ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297033
10 | ASPIRE , Fall 2012 P E R S P E C T I V E The average age of bridges in the United States is nearing 50 years. This means that agencies are spending a greater proportion of the limited transportation funding maintaining t h e s e a g i n g b r i d g e s a n d w h e n necessary, replacing them. The graph shows the number of bridges built during various decades and those remaining in Oregon. This chart shows that there are many bridges in service that are over 40 years old with growing needs. In Oregon the elements that consume the most maintenance resources are • decks (patching, sealing, and overlays), • steel girders (painting and fatigue mitigation), • e x p a n s i o n j o i n t s ( r e s e a l i n g , patching, and replacements), and • bearings (cleaning, painting, and replacements). While only some areas of the country have major issues with corrosion on substructures and scour issues, maintenance crews throughout the nation spend significant resources preserving decks, joints, and steel bridges. Extending Service Life of New and Existing Bridges Many questions are raised when considering design lives of bridges: • Why doesn't a bridge deck or deck joint last as long as most superstructures and substructures and is that a realistic goal? • Would it be more cost effective to design and construct a deck or expansion joint that would last 100 years without major rehabilitation? • What would the design specifications for a 100-year deck joint look like? • Would the stresses need to be limited to some very low value? • Would you need to design three or more redundant deck sealing systems to avoid corrosion and the constant maintenance needed to maintain cathodic protection systems? • How can you limit the build up of debris that is the killer of all sealing systems without consistent maintenance actions? • Should we just be more explicit about the need for scheduled maintenance and preservation actions to achieve the service life we want? T h e a v i a t i o n a n d n u c l e a r e n e r g y industries have concluded that both service-life design and preservation actions are the only way to ensure desired performance and safety. Is it time for the civil engineering discipline to take a similar structured approach to service life and preservation? Two very different approaches can be considered to extend service life. One approach is to design and construct using indestructible materials at a greatly increased first cost. Bid prices to construct a 100-year bridge element would likely be higher than one intended for a 50-year life. The other approach is to design and construct with ordinary materials, and require inspection and maintenance activities at specified intervals to keep the structure safe and in serviceable condition throughout the service life. This practice is consistent with the aviation and nuclear industries, where safety is paramount but indestructible materials sometimes cannot be used due to their cost (or weight). Several agencies are working to develop a systematic, rational method of designing bridges with elements that have a uniform service life. Bridge designers do this to a certain extent today, but for the most part it is done subjectively. Decisions regarding deck joints, materials such as high-performance concrete, high- performance reinforcement, and bridge type are sometimes justified by seeking longer service life, but normally without developing detailed life-cycle costs over the desired years of acceptable service life. One challenge in taking a more- structured approach to design for service life is that we do not have readily available, easy-to-use tools for analysis. by Bruce V. Johnson, Oregon Department of Transportation Uniform Service Life of Bridge Elements through Design and Preservation Ser vice -life - design plans may be the key 55% 63% 86% 100% 95% 100% 100% 100% 0 100 200 300 400 500 600 700 800 <1950 1950-1959 1960-1969 1970-1979 1980-1989 1990-1999 2000-2009 2010- Number of Bridges Decade State Bridges Built/Remaining Historic Data 2012 Data Percent Remaining Percent of state-owned bridges still in service in Oregon by decade. Figure: Oregon Department of Transportation. AspireBook_Fall12.indb 10 9/18/12 8:57 AM