Engineering
The Structural Secret of Long-Span Cable-Stayed Bridges
Quick fact
In a cable-stayed bridge, every stay cable is tensioned so that the deck is kept in compression along its length, much like the string of a bow. This pre-stressing effectively turns the deck into a continuous beam that rests on elastic supports provided by the cables, allowing spans of over a kilometer with a surprisingly thin deck.
Why this is interesting
You’ve probably crossed a cable-stayed bridge without a second thought, but the way it lets a thin ribbon of concrete and steel leap across a vast river is almost magical. How can those slender cables hold up an entire roadway?
Read the full explanation
Understanding The Structural Secret of Long-Span Cable-Stayed Bridges
Imagine holding a heavy book with a trampoline stretched across your two hands. The trampoline is the deck, and your hands are the towers. Now, your arms are the stay cables—they pull the trampoline upward at many points along its span. In a real cable-stayed bridge, the cables attach directly from the tower to the deck at regular intervals. Each cable takes a share of the load and transfers it to the tower, which then passes the load down to the foundation. The key is that the cables are always in tension, the deck is partly in compression, and the tower is in compression. By balancing these forces, the structure starts to act like a continuous beam resting on many small columns (the cables), which lets the deck be much thinner than a conventional girder bridge of the same span. The cables are not just holding the deck; they are actively pulling it inward, compressing it along its length, which adds stiffness and prevents the deck from sagging excessively under heavy traffic or wind.
A deeper explanation
The mechanical secret lies in the geometry and pre-stressing of the stay cables. Each cable forms the hypotenuse of a triangle with the deck and the tower. When a vertical load (like a truck) sits on the deck, it causes the cable to want to stretch. But because the cable is already pre-tensioned, it resists this by exerting an upward component of force on the deck at the anchor point. This upward force reduces the net bending moment in the deck, because the deck is now supported at many closely spaced points rather than just at the ends. The horizontal component of the cable tension pulls the deck inward, placing it in compression. Concrete and composite decks are very strong in compression, so this is beneficial: it allows the deck to be very shallow, reducing dead weight, which in turn reduces the load on the cables and towers. The tower itself is also an essential part: it must be stiff enough to resist the unbalanced pull of the cables on either side, especially during construction. The overall system is a statically indeterminate structure, meaning internal forces are distributed based on relative stiffness. This is why cable-stayed bridges can achieve spans like the Russky Bridge (1104 m) or the Sutong Bridge (1088 m): by using the entire depth of the structure—from cable to tower—to resist loads, rather than just a massive girder. The result is a structure that is both extraordinarily strong and visually light, a perfect marriage of form and function.