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/306900
44 | ASPIRE , Summer 2007 S TAT E T he broad spectrum of landscape, from majestic mountain ranges to broad cultivated valleys and legendary coastline, defines the uniqueness of California. The state is situated at the junction of the Pacific and North American plates—two active tectonic crustal plates forming part of the "Rim of Fire." This is one of the most active volcanic and seismic regions in the world today. The constant shifting of the plates forms some of the most tantalizing landscape in the world, but also presents tremendous engineering challenges in one of the most heavily populated states. Geography, climate, and history have come together in California to produce dense population centers, with a high concentration of bridges in regions of high seismicity. Bridge designers in California face the somewhat unique challenge posed by the ever-present threat of earthquakes. A close inspection of fault maps reveals that the state is fragmented not unlike a textbook example of a shattered crystalline structure. Engineers a n d a c a d e m i c i a n s c o n t i n u e s t u d y i n g earthquake damage and recorded seismic events to update design philosophies and details in an effort to mitigate damage. The basic underlying premise of bridge design in California is to prevent structural collapse. To ensure consistent behavior, the California Department of Transportation (Caltrans) has developed and continuously updates a seismic design manual—the Seismic Design Criteria (SDC). The SDC assumes that some structures or elements therein will require rehabilitation or replacement, but life preservation during the event is the primary consideration. However, California's most critical bridges have been designed to higher standards to provide some level of post-earthquake serviceability due to their importance in recovery activities and the economic impact of the closure. T h e m a j o r i t y o f b r i d g e s t r u c t u r e s i n California are cast-in-place, post-tensioned (CIP/PS) concrete box girders. The advantages afforded from precast construction are also realized in many instances, particularly when considering accelerated bridge construction. CIP/PS box girder structures have long held favor in California as they provide the following advantages: • Monolithic connections, which are not susceptible to support loss due to ground motions; • Excellent torsional resistance to seismic loading; • Efficient designs for shear transfer and moment distribution; • Historically, the most cost-effective structure type for many applications; and • The preferred structure type of the local construction industry. ductile design detailing Designing for purely elastic behavior in a seismic event is generally considered unnecessary as well-detailed concrete elements can resist loading through ductile response. Based on substantial research, Caltrans relies on ductile design detailing as a fundamental tenet of seismic bridge design. Ductile detailing in seismic zones is important because: • I t d o e s n o t r e l y o n h i g h l y a c c u r a t e prediction of seismic demands; • It ensures ductile response in predetermined plastic hinge zones resulting in an extended inelastic region with significant energy absorption; • Ductility can be enhanced in a cost-effective manner through the addition of column confinement steel; and • It yields a more cost effective foundation design by limiting the transmitted forces. Elastic design produces stiffer bridges, which attract more seismic loads, increasing the cost of survivability and post-event serviceability. Substantial research in confined concrete sections has led to proven design details to limit damage and force plastic hinging to occur at predetermined locations in the columns. Poor detailing in columns leads to brittle failure. Caltrans design criteria establishes a minimum displacement ductility capacity of three. This ensures a level of post-elastic performance to address inherent seismic uncertainty, even in regions of low seismicity where seismic demands may not control the design. dynamic design Considerations Experience has highlighted the importance of balancing the overall structure to enhance its performance when subjected to seismic excitation. This is accomplished through providing a measure of "effective" stiffness Bridging Faults— California's Unique Engineering Challenges by Raymond W. Wolfe and Michael D. Keever, California Department of Transportation Little Sycamore Trail Bridge, Orange County (Reinforced concrete box girder arch becoming a slab at the apex). Adams Avenue Overcrossing, San Diego County (Three-span box girder bridge on pier columns). Mission Valley Interchange, San Diego County (Multi-span box girders on single column bents). ASPIRE_Summer_2007.indb 44 5/15/07 11:44:03 AM