Structural design for seismic effects have always been my area of interest ever since I took up Civil Engineering as my major during my undergraduation. My journey into research in seismic engineering began with the devastating earthquake that rocked Gujarat on 26th January 2001.
My first experimental research on seismic mitigation systems was develop a cheap and effective base isolation systems in seismically active areas. Base isolation in lay man terms is isolating the base of a structure from the earthquake ground motions. Non-believers can see this and people who understand the jargon can visit here.
One of the types of base isolation systems used consist of elastomeric bearings, the elastomer made of either natural rubber or neoprene In this approach, the building or structure is decoupled from the horizontal components of the earthquake ground motion by interposing a layer with low horizontal stiffness between the structure and the foundation.
Natural rubber is a material known to have a low horizontal stiffness and this is a property which we chose to exploit in developing a cheap means of base isolation systems. Automobile tires provided a simple and elegant solution to finding a cheap material which has a low horizontal stiffness. Used tires prove to be a environmental hazard due to problems related to its disposal as they are non-biodegradable. The tires have a estimated half life period of more than 30,000 years, which means they will be around for a long long time. The results of the research were very encouraging, with energy dissipations of about 70-75 % observed.
My current research involves the investigation of the hyperelastic properties of rubber and its implementation in a structural system. Specifically the influence of the hyperelastic device on base shear and interstory drift will be investigated as the main parameters of interest.
Dynamic instability is a structural behavior of particular concern because of its highly sensitive nature. Many structural devices are available for increased structural stability and dissipation of dynamic energy. These devices include dampers, base isolators, and braces. Nonlinear dynamic analysis of structures is very complex and requires advanced modelling and analysis techiniques.
Hyperelastic devices are nonlinear elastic devices that have a stabilizing potential at high displacement with regard to MDOF systems near collapse. The theoritical function of these systems is to add increasing stiffness to a structure as deformation increases to prevent instability at high levels of acquired displacement. Increased stiffness at low levels of displacement is not desired due to the increase of system forces that would occur for service-level loads. Hyperelastic devices behave elastically along a nonlienar stress-strain relationship defined by a cubic polynomial, and may be analytically implemented through the use of a hyperelastic material.
Rubber is a hyperelastic material, which behaves differently in tension and in compression. A hypothetical hyperelastic material would have a stress-strain relationship as shown below.
Hyperelastic braces differ from other nonlinear devices in that they do not dissipate energy and they are designed to only influence the structural behavior nearing instability. Specific hyperelastic relationships may be formed to suit a particular system based on yield strength, stiffness and ductility demand. This type of behavior is beneficial for structural engineering due to the ability to avoid increased system forces at low levels of dynamic excitation and to increase structural predictability. Hyperelastic braces maybe beneficial to any structure that experiences nonlinear dynamic behavior and not just unstable systems.