Creating NURBSpline Curve-Shaped Steel Bars in Revit: A Guide to Secondary Development

In Revit’s API, a Nurbspline curve is represented as a series of straight line segments.

Since Revit only supports converting steel bars that are either straight or curved, a Nurbspline curve cannot be directly converted into steel bars. To work around this, you can first convert the Nurbspline model line into a sequence of continuous straight lines, and then convert those lines into steel bars.

Below is the sample code demonstrating how to convert a Nurbspline model line into continuous straight lines:

using System;
using System.Collections.Generic;
using Autodesk.Revit.UI;
using Autodesk.Revit.DB;
using Autodesk.Revit.UI.Selection;

// Namespace for converting Nurbspline curves to continuous straight lines
{
    [Autodesk.Revit.Attributes.Transaction(Autodesk.Revit.Attributes.TransactionMode.Manual)]
    [Autodesk.Revit.Attributes.Journaling(Autodesk.Revit.Attributes.JournalingMode.NoCommandData)]
    [Autodesk.Revit.Attributes.Regeneration(Autodesk.Revit.Attributes.RegenerationOption.Manual)]
    public class Class1 : IExternalCommand
    {
        public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements)
        {
            Document revitDoc = commandData.Application.ActiveUIDocument.Document;  // Get the active document
            Application revitApp = commandData.Application.Application;            // Get the Revit application
            UIDocument uiDoc = commandData.Application.ActiveUIDocument;            // Get the active UI document

            Selection sel = uiDoc.Selection;
            Reference ref1 = sel.PickObject(ObjectType.Element, “Select Nurb Model Line”);
            Element elem = revitDoc.GetElement(ref1);
            ModelNurbSpline nurbSpline = elem as ModelNurbSpline;

            IList<XYZ> listPoint = nurbSpline.GeometryCurve.Tessellate();

            XYZ normal = new XYZ(0, 0, 1);

            using (Transaction transaction = new Transaction(revitDoc))
            {
                transaction.Start(“Convert Nurbspline to Lines”);
                SketchPlane modelSketch = SketchPlane.Create(revitDoc, revitApp.Create.NewPlane(normal, XYZ.Zero));
                for (int i = 0; i < listPoint.Count – 1; i++)
                {
                    ModelCurve modelCurve = revitDoc.Create.NewModelCurve(Line.CreateBound(listPoint[i], listPoint[i + 1]), modelSketch);
                }
                revitDoc.Delete(nurbSpline.Id);
                transaction.Commit();
            }
            return Result.Succeeded;
        }
    }
}

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