Analytical Model Validation of a Hybrid Precast Concrete Wall for Seismic Regions

This paper presents an ongoing research project on the validation of “hybrid” precast concrete wall structures for use in seismic regions. Hybrid precast walls utilize a combination of mild (e.g., Grade 60) steel and high-strength unbonded posttensioning (PT) steel for lateral resistance across horizontal joints. The mild steel reinforcement is designed to yield and provide energy dissipation. The unbonded PT steel provides self-centering capability to reduce the residual lateral displacements of the wall from a large earthquake. Both the PT steel and the mild steel contribute to the lateral strength, resulting in an efficient structure. The measured behavior of a 0.4-scale hybrid precast concrete wall test specimen is compared with an analytical model, focusing specifically on the applied lateral load and displacement, energy dissipation, behavior of the steel reinforcement, and behavior along the horizontal base-panel-to-foundation joint. The results from the analytical model were found to be consistent with the results from the testing of the wall. INTRODUCTION AND BACKGROUND As shown in Figure 1, the hybrid precast concrete wall system investigated in this research utilizes a combination of mild (e.g., Grade 60) steel and high-strength unbonded post-tensioning (PT) steel for lateral resistance across horizontal joints. The PT steel is provided by multi-strand tendons placed inside un-grouted ducts to prevent bond between the steel and concrete. Thus, the tendons are connected to the structure only at end anchorages. Under the application of lateral loads into the nonlinear range, the primary mode of displacement in these walls occurs through gap opening at the horizontal joint between the base panel and the foundation. Upon unloading, the PT steel provides a restoring force to close this gap, thus reducing the residual (i.e., permanent) lateral displacements of the wall after a large earthquake. The use of unbonded PT tendons delays the yielding of the strands and reduces the tensile stresses transferred to the concrete (i.e., reduced cracking) as the tendons elongate under lateral loading. The mild steel bars crossing the horizontal joint at the wall base are designed to yield in tension and compression and provide energy dissipation through the gap opening/closing behavior. A pre-determined length of these bars is unbonded at the bottom of the base panel (by wrapping the bars) to prevent low-cycle fatigue fracture. Both the PT steel and mild steel contribute to the lateral strength of the wall, thus, resulting in an efficient structure. Hybrid precast wall structures can offer high quality production, simpler construction, and excellent seismic characteristics. However, these walls are currently not allowed by ACI 318 (2008) unless their lateral performance is demonstrated through experimental evidence and analysis. To address this limitation, the primary objective of this ongoing research project at the University of Notre Dame is to experimentally and analytically validate hybrid wall structures for code approval according to the guidelines, prerequisites, and requirements in ACI ITG-5.1 (2007) and ACI 318. The specific project objectives are to develop: (1) a validated seismic design procedure for the hybrid precast wall system; (2) validated analytical models and design tools; and (3) practical guidelines and experimental evidence demonstrating the performance of these structures under lateral loading. In accordance with these objectives, the current paper compares the post-test analysis results with the measured behavior of a wall test specimen. The procedure that was used to design the specimen and the results from a pre-test analytical study can be found in Smith and Kurama (2009). OVERVIEW OF VALIDATION AND TESTING REQUIREMENTS The roadmap to the code validation of hybrid precast concrete walls is provided by ACI ITG-5.1, which lays out the minimum experimental evidence needed for the classification of these walls as “special” reinforced concrete (RC) shear walls based on ACI 318. Specific requirements are given with regards to the tested wall roof drift, ∆w, measured wall lateral strength to the predicted strength ratio, PT strand stresses and strains, amount of energy dissipation, wall strength degradation, and shear slip along the horizontal joints, among other requirements. The design is conducted at two levels of wall drift as follows: (1) the design-level drift, ∆wd, which is determined according to the requirements of ASCE 7 (2006); and (2) the validation-level drift, which is defined by ACI ITG-5.1 as: Figure 1. Elevation, Exaggerated Displaced Position, and Cross-Section of Hybrid Wall System ( ) % 0 . 3 5 . 0 8 . 0 % 9 . 0 ≤ + ≤ = Δ w w wm l h (1) where, hw is the height to the top of the wall, and lw is the length of the wall. The wall drift, ∆w is defined as the lateral displacement at the top of the wall divided by the wall height. Prior to the validation testing, ACI ITG-5.1 requires that a pre-test design/analysis procedure for the specimen be established. A few key ACI ITG-5.1 requirements include: (1) the use of a minimum of two wall panels in the test structure (in order to model a representative panel-to-panel joint as well as the basepanel-to-foundation joint) unless the prototype structure uses a single panel for the full height of the wall; (2) a minimum specimen scale of one-third; (3) a minimum wall height-to-length aspect ratio of 0.5; and (4) the use of similar reinforcement details and representative building materials as in the full-scale prototype structure. OVERVIEW OF TEST SET-UP, PROCEDURE, AND SPECIMEN A photograph of the test specimen and a schematic of the test setup are shown in Figure 2. As described in Smith and Kurama (2009), the specimen was designed for a 4-story prototype parking garage with an approximate footprint of 42,000-sq-ft. The test was conducted at 0.4-scale, which satisfies the minimum scaling limit of ACI ITG-5.1. The lateral load was applied at the resultant location of the 1 mode inertial forces (12-ft from the wall base), resulting in a wall base moment to shear ratio of Mb/Vb=1.5lw. An external downward axial load of about 73 kips was applied at the centerline of the wall at the top to simulate the gravity loads acting on the prototype structure. The test wall featured two panels: the base panel representing the 1 story and the upper panel representing the 2 through 4 stories, thereby satisfying the ACI ITG-5.1 requirement for testing multi-panel walls. It was possible to model the upper story panels of the prototype wall as a single panel since the joints between these panels were designed not to have any significant gap opening. For the subject test wall, the length, lw, was 96-in, the height of the base panel, hpb, was 57.5-in, and the wall panel thickness, tw, was 6.25-in. The PT steel consisted of two tendons located 9-in north and south from the wall centerline. Each tendon included three 0.5-in diameter strands (design ultimate strength, fpu=270-ksi) with an unbonded length from the top of the wall to the bottom of the foundation beam of about 18-ft. The average initial stress in the tendons, calculated from the measured strand forces prior to the application of the lateral load, was fpi=0.55fpu. The mild steel (i.e., energy dissipating steel) crossing the base joint consisted of four No. 6 bars (design yield strength, fsy=65-ksi), with one pair of bars placed 6-in north and south from the wall centerline and the other pair 3-in north and south from the centerline. The energy dissipating bars were unbonded over a length of 10-in at the bottom of the base panel. Across the panel-to-panel joint, only two No. 6 bars were used, with one bar located at each end of the wall. This reinforcement was not designed to yield or dissipate energy, but to control any gap opening along the panel-to-panel joint. To prevent strain concentrations in the panel-to-panel joint reinforcement, a short 3-in length of the bars was unbonded at the bottom of the upper panel. The design unconfined concrete strength for the wall was 6.0-ksi and the design confined concrete strength (within the toes of the base panel) was 9.1-ksi. However, the measured unconfined concrete strength for the base panel was only 4.8-ksi on the day that the wall was tested. (Front Face of Wall) NORTH