Why Bridges Have Small Gaps in the Road

Twelve feet. That's how much lower the roadway of New York's Verrazzano-Narrows Bridge sits in summer than it does in winter, according to the Metropolitan Transportation Authority, which operates the span. Heat makes the bridge's steel cables expand and sag; cold makes them contract and pull the deck back up. Nobody driving across notices the shift happening in real time, because it takes months, not minutes. But the bridge is quietly breathing in and out all year, and the narrow metal gaps built into its roadway are what keep that movement from tearing the structure apart.

Why do bridges have gaps in the road?

Those gaps are called expansion joints, and they exist because bridges are not the rigid, unmoving objects they appear to be. Steel and concrete both expand when heated and contract when cooled, and on a structure that can stretch for hundreds or thousands of feet, even a small percentage change in length adds up to real inches. Add in concrete shrinkage as it cures, gradual settlement of the supports, the weight of passing trucks flexing the deck, and — in earthquake country — sudden seismic movement, and a bridge without room to move would eventually crack.

The Arizona Department of Transportation puts it plainly: bridges made of concrete expand and contract for a number of reasons, including temperature swings, shrinkage, settlement, ice and vehicle loads, and the joints are what let that happen without the concrete cracking. Leave a bridge deck with nowhere to expand into, and the material pushes against itself instead — the kind of internal stress that eventually shows up as spalling, buckling or structural fatigue.

How much do bridges actually move?

More than most drivers would guess. Engineering firm Foster International, which designs joint systems for infrastructure projects, breaks the hardware into a handful of categories based on how much movement they're built to absorb. Compression seal joints, made of pre-compressed neoprene, handle small shifts of under 2 inches and show up mostly on short-span structures. Strip seal joints — a flexible seal locked into metal extrusions — manage up to about 4 inches and are common on highway bridges because they ride smoothly. At the far end, modular bridge expansion joints use multiple steel beams linked by rubber seals to absorb more than 12 inches of movement in any direction, the kind of range needed on the longest spans or in places with extreme seasonal temperature swings.

Finger plate joints work differently — interlocking steel teeth that slide past each other as the deck shifts — and show up where engineers need a lot of movement capacity but are willing to trade some ride comfort for it; they tend to be noisier and more noticeable underfoot than a sealed strip joint.

Why does my car bump when I cross one?

That thud is the joint itself, and on a well-maintained bridge it shouldn't be much more than a minor jolt. Tim Wolfe, a maintenance district engineer with the Arizona DOT, told the agency's blog that crews can weld steel plates over smaller joint openings to smooth the transition, or mill the surrounding asphalt so it "feathers" down toward the gap instead of dropping off sharply. The bump tends to get more noticeable in winter, when contracting concrete pulls the joint a little wider open, and less noticeable in summer as everything expands back toward flush. On bridges with the largest joints, ADOT says there's really only one long-term fix once a joint wears out: pull it and install a new one — a costly repair, but one crews frame as a maintenance issue rather than a safety one, since properly inspected joints don't pose a hazard at posted speeds.

The same expand-and-contract logic that governs bridge decks governs rail lines, too, just with a different fix. Trains solve their version of the thermal-movement problem with conical wheels rather than joints, letting the wheel shape itself correct for small shifts instead of leaving gaps in the rail. Bridges can't borrow that trick, because a road deck has to stay flat and driveable in both directions, which is exactly why the joint — not the material underneath it — ends up doing all the work.

None of this is a flaw engineers are quietly working around. It's closer to the opposite: a joint that's visibly doing its job, flexing a little wider in January and a little tighter in July, is usually the sign of a bridge aging exactly the way it was designed to. The next time a car thumps over one, on the Verrazzano-Narrows or any bridge with a horizon long enough to expand and contract with the seasons, that bump is a small piece of structural engineering working precisely as intended.