Harnessing the Mechanics of Thin-Walled Metallic Structures: from Plate-Lattice Materials to Cold-Formed Steel Shear Walls

Thin-walled structures have received a lot of interest during the last years due to their light weight, cost efficiency, and ease in fabrication and transportation, along with their high strength and stiffness. This dissertation focuses on the mechanical performance of thin-walled metallic structures from cold-formed steel shear walls and connections (PART I) to plate-lattice architected materials (PART II) via computational, experimental, and probabilistic methods. Cold-formed steel (CFS) shear walls subjected to seismic loads is the focus of PART I of this dissertation. An innovative three-dimensional shell finite element model of oriented strand board (OSB) sheathed CFS shear walls is introduced and benchmarked by nine different experimental studies. Particular attention is given to the fastener behavior since they are governed by significant inherent variability and they represent a dominant failure mechanism in CFS shear walls. Shear fastener behavior is experimentally determined and introduced into the finite element approach. To further address the connection variability, an extensive parametric analysis accompanied by Monte Carlo simulations are conducted. Design recommendations for higher capacity sheathings (fiber cement board (FCB) and steel-gypsum (SG) composite board) that are not currently enabled in design specifications are also introduced. Architected plate-lattice materials subjected to uniaxial compression is the focus of PART II of this dissertation. Architected materials are structures whose mechanical performance is governed by their geometry rather than their constituent material. Plate-lattices are composed of plates along the planes of crystalline structures. They represent the stiffest and strongest existing materials, since they can reach the Hashin-Shtrikman and the Suquet upper bounds. The stability and imperfection sensitivity of plate-lattices are evaluated in this work via elastic and plastic shell finite element analyses. Plate-lattice geometries of cubic symmetry are examined, such as the simple cubic (SC), the body-centered cubic (BCC), the face-centered cubic (FCC) structures and their combinations (SC-BCC, SC-FCC) over a range of relative densities between $\rho$*=0.5\% and $\rho$*=25\%. Imperfections are characterized by modal shapes at five different imperfection amplitudes. Finally, knockdown factors are recommended for these metamaterials

NONLINEAR FASTENER-BASED MODELING OF COLD-FORMED STEEL SHEAR WALLS UNDER LATERAL LOADS

As cold-formed steel (CFS) has increasingly been used in low- and mid-rise construction across United States, it becomes necessary to capture and evaluate its lateral response in both, sub-system/member level and system level. The main lateral resisting system in cold-formed steel construction is shear walls; shear walls are the focus of this work. In particular, the present study aims to shed light on the response of wood sheathed coldformed steel (CFS) shear walls exposed to earthquake events through nonlinear high fidelity fastener-based modeling. The numerical approach is fastener-oriented including nonlinear experimental-determined connector elements for steel-to-sheathing connections, orthotropic oriented strand board (OSB) modeling for sheathing material, contact implementation and linear spring hold-down simulation for preventing uplift. The numerical results are compared and validated by a previous experimental study, assessing the efficiency of fastener-based modeling to capture the peak load and displacement, the failure mechanisms and the overall structural behavior of sheathed cold-formed steel shear walls. Furthermore, cold-formed steel to sheathing shear fastener response is computationally examined and validated by a previous experimental work. The main goal of this work is to introduce a robust computational tool capable of demonstrating how wood sheathed cold-formed steel framed shear walls behave during a lateral load event with potential use in any cold-formed steel screw-fastened connection system, such as diaphragms and in any fastener-based cold-formed steel full building simulation

Defect-Defect Interactions in the Buckling of Imperfect Spherical Shells

We perform finite element simulations to study the impact of defect-defect interactions on the pressure-induced buckling of thin, elastic, spherical shells containing two dimpled imperfections. Throughout, we quantify the critical buckling pressure of these shells using their knockdown factor. We examine cases featuring either identical or different geometric defects and systematically explore the parameter space, including the angular separation between the defects, their widths and amplitudes, and the radius-to-thickness ratio of the shell. As the angular separation between the defects is increased, the buckling strength initially decreases, then increases before reaching a plateau. Our primary finding is that the onset of defect-defect interactions, as quantified by a characteristic length scale associated with the onset of the plateau, is set by the critical buckling wavelength reported in the classic shell-buckling literature. Beyond this threshold, within the plateau regime, the buckling behavior of the shell is dictated by the largest defect

Impact of Fastener Spacing on the Behavior of Cold-Formed Steel Shear Walls Sheathed with Fiber Cement Board

As cold-formed steel (CFS) is progressively used in high seismic regions as part of the designated lateral force resisting system, it is necessary to explore higher capacity systems. Furthermore, these systems must be fully enabled within relevant design specifications. The objective of this work is to provide design guidelines and performance benchmarks for cold-formed steel shear walls sheathed with fiber cement board (FCB). High-fidelity three-dimensional finite element modeling is introduced by investigating wall aspect ratio, and fastener spacing pattern. Herein, fasteners, which represent the critical load path in CFS shear walls, are modelled via experimentally-derived phenomenological models. An experimental program of monotonic and cyclic connection testing is conducted aiming to shed light on the response of cold-formed steel to fiber cement board sheathing connections. Connection backbone parameters are extracted from the experimental results and are implemented in the finite element model. As fastener spacing is decreased, failure mode shifts from fastener-dominant to the steel framing itself. This work aims to characterize this change in limit state and provide recommendations for design. Updates to AISI S400 are proposed, specifically in providing prescriptive design aids for the practicing engineer. Furthermore, the high-fidelity modeling approach expanded upon herein provides an analytical approach to explore the impact of detailing on wall performance. Fiber cement board-sheathed shear walls represent a wealth of design potential in increasing the lateral capacity available in cold-formed steel shear wall systems. This work provides the fundamental behavioral and limit state analysis towards eventual enabling within national specifications.The authors would like to acknowledge Dr. Benjamin W. Schafer and Dr. Tara C. Hutchinson for the collaborative discussions and advice within CFS-NHERI project. We would also like to thank the CFS-NHERI research group graduate students: Zhidong Zhang, Amanpreet Singh, Hernan Castaneda, and Xiang Wang

Impact of Fastener Spacing on the Behavior of Cold-Formed Steel Shear Walls Sheathed with Fiber Cement Board

As cold-formed steel (CFS) is progressively used in high seismic regions as part of the designated lateral force resisting system, it is necessary to explore higher capacity systems. Furthermore, these systems must be fully enabled within relevant design specifications. The objective of this work is to provide design guidelines and performance benchmarks for cold-formed steel shear walls sheathed with fiber cement board (FCB). High-fidelity three-dimensional finite element modeling is introduced by investigating wall aspect ratio, and fastener spacing pattern. Herein, fasteners, which represent the critical load path in CFS shear walls, are modelled via experimentally-derived phenomenological models. An experimental program of monotonic and cyclic connection testing is conducted aiming to shed light on the response of cold-formed steel to fiber cement board sheathing connections. Connection backbone parameters are extracted from the experimental results and are implemented in the finite element model. As fastener spacing is decreased, failure mode shifts from fastener-dominant to the steel framing itself. This work aims to characterize this change in limit state and provide recommendations for design. Updates to AISI S400 are proposed, specifically in providing prescriptive design aids for the practicing engineer. Furthermore, the high-fidelity modeling approach expanded upon herein provides an analytical approach to explore the impact of detailing on wall performance. Fiber cement board-sheathed shear walls represent a wealth of design potential in increasing the lateral capacity available in cold-formed steel shear wall systems. This work provides the fundamental behavioral and limit state analysis towards eventual enabling within national specifications.The authors would like to acknowledge Dr. Benjamin W. Schafer and Dr. Tara C. Hutchinson for the collaborative discussions and advice within CFS-NHERI project. We would also like to thank the CFS-NHERI research group graduate students: Zhidong Zhang, Amanpreet Singh, Hernan Castaneda, and Xiang Wang

Steel Sheet Sheathed Cold-Formed Steel Framed In-line Wall Systems. II: Impact of Nonstructural Detailing

Although cold-formed steel (CFS) framing systems have the potential to support the need for resilient housing, the use of CFS has been restricted due to gaps in understanding its structural behavior and by the limited guidelines provided in design standards. In particular, the contribution from nondesignated lateral systems and portions of the building system not specifically designated by the design engineers has not been substantially investigated through experiments. To address these shortcomings, a two-phased experimental effort was undertaken to assess the impact of gravity walls, finish application, window openings, and their relationship with the designated lateral force-resisting system. The wall-line assemblies tested, which have shear walls placed in-line with gravity walls, adopted chord stud packs with a tie-rod assembly and were either unfinished or finished, and laid out in a symmetrical or unsymmetrical fashion. In addition, both Type I and Type II shear wall and anchorage detailing were investigated. In this paper, the impact of test variables governing the nonstructural detailing of CFS-framed walls has been quantified, and a companion paper presents findings regarding the impact of structural detailing

Steel Sheet Sheathed Cold-Formed Steel Framed In-line Wall Systems. I: Impact of Structural Detailing

The North American construction industry has seen substantial growth in the use of cold-formed steel (CFS) framing for midrise buildings in recent years. In seismic zones, CFS-framed buildings utilize shear walls to provide the primary lateral resistance to earthquake induced loads. Although oriented strand board (OSB) and plywood panels have been traditionally used as the sheathing material for these essential components, more recently, steel sheet sheathing has emerged as a novel strategy due to its strength, ductility, ease of installation, and use of noncombustible material, among other benefits. To address the paucity of data regarding CFS-framed shear wall response within actual wall lines of buildings, a two-phased experimental effort was conducted. Wall-line assemblies were fabricated and tested with shear walls placed in-line with gravity walls. The shear walls chord stud packs include tie-rod assemblies consistent with multi-story detailing. Specimens were either unfinished or finished, and the shear walls were laid out in a symmetrical or unsymmetrical fashion within in the wall line. In addition, both Type I and Type II shear wall and anchorage detailing were investigated. In this paper, the impact of test variables governing the structural detailing of CFS-framed walls are quantified through dynamic and quasi-static tests, and a companion paper presents findings regarding the impact of architectural variations on seismic performance.(c) 2022 American Society of Civil Engineers

Proceedings of Summer Symposium for Students 2020

CFSRC student and postdoctoral researchers shared their latest research at the CFSRC Summer Symposium on 26 and 27 May 2020. Thirty-four participants enjoyed a deep dive into cold-formed steel and related research – focusing the first day on earthquake engineering and the second day on broader topics in thin-walled structural research. The Symposium was held online and hosted by CFSRC Director Ben Schafer and moderated by CFSRC Investigators: Hannah Blum, Matt Eatherton, Kara Peterman, and Cheng Yu. The idea for the Symposium was borne out of the loss of conferences and sharing of research due to the global pandemic. The notion was enthusiastically received by the CFSRC investigators and led to a large degree of interest from the CFSRC student researchers. The interest from the students led to an exceptional list of talks. Editors: Benjamin W. Schafer and Deniz Ayhan Moe

Proceedings of Summer Symposium for Students 2020

CFSRC student and postdoctoral researchers shared their latest research at the CFSRC Summer Symposium on 26 and 27 May 2020. Thirty-four participants enjoyed a deep dive into cold-formed steel and related research – focusing the first day on earthquake engineering and the second day on broader topics in thin-walled structural research. The Symposium was held online and hosted by CFSRC Director Ben Schafer and moderated by CFSRC Investigators: Hannah Blum, Matt Eatherton, Kara Peterman, and Cheng Yu. The idea for the Symposium was borne out of the loss of conferences and sharing of research due to the global pandemic. The notion was enthusiastically received by the CFSRC investigators and led to a large degree of interest from the CFSRC student researchers. The interest from the students led to an exceptional list of talks. Editors: Benjamin W. Schafer and Deniz Ayhan Moe