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
by M. Myint Lwin F H WA C oncrete possesses some very desirable properties for the construction of bridges. Concrete is adaptable to a variety of shapes, forms, and colors, allowing engineers and architects to showcase their bold and imaginative expressions. Concrete has high compressive strength and resistance to environmental and chemical effects. It is relatively low cost, readily available, and easy to use in construction. Cast-in-place concrete bridges can be made continuous and monolithic for improved d u r a b i l i t y, s t r u c t u r a l p e r f o r m a n c e , a n d seismic resistance. Concrete is most suitable for superstructures with curved alignments and superelevations, piers with skews, ramps with various configurations, and structures with complex geometric forms and architectural features. Many concrete arch and tied-arch bridges built in the late 1800s and early 1900s are still in service. Some of them have undergone strengthening and rehabilitation to extend their service lives. One example is the Taft Bridge, which carries Connecticut Avenue over Rock Creek in Washington, D.C. It is an arch bridge with unreinforced concrete for the five arches and reinforced concrete for the deck. The large lion sculptures on the bridge were also made of concrete. The construction of the Taft Bridge started in 1897 and was completed in 1907. The bridge was renovated in 1990 and is expected to serve the communities for many more years. Concrete is strong in compression but weak in tension. The true potentials of concrete were not realized until the industrial revolution in the 1800s, when structural steels were produced and readily available in large quantities and portland cement was produced in significant amounts in the United States. Ernest Ransome of California received a patent in 1884 for his invention of a twisted reinforcing steel bar. Ransome went on to design and build the first reinforced concrete bridge in the United States in 1884. The bridge is known as the Alvord Lake Bridge in Golden Gate Park, San Francisco, California. The bridge is still in service. The successful use of reinforced concrete in the Alvord Lake Bridge has led to the construction of many reinforced concrete arch bridges in other parks around the country. Robert Maillart and Eugene Freyssinet have been credited as the pioneers and champions in reinforced and prestressed concrete bridges, respectively. They set the trend for modern design and construction of cast-in-place, precast, and prestressed concrete bridges—arches, beams, box girders, segmental construction, and others. Precast and prestressed concrete members are often an integral part of modern cable-stayed and suspension bridges. The common goal of the FHWA bridge community is to work together with our state partners, industry, and academia to continuously improve the condition and durability of the nation's bridges and tunnels. Durability of bridges and tunnels may be considered as meeting the design life and serviceability requirements with minimal systematic preventive maintenance and low life-cycle costs. The design life, based on the AASHTO LRFD Bridge Design Specifications, is 75 years. For major bridges, the owners may specify that the bridges be designed and built for a design life of 100 years or more. The AASHTO LRFD Specifications imposes four limit states to be satisfied by the design to achieve durability, serviceability, constructability, and safety. The limit states serve as a systematic approach to structural design to ensure low maintenance in the short- and long-term. The four limit states are: Service Limit State: This limit state imposes restrictions on stress, deformation, and cracking under regular operating conditions. Fatigue and Fracture Limit State: This limit state imposes restrictions on the stress range due to a design truck occurring at the number of expected stress cycles. Strength Limit State: This limit state stipu- lates the strength and stability requirements to resist the specified statistically significant load combinations expected to be experienced by a bridge over its design life. Extreme Event Limit State: This limit state ensures the structural survival of a bridge during events such as earthquakes, ice load, Integrating Research, Theory, and Practice into Building Concrete Bridges that Last Concrete lions stand guard over the Taft Bridge. The Taft Bridge, completed in 1907, has achieved a 100-year service life. ASPIRE_Summer_2007.indb 42 5/15/07 11:43:59 AM