Bon Air Road Bridge Emergency Repair Project
Caltrans Structure Maintenance & Investigations conducted a field inspection of Bon Air Road Bridge on September 27, 2012. The results of their inspection were documented in a Bridge Inspection Report and a Record of Critical Findings, dated October 1, 2012, which described the observed problem:
The bent cap at Bent 4, on the Span 4 side, has spalled and lost bearing capacity below Girders 2, 3, 4 and 5. There is one inch settlement along the concrete deck between the Span 3 and Span 4 side of Bent 4. Similarly the bent cap at Bent 6 under Girders 2, 3 and 4 has spalled and lost bearing capacity. The seismic continuity plate between the girders at these locations and opposite bearing plate on these bents are carrying loads that should be supported by the failed bearings.
The Record of Critical Findings recommended action is:
Restore the structural capacity of the bearings on the caps at Bent 4 and 6 or provide temporary shoring below Girders 2, 3, 4 and 5 at Bent 4 and Girders 2, 3 and 4 at Bent 6.
The recommended temporary shoring is to be in place within three months (by January 1, 2013).
Temporary shoring will be installed at Bents 4, 6 and 8 as shown on Figures 1 and 2. The shoring will consist of structural steel members, 12-inch wide flange beams and 10-inch “C” channels, attached to the existing pile caps and bent caps. Timber blocking will be installed between the new steel members and the existing girders.
The work will be performed from a floating work platform secured in position by temporary attachment to the existing bridge piers with rope. No piles would be required. The floating work platform would be approximately 5’ wide and 20’ long and would be constructed from untreated 12” x 12” Douglas fir timber with pontoons attached to the bottom. The floating work platform would be assembled and launched from the City of Larkspur’s boat dock at Piper Park downstream of Bon Air Road or from other downstream boat ramp locations. The floating work platform would be towed into place using a small skiff. Small handheld electric power tools would be powered by a 500w gasoline generator that would be located on the floating work platform, skiff or in the back of a truck parked off the bridge. Construction materials may be lifted down to the floating work platform from the top of the bridge. This would require temporary closure of the bicycle/pedestrian path on the northwest side of the bridge while the materials are being hoisted below to the floating work platform. The temporary closure of the bicycle pedestrian path would be for no more than 4 hours at a time on no more than 4 days. The total duration of work is not expected to exceed two weeks.