Bridge model for SR 418 over Heron Creek, Heron Falls, Tennessee

New steel plate girder bridge with a composite concrete deck carrying SR 418 over Heron Creek, modeled from the edge-of-deck polylines and merged with the fabricator’s girder and bolt models and newly modeled bridge rail into one IFC file for coordination and 3D printing. The existing bridge is modeled from the dimensions in TDOT’s bridge inspection report and checked against 22 site photos. The curbs follow the polylines with no deviation, but the bolt model is missing 40 bolts and the photos show a sidewalk the inspection report doesn’t list.

Edge-of-deck polylines (DXF), plan and profile sheetsConcrete, girder and bolt models (IFC)22 site photos, Sep 17, 2026Sta. 14+20 to 15+40NAD83(2011) Tennessee State Plane (ftUS), NAVD 88IFC 4.3 and DXFPrepared Nov 5, 2026

Summary

Curbs

All 62 vertices fall on the edge-of-deck polylines

The polylines in SR418_HeronCreek_Edge_of_Deck.dxf are flat (Z = 0). Elevations were set from the +0.62% grade and 2.0% superelevation on profile sheet 4 and are within 0.004 ft of the sheet’s elevations, which are given to 0.01 ft. Horizontal deviation is 0.000 ft in both the IFC and the DXF export.

Bolts

40 bolt holes have no bolt at Field Splice 2, Girder G3

The fabricator’s girder model has 1,400 bolt holes and the bolt model 1,360 bolts. All 40 missing bolts are in the half of the web splice toward Abutment 2 at FS2 on G3; the same splice on the other three girders has a bolt in all 80 web holes.

Ask the fabricator to complete the bolt model before the coordination model goes to 3D printing and erection planning.

Existing bridge

16 of 22 photos show a sidewalk the inspection report doesn’t list

The SI&A sheet for Bridge No. 41SR4180005 gives a deck width of 24.0 ft out-to-out. The photos show a 5-ft sidewalk on steel brackets on the upstream side and a pipe beneath it. Horizontal clearance to the new bridge’s curb at midspan is 1.6 ft, not 6.6 ft.

Confirm the sidewalk and the utility with TDOT and the utility owner before the construction staging is set.

The model

Rotate and zoom in 3D, or switch to plan and elevation. Click any part to see which file it came from and how it was checked. The tabs below show the curb check, field splices and bolts, the photos against the existing bridge, and how the models were merged.

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Model

74vertices on the polylines, 0.000 ft horizontal

0 / 0bolts vs. bolt holes in the girder model

16 of 22photos differ from the inspection report

0objects in the coordination model

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Polylines EDGE-DECK-LT and EDGE-DECK-RT have 31 vertices each. The table shows the vertices read back from the IFC export and compared with the DXF, in Tennessee State Plane (US survey feet) and NAVD 88.

Elevation check against the profile sheet

Top of curb from the +0.62% grade and 2.0% superelevation toward the inside of the curve (R = 2,000 ft), compared with the elevations on profile sheet 4.

Four source models were merged into one IFC 4.3 file. The girder and bolt models were moved from the fabricator’s local origin and converted from inches to feet.

IFC structure and georeferencing

Adjustments for 3D printing at 1:120 (1 in. = 10 ft)

The print is filtered by layer from the same IFC file. The coordination model is not changed.
Webs, flanges and splice plates are thickened to at least 0.04 in. in print (about 5 in. at full scale). The 1/2-in. web would otherwise print at 0.004 in.
Bolts are left out, since a 7/8-in. bolt prints at less than 0.01 in. They stay in the coordination model.
Site, creek and the existing bridge are removed, and the abutments sit on a base about 1/8 in. thick.
The print is about 13 by 4 in.

Sources and method

Sources

SR418_HeronCreek_Edge_of_Deck.dxfEdge-of-deck polylines, layers EDGE-DECK-LT and EDGE-DECK-RT, NAD83(2011) Tennessee State Plane (ftUS), no elevation2 × 31 vertices
SR418_Plan_and_Profile_Sheets.pdfRoadway plan and profile, Sta. 13+00 to 17+00: 2,000-ft curve, +0.62% grade, 2.0% superelevation (sheet 4)6 sheets
HeronCreek_Bridge_Concrete.ifcDeck, curbs and abutments from Heron Falls Bridge Engineering, PLLC, IFC4, georeferenced5 objects
HeronCreek_Girders_Tekla.ifcPlate girders, splice plates and cross-frames from Heron Falls Steel Fabricators, Inc., IFC2x3, local origin at Abutment 1, inches1,400 bolt holes
HeronCreek_Bolts.ifc7/8-in. ASTM F3125 Grade A325 bolts as IfcMechanicalFastener, IFC2x3, same local origin as the girder model1,360 bolts
Bridge_Rail_Details.pdfTwo-tube steel bridge rail on curb, MASH TL-3, posts at 6'-0", first post 3'-0" from end of deck2 sheets
Bridge_Inspection_Report_41SR4180005.pdfTDOT bridge inspection report with SI&A sheet for the existing bridge: reinforced concrete slab built in 1956, 40-ft span, deck width 24.0 ft out-to-out, slab depth 1'-9"18 pages
Site_Photos_2026-09-17.zipPhotos of the existing bridge and both bridge sites, 4032 × 3024 px22 images

Method

The polylines were read from the DXF and used as control lines for the outside edge of the curbs. Since they carry no elevation, elevations were set from the grade and superelevation on profile sheet 4.
The deck and curbs were swept along the control lines. The abutments came from the concrete model, which was already georeferenced.
The girder and bolt models were moved from the fabricator’s local origin at Abutment 1 to Tennessee State Plane and NAVD 88, rotated to the bridge alignment and converted from inches to feet. Girder positions were checked against the curbs and abutments.
Bolt holes and bolts were counted per splice and part (flanges and web) and compared.
The bridge rail was modeled from the rail details as IfcRailing with posts on the curbs.
Everything was written to one IFC 4.3 file with IfcBridge, IfcBridgePart and IfcMapConversion, and a DXF was exported from the same model. Vertices were read back from both files and compared with the polylines.
The existing bridge was modeled from the inspection report dimensions and overlaid on each photo from a camera position solved from the image. What the photos show but the report doesn’t was added and tagged with the photo as its source.

Assumptions and limitations

The polylines are taken as the top outside edge of the curbs.
Elevations on the profile sheet are given to 0.01 ft, so differences under 0.005 ft are rounding.
The 5-ft sidewalk width is scaled from the photos against the 1'-9" slab depth in the inspection report, to about ±0.3 ft.
The pipe’s purpose and owner can’t be determined from the photos.
Cross-frame bolts are counted but not drawn.
The print version doesn’t affect the coordination model; it is filtered from the same IFC file.
The bridge rail is modeled as MASH TL-3, as shown in Bridge_Rail_Details.pdf. Whether TL-3 is adequate for traffic on SR 418 has not been checked; the structural engineer of record should confirm it against TDOT’s bridge rail requirements.