EFFECT OF TEMPERATURE GRADIENTS AND GIRDER SUPPORT CONDITIONS ON THE BEHAVIOR OF BRIDGE DECK LINK SLABS
Creators: name: Cardona, Sandra Ximena Villamizar
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 400510
subject: Structural engineering

description: Link slabs offer a cost-effective solution for eliminating deck expansion joints in multi-span bridges. A link slab is the cast-in-place concrete portion that makes only the deck slab continuous while the girders remain simply supported between two adjacent deck spans. By closing the expansion-joint opening, link slabs can reduce the costs of repairing and rehabilitating leaking joints and improving the bridge riding surface. Link slabs are designed to resist the bending moments imposed by girder end rotation due to live load plus impact, assuming the bridge spans are simply supported at the joints. The continuity provided by the link slab under live load is neglected, based on the assumption that its stiffness is lower than that of the girders. Furthermore, structural elements capable of load transfer (e.g., stirrups and shear stud connectors) within the limits of the deck joint elimination are often removed to reduce induced stresses in the link slab. A bond breaker is placed between the top of the girders and the bottom of the link slab to mitigate stresses. The debonded length, typically set at 5% of each span length, defines the total length of the link slab. Practices may vary among states, such as Indiana, where a composite action between the link slab and supporting girders is maintained. However, increased cracking observed in the field is the primary concern about debonded link slabs. Once the cracks form, they allow the entrance of corrosive chemicals and debris, causing deterioration of concrete bridge components. The causes of the increased cracking and resulting leakage at the link slabs have been associated with the limitations of the existing design approaches in considering the effects of thermal loads and support conditions. This study presents a comprehensive finite element analysis to evaluate the behavior of bridge deck link slabs under the combined effect of traffic loads and vertical temperature gradients. The link slabs are subjected to HL-93 loading and temperature gradients following AASHTO LFRD Bridge Design Specifications. A finite element model of the Plott Creek Bridge in Haywood country in North Carolina, instrumented by Wing &amp; Kowalksy (2005), is developed using ABAQUS/Standard software. The numerical model is validated against test data from previous studies available in the literature.The results of the numerical investigation reveal that vehicular traffic loading is the primary factor contributing to the cracking of the link slabs. However, vertical temperature gradients are also identified as significant factors inducing stresses within the link slabs. Specifically, the combination of live load and a negative temperature gradient is the most influential loading condition contributing to cracking at the top surface of the link slabs. It is important to note that the rotation of the girder ends due to live load induces a negative moment (tension at the top) on the link slab. A negative temperature gradient, where the temperature on the top deck surface is lower than that on the web of the beams, results in an additional negative moment on the link slab due to its addition to the rotation from the live load. The temperature gradients are observed to increase the girder end rotation obtained from live load analysis for simply supported beams by approximately 20% in the range of parameters considered in the present study. This finding underscores the importance of considering temperature effects in link slab design to ensure structural integrity. Furthermore, parametric studies are conducted to assess the impact of various factors such as girder support conditions, span length, debonded zone length, and material properties on crack initiation in link slabs. The analyses show that the primary factors affecting the tensile stress developed in the link slabs are the span length and the girder support conditions. This highlights the importance of considering these factors when designing link slabs. Based on the findings, design recommendations are proposed to enhance the current practices for link slab design. These recommendations include considering temperature gradients alongside live loads, adopting distributed bar spacing for crack control, and incorporating an allowable stress limit of 0.60fy for steel reinforcement following AASHTO LFRD Bridge Design Specifications. Given that link slabs exhibit cracking under service conditions, it is advisable to determine the amount of longitudinal tension reinforcement based on cracked section analysis rather than simply providing the minimum reinforcement. Furthermore, incorporating a debonded zone within 5% to 7.5% of the span length at each side of the link slab is recommended to reduce stresses. The use of roller support is not recommended for link slab applications, while hinge supports can be effective if the span length is less than 15~m (50~ft.).