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Concrete U-Girders

Introduction

A concrete post-tensioned U-girder is an exceptional structural component widely employed in the construction of bridges and specifically on elevated metro networks. It boasts a precast concrete configuration meticulously constructed to bear substantial loads and span extensive distances.

Cross-Sectional Geometry


The U-girder has a distinct shape, resembling the letter "U" when observed from the side. The U-girder is divided into mainly two components:

· Webs

The girder has two vertical webs on either side of the ends as shown. Geometrically, these members are OF uniform WIDTH AND THICKNESS unless otherwise specified.

· Bottom flange

The cross-section consists of a bottom flange that horizontally orients, and which thickens at the supports and thins at the mid segment. This geometry will determine the cable profile and the save on material costs.


The girder can be single span or double spanned in which case the pier at the middle will be at a single plane of support at the bearing location. The singly supported span is supported at both ends by means of being suspended on pier bearings simply where the load is transferred via the bearings to the footing below.


Post-Tensioning System

A distinguishing feature of the U-girder is its incorporation of post-tensioning tendons that run throughout the girder span. These tendons, fabricated from high-strength steel strands or bars, are meticulously encased within conduits or tubes embedded within the concrete during the precasting phase. Following concrete curing, hydraulic jacks expertly stress the tendons to premeditated levels. Specialized anchorages secure the tendons, meticulously transferring the induced tension to the concrete matrix.



The ducts housing the post-tensioning tendons are diligently fashioned from corrosion-resistant materials, such as plastic or galvanized steel, safeguarding them against environmental degradation. Once the tendons are stressed, these ducts are meticulously infused with grout—a cementitious substance that provides both protection and robust bonding between the tendons and the surrounding concrete. This meticulous grouting procedure ensures optimal load transfer and durability.


Bridgehawk® video on U-girder Modelling



Construction Considerations

U-girders are precast in a controlled environment, such as a precasting yard or factory. The precasting of U-girders enables meticulous quality control and efficient manufacturing. These girders are expertly cast into customized molds,using high-strength concrete meticulously composed with carefully selected materials and precise mix designs. Casting in the yard allows for better control on the overall finish of the girder. This is enhanced by the use of hydraulic type of formwork. Rigorous quality control measures, encompassing concrete strength testing and tendon stressing verification, ensure the achievement of desired performance characteristics.


Design Codes and Standards

The design of concrete post-tensioned U-girders adheres meticulously to well-established codes and standards specific to bridge design, such as the AASHTO LRFD Bridge Design Specifications. These meticulous guidelines provide comprehensive directives concerning loadings, material properties, prestressing design, and construction practices, ensuring utmost structural reliability.


Transportation and placement

Once the concrete has cured to the desired strength, the precast U-girders are prepared for transportation to the construction site. A mobile crane is typically employed for the lifting and placement of the U-girders. Rigging crews connect the lifting equipment, such as slings or lifting lugs, to the precast U-girders in a manner that ensures proper weight distribution and structural integrity. Careful attention is paid to securely fasten the rigging to the girder's designated lifting points, which are typically predetermined during the design phase.


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