Date of Award
5-2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical and Civil Engineering
First Advisor
Nakin Suksawang
Second Advisor
Paul Cosentino
Third Advisor
Jean-Paul Pinelli
Fourth Advisor
David Fleming
Abstract
A mathematical model is proposed for the confinement of rectangular confined reinforced concrete elements under compression. The model was modified from existing models applied to rectangular confined concrete specimens. The present model applies to rectangular reinforced concrete cross-sections with a core aspect ratio of at least 2.0 and a high effective confinement index. The model applies to normal and high-strength concrete and incorporates the spacing of the transverse reinforcing hoops, the strength of the reinforcing bars, the reinforcing quantity, and the core aspect ratio of the section. The proposed model was verified against extensive experimental data, and there was a good agreement.
Double composite steel box girder bridges utilize the excellent compressive properties of concrete and replace traditional bottom flange stiffeners with concrete in the negative moment region of continuous girder bridges. This allows for a more efficient and economical design. It is hypothesized that the discontinuity in the bottom concrete slab, resulting from steel diaphragms at the bearing location, leads to an improper stress transfer between one side of the composite girder and the other at the support. To remedy this issue, it is proposed that a continuous concrete bottom slab is used to better transfer the forces in the girder in the negative moment region. Furthermore, it is suggested that the bottom concrete slab should be highly confined using reinforcing steel in the bearing region. The confined concrete will increase the slab's maximum compressive strength and ductility, which can be an issue of high strength concretes. The confined concrete will transfer bending, shear, and torsional forces between the bridge section and the supports.
Double composite steel box girder bridges present many advantages, especially in the negative moment region, over single composite steel box girder by increasing the load-carrying capacity and stiffness. This leads to cost savings in structural steel, simplified fabrication details, and reduced cross-frame requirements. However, significant drawbacks with current practices are the added complexity of casting the bottom slab and the increased weight of the superstructure that needs to be supported by the substructure. To mitigate these drawbacks, a confined concrete bottom slab is proposed. The confinement of the concrete increases the strength and ductility of the bottom concrete slab so that it can match the high strain the steel flange experiences. This enables the utilization of high-performance steel with 485 MPa (70 ksi) yield strength, reducing the cross-sectional area of the steel box girder. The confined concrete also eliminates the intermediate diaphragms, making the bottom slab's construction easier and continuous since adequate bearing capacity can be achieved with the confinement. The proposed method significantly reduces the cost of the superstructure section, reduces the number of bottom flange plates that need to be fabricated, and allows for easier placement of the reinforcing steel to the bottom of the box girder. Recommendations are made on the pattern and spacing of confinement to achieve the required concrete strength and ductility. General recommendations on dimensioning double composite steel box girder bridges are also recommended.
Recommended Citation
Craik, Harry, "Utilization of Confined Concrete in the Bottom Slab of Double Composite Box Girder Bridges" (2024). Theses and Dissertations. 1421.
https://repository.fit.edu/etd/1421