THE CONCRETE BRIDGE MAGAZINE

WINTER 2018

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/922349

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H i g h - p e r f o r m a n c e / h i g h - s t r e n g t h concrete (HPC/HSC) is now a standard m a t e r i a l f o r t h e f a b r i c a t i o n a n d construction of precast concrete girders. Among many other advantages, HPC/ HSC offers higher concrete strength that allows girders to accommodate a g r e a t e r p r e c o m p r e s s i o n f o r c e from pretensioning. The increased precompression force, achieved by utilizing 0.6-in.- or 0.7-in.-diameter strand, translates to the design of significantly longer girder spans. Some owners are extending the spans of typical AASHTO-type girders. These shallow Type II, III, and IV girders have relatively narrow flanges and may not have adequate lateral stiffness to resist lateral bending forces during handling. Other bridge owners have developed wide-flange girders to accommodate precompression forces commensurate with the strength of HPC/HSC and to provide greater lateral stiffness. These girders offer span lengths in excess of 200 ft. Long-span girders fabricated with HPC/HSC present many challenges. Of primary concern are the lateral stability of long, slender girders and fabrication of these girders in existing stressing beds that were not designed for the larger pretensioning forces and girder sections. Working with local fabricators, WSDOT developed a n e w d e s i g n m e t h o d o l o g y. T h e primary goal is to support optimized fabrication by determining the least required concrete strength at transfer and lifting, while achieving adequate stability of the girder during lifting and hauling operations. Brice, Khaleghi, and Seguirant 2 give a detailed description and example of this design procedure. The design outcomes include support locations during lifting and hauling, m i n i m u m c o n c r e t e s t r e n g t h s a t lifting and shipping, and, if necessary, temporary top-strand requirements. Stability Design Parameters The key stability design parameters are camber, sweep, dynamic loading effects, hauling vehicle characteristics, and maximum superelevation along the haul route. Other parameters can include lifting device rigidity and eccentricity, and bunking locations and eccentricity, depending on testing or experience. A common argument against consideration of lateral girder stability by the design engineer is that the contractor's means and methods, and thus many of the key parameters, are unknown at the time of design. Successful past practices, contractual requirements, measurements, and conservative estimates are reasonable b a s e s f o r s e t t i n g t h e n e c e s s a r y parameters. Design engineers routinely e s t i m a t e c a m b e r. C o n s t r u c t i o n s p e c i f i c a t i o n s t y p i c a l l y p r o v i d e tolerances for sweep, lifting device placement, and bunking locations. The maximum superelevation can be determined for likely project-specific haul routes or, more generally, the e x i s t i n g r o a d w a y i n f r a s t r u c t u r e t h ro u g h o u t a re g i o n . F o r o t h e r parameters, the design engineer can assume conservative values that increase predicted stresses and reduce factors of safety related to lateral stability. Haul-truck characteristics are the most difficult parameter to establish. Fabricators in Washington state have been proactive regarding stability and have probable values of hauled weight per axle, overhangs, rotational stiffness per axle, and height of girder center of gravity above axle for typical girder transport vehicles. The WSDOT Bridge Design Manual incorporates these values as standard stability design parameters. 3 (A standard method for measuring rotational stiffness is not available. Mast, 4,5 and Seguirant 6 report placing girders on haul trucks and measuring rotations to determine stiffnesses.) WSDOT recently collaborated with local fabricators and haulers to develop a n e w m e t h o d f o r i n c o r p o r a t i n g haul-truck rotational stiffness into lateral-stability design. Rather than using measured values for a specific hauler's equipment, engineers instead e s t i m a t e t h e m i n i m u m ro t a t i o n a l stiffness needed to satisfy hauling design requirements. This method results in a proposed hauling scheme that is compatible with a variety of hauling equipment. Communicating Assumptions and Responsibilities At the time of design, engineers do not know the means and methods for lifting and transport of girders. Reasonable estimates are the basis for stability design. WSDOT provides a proposed lifting and hauling scheme in contract documents and lists all relevant assumptions, including the estimated minimum haul truck rotational stiffness. When contract documents provide design assumptions, bidders can plan alternative lifting and shipping schemes and account for cost and schedule implications, leading to more accurate bids. Based on this experience, other owners are encouraged to work with local fabricators, haulers, erectors, and contractors to establish reasonable stability design parameters. WSDOT provides design assumptions in contract documents by listing assumed stability parameters common to all projects, such as lifting device rigidity and eccentricity, girder lateral sweep, and dynamic loading factors, in the WSDOT Standard Specifications for Road, Bridge and Municipal Construction. 7 Project- ASPIRE Winter 2018 | 11 Maximum midspan vertical deflection at shipping L L 1 L 2 K q Minimum shipping support rotational spring constant W cc Minimum shipping support center-to-center wheel spacing 3 7 / 8 in. 5 ft 0 in. 10 ft 0 in. 10 ft 0 in. 50,000 k-in./rad 72 in. Example of a girder schedule showing stability parameters. Figure: Washington Department of Transportation. Note: L = Distance from end of girder to lifting point; L 1 = Distance from one end of girder to support point during shipping; L 2 = Distance from other end of girder to support point during shipping.

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