THE CONCRETE BRIDGE MAGAZINE

SUMMER 2007

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

Contents of this Issue

Navigation

Page 46 of 51

ASPIRE , Summer 2007 | 45 equity between adjacent bents in a frame and/or adjacent columns in a bent, and tuning the fundamental periods of vibration of adjacent frames. The above is analogous to an appealing aesthetic design wherein proportioning yields enhanced visual flow; seismic forces are better resisted through balanced geometries. The concept of balanced "effective" column stiffness precludes the danger of substantial damage to stiffer elements from localized shear demand. An example of this is unbalanced i n e l a s t i c r e s p o n s e o r i n c r e a s e d c o l u m n torsion demand due to rigid body rotation in the superstructure. Empirical data has led to target column "effective" stiffness ratios of k i e / k j e ≥ 0.75, where k i e is defined as the smaller "effective" bent or column stiffness, and k j e is the larger bent or column stiffness. The calculated "effective" stiffness should consider column heights and diameters, framing effects, end conditions, longitudinal and transverse steel ratios, and foundation flexibility. Long concrete bridges in California often incorporate in-span hinges to accommodate superstructure thermal movement without substantially increasing column stresses. Design of structures containing in-span hinges requires careful attention to the potential for out-of-phase response between adjacent frames. Neglecting this leads to increased probabilities of localized failure due to unintended collisions between adjacent elements excited by seismic events at different fundamental periods. Unseating of in- span hinges, an event leading to partial or total collapse mechanisms, was first identified after the 1971 San Fernando Earthquake. Modern designs incorporate a minimum hinge seat of 2 ft. Retrofit strategies for narrower seats primarily employ double extra-strong steel pipe seat extenders to accommodate larger longitudinal displacement excursions under extreme seismic events. Restrainer cables attached to concrete bolsters were employed in California's Phase I seismic retrofit program, with pipe seat extenders gaining favor later. Another innovation developed to address this vulnerability is the "seatless" hinge. It consists of cantilever end spans, emanating from adjacent frames, butted together without restraint as in typical seated hinge configurations. It is also important to limit the ratio of longitudinal and transverse fundamental periods of vibration between adjacent frames. Based on analytical studies, the SDC caps the ratio of the natural period of the less flexible frame to that of the more flexible frame at a minimum of 0.7. Effective strategies to accomplish this include adjusting "effective" column lengths, modifying end fixities, reducing/redistributing s u p e r s t r u c t u r e m a s s , m o d i f y i n g c o l u m n reinforcement ratios, etc. Column length adjustments are typically the most easily incorporated, and involve simple footing depth modifications or isolation casings. The latter employs steel casings designed to provide a gap between the column and the surrounding soil mass to effectively lengthen the column. Seismic response modification devices Isolation from extreme event forces such as earthquakes is a proven means of cost-efficient design. This strategy has been employed in civil infrastructure design worldwide, from Japan to Italy and California. Rather than relying solely on ductile behavior, the premise is to limit the forces transmitted into the structure by isolating portions from ground motions, or reducing the input magnitude. Numerous devices exist today to accomplish this goal, from large self- centering bearings to viscous dampers, hysteretic damping mechanisms, and lock-up devices. These are particularly useful when considering retrofitting existing structures not previously detailed to respond in a ductile fashion to large displacement demands. As primary reinforcement confinement is crucial to ductile behavior, it may be difficult to incorporate into existing structures, particularly on large-scale structures, and thus isolation strategies become more enticing for retrofits. Most of the major long-span toll structures have some type of seismic response modification device to enhance performance and limit damage. However, because of their cost and long-term maintenance needs, these devices are not typically used on "standard" structures designed with today's criteria, or those that can be retrofitted with simpler solutions. Future Innovation Accelerated bridge construction has received much attention in recent years, largely through a concerted effort by the Federal Highway Administration and public demand for less interruption from highway construction projects. Research projects contemplating substantial diversions from historical design norms such as re-centering precast column/bent elements may prove viable for future bridges. The long- term vision for transportation projects across the Golden State includes incorporation of design features leading to rapid on-site construction. Increased structure durability and enhanced post-earthquake serviceability must be provided to effectively meet the motoring public's demands, maintain California's prominence in the global economy, and promote good environmental stewardship. Conclusion California has and continues to expend t r e m e n d o u s r e s o u r c e s d e v e l o p i n g d e s i g n criteria and pursuing research in an effort to counter seismic effects on bridges. The SDC is performance-based. In simple terms, the element and system capacities are selected to exceed the imposed demands. The strong beam- weak column approach focuses the damage at predefined locations, otherwise known as plastic hinge zones, in column elements. System redundancy is required to provide alternate load paths and prevent local failure from resulting in collapse. Targeting a minimum ductility prevents brittle failure modes, which are often sudden and catastrophic. Some solutions employ seismic response modification devices to isolate or limit exciting forces from specific areas of structures. The goal is to prevent collapse, while localizing damage to accommodate repairs. Neprud Wash, San Bernardino County (Three-span precast, prestressed concrete I-beam bridge). Russian River Bridge, Sonoma County (Precast, prestressed and post-tensioned double-tee superstructure used as an emergency replacement). Donner Park Overcrossing, Nevada County (Two-span cast-in-place post-tensioned box girder). ASPIRE_Summer_2007.indb 45 5/15/07 11:44:06 AM

Articles in this issue

Archives of this issue

view archives of THE CONCRETE BRIDGE MAGAZINE - SUMMER 2007