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/306858
S A F E T Y AND S E R V I C E A B I L I T Y This advisory has been prepared to inform owners, designers, fabricators, and girder erectors about issues affecting long-term sweep of pretensioned concrete girders during manufacturing, shipping, and erection. Jobsite incidents have been reported involving excessive deformations and, in extreme cases, failure. The use of increasingly long, slender girders made possible by stronger concrete and larger prestressing strands may render the issues discussed below more significant in modern design and construction of I-girder bridges. This alert is not intended to alarm or discourage bridge designers from using long, slender I-girders. They have proven to be significantly less expensive than alternative steel plate girders, which are even more slender. When concrete I-girders are constructed with due care and in accordance with published guidelines, there should be no concern about stability problems. Numerous examples exist of good practices. A bridge in Calgary, Alberta, Canada, utilized 65-m (213-ft)-long, 2.8-m (9-ft 3-in.)-deep girders. In many states including Nebraska, Oregon, and Washington, long-slender girders are increasingly being used. For the purposes of this discussion, two fundamental terms must be understood. Initial Sweep (IS)—lateral midspan deviation from a straight line measured at or close to the time of strand release and removal from the form. Long-Term Sweep (LTS)—lateral midspan deviation from a straight line occurring after IS and until the time the deck is cast on the girders. Effects of Sweep Sweep should be considered for all precast concrete products, keeping in mind that the effects are magnified as girder length and slenderness increase. The core issue is the performance of the system consisting of girders seated on bearings in any given span, before the deck is placed and cured. The interaction between girders and bearings must be considered for all projects. Recent Events I-girders are much weaker and more flexible about their vertical axis than about their horizontal axis. In some instances, the ramifications of IS and LTS have been experienced with a general lack of understanding of this behavior. There are examples where girders have been rejected on arrival at the jobsite because sweep was measured while the girder was on a truck sitting on unlevel ground. In other cases, girders have been rejected and removed from the bridge. In other instances, remedial action at the jobsite has been permitted to straighten girders. In rare situations, girders have become unstable and fallen from their supports after being erected. This is obviously a potential danger to people and property. Bearing Design Concerns Some bearing types such as cotton duck pads and relatively stiff steel-reinforced elastomeric pads (SREPs) have demonstrated superior performance. They have a minimal first cost impact on the overall project and require practically no maintenance. However, current design specifications for SREPs can produce a wide range of solutions, some of which can lead designers to select relatively soft and thick bearings that could be also be considerably narrower than the width of the girder's bottom flange. Any IS or LTS that is present in a girder will likely be amplified when seated on such bearings. When a girder is placed on a narrow bearing and not exactly centered, slight eccentricity would create significant lateral stress about the girder's weak axis, and significant additional LTS. This situation would be magnified if the girder web were not plumb. PCI Advisory on I-Girder Stability during Handling and Construction by Chris D. Hill, John S. Dick and Maher K. Tadros A 9-ft 3-in.-deep, 213-ft-long girder being shipped in Calgary, Alberta, Canada, using a reinforcing steel truss attached to the top flange for additional stiffness. Photo: Con-Force Structures.