Shape Memory Alloy

Description

Field of the invention

The field of this invention is related to the building structures.

Background of the invention

When the earthquake strikes, within a short time, a huge amount of energy is released simultaneously or with a little delay (according to the distance of seismic source and ...) and the under structure's ground is influenced by the vibration with a variation of frequencies. These vibrations largely create basic motions (different types of vertical and horizontal motions in various directions) in the structure. It is obvious that mass, stiffness, damping,... in the structure and the earthquake specification directly impacts the amount of forces created in the elements of the structure, Even though it is impossible to control and intervene in the content of earthquake (time, specification,... etc.), it is possible to depreciate and reduce the effect of earthquake on the structure by controlling the mass, stiffness and damping and other dynamic specifications of structure.

Prior Art

Moment Resistant Frame (MRF) as one of the earthquake-resistant structural systems.

Lateral Load-resisting systems are a part of all the structures which tolerate the lateral loads caused by wind, earthquake, even explosion, etc, and are applied on the whole structure. MRF is one of the lateral Load-resisting systems (in addition to tolerating the heavy weights). MRF is a kind of structural system in which the beams, columns and their connectors form a Moment resistant Frame (MRF). In this type of structural system, the lateral force exerted on the structure is controlled through benefiting from the fixity of connection between the beams and columns and the columns gain the ability to transfer their moment to the foundations by anchors in the mentioned connection.

The conventional methods are currently applied to reduce the effect of basic movements on the structures including the use of seismic isolators, like seismic dampers separating the foundation from the ground, using different elements and methods to dissipated the energies in the structures including different types of active and passive vibration control systems and energy dissipater or a combination of all the above mentioned systems are applied to decrease the seismic ground motion on the structure.

Recently beam connections and the control of the place of plastic joints in the beams and the kind of beam connection to the columns for controlling the reflection of bending moment has been changed but the column connection to the foundations has been less considered by the researchers.

Summary of the invention

The behavior of steel moment resistant frame (SMRF) strongly relies on the function of beam and column and practically the connections between them and also depends on column- foundation connection. It is obvious that structural connections should be designed in such a way that the structural members and connections (generally the structural system) can provide the required resistance and flexibility against lateral loads. Basically the lateral resistance and the capacity of flexibility of connections in the MRFs are the most essential factors in the seismic capacity of such structures. In addition to the connection of beams to the columns, the way the columns are connected to the foundations in the MRFs has a considerable effect on carrying the lateral force by the moment resistance frame. If the connections can have the required flexibility and resistance like the beam connections against the lateral forces, practically the seismic capacity of such structures will be related to the function of these connections. Hinged connection and fixed connection are conventional connections used to connect the column to the foundation in the steel structures.

In this invention the function of the connection is in a way that the connection angles of the two sides of column web are responsible for carrying, shear force and creating joints for the rotation of the column and the connection rods of two sides of the column are responsible for tolerating the bending moment (as couple axial forces). The rods of connecting in two sides of the column are not connected to the base plate and are directly connected to the controlled anchor bolt in the foundation concrete by the couplers. To do the connection, the stiff ener plates (cut and drilled) are welded to the place of connection to the column and the location of placing connection couplers to the anchor bolts in the concrete on the base plate is specified and the holes are created and after connecting the couplers to the control rod (after adjusting with the grout), balancing on the base plate continues. The angles of column sides are connected on the base plate and on their own place and the column is plumbed on its own place and between the angles and then the necessary welding is performed. Height adjustment coupler is installed at its own place and then the SMA connection rods with the required length and diameter are screwed on its two heads and are located at their own place along with the height adjustment coupler. After adjusting all of the considered lengths, the nuts which are equipped with the spring washers are tightened at their own place.

Brief description of the drawings

Figure 1: the schematic of different kinds of column connection to the foundation and controlled connection and the area of using controlled connection

Figure 2: the schematic model of connection (with one or two connection rod at the sides of the column) and the parameters of connecting rods with the column profile

Figure 3 - Stress-strain curve of the super elastic classic Shape Memory Alloy (SMA)

Figure 4 - schematic model of the connection equipped with the super elastic Nitinol Shape Memory Alloy (SMA)

Figure 5 - the upper view (plan) of the connection and its details Figure 6 - the side and front view of the connects and its details

Figure 7 - the exploded view of the control rod made of Shape Memory Alloy along with the connection tools like nuts, washers, connection and height adjustment couplers.

Figure 8 - the three-dimensional model of connection and the connection tools (including the couplers, SMAs, stiffeners, bolts,)

Detailed description of the drawings The analyses conducted on the investigated frames (push over and cycling analysis) confirm that these frames have shown two different capacity functions due to the difference in the method of connecting column to the foundation. Figure 1 which tends toward the fixed connection in case of having the rotational capacity and unlimited stiffness. If not, it plays the role of hinged connection of column to the foundation and displays appropriate energy depreciation performance in the cyclic and seismic loading by using proper materials in the above area.

The special performance of controlled connection requires having the special qualities of elasticity, high strain tolerance, energy depreciation, reversibility and etc. Therefore, the controlled dynamic behavior of these connections strongly requires proper details and more importantly using special materials. These days, the materials are named Shape Memory Alloy (SMA) and are known as smart material.

In the schematic connection shown in Figure 2, it is assumed that fixation in the connection of column to the foundation is provided and axial stress caused by the axial force in the column is tolerated by the base plate and foundation and the available bending moment on the column on the supporting place (in the end of column) is tolerated by the coupler and controlling rods shown in the figure. In this manner, the column has the possibility of rotation and the lateral shear force is tolerated by the appropriate weld stud (web) and the controlling rods are responsible for tolerating the bending moment.

Assuming IPB profile for the column and considering figure 3 and assuming the use of different material for the columns and rods and assuming that the whole surface of the column is plastic, so, the diameter of the rods equals Φ _¾αΓ replaced by column for anchor bolt is calculated as follows:

(1) =

In the above equation, the used parameters include:

<t>b _ar the diameter of bar used for tolerating the anchor exerted on the connection. A _bar Cross section of the bar used for tolerating the anchor exerted on the connection. Oy yield stress of the bar used for tolerating the anchor exerted on the connection. N the number of bars for tolerating the anchor exerted on the connection on each sides of the connection.

B the height of IPB profile (The height of web and the thickness of flanges)

D the height of IPB profile along with the distance of bars from the flange D = B + 2 X (5~10) ^cm bf- Width of used IPB profile flange t _j - thickness of used IPB profile flange t _w - thickness of used IPB profile web ^σ νΐΡΒ ~ ^use< ^ profile yield stress

Connection equipment with the Shape Memory Alloy (SMA)

The connection is equipped with using SMA bars made of super elastic Nitinol, with Stress- strain curve as in Figure 3 and through using the relation between the cross section of steel bars and Shape Memory Alloy's bars and used as a schematic like Figure 4.

In figure 3, the horizontal axis is Strain 1 and the vertical axis is stress 2. Point 3 shows the stress of the start of Austenite phase in turning into the martensite phase. Point 4 shows the stress of the end of Martensite phase in turning into the Austenite phase. Point 5 shows the beginning of Martensite phase in turning into the Austenite phase. Point 6 shows the end of Martensite phase in turning into the Austenite phase. Point 7 shows the reversible strain. Point 8 shows modulus of elasticity.

Figure 4

The parts included in this figure include:

9- Foundation

10- Anchor

11 - Coupler

12- Base Plate

13- SMA Rod (NiTi) Superelastic 14- Column

The details of connection

By equipping the connection with alloy and precise design of connection details (invention), the executive part of it including the couplers, bolts, SMA bars, nuts, washers, stiffener plate, and etc are shown in figures 6 and 7.

Figure 5

The parts included in this figure include

15. Base plate

16. Controlling connection bars of base plate to the foundation

17. Stiffener plates

18. IPB profile

19. Control bars made from Shape Memory Alloy (SMA) along with the connection tolls including the nuts, washers, connection couplers and height adjustment couplers

Figure 6:

20. The front view of connection and its details

21. The side view of the connection and its details

22. Control bar involved with the concrete

23. The concrete control coupler connection to the height adjustment coupler

The advantages of the invention a. The proposed connection through using steel bars behaves exactly like the fixed connection of the column to the foundation and verifies the accuracy of connection in its performance. b. The proposed connection is equipped with Shape Memory Alloy and is effective in the reflection (base incision) and the extent of reducing reflection in two used records (Tabas and Lomma) is on average 20%.

c. One of the outstanding points in the connection of seismic loading is the self- centering ability of the connection. This issue is very obvious in comparing the change of place remained from the column side in the reflection under the record of Tabas and Lomma after the earthquake finishes. Therefore, the proposed connection can be considered among the low damaging ones.