Adjacent Box Beam Connections: Performance and Optimization

CHAPTER 1. INTRODUCTION

BACKGROUND

Precast prestressed concrete adjacent box beams are widely used throughout the United States. Box beams are normally fabricated by a precast concrete producer in a controlled environment, enabling production of durable structural elements. They are then transported and assembled on the job site using field-cast shear key connections, transverse ties, and potentially a structural overlay to form a complete bridge system. This type of bridge superstructure is considered to have a lower erection cost and be easier to construct compared to other systems. One of the recurring issues for this bridge system is degradation of the shear key connections, which can compromise both the strength and serviceability of the bridge. When the ability for shear key connections to transfer loads to adjacent beams is affected by degradation, the live load may remain concentrated in the few beams under the wheels. This can potentially lead to damage caused by exceeding the designed allowable load of those beams. Beams do not deflect uniformly under live loads when the shear keys have failed. Excessive differential deflection (Δ_δ) between adjacent beams may lead to widening of existing cracks in the shear keys as well as reflective cracking in the overlay if one is present. These cracks can allow chloride-laden water to infiltrate the structure and result in corrosion of the tie bars, prestressing strands, and embedded steel reinforcement. Decades of experience have demonstrated that the field-cast shear key connections are a weak link in the box beam system that can lead to substandard performance of the overall bridge system.^(1,2–4)

Most shear key connections are designed using regional standard details that are of uncertain origin with neither information on the magnitude of forces transferred through the shear key nor the ability of a given detail to resist these loads.⁽²⁾ Neither the American Association of State Highway and Transportation Officials’ (AASHTO) Standard Specifications for Highway Bridges, the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications, nor the AASHTO LRFD Bridge Construction Specifications provides specific guidance for the design or construction of the connection between adjacent box beams.^(5–7) The AASHTO Standard Specifications for Highway Bridges states that “the interaction between the beams is developed by continuous longitudinal shear keys used in combination with transverse tie assemblies which may, or may not, be prestressed” (p. 34).⁽⁵⁾ The shear key design details and the calculation of the transverse forces are not provided. In the AASHTO LRFD Bridge Design Specifications, a minimum of 0.25 ksi (1.7 MPa) transverse prestress is suggested, but no further guidance on the connection geometry or prestressing forces is provided.⁽⁶⁾ The Precast Prestressed Concrete Bridge Design Manual presents an empirical design based on the State of Oregon’s experience and suggests a design procedure based on research conducted by El-Remaily et al. and Hanna et al.^(3,8,9)

This procedure assumes that post-tensioning (PT) transverse diaphragms are the primary mechanism for the distribution of wheel loads across the bridge; longitudinal shear keys are not required for the structural performance of the bridge. In the design, the required transverse PT force at diaphragms is calculated so that diaphragm concrete stresses due to the combination of wheel loads and PT forces are maintained within allowable limits. The limit for compression is 0.6f′_c, where f′_c is the specified compressive strength of the concrete. No tension is permitted.

This design methodology was initially developed based on practices in Japan, where cast-in-place concrete is placed in relatively wide and deep connections at diaphragms in conjunction with high levels of PT.⁽⁸⁾ The Canadian Bridge Design Code assumes that the load is transferred from one beam to another primarily through transverse shear; transverse rigidity is neglected.⁽¹⁰⁾ Figure 5.7.1.8 in the Canadian Bridge Design Code provides charts to determine the shear force, and a reinforced concrete structural slab is required to provide the shear transfer between beams.⁽¹⁰⁾

Shear key connection deterioration can be caused by many factors, such as shrinkage during curing of the grouting material, the live load being transferred through the shear key to/from adjacent beams, thermal effects, misalignment, and general poor construction practices. Prior research on shear key connections has investigated the possibility of using a variety of grout materials (e.g., magnesium ammonium phosphate grout and epoxy grout) and shear key designs (e.g., partial- and full-depth shear keys) as well as improving the mechanical behavior of the connection (e.g., providing increased transverse PT).^(11–13)

This research compared and evaluated four connection designs. Two of the connection details are currently considered to be operating in good practice by utilizing high-strength, non-shrink grout in combination with transverse PT. The other two details utilize ultra-high performance concrete (UHPC). Full-scale tests were conducted to investigate the factors affecting shear key performance, including simulated traffic loads, thermal loads, and the magnitude of transverse PT forces.

OBJECTIVE

This research evaluated and compared the performance of four connection designs. Based on the results, quantitative measures to assist bridge owners in evaluating existing shear key performance are suggested. Three critical design parameters were investigated: (1) the magnitude of the transverse PT force, (2) the transverse shear strength of the shear key, and (3) the condition of the interface between the connection material and box beam concrete. Design suggestions are provided for each of these parameters that can be used to assist bridge owners in the use of this economical bridge system.

REPORT ORGANIZATION

This research investigated different connection designs for precast prestressed box beam bridges. The results of full-scale testing of four connection details are included in this report, which is divided into six chapters and an appendix. Chapter 1 provides an introduction and the objective of the research. The experimental test setup is presented in chapter 2. Chapter 3 presents the analytical models and assumptions used to compare the specimens. Chapter 4 includes the results from the thermal and cyclic loading. Discussion of the effects of different design parameters are presented in chapter 5. Chapter 6 delivers the summary and conclusions of this research. Finally, the appendix provides drawings associated with the fabrication of the box beams.

A TechBrief was published that provides an executive summary of the information contained in this report.⁽¹⁾ Additionally, a peer-reviewed journal paper by Yuan and Graybeal also presents the results of this study.⁽¹⁴⁾