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/657823
C O N C R E T E B R I D G E T E C H N O L O G Y 42 | ASPIRE Spring 2016 limits, such as 60 degrees, and modifications to the lifting system are often required. The girder stability must also be evaluated for the additional horizontal compressive forces. Horizontal spreader beams can be used to eliminate the horizontal forces delivered to the girder. Bearing elevations that are different at each end of the girder can lead to challenges as the statics of the system change when one end of the girder rests on the support bearing before the other end does. Passing Girders between Two Cranes Another technique is to pass the girder from one crane to another crane located on the opposite bank of the crossing. The long girder is shipped using a rear steering unit and the front jeep or tractor. The steering unit is backed into the site and one crane lifts one end of the girder off the rear steering unit. The rear steering unit is then disconnected and moved out of the way. The other end of the girder remains securely attached to the front jeep or tractor. The crane rotates as the girder/truck assembly backs up, moving the girder toward the other bank. As the first crane reaches its radius capacity, the load is transferred to a second crane located on the opposite bank. The first crane then swings into position to pick the other end of the beam off the truck. The two-crane pick completes the erection. Load transfer between cranes may involve a load triangle with the two cranes attached to the top two corners of the triangle. The triangle rotates as the load is transferred to the second crane. It is important to not attempt to change the position Improved materials, handling equipment, and shipping equipment continue to push the fabrication limits of precast, prestressed concrete girders. Concrete compressive strengths at transfer of prestress and at 28 days may exceed 7 and 10 ksi, respectively. Precast, prestressed concrete fabricators have constructed stressing beds capable of jacking over 3 million pounds. Many states or regions have adopted bulb- tee cross sections to receive the high pres tressing loads and improve the response to handling, shipping, and erection stability. Design engineers take advantage of these improvements to increase span lengths. The project environmentalist, biologist, a n d h y d ro l o gi s t r e s t ri c t w a t er w a y construction and limit in-water activities such as work bridges and foundation cons truc tion. With thes e time and geometry restrictions, contractors find unique ways to erect these long spans. Single Pick One approach to erect a long, heavy girder is to mobilize a large crane and utilize a single pick. Sling orientation Unique Approaches to Erecting Long-Span Girders by Dr. Keith Kaufman, Knife River Corporation Decked bulb-tee being erected by a single pick. Photo: Morrison Structures Inc. Passing a girder between two cranes using a load triangle (inset). Photos: Knife River Corporation.