Steven M. Barbachyn

Application of multiple digital image correlation sensors in earthquake engineering

As structural testing has become more expensive, researchers are pushing to capture more and better data with each structural test conducted. Digital image correlation (DIC) is a tool that is gaining popularity as one way to capture more detailed information in testing programs. In threedimensional DIC (3D-DIC), the measured object is photographed with a pair of digital cameras before and after a load event and a stochastic pattern marked on the object is tracked through the images such that a near full field of displacements is derived. Setup of the system involves mounting the two cameras securely to a rigid bar, and capturing photographs of a NIST certified calibration object. These calibration images are used to determine the orientation of the cameras with respect to one another, and then during testing, 3D locations of pattern sub-regions (called facets) are determined via photogrammetric triangulation principles. Historically, the cost of the system, knowledge of how to correctly specify appropriate testing protocol, and how to correctly interpret the results have limited the application of this promising technology to structural testing. The current paper focuses on three main aspects of DIC technology. First, a treatment of the basic theory behind the method is provided. Included are recommendations and guidelines for accuracy and other lessons learned during deployment over a broad range of projects. The final portion of the paper is a case study of the deployment of DIC on the large scale lateral load testing of a novel reinforced concrete coupled wall system. The bottom three stories of an eight story building were constructed in the laboratory – the top five stories were simulated using hydraulic actuators at the top of the test specimen. Fourteen DIC sensors were deployed simultaneously during the test, believed to be the largest number of simultaneous deployment for structural testing.

LATERAL LOAD BEHAVIOR OF A POST- TENSIONED COUPLED CORE WALL

A 40%-scale multi-story reinforced concrete coupled core wall structure with unbonded posttensioned coupling beams was recently tested under quasi-static reversed-cyclic lateral loading. This paper provides an overview of the design and experimental results from this test. Conventional reinforced concrete coupling beams in seismic regions are often designed with two intersecting groups of diagonal reinforcing bars crossing the beam-to-wall joints. The placement of these reinforcing bars is a major challenge during construction. The new system eliminates the diagonal reinforcement by using a combination of high-strength unbonded post-tensioning (PT) steel and top and bottom horizontal mild steel reinforcing bars crossing the beam-to-wall joints to develop the coupling forces. The coupled wall specimen that was tested represented the most critical bottom three stories of an eight story prototype structure, consisting of two C-shaped wall piers, six post-tensioned coupling beams (two beams at each floor because of the C-shaped piers), tributary post-tensioned slabs at each floor, and the foundation. The less critical upper stories of the prototype structure were simulated analytically to obtain the axial forces and overturning moments imposed at the top of the bottom three stories. In addition to a dense array of conventional sensors, the deformations of the test specimen were monitored using a total of 14 two and three-dimensional digital image correlation (DIC) sensors, providing near-full-field response data of the most critical regions of the structure in the wall piers, floor slabs, and coupling beams. Ultimately, the high-fidelity measured data from the test specimen will be used to validate seismic design procedures and modeling/prediction tools for post-tensioned coupled wall structures. These procedures and tools may form the basis for the future implementation of this novel structural system as “special” reinforced concrete shear walls in medium and high seismic regions of the U.S.

Testing and behavior of a coupled shear wall structure with partially post-tensioned coupling beams

A 40%-scale coupled core wall structure with C-shaped wall piers and novel unbonded post-tensioned (PT) coupling beams was tested under quasi-static reversed-cyclic lateral loads combined with tributary gravity loads. This paper describes the design, analysis, and testing of this specimen, which included the bottom three stories, the tributary floor slabs, and a large portion of the foundation from an eight-story prototype structure. The upper five stories of the prototype structure were simulated analytically to impose forces and moments at the top of the test specimen. In addition to conventional sensors, the specimen was monitored using 14 digital image correlation (DIC) sensors, providing near-full-field response data of the most critical regions. Overall, the structure performed as predicted, validating the design approach. Strength loss at the end of the test was largely caused by the fracture of the vertical reinforcing bars in the wall pier toes at the base. The coupling beams performed well, demonstrating the advantages of the new PT system.

Experimental Evaluation of a Multi-Story Post-Tensioned Coupled Shear Wall Structure

This paper discusses the design and experimental evaluation of a novel seismicresistant reinforced concrete (RC) coupled shear wall system. In this system, the widely-used unbonded post-tensioned floor slab construction method is adapted to couple (i.e., link) two RC wall piers, providing significant performance and construction benefits over conventional RC coupling beams in high seismic regions. Previous experiments of post-tensioned coupled wall structures are limited to floorlevel coupling beam subassemblies. The current paper extends the available research to multi-story structures by presenting the design of an 8 story prototype test specimen consisting of two C-shaped shear walls. The design is validated through the testing of a simplified 15% scale specimen in the laboratory. The experimental specimen includes the foundation, the first three floors of the shear walls, and the associated coupling beams. The upper stories of the building are simulated with hydraulic jacks that supply the appropriate bending moment, shear, and axial forces at the top of the laboratory structure. This paper compares the measured load displacement response of the laboratory structure with predictions from design models. Experimental and design predictions of several key behavior parameters are shown to match well. Future work involves the construction and testing of large scale (40%) specimens to validate the approach. Ultimately, the measured information from the test specimens will be used in the development of validated design procedures and modeling/prediction tools for multi-story post-tensioned coupled wall structures.

Advanced Sensing Techniques for Damage Detection in Reinforced Concrete Structures

This paper primarily presents a comparison of traditional and advanced sensing techniques in the field of Structural Health Monitoring for use in damage detection in reinforced concrete (RC) structures. The accuracy of these methods is evaluated through standard laboratory tests on concrete cylinders. Furthermore, a damage detection method for RC structures is introduced where strains measured from densely clustered sensors are used to develop damage sensitive features. This method is verified through simulation data from a Fiber Element model of a new earthquake resistant RC coupled shear wall system. A large scale specimen of this system with a dense network of embedded strain gauges, displacement and rotation transducers, as well as Digital Image Correlation systems was recently tested. The data collected through this experiment will be used to experimentally validate the proposed damage detection method.

Testing of a Post-Tensioned Coupled Shear Wall Structure

A 40%-scale reinforced concrete coupled core wall structure with novel unbonded post-tensioned coupling beams was recently tested under quasi-static reversed-cyclic lateral loading combined with tributary gravity loads. This paper provides an overview of the design, analysis, and testing of this specimen. The specimen represented the most critical bottom three stories of an eight story prototype structure, consisting of two C-shaped wall piers, six post-tensioned coupling beams, tributary post-tensioned slabs at each floor, and the foundation. The less critical upper stories of the prototype structure were simulated analytically to impose lateral forces, axial forces, and overturning moments at the top of the laboratory specimen. Overall, the structure performed as predicted and validated the design approach, which was conducted for a maximum roof drift ratio of 3% for the full 8-story structure. Strength loss during the final cycles of the test was largely caused by the fracture of the vertical reinforcing bars in the wall pier toes at the base. The post-tensioned coupling beams performed well, demonstrating the advantages of the new system.

Economic Evaluation of High-Strength Materials in Stocky Reinforced Concrete Shear Walls

AbstractPrevious numerical research has shown that high-strength steel reinforcing bars (rebar) combined with high-strength concrete can increase the lateral strength of stocky (i.e., low height-to...

Measured Behavior of a Reinforced Concrete Coupled Wall with Fully Post- Tensioned Coupling Beams

Recent results from the large-scale experimental evaluation of a multi-story coupled shear wall system with fully post-tensioned coupling beams are discussed. In this novel system, high-strength unbonded post-tensioning (PT) strands are used to couple (i.e., link) reinforced concrete wall piers. To validate the new system, reversed-cyclic quasi-static testing of a 40%-scale coupled wall structure with the proposed details was conducted. The laboratory specimen represented the most critical bottom three stories of an eight-story prototype structure, consisting of two Cshaped wall piers, six coupling beams (two beams at each floor level), tributary slabs at each floor, and the foundation. The other (less critical) regions of the structure were simulated analytically. The test specimen performed well, demonstrating ductile behavior through the completion of three full cycles at a lateral drift greater than the 3.0% roof drift demand used in design, thus supporting the design approach, assumptions, approximations, and tools. Ultimately, the test results also support the ACI classification of these structures as “special” reinforced concrete shear walls in moderate and high seismic regions of the U.S.

Nominal Shear Strength Limits for Short Diagonally-Reinforced Concrete Coupling Beams

Sections 21.9.4.5 and 21.9.7.4 of ACI 318-08 limit the nominal shear strength, Vn of concrete coupling beams, including short beams with diagonal reinforcement, to Vn≤0.83Acw√f’c (with f’c in MPa units, or 10Acw√f’c with f’c in psi units). Many experimental studies have been conducted on the design and detailing of diagonallyreinforced coupling beams for ductile behavior under seismic loading. The primary objective of the current ACI strength limit is to prevent the splitting and/or crushing of the diagonal strut in these beams. Through an analytical parametric investigation of isolated floor-level subassemblies, this paper shows that the boundary conditions utilized in the previous coupling beam experiments may not be adequate to investigate this important failure mode. The analytical model that forms the basis of the study uses a strut-and-tie representation of the forces developing inside a coupling beam under lateral loading. Multi-story coupled wall analyses are also conducted.