nowhere to describe them, so give the outstand, the thickness, the spacing, one face or both, and whether the girder ends carry one. They are spaced along the whole girder rather than along each element, stand on the web’s own mesh nodes so the model stays one connected body, and are kept out of the volume check and the load conversion because they are steel the published section never had.
Two checks on every section — Read compares the outline built from the section data with the area and second moments CIVIL NX publishes, so a misread dimension is caught before anything is written; Plates reports how far the plate model sits from the section, which is idealisation, not error. A section that fails is held back until you accept it.
Beam stiffness matched — where a shape has to be idealised, the plate thicknesses are adjusted smoothly, by the smallest change that will do it, until the mesh has the section’s own area, centroid, Iyy and Izz. Where the geometry cannot carry a condition, the plugin says so.
Loads carried across with the resultant preserved — uniform, trapezoidal, concentrated, moment, pressure, eccentric and projected beam loads, and the nodal loads on the beams’ own nodes, become nodal loads with the same force and the same moment, checked against the reactions CIVIL NX reported for the same loads. Choose where they land: on the section centreline, spread over the whole section, or spread over the deck. A distributed torsion becomes a pair of forces rather than a smear over the section, and a load is never placed on a node that its construction stage has not built yet. Element temperatures follow too.
Virtual beams for reading the forces back — a virtual beam is created for every row of plates, so a converted girder still reports axial force, shear, moment and torsion section by section, and the plate model can be checked against the beam model it came from.
Construction stages keep working — the plates join the beams’ structure groups, the rigid links go into a boundary group for each stage that needs them, and a deck cast later follows the composite section for construction stage data: its plates activate in the stage the section names, at the age it gives, with the material it gives them.
The rest of the model stays connected — supports and elements meeting a converted member are tied in with rigid links, beam end offsets are respected, and a tapered section group tapers smoothly along its whole run rather than restarting at every element.
Clear about what cannot follow — tendons, lanes, end releases, stiffness scale factors and other beam-only data are listed against their elements before you write, because deleting a beam takes them with it. A section plates cannot honestly carry is refused with the reason in plain words: a steel section cased in concrete, whose two materials act as one; two tees with a clear gap between them; a section so thick-walled or so solid that plates would misstate its stiffness.
Safe to try — nothing is written until Write is confirmed, the model is re-read and checked for changes first, new node ids are read back rather than assumed, and Undo removes the plates, nodes, links, virtual beams and converted loads, restores the construction stages it touched, and puts the beams back with their loads.

How to use this plugin?

Open the model in CIVIL NX and start the plugin — the connection details are supplied automatically, and the length unit is read from the model, so every size is entered in the model’s own units.
Choose the beams — select them in CIVIL NX and keep Selected, or pick them in the plugin by section, by structure group or by element id.
Set the mesh and the write options — the mesh size along the member and across each wall, and whether beam stiffness is matched where the shape is idealised, for every section, or never. Under Write, name the structure and boundary groups, choose where beam loads are placed, and whether a virtual beam is created for each converted beam.
Read model & plan — nothing is written. The Overview reports the beams converted, plates, nodes, thicknesses, rigid links, loads moved and virtual beams, with a volume check, a preview of the mesh from four views, and everything that will be left behind.
Review the sections — the Sections tab shows each section as it was read, where its geometry came from, and its Read and Plates checks. Correct a dimension, choose the deck slab material of a composite section, or accept a section that was held back.
Add the transverse stiffeners, if the girder has them — under Advanced, switch them on and give the outstand, the thickness and the spacing, one face of each web or both, and whether each end of the girder carries one. The plan reports how many were built and how far the nearest mesh station moved them.
Write — Write to model confirms the counts and whether the source beams will be deleted, then writes the plates, links, virtual beams and loads, and the Result tab reports what reached the model. Undo the conversion restores the beams and their loads whenever you want the beam model back.

Conclusion

Beam to Plate turns a beam model into a plate model without the days of rebuilding: every section read as CIVIL NX builds it and checked against the properties CIVIL NX computed, the stiffeners meshed with it, the mesh laid and connected, and the loads, links, groups, stages and virtual beams carried across. Convert a girder, a box or a whole deck and compare the plate results with the beams — the reactions match, and the difference in deflection is the shear lag and distortion the beam model could not show — then undo it just as easily, leaving the engineer’s time for interpreting the behaviour rather than rebuilding the model.