Using NiTi SMA tendons for vibration control of coastal structures

Abstract

Hurricane damage inflicted upon coastal structures, particularly residential structures, results in millions of dollars in financial damage and loss of life each year. A major cause of this damage usually begins with roof uplifts of coastal structures; prevention of roof uplift helps mitigate damage to coastal structures by hurricanes. Development of more effective fastening mechanisms for the connections between the walls and the roofs of these structures will aid in damage reduction to coastal structures. Recent developments in the new field of auto-adaptive materials offer promising opportunities for developing radically new fastening mechanisms. One of the classes of materials in this category is shape memory alloys (SMAs). SMAs are very attractive for structural application because of their major constitutive behaviors such as pseudoelastic characteristics. The pseudoelastic behavior of NiTi SMAs is a unique hysteretic energy dissipation behavior which, combined with a very long fatigue life, makes NiTi a viable candidate for developing new fasteners. However, as a first step it is important to develop an in-depth understanding of NiTi behavior under dynamic loads. Research carried out in this area has been very limited in scope. Therefore, in this paper, eight different configurations of bracing systems, divided into two categories, are explored on a single degree of freedom (SDOF) structure to investigate the feasibility of developing devices for the mitigation of hurricane damage. These bracing devices basically utilize the hysteretic energy dissipation of NiTi resulting from its pseudoelastic characteristic. Since the main goal of this ongoing research is to develop a thorough understanding of the pseudoelastic and hysteretic behavior of SMAs under severe dynamic loading/excitation, a series of earthquake data has been considered as the source of excitation. Through this analysis both the damping and stiffening characteristics of NiTi wires and the effect of these dynamic characteristics on changing the dynamic response of the structure are studied. In the first category the NiTi wires are not pre-strained, while in the second category they are pre-strained. In each category, four different combinations of wire length and modeling of pseudoelastic behavior of NiTi wire are considered. A bilinear stress-strain model is used for representing the pseudoelastic behavior of NiTi tendons, capable of representing internal yield, internal recovery and trigger line concepts. This study establishes that hybrid tendons have the highest damping and stiffening effects on the structure. It is also concluded that, when the amplitude of excitation is small, tendons act as stiffening devices. Once the amplitude of the excitation is large enough to initiate stress-induced phase transformations, tendons act as energy absorption devices. These findings provide very useful information for the development of more effective fastening devices that can withstand severe dynamic loads, such as hurricane loadings.