Math error causes bridge accident

During bridge construction in Tacoma, Washington, a ship bringing road-deck sections hit the existing Tacoma Narrows Bridge, due to a math error. A large one. The distance from the water to the under side of the existing bridge is 135 feet - at high tide I assume; the tides run over 15 feet. Their calculation said that the loaded barge would clear by 36 feet. So they tossed some scaffolding on top. When going under the bridge the scaffolding hit it. An error of 36 feet out of 135 feet? That's 26.6667 per cent. That's a huge error. Who made it? The Washington State Department of Transportation. Seattle Times