THE CONCRETE BRIDGE MAGAZINE

Spring 2019

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/1093138

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C O N C R E T E B R I D G E T E C H N O L O G Y 38 | ASPIRE Spring 2019 by Dr. Bruce W. Russell, Oklahoma State University Sweep in Precast, Prestressed Concrete Bridge Girders Sweep is defined in the Manual for Quality Control for Plants and Production of Structural Precast Concrete Products (PCI MNL-116) 1 as "a variation in horizontal alignment from a straight line parallel to centerline of member (horizontal bowing)." A generalized representation for sweep is shown in Fig. 1. Decades of industry experience inform us that very few girders are produced with noticeable or out-of-tolerance sweep. However, when sweep is large enough to be noticed—or when the amount of sweep exceeds tolerance—producers, contractors, and owners may be unders tandably concerned about how sweep could occur in production, and what corrective actions can, or should be taken. Regardless the amount of sweep, the truth is this: most prestressed concrete bridge girders, once they are in place with composite concrete decks cast upon them, will function effectively for the design life of the bridge without concern for limits on strength, durability, or serviceability. Prestressed concrete bridge girders that are built in accordance with our design standards, and are made from conforming materials (concrete and steel) within a geometry described by the contract documents, will support the intended design loads for millions of cycles, in all kinds of climates, for the design service life of the structure. This article focuses on the sweep of bridge girders that may occur during initial fabrication, lifting, or storage in the yard. One probable cause is explored with a hypothetical example and corrective actions that can be taken while the girder is still in the precast producer's yard. Subsequent ar ticles will cover transportation, erection, and construction issues and discuss whether permanent sweep in girders should cause concern for long-term stresses, strength, or performance. Measurements and Tolerances The initial measurement of sweep is made after the side forms are removed and prestressing force is transferred, or after the girder is removed from the forms and is placed on dunnage in the precast producer's yard. When measuring sweep, it is important that the girder is level, the sun is not shining on one side, and the actual flange widths have been verified at the ends and middle. Commonly, sweep is measured at the midspan of the girder. Admittedly, accurate measurement may be difficult for long girders. However, a measurement to ¼ in. precision should be sufficient ( 1 ⁄ 8 in. would be preferable). The measurement(s) should be recorded even if zero. Alternatively, it may be easier to measure the sweep at approximately 20 ft increments. In this case, measurements should be made accurate to ± 1 ⁄ 16 in. A sweep measurement made at the center of a 20.0-ft length, or the average of several such measurements, can be converted to an overall sweep by the following: f = f 20 L 20.0 ft ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 2 where f = sweep projected over the entire girder length L f 20 = sweep measured over a 20.0 ft length Tolerances for precast, prestressed concrete members have been established to allow for production and erection variances, as well as the interface of the member with the overall construction of the structure. Tolerances are to be used for guidelines and not as reasons for rejection. If the girder can be brought within tolerance in an acceptable manner, or if exceeding the tolerance does not affect the integrity or performance of the girder, the girder should be accepted. Later articles will show that girders with sweep in excess of tolerances can function without compromising the completed bridge system. The tolerance for sweep for bridge girders given in MNL 116-99 1 is 1 ⁄ 8 in. per 10 ft. For example, a bridge girder with a length of 130.5 ft would have a sweep tolerance of 1.63 in. The calculation for sweep tolerance follows: Sweep Tolerance = 130.5 ft 1 8 in. 10 ft ⎛ ⎝ ⎜ ⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ ⎟ = 1.63 in. C L Sweep L Figure 1. A generalized representation for sweep in a girder. Figure: Dr. Bruce Russell.

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