Why do engineers love bridges? They're outstanding in their field.
Why do engineers love bridges? They're outstanding in their field.
The simplest bridge form: a flat concrete slab spanning between supports, common for short crossings.
Total in US
82,465
Poor Condition
6.0%
Avg Sufficiency
63.9
Average Age
49 yrs
Slab bridges are the simplest structural form a bridge can take: a flat plate of reinforced or prestressed concrete spanning between two supports. Despite their simplicity, slab bridges are remarkably common, particularly for short spans over small streams, drainage channels, and minor road crossings. Their straightforward design makes them quick and economical to build.
The concrete slab acts as both the structural element and the driving surface, eliminating the need for separate beams and a deck. This gives slab bridges an extremely shallow depth, which is a significant advantage where under-bridge clearance is limited. Many of the small bridges that drivers cross without even noticing are slab bridges.
Slab bridges can be solid or voided (with circular or rectangular voids cast into the slab to reduce weight while maintaining depth and strength). Voided slabs extend the practical span range beyond what a solid slab can achieve. They are a staple of county and municipal bridge inventories across the country.
A slab bridge works like a wide, flat beam. When vehicles drive over it, the slab bends between its supports, with the top surface going into compression and the bottom into tension. Reinforcing steel bars (rebar) embedded near the bottom surface carry the tensile forces, while the concrete handles the compression. The slab distributes concentrated wheel loads across a wide area, which is one of its structural advantages. For longer spans, prestressing is used: high-strength steel strands are stretched before the concrete is cast (pre-tensioned) or after (post-tensioned), creating compressive forces in the concrete that counteract the tensile stresses from bending. Voided slabs use hollow cores to reduce the dead weight while maintaining the structural depth needed for bending resistance.
Ancient Roman road engineers built some of the earliest slab bridges using large flat stones laid across stream beds. Some of these "clapper bridges" still exist in England's Dartmoor region, where granite slabs up to 15 feet long span small rivers.
Numerous county road crossings throughout the US
Tarr Steps (England, ancient stone clapper bridge)
Many Interstate highway drainage crossings
Post Road Bridge types common across New England
| Rank | State | Count | % of Type |
|---|---|---|---|
| 1 | California | 5,968 | 7.2% |
| 2 | Louisiana | 5,877 | 7.1% |
| 3 | Iowa | 5,679 | 6.9% |
| 4 | Kansas | 5,289 | 6.4% |
| 5 | Wisconsin |
There are 82,465 slab bridges in the United States.
Iconic bridges with a deck hung from cables draped over tall towers, capable of spanning enormous distances.
Modern bridges where cables run directly from towers to the deck, creating a dramatic fan or harp pattern.
One of the oldest bridge forms, using a curved structure to transfer loads outward to abutments at each end.
Bridges built from interconnected triangular elements, combining strength and material efficiency.
| 4,921 |
| 6.0% |
| 6 | South Carolina | 4,252 | 5.2% |
| 7 | Texas | 4,159 | 5.0% |
| 8 | North Carolina | 3,939 | 4.8% |
| 9 | Indiana | 3,623 | 4.4% |
| 10 | Ohio | 3,493 | 4.2% |