North Fork Bridge Seismic Retrofit*

The retrofit design required work around four river piers in a channel with seasonal flows that rose from 600 cfs to more than 4,000 cfs during spring runoff. The bridge was the only crossing for a rural community of 3,200 people and could not be closed.

Original drawings were incomplete, and exploratory work revealed corroded bearing seats and unmarked reinforcement locations that affected the jacket design.

We worked the piers in a low-flow sequence from July to October, using sheet-pile cofferdams driven with vibratory hammers and dewatered with sumps. A scanned reinforcement survey of each pier was completed before drilling so that the retrofit anchors avoided steel, and the engineer issued revised details within five working days of each discovery.

Bearings were replaced one girder line at a time on synchronized jacks, with live-load monitoring and a single lane open under flagging control. The deck overlay was placed in half-width sections over eight weekends.

The bridge was rated to withstand a 1,000-year seismic event and reopened to full capacity 22 days ahead of schedule. The project closed with change orders totaling 2.7 percent of the contract, most of which traced to the incomplete as-builts.

Crews worked 94,000 hours with 0 lost-time incidents, and the county used our as-built survey as the basis for retrofitting two similar bridges.