THE CONCRETE BRIDGE MAGAZINE

FALL 2017

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/879956

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PERSPECTIVE Lateral Stability: Who Owns the Risk? by J.P. Binard, Precast Systems Engineering LLC, and Glenn Myers, Atkins. Prestressed concrete girders are widely used for construction of bridges in the United States, and have been used successfully since their introduction. The girders are generally designed for initial conditions at the transfer of the prestress force and for final conditions with the girder incorporated into the bridge. One of the features of prestressed concrete girders is that they are fabricated in a manufacturing facility rather than at the project site. Therefore, they must be handled several times and transported prior to erection in their final locations in the bridge. Accidents or near-miss incidents have often been traced to assumptions that are made with a lack of complete control or complete information or awareness of interdependencies of variables regarding what is required to achieve the final positive outcome. In other words, the designer does not know the route or the equipment being used by the contractor and producer. With newer types of girder sections and advances in materials, designers can now design girders that are significantly longer than those of the past. With the increased length comes an increased risk of lateral instability of the girder during handling or transportation, which can lead to damage or collapse of a girder. While such events are rare, those that have occurred are reminders that lateral stability of girders is an issue that needs to be considered by designers, contractors, and precast manufacturers. The industry has recognized the need for improved methods for analysis of lateral stability of prestressed concrete girders at each stage of handling and transportation. When such an analysis is made, it is important that assumptions made by the designer in assessing the lateral stability of a girder and design modifications required to address lateral stability are clearly presented in the contract documents. This alerts the contractor and precast producer to potential issues, and allows them to assess their risks during the bidding process. Behavior of the girder during each of the following stages should be considered: -transfer of prestress force, -lifting the girder from the casting bed, -transportation to the storage yard, -support conditions in the storage yard, -transportation to the project site, -erection at the project site, and -bracing requirements during the construction of the deck or posttensioning operations. Early work done by Mast has now been extended and refined in the recently released publication from PCI: Recommended Practice for Lateral Stability of Precast, Prestressed Concrete Bridge Girders.3 This comprehensive document has established the methods for analysis. This is a step forward in addressing the lateral stability of girders. A major issue, however, remains to be answered, which is who owns the risk for each of the activities listed above: designer, contractor, owner, or precast producer? There is no consensus and the question remains controversial. A common issue with many contracts is that there is no assignment of responsibility for stability analysis during each of these construction phases. With this lack of definition, many contractors elect to take responsibility for only the areas that are clearly within their scope of work and pass the remainder to the precast producer. The primary problem with this approach is related to technical competency, time, and transparency. The procurement procedure typically results in selection of the producer at a late stage in the project. If changes are made to the girder design to accommodate any of the construction phases, they can have significant schedule and coordination impacts. PCI's Recommended Practice for Lateral Stability of Precast, Prestressed Concrete Bridge Girders offers guidance on how to address each step in the fabrication, delivery, and bridge completion cycle. The document assigns the role of responsible charge to a "stability engineer." This category of engineer is necessary because clear lines of responsibility have yet to be identified. The AASHTO LRFD Bridge Design Specifications are unclear: Section 5.5.4.3 states that stability shall be investigated; however, no responsible party is identified. The AASHTO Technical Committee T-10, Concrete Design, is currently considering how to address this matter and provide more explicit direction in the AASHTO LRFD specifications. It is the opinion of the authors that designers must consider aspects of the construction process within the scope of their work and indicate on the contract plans their assumptions regarding lateral stability that they believe will provide a constructible solution. All modifications to the concepts presented by the designer in the plans are the responsibility of the contractor and producer. The stated assumptions for the transportation vehicle, maximum sweep, and route

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