Stahl gegen Beton Pt. 2 [OC]

[OC] Using the 2025 U.S. National Bridge Inventory, I mapped the proportion of different bridge structural materials at the county level across the continental U.S. Each map shows the percentage of bridges within a county that are constructed using a specific material type.

I processed the dataset in R, calculated material proportions by county, and visualized the results using Datawrapper.

There are 12 maps total:

**All Concrete Bridges** (includes types 1, 2, 5, 6)

**All Steel Bridges** (includes types 3 and 4)

One map for each of the 10 distinct structure materials.

Each bridge is assigned a material/design code in the datafile. Here’s the mapping:

0 – Other
1 – Concrete
2 – Concrete, Continuous
3 – Steel
4 – Steel, Continuous
5 – Prestressed Concrete
6 – Prestressed Concrete, Continuous
7 – Timber
8 – Masonry
9 – Aluminum, Wrought Iron or Cast Iron

**What Qualifies as a Bridge in the NBI?**

A structure is classified as a bridge and included in the NBI if it meets both of the following criteria:

Function: It must carry traffic (vehicular or pedestrian), or span over traffic routes (like roads, railways, or navigable waterways). Snowmobile paths are generally not counted unless maintained year-round.

Size: It must have a total span length greater than 20 feet (6.1 meters) over the feature it crosses.

This means from abutment to abutment or bearing to bearing.

Structures shorter than 20 feet may still exist in datasets, but they are not subject to federal inspection requirements or NBI reporting unless part of a grouped structure.