Special Reports

Similar puzzle faced engineers 64 years ago

This is not the first time bridge builders have scratched their heads over removing the bottoms of caissons.

According to 64-year-old construction records, builders of the first Tacoma Narrows Bridge puzzled over the same situation.

The "Report on Construction of the Substructure," prepared in 1940 by the bridge's chief inspector, lays out a sequence of caisson construction generally similar to today's efforts.

One notable exception was that the bottoms of the caissons were made of wood instead of steel.

In the old bridge, each caisson section was floored with 8-inch-by-12-inch timbers, reinforced with 4-inch-by-12-inch cross pieces attached to knee braces on the caisson walls.

Engineers designed the floors so that pulling on cables attached to key members of the floors would spring the whole works loose.

But when the caissons touched down and workers pulled on the cables, the supporting braces wouldn't budge. When they pulled harder, the cables broke loose.

As an alternative, workers lowered a long-toothed clam bucket down the caisson wells and tried to grab onto the floors, opening the bucket and then closing it on whatever it could grab.

They were able to remove many of the 4-by-12s and braces, but the floors refused to yield. As a last resort, workers lowered a battering ram and bashed the floors until they broke free.

Even then, the chief inspector reported, it was necessary to send divers down to work the most difficult floors loose.