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Yep that's why armies always broke stride when marching across bridges.

There's a big difference between a third world wooden "peasant, horse, and cart"-grade bridge, and a domestic bridge designed by expert engineers. Try driving an 18-wheeler loaded with three rolls of steel over one of those "break cadence when crossing" bridges in any manner you choose, and see if you can keep it out of the water.

"The Hoan Bridge was temporarily closed on December 13, 2000 after two of three support beams failed, causing the north-bound lanes to buckle and sag by several feet, leaving the span in a near collapsed state. Incredibly, no motorists were injured when the bridge failed. On December 28, 2000 engineers used explosives to remove the damaged section. After reconstruction of the failed span (and extensive rehabilitation and retrofitting of the remainder of the bridge), the Hoan Bridge fully reopened in November 2001. A total of $19 million was spent to demolish and replace the damaged section and retrofit the remainder of the bridge."

The Hoan Bridge was inspected only a few days nefore the partial collapse in 2000.

I realize that -- but these bridges are also carrying a much bigger load and constantly rather than intermittenly.

I found this interesting page based on a report of the bridge's condition in 2003

http://www.visi.com/~jweeks/bridges/pages/ms16.html

The National Bridge Inventory contains a report on this bridge from 2003. It reports the following items:

Deck Condition: Fair.
Superstructure Condition: Poor.
Substructure Condition: Satisfactory.
Scour: Foundations determined to be stable.
Bridge Railings: Meets currently acceptable standards.
Structural Evaluation: Meets minimum tolerable limits to be left in place as-is.
Water Adequacy Evaluation: Superior to present desirable criteria.
Bridge Sufficiency Rating: 50%

A University of Minnesota Civil Engineer in a report to MN-DOT recently noted that this bridge is considered to be a non-redundant structure. That is, if any one member fails, the entire bridge can collapse. A key factor is that there are only four pylons holding up the arch. Any damage to any one pylon would be catastropic. The textbook example of a non-redundant bridge is the Silver Bridge over the Ohio River. It failed shortly before Christmas in 1967 resulting in 46 deaths. A single piece of hardware failed due to a tiny manufacturing defect. But that piece was non-redundant, and the entire bridge collapsed into the icy river. Today, bridge engineers design bridges so that any single piece of the bridge can fail without causing the entire bridge to collapse. It is tragic that the I-35W bridge was built a few years too early to benefit from that lesson.