THE CONCRETE BRIDGE MAGAZINE

SPRING 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/957336

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CONCRETE BRIDGE TECHNOLOGY Practical Solution for Skewed Geometry on Decked-Girder Bridges by Susan M. Kovich and Jerome J. Nicholls, Nicholls Kovich Engineering PLLC Decked-girder bridges with precast, prestressed concrete girders are used extensively throughout Washington state, particularly for local agencies with lowvolume roadways. Decked girders are plant-fabricated and transported by truck to a bridge site for side-by-side placement. The deck, or driving surface of the bridge, is the top flange of the girder and is typically at least 6 in. thick. Decked girders can be fabricated in various cross sections, but the most common type in the Northwest is the decked bulb-tee girder. These sections are versatile, with overall depths ranging from 35 to 65 in. and top flange widths ranging from 4 to 8 ft. These bulb tees have the capability to span up to 160 ft. One challenge of decked-girder construction involves a geometric issue with skewed bridge alignments. This issue can be overcome with some forethought and planning during the design phase. Advantages to Decked Girder Construction There are many advantages to deckedgirder construction. First, decked systems are cost-effective. Both design and construction costs are lower when compared to the costs of cast-in-place deck construction. Second, construction time is reduced when erecting decked girders because the deck is fully precast. Typically, a single-span prestressed concrete superstructure can be shipped, erected, and grouted within one week. Finally, a decked-girder bridge, when properly designed and constructed, provides for easy construction and durability, good structural performance, and low maintenance for the life of the bridge. Each bridge site should be evaluated for feasibility of this system. Not all sites are suitable for decked girders. For example, bridges with high traffic volumes or superelevation transitions are better accommodated by cast-in-place decks. Additionally, the engineer must consider access to the site and a source for the precast, prestressed concrete girders. Consideration of Girder Camber For precast, prestressed concrete girders, it is imperative to predict the estimated girder camber during the design phase. Camber is the upward deflection of the girder due to effective prestressing force and dead load. Although determining camber is not an exact science, camber prediction methods have a history of reasonable accuracy. Camber can be predicted using published formulas or, girder design software, or a girder manufacturer can be consulted. Camber does change over time, so camber at the time of girder setting is of particular importance. It is advantageous to design the vertical profile of the roadway to fit the camber of the girder. If this method is not possible at a particular bridge site, the girder flanges can be thickened at the ends to result in a flat grade even when the girder is cambered. To design the vertical profile of the roadway to fit the camber of the girder, the girder end slopes can be determined by using the following equation for a parabolic curve: G = 4C/12L where G = tangent slope at girder ends C = net girder camber (in.) at the time of girder setting L = span length (ft) 2G = change in slope over span length of girder For example, a 100-ft span girder with a camber of 4.5 in. has 1.5% slope at each girder end, resulting in a 3% change in grade over the girder span. To align the roadway profile grade with girder camber, the engineer would need to ensure that the vertical curve length extends the entire length of the skewed bridge, from beginning to end, including the skewed corners. The overall grade change of the vertical curve would need to be greater than 3%. Skewed Decked Girders Due to roadway or stream alignment, a bridge may need to be skewed at the ends. A geometric anomaly in the deck, known as the “sawtooth” effect, can result if the skew angle is not properly accounted for in the beam seat elevations. Let’s look more deeply at this issue and visualize what can occur. Each prestressed concrete girder in the bridge will have camber, or upward deflection. Should the girder have skewed ends, the acute corner of the girder end will be lower on the camber curve than the obtuse corner. Now, imagine setting an adjacent girder if the abutment elevations were level. Due to camber, the adjacent girder corner would not have the same

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