Hidden cables make for a strong, flexible bridge

From ODOT-

After
the fresh concrete on the Whilamut Passage Bridge hardens and cures, we post-tension
bridge beams and decks to make them stronger.  

Post-tensioning
involves stretching interior steel cables like a rubber band to give bridge
beams and decks an inner link of strength and flexibility.  

Steel
cables are threaded through ducts in the bridge and stretched tight using
hydraulic jacks. Workers must tighten the cables in a specific order to
maintain a balance of pressure. Special wedges keep the cables from slipping
while being stretched.  

In
the photo below, you can see:

• Numerous post-tensioning cables.


• Three ducts, or passageways, for
  the cables.


• Three round anchor plates that
  are fixed at the end of the ducts, each with smaller openings that the
  cables pass though.

 

Post-tensioning
the concrete bridge beams and decks makes them stronger and more flexible than
concrete alone, ensuring the bridge can easily carry heavy traffic and better
resist an earthquake for decades to come.