3D strut-and-tie design recommendations for column and drilled shaft anchorages in drilled shaft footings

A drilled shaft footing is a deep structural member subjected to nonlinear strain distribution, known as D-regions, and therefore recommended to be designed using the strut-and-tie method (STM). Drilled shaft footings supported on four drilled shafts require a three-dimensional (3D) configuration of struts and ties to reproduce the internal force flow of the footings. However, a lack of clear design guidelines and experimental verification for the anchorages of the column and drilled shaft reinforcement in drilled shaft footings hinder the practical use of the 3D STM in the design of these types of components. This dissertation covers part of a research project on drilled shaft footings subjected to various loading scenarios (Phase I: uniaxial compression-only; Phase II: axial compression combined with moderate uniaxial flexure; and Phase III: axial compression combined with large uniaxial flexure). A comprehensive study, comprising large-scale tests and finite element analyses, presented here is specifically focused on drilled shaft footings subjected to combined axial compression and two different uniaxial bending moments: moderate (Phase II) and large uniaxial bending moment (Phase III). The anchorage behavior of column and drilled shaft reinforcement subjected to tension in drilled shaft footings loaded under the combined axial compression and uniaxial bending moments were investigated experimentally and numerically. Phase II large-scale tests were conducted on four footing specimens designed with different column bar anchorage details: straight bars, hooked bars with two different hook orientations, and headed bars. All column reinforcement in Phase II specimens could yield during the tests regardless of the anchorage types. Furthermore, all anchorage types developed reinforcing bar stresses in the vicinity of the anchorage region, except for the hooked bars oriented outwards to the column. The properly-oriented hooked bars, considering the internal force flow of the strut-and-tie model, and the headed bars developed a relatively uniform reinforcing bar stress distribution throughout their length than the straight bars. Based on experimentally-measured stress distributions for the column reinforcement, a critical section was also proposed to establish the anchorage requirement for the column reinforcement in a 3D strut-and-tie model. Additionally, four large-scale tests were conducted on drilled shaft footing specimens employing an equivalent loading condition of Phase III loading scenario by introducing tension in the drilled shaft reinforcement. Three different anchorage details were tested: straight bars, hooked bars, and headed bars. The drilled shaft reinforcement was capable of developing its full yield strength in tension in all the tests, regardless of the anchorage detail. The tensile stresses in drilled shaft bars were primarily developed in the region of the embedment length closest to the interface between the drilled shaft and the footing, while negligible stress and slip were measured in the vicinity of the unloaded end of the bars. Based on the findings of the experimental program, a critical section was also proposed to establish the anchorage requirement for the drilled shaft reinforcement in a 3D strut-and-tie model. Numerical parametric studies were also conducted to include more design parameters that can affect the position of the proposed critical sections for the column and drilled shaft reinforcement to solidify the proposed critical sections. The analysis results could verify the conservativeness of the proposed critical sections for the column and the drilled shaft reinforcement. Lastly, a series of the 3D STM design guidelines were proposed by refining the current two-dimensional (2D) STM design guidelines on the basis of the test data and insights obtained from the experimental and numerical parametric studies. To the author’s knowledge, this study firstly established the database of drilled shaft footings subjected to uniaxial flexural compression loading scenarios; therefore, only a total of nine test specimens in this study could be evaluated using the refined guidelines. As a result, the accuracy of the ultimate strength predictions was improved without any unconservative or overly conservative predictions. A design example of a drilled shaft footing subjected to various uniaxial loading scenarios is also provided.