NEESR-II: Inelastic Web Crushing Performance Limits of High-Strength-Concrete Structural Walls

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AbstractNEESR-II: Inelastic Web Crushing Performance Limits of High-Strength-Concrete Structural WallsPI: Rigoberto Burgueno, Michigan State University; Co-PI: Eric M. Hines, Tufts UniversityMotivation & Scope: Significantly lighter members for structural walls in moderate seismic zones are a viable possibility by using high-strength concrete and incorporating ductile shear failures as a new genre of ductile failure mechanisms. Recent research on the seismic design of hollow piers has provided new insights on the accurate assessment of elastic and inelastic web crushing shear capacity of structural walls with boundary elements. Ductile shear failures, displayed as web crushing failures or yielding of the transverse reinforcement at relatively high levels of displacement ductility, allow for easy repair since damage to the boundary elements can be minimal. The advent of high-strength concrete has generated great interest in the promise that it may provide for cost-effective seismic design. However, its potential cannot be fully realized due to current outdated and prescriptive design criteria. Rational assessment models show that web crushing is linearly related to concrete compressive strength, indicative of new possibilities for increased shear capacities of lighter members with increased concrete strength. This project will verify this promise by establishing the inelastic web crushing limits for structural walls. Objectives: The goal of this project is to investigate and establish rational performance levels for the development of seismic assessment and design approaches to high-strength-concrete (HSC) structural walls based on ductile shear failure mechanisms. Specifically, the project will: (1) investigate and establish the web-crushing performance limits of HSC structural walls at moderate ductility, (2) investigate the bi-directional seismic performance of structural wall assemblies in the context of hollow piers, (3) develop analytical modeling and analysis procedures for structural walls with boundary elements, and (4) develop simple assessment models for HSC structural walls.Approach: The research objectives will be achieved through integrated experimental and analytical investigations. The first part of the experimental investigation will focus on the determination of dependable limits to web crushing failures for ductile shear response in HSC structural walls through 8 quasi-static monotonic and cyclic tests on 1/4-scale walls with concrete strengths of 34, 69, 103, and 137 MPa. Parallel analytical investigations will focus on the development and validation of assessment tools for structural walls loaded in their principal and diagonal directions through 3D nonlinear finite element models and simpler sectional analyses. Using the improved assessment models, two 1/4-scale HSC (137 MPa) wall assemblies analogous to hollow piers will be designed and tested under bidirectional loading. One assembly will be designed to obtain a web crushing failure at low ductility levels to validate the established limits for systems under combined loading. The second unit will be designed to fail in a ductile shear failure mode at high ductility levels. Conventional and advanced non-contact strain measurements will be correlatedwith analysis results to fundamentally understand the associated deformation limits. Rational, yet simple, assessment models will be developed to provide designers with practical tools for the design of HSC structural walls with reliable ductile shear failure modes. A website will be developed to disseminate results to researchers, designers, educators and students. Transition to practice will be pursed by active participation of the PIs in technical committees.NEES Use: The research plan will strategically combine the experimental resources of Michigan State University's Civil Infrastructure Laboratory to conduct the required conventional pseudo-static investigations, and the new capabilities provided by the Multi-Axial Subassemblage Testing (MAST) NEES facility at the University of Minnesota-Twin Cities to evaluate the bi-directional performance of structural wall assemblies.Collaborative Elements: The research team consists of collaboration between Michigan State University and Tufts University, which combines strengths and resources in experimentation, analysis, design practice and education. The research effort will be assisted by an external advisory board with significant project-related experience.Intellectual Merit: Increased understanding of earthquake design principles has become so robust that seismicsafety is rarely compromised. Rather, advancement in materials science, increased knowledge on structural behavior and the availability to perform complex computational simulations indicate a proper moment to migrate from current conservatism towards improvements in immediate and long-term cost and enhancement of structural elegance without sacrificing safety. The establishment of performance limits for high-strength-concrete structural walls behaving in alternative ductile modes of failure is expected to contribute to the groundwork of the next stage in earthquake engineering design of thin-webbed elements and systems.Broad Impact: The project will integrate the research efforts to the educational missions of both collaborative institutions by fostering knowledge in earthquake engineering through: (a) training of two graduate students, (b) research experiences for two undergraduates from underrepresented groups, and (c) enhanced teaching curricula. A determined attempt will be made to recruit underrepresented graduate students using university programs. The research will contribute to the groundwork of the next stage in seismic design by migrating from conservative approaches through establishment of rational performance limits for HSC in ductile shear failure modes.