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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12 | ASPIRE Winter 2018 specific parameters, such as lift and bunk locations, camber, and haul-truck rotational stiffness and wheel spacing, are shown in a girder schedule in the contract plans. The figure on the previous page shows an excerpt from WSDOT girder schedule. The design engineer, when considering lifting and hauling stability concerns, does not bear full responsibility for safety throughout the duration of the construction project. Means and methods of girder handling, transport, and erection are well within the scope of work of the fabricator, hauler, erector, and contractor. Contract documents must clearly assign responsibilities to these parties. Section 6-02.3(25)L1 of the WSDOT Standard Specifications for Road, Bridge and Municipal Construction states that "the contractor is responsible for safely lifting, storing, shipping and erecting prestressed concrete girders." Recent amendments to these specifications provide parameters for performing lateral-stability analyses as well as clarification of requirements and responsibilities. The contractor and subcontractors, the fabricator, haulers, and erectors then assign responsibilities among themselves. Consequences of Ignoring Stability During Design Designs that do not consider girder stability are more likely to require postbid design modifications. These modifications are highly undesirable. They can lead to extensive redesign of girders, lengthy review cycles, delays in schedules, and significant changes to material quantities and geometric aspects of the bridge. Design engineers are required to check girder stresses at transfer and to indicate the minimum concrete release strength on plans. If a lack of stability prevents the lifting of the girder from the casting bed, these computations become meaningless. Higher concrete strengths and restranding may be necessary. Other than the cross section, the revised girder may not bear any similarity to the original design. Stability improves when handling support locations are moved in from girder ends. Temporary strands reduce tensile stresses at the support and harp-point locations. However, temporary strands affect long-term camber, and changes to camber affect the slab haunch buildup and bridge geometric elements such as bearing seat elevations. Significant changes in slab haunch concrete quantities can be the result for girders with wide top flanges. Additional prestressing force may be needed to accommodate an increased dead load. WSDOT approach adopts the practice that the first line of defense against these types of postbid changes is the design engineer. Implementing Stability Design State-of-the art analysis techniques, b e s t p r a c t i c e s , a n d i n d u s t r y recommendations have been developed and published. 2,8 Girder stresses and stability at initial lifting and hauling are integral elements of the design process. Lifting and hauling conditions are often a governing design case. Designing for optimized fabrication and girder stability involves complex, iterative analytical procedures. Properly implemented tools are essential for lateral-stability design to be a common and routine practice. WSDOT has successfully implemented the practice of lateral-stability design with sophisticated software tools. WSDOT's prestressed concrete girder d e s i g n s o f t w a re — P G S u p e r ™ f o r pretensioned girders, PGSplice™ for post-tensioned spliced girders, and PGStable™ for general precast concrete girder stability analysis—incorporates the necessary analytical procedures to enable engineers to arrive at acceptable design solutions quickly. These tools are part of the BridgeLink™ suite of bridge engineering software that provides the critical link between the state-of-the-art and everyday practice (see http://www. wsdot.wa.gov/eesc/bridge/software/ i n d e x . c f m ? f u s e a c t i o n = s o f t w a r e _ detail&software_id=69). Summary As bridge owners make use of longer, more slender girders, stability becomes a serious concern. Engineers should become familiar with these concerns and address them during design. Stress and stability considerations during lifting and hauling should become a routine part of precast, prestressed concrete bridge girder design. Excellent design tools and resources are available (see references). Providing complete design assumptions in contract documents enables prospective bidders, fabricators, h a u l e r s , a n d e re c t o r s t o a d d re s s and account for possible changes to proposed lifting and hauling schemes during the bidding process. References 1. AASHTO (American Association of State Highway and Transportation Officials). 2014. AASHTO LRFD Bridge Design Specifications, 7th ed. Washington, DC: AASHTO. 2. B r i c e , R . , B . K h a l e g h i , a n d S . S e g u i r a n t . 2 0 0 9 . " D e s i g n Optimization for Fabrication of Pretensioned Concrete Bridge Girders: An Example Problem." PCI Journal 54(4): 73-111. 3. Washington State Department of Transportation (WSDOT). 2017. Bridge Design Manual. M23-50.17. Olympia, Wash: WSDOT. 4. Mast, R. F. 1989. "Lateral Stability of Long Prestressed Concrete Beams: Part 1." PCI Journal, 34(1): 34-53. 5. Mast, R. F. 1993. "Lateral Stability of Long Prestressed Concrete Beams: Part 2." PCI Journal 38(1): 70–80. 6. Seguirant, S. J. 1998. "New Deep WSDOT Standard Sections Extend Spans of Prestressed Concrete Girders." PCI Journal 43(4): 92-119. 7. W S D O T ( W a s h i n g t o n S t a t e Department of Transportation). 2016, 2017. Standard Specifications for Road, Bridge and Municipal Construction. M41-10, amended April 3, 2017. Olympia, WA: WSDOT. 8. PCI (Precast/Prestressed Concrete Institute). 2016. Recommended Practice for Lateral Stability of Precast, Prestressed Concrete Bridge Girders. CB-02-16-E. Chicago, IL: PCI. EDITOR'S NOTE A Perspective article on this important topic written by a consulting engineer and a precast plant engineer, both of whom were instrumental in the writing of the PCI Recommended Practice for Lateral Stability of Precast, Prestressed Concrete Bridge Girders, appeared in the Fall 2017 issue of ASPIRE ® .

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