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This paper presents an experimental study on the anchorage behavior of drilled shaft reinforcement subjected to tension in drilled shaft footings loaded under combined axial force and uniaxial bending moment. Four large-scale tests were conducted on drilled shaft footing specimens employing an equivalent loading condition 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 for all three anchorage details. Finally, a critical section is also proposed in this study to perform the anchorage check for the drilled shaft reinforcement in drilled shaft footings designed with the strut-and-tie method. The definition of the critical section provides a safe estimate of the available development length of the drilled shaft bars according to the findings of the experimental program., Peer Reviewed, Postprint (author's final draft)

This paper presents an experimental study on the structural behavior of reinforced concrete drilled shaft footings, also commonly referred to as pile caps, supported on four drilled shafts. The test program investigated the effects of three design parameters related to reinforcement detailing: layouts of bottom mat reinforcement (grid versus banded), anchorage details for bottom mat reinforcement (straight versus hooked bar), and ratios of side-face reinforcement to control shrinkage and temperature cracking. Seven large-scale drilled shaft footing specimens, each constructed with a center-located square column stub, were tested under uniform axial compression column loading. The specimens represent typical drilled shaft footings, which can generally be considered as three-dimensional (3-D) deep (D-region) elements. The test results indicate that the ultimate strengths of the footings were comparable when the bottom mat reinforcement was properly developed, regardless of the type of layouts and anchorage details. Nevertheless, different crack patterns were obtained for different bottom mat reinforcement layouts. The absence of side face reinforcement led to lower capacity and more brittle failure as compared to footings having a minimum amount of side-face reinforcement. Finally, some refinements are proposed to existing 3-D strut-and-tie modeling guidelines to more accurately reflect the ultimate strengths of the test specimens., Peer Reviewed, Postprint (author's final draft)

This paper presents an experimental study on the anchoragebehavior of column reinforcement subjected to tension in drilled shaft footings loaded under combined axial force and uniaxial bending moment. Large-scale tests were conducted on four footing specimens that were constructed with different column bar anchorage details: straight bars, hooked bars with two different hook orientations, and headed bars. All tension-loaded column reinforcement was shown to yield regardless of anchorage type. Further, all anchorage types developed stresses in the vicinity of the anchorage region except for bars with end hooks that were oriented outwards from the base of the column. Properly oriented hooked bars, considering the internal force flow of the strut-and-tie model, and headed bars developed more uniform stress distributions over their lengths as compared to straight bars. Based on developed stress distributions for the column reinforcement estimated fromstrain measurements, a critical section was also proposed to establish the anchorage requirement for the column reinforcement in a three-dimensional strut-and-tie model., Peer Reviewed, Postprint (author's final draft)

Project performed in cooperation with the Texas Department of Transportation and the Federal Highway Administration at the Center for Transportation Research of The University of Texas at Austin., A comprehensive study was conducted to characterize the structural response and develop design guidelines for drilled shaft footings. The study included large-scale testing and numerical analyses of footing specimens subjected to various loading conditions. A database of 35 drilled shaft footings constructed in Texas by TxDOT was established and analyzed for designing test specimens. A total of 19 large-scale specimens were designed and tested to study various design parameters and loading scenarios including vertical compression and uniaxial bending. A series of numerical analyses employing experimentally-verified models were also performed to account for the effect of additional design parameters that could not be covered in the experimental program. Based on the data and insights obtained from the experimental and numerical studies, 3D strut-and-tie modeling guidelines for drilled shaft footings are proposed by refining current provisions for 2D strut-and-tie models in AASHTO LRFD (2020). The new guidelines include the definition of the 3D nodal geometry at bearing faces, refinements for strength modification factors, critical section definitions for development of horizontal and vertical ties, and recommendations for bottom mat reinforcement configuration. Project findings have indicated that the proposed recommendations improve the accuracy of the ultimate strength predictions for a database including drilled shaft footing tests from the literature and the current study, without generating unconservative or overly conservative predictions. This represents an improvement of the accuracy achieved using the recommendations of TxDOT Project 5-5253-01. Lastly, a design example of a drilled shaft footing subjected to various loading scenarios is provided., Preprint

0-7039, The strut-and-tie method (STM) is a reliable and widely used approach for designing reinforced concrete members, but there are still gaps in knowledge that need to be addressed to optimize its use in common design scenarios. Addressing these gaps would lead to more efficient and cost-effective design solutions. This requires additional experimental data to inform design codes and practices related to key areas, such as tri-axial tension anchorage, curved-bar nodes, confining reinforcement in nodal zones, and crack control reinforcement spacing. Chapters 3-6 of the research report describe the experimental investigations conducted to address some of these knowledge gaps. Chapter 3 presents insights into the behavior of drilled shaft reinforcement under tri-axial tension and the validity of the design recommendation. Chapter 4 investigates the effect of the bend radius on the behavior of closing knee joints and recommends an equation for designing curved-bar nodes. Chapter 5 examines the effect of nodal zone confinement on nodal strength, concluding that effective confining reinforcement can increase nodal strength. Lastly, Chapter 6 investigates the possibility of relaxing crack control reinforcement spacing and recommends a new maximum spacing. Overall, the research findings provide valuable insights into the behavior of reinforced concrete members and suggest improvements for STM design practices.

0-7039, The strut-and-tie method (STM) is a simple and widely used method for designing D-regions in reinforced concrete members. Despite its incorporation into major design codes and continuous refinement over the past century, there are still areas that require further research. These include the use of 3D strut- and-tie models for designing drill shaft footings under eccentric axial loads, the use of curved- bar nodes in closing knee joints, the provision of appropriate confinement to enhance nodal strength, and the relaxation of the “d/4 limit” on crack control reinforcement spacing based on experimental data. Addressing these knowledge gaps would improve the applicability and cost- effectiveness of STM in reinforced concrete deep member design.