Smith Point Bridge is a 9 span, 1424 foot long structure located in Humboldt County 5 miles south of Garberville, California on US Highway 101.  Built in 1980, it is two conventionally reinforced 2 cell concrete box girder bridges with common pier walls.  There are two hinges per separated superstructure for a total of four hinges.    The bridge has some unique features; however, it has the common problem of early failed hinge elastomeric bearing pads.   These elastomeric bearing pads allow the bridge to expand and contract with temperature changes and prevent concrete-to concrete bearing.   Within 2 years after construction the bearing pads permanently compressed over 1" at the south hinges and 1" at the north hinges.  Caltrans bridge reports noted and monitored the problem, concluding that the bearing pads had stabilized without any further evidence of continued bearing pad compression by 1991.  In June of 1996  Smith Point Bridge was 1 of 4 bridges to be retrofitted under a $565,000.00 Seismic Retrofit contract.   The retrofit strategy for the Smith Point Bridge included pipe restrainers and bolsters at the hinges, but excluded bearing pad replacement due to excessive cost estimated at an additional $400,000.00.

As part of the new retrofit work, 8- 5/8" diameter pipe restrainers were planned to be inserted in 9" diameter cored holes through the hinges.  There were two pipe restrainers per box girder cell for a total of 16 pipe restrainers. One end of the pipe restrainer was planned to be cast in a newly constructed bolster, while the opposite end extended 10" beyond another newly constructed bolster.  Early in the construction the problem of replacing the hinge bearing pads was once again addressed.  The CALTRANS Office of Structure Design relayed the past considerations during the retrofit strategy and the rationale ( an additional cost of $400,000.00 for bearing pad replacement and the bearing pad compression had stabilized ) for excluding bearing pad replacement from this retrofit project.   I had reviewed the as-builts at the beginning of the project and determined that the hinges were bearing concrete-on-concrete and that it appeared that the south hinge shear keys were crushed.  The Standard Drawing in the as-builts only required 1" of expanded polystyrene on top of the concrete shear keys.  This accounted for the "stabilized compression".   Crushed shear keys would account for the compression over 1".  This fact, although obvious today, had been overlooked until now.  

The patent demonstration resulted in replacement of the exterior bearing pads and installation of belting material from inside the cored holes at a fraction of the cost of conventional methods and without the need for expensive temporary supports, right-of-way, or environmental permits.  However, during the demonstration existing hinge damage was identified that had gone unnoticed until this time.   An additional benefit to utilizing this patented technology, assessment of the actual condition of concrete hinges, was realized.

Seismic Retrofit Hindsight - the BIG mistake

 By 1997 the Seismic Retrofit strategy for concrete hinges had changed by enlarging the 9" cored hole to 10".  This allows for about 5/8" clearance around the pipe restrainer.  Caltrans Structures Design originally detailed minimal clearance ( 1/8" ) around the pipe restrainer on Seismic Retrofit contracts.  The Smith Point Bridge Project revealed daily hinge deterioration due to the cyclic deflection of the bridge simply from normal traffic loading.   Consequently, when the hinge seismic pipe restrainer clearance in the planned cored holes is less than the deflection of the span due to truck traffic, the retrofitted hinge will eventually destroy all of the hinge Seismic Retrofit work due to the pipe restrainer pounding against the hinge interior every time a truck crosses the bridge.  Instead of solid concrete shear keys and solid concrete surrounding the hinge pipe restrainers, Caltrans has introduced a design flaw that replaces the solid concrete with essentially gravel.

For the complete article, licensing information, and how our patent can help you, please e-mail:  elcock@northcoast.com

Stanley E. Elcock is a Professional Civil Engineer, owner of SEE Civil Engineering, and co-owner of patent 5,920,938 with over 20 years experience in highways and bridges.  Recent publications include collaboration on the Rock Creek Bridge Project.

Stanley D. Elcock is a retired materials testing engineer, NICET certified Senior Engineering Technician with NICET Level IV certifications in soils, concrete and asphalt.  His engineering roots can be traced back to Boulder City, where his father Stanley T. Elcock worked as a Civil Engineer for the Bureau of Reclamation on the Hoover Dam and later on Shasta Dam.   He is also co-owner of patent 5,920,938.