# Multi-Step Prestressing with Hybrid SMA Wires
## 2 Prestressing with Hybrid SMA Wires  
For applying prestress with hybrid SMA wires, we used two different types of prestrained SMA wires as prestressing members instead of conventional tendons. The prestrained hybrid SMA wires were used to apply prestressing force rather than direct tensioning by using hydraulic jacks. When subjected to temporary heating, the prestrained hybrid SMA wires undergo recovery, and the recovery stress in the hybrid SMA wires is converted into a prestressing force as the temperature reaches the austenite temperature of one of the several materials constituting the hybrid SMA wires. In particular, heating is performed by dividing the hybrid SMA wire into multiple sections and heating each section to a different temperature range.  
Figure 1 shows the general procedure of prestressing with hybrid SMA wires. The arrangement of the SMA wire and the manner in which prestressing force is transmitted to the structure are the same as those in the conventional pre-tensioning method. Figure 1a shows an RC structure prestressed using SMA1 and SMA2, which have different austenite temperatures. If the austenite temperature of SMA2 is higher than that of SMA1, SMA2 will not exhibit the SME when heated to the austenite temperature of SMA1, T1. As illustrated in Figure 1b,c, the hybrid SMA wires are heated to different temperatures. After the temperature reaches the austenite temperature, the SMA wires in the heated sections induces prestress in the RC beam. Upon differentiating the composition of the hybrid SMA wires and heated sections, the prestress induced by the SMEs shows a multi-step distribution, as illustrated in Figure 1d.  
Figure 1: General application procedure of prestressing method using hybrid shape memory alloy (SMA) wires: (**a**) prestressed concrete girder with hybrid SMA wires; (**b**) heating of hybrid SMA wire to temperature 1; (**c**) heating of hybrid SMA wire to temperature 2; (**d**) final prestressing force distribution and multi-step prestressing force in section A.  
Figure 2 shows the general stress distribution diagram at the bottom of a simply supported beam due to prestressing with hybrid SMA wires. Curve (1) in Figure 2 represents the stress distribution without prestressing and curve (2) represents the stress distribution when the beam is prestressed with SMA1 only. Curve (3) represents the final stress distribution when the beam is prestressed with SMA2. These figures show that the stress is distributed uniformly over the length of the beam when it is prestressed with hybrid SMA wires. The proposed method offers the following advantages compared to the prestressing method that employs conventional tendons.  
(1) If a member has two or more types of austenite temperatures, the SME can be controlled in steps, where the number of steps is equal to the number of austenite temperatures. Efficient prestressing is possible if it can be performed in several stages as needed during the construction phase or during the service phase according to the additionally required load-carrying capacity.  
(2) The conventional pre-tensioning method introduces prestress through the elimination of tendon restraints after the design strength of the structure is attained. Therefore, re-tensioning is impossible. However, with the proposed prestressing technique, the timing of prestressing can be selected by adjusting the stage of SME expression. In addition, strains occurring in the SMA wires during service can be used for further re-tensioning.