1. Midshipman Research Project: Structural Performance of Gridshells under Dynamic Loading
Problem Statement
Gridshell structures are an efficient solution for rapidly deployable shelters because they are lightweight, affordable, adaptable and easily constructed.. This proposed research project will assess the structural performance of gridshell structures under dynamic loads using finite element analysis.
Background
Gridshells are shell structures created from a lattice of linear members bent into a curved surface [1]. They have mostly been used as permanent curved roofs to enclose existing buildings. Gridshells have an unexplored potential to be used as rapidly deployable, reusable structures because of their unique construction process. Gridshells are built from easily transportable linear members and joints. The members are then laid out on the ground and formed on-site into a lattice. The lattice can then be lifted, pushed, dropped, or inflated into the curved structure.
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| Figure 1: The construction process of a "dropped" gridshell [2] |
Because gridshells can be transported compactly and easily erected on-site, they are ideal for use as emergency shelters in areas ravaged by natural disasters. Presently gridshells have only been analyzed under static loads. Their potential for use in natural disaster environments necessitates testing their structural performance under dynamic loading conditions, e.g. earthquake and wind loading.
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| Figure 2: Example of a finite element spherical cap gridshell [3] |
Methodology
The first part of the project will be a literature review. The purpose of this review will be to gain initial exposure to existing gridshell testing, the concept of gridshells, computational modeling, and finite element analysis. Once we move past the core concepts, the next step will be the selection of a finite element software package to use for the modeling and analysis of the gridshell. The software package will be chosen based on coordination between USNA and Naval Postgraduate School (NPS), to facilitate familiarity when moving between the NPS internship and the research at USNA, all while being powerful enough to meet the needs of both organizations.
The concepts and computational skills will then be used to create the model of a gridshell. The project will focus on one global gridshell form, either a spherical cap or a barrel vault. The material and member geometry will be selected based on typical values of existing gridshells. The computational model will then be validated under static loads using results from the literature.
With the model created and validated, the next step will be to determine the relevant dynamic loads to use for testing the model. First, we will select a geographic region where these gridshells would be used. Then we will search building codes for standard loads (e.g. wind, earthquake) to provide a relevant, meaningful, and repeatable analysis based on that geographic region.
At this point the model will be ready for dynamic load testing. The testing will be run for each type of standard dynamic loading. The results of interest will be the deflection of the structure and the stresses in the members. Analysis of the stress and deflection data will give insight into the material and shape choices for gridshells under each type of loading. This insight could someday drive design decisions for real-world gridshells.
2. Internship Opportunity: Composite Structural Technology at Naval Postgraduate School
The internship will be my first exposure learning and using the finite element method which will be used in the proposed research project at USNA . The internship involves computational modeling of composite structures, which will give me skills that I can take back to USNA and apply throughout the research. There is also possibility for gridshells made of composite materials, so both the experience in modeling composites and the results from the computations at the internship could be useful in choosing a material for a prototype gridshell structure. The internship will also be my first research experience and will give me invaluable research skills to apply during 1/C year.
The USNA point of contact (POC) for the internship is CDR Blair, USN in the Mechanical Engineering Department, and the NPS POC is Professor Kwon. The internship is funded by the federal host, NPS. Professor Kwon has already shown willingness to tailor the internship to meet the needs of this research, and we will meet to discuss the details of the internship when he is in Annapolis on March 4th. I intend to participate in the internship during first block.
References
[1] Happold E, Liddell WI. Timber lattice roof for the Mannheim
Bundesgartenschau. Struct Eng 1975;53(3):99-135.
[2] Harris R, Romer J, Kelly O, Johnson S. Design and construction
of the downland gridshell. Build Res Inform 2003;31(6):427-54.
[3] Malek, S., Wierzbicki, T., & Ochsendorf, J. (2014). Buckling of
spherical cap gridshells: A numerical and analytical study revisiting
the concept of the equivalent continuum. Eng Struct, 2014;75:288-298.
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