Lamine Dieng

SMA in Mitigation of Extreme Loads in Civil Engineering: Damping Actions in Stayed Cables

Reliable use of Shape Memory Alloys (SMA) in mitigation of extreme load effects requires a deep study of the SMA behavior according to the necessities of the application. Damping the oscillations induced by one storm of three or four days with a strong wind and/or rain usually requires more than one million of working cycles. A SMA damper is checked in to two realistic cables of ELSA and of IFSTTAR. The measurements establish that the SMA damping device reduces drastically the oscillation amplitude. Technical suggestions for the preparation of the dampers built by several SMA wires of NiTi with 2.46 mm of diameter are included. The choice of appropriate length and the choice of the number of required SMA wires are suggested. Moreover, a suitable simulation by proprietary SMA routine inside ANSYS is included.

The SMA: An effective damper in Civil Engineering that smoothes oscillations

The properties of SMA (Shape Memory Alloys, smart materials) are associated to a first order phase transition named martensitic transformation that occurs between metastable phases: austenite and martensite. At higher temperature phase or at lower stress the austenite is the metastable phase. The martensite appears at lower temperature phase or higher stresses. The hysteresis of the transformation permits different levels of applications, i.e., in their use as a damper. Two types of applications can be considered in damping of structures in Civil Engineering. The first one is related to diminishing the damage induced by earthquakes. The second one is a reduction of oscillation amplitude associate to an increase of the lifetime for the stayed cables in bridges. Different fundamental behavior of the SMA needs to be guaranteed in each case.

A New Shape Memory Alloy-Based Damping Device Dedicated to Civil Engineering Cables

Most of civil engineering cable structures are subjected to potential damages mainly due to dynamic oscillations induced by wind, rain or traffic. If vibration amplitudes of bridge cables for example are too high, it may cause a fatigue phenomenon. Recently, researches had been conducted dealing with the use of damping devices in order to reduce vibration amplitudes of cables. Thin shape memory alloy (SMA) NiTi (Nickel-Titanium) wires were used as a simplified damping device on a realistic full scale 50 m long cable specimen in Ifsttar (Nantes - France) laboratory facility, and its efficiency was shown. It has been done using finite element simulations, as well as experimental test methods. The aim of this work is to link the wire material behavior with the local damping induced along the cable qualitatively. Indeed, thermomechanical energy dissipation of the NiTi-based wires enables their damping power. The hysteretic behavior in NiTi-based alloys demonstrates a consequent dissipation because of an exothermic martensitic transformation and then an endothermic reverse transformation.