Ground Motions and Structures - Earthquake Engineering for Structural Design - page 339

Ground Motions and Structures 339

8.4.3 Steel

In modern design practice it is generally recognized that steel is an excellent material
for seismic-resistant structures because of its performance in terms of strength and
ductility, being capable of withstanding substantial inelastic deformation. In general this
is true, but in the last few decades the strong earthquakes have seriously compromised
this ideal image of steel as the perfect material for seismic areas. This is due to the fact
that, in some cases, the good ductility of the material is lost when it is used in
inadequate structural types. As a consequence, an important field of research works has
been developed in the last decades, including also the improving of the material
properties. This aspect can be framed in the field of high strength steels, low yield steels
and aluminium alloys.
The high strength steels are used in structures for non-dissipative elements, which
must remain in elastic range during the earthquake, due to their limited ductility, when
compared to mild steels. It is generally accepted that the transition from mild steel to
high strength steel occurs at the yield strength of about 350 MPa. (Bjorhovde, 2002).
Figure 8.37a shows the stress-strain curves for steels exceeding yield strength of about
350 MPa and having convenient rupture strain.
The lowyield steel is proposed to be used for dissipative elements, as the shear wall
panels in moment resisting frames. This system has found a large consensus in recent
years. Due to the small amount of carbon and alloying elements, the nominal yield
stress is about 90 to120 MPa and the nominal ultimate stress over 50%. This enables
the panels to undergo large inelastic deformations at the first stages of the loading
process, enhancing the energy dissipation capability (De Matteis and Mistakidis, 2003).
Recently, a new material has been proposed at the University of Naples for shear
panels, the pure aluminium (Formisano et al, 2006). A comparison of the stress-strain
curves for mild steel, aluminium alloy, low yield steel and pure aluminium, is presented
in Figure 8.37b.

8.4.4 Shape-memoryAlloys

The shape-memory alloys, also known as smart alloys, are materials which remember
their initial shape. It is considered as one promising material for special devices in
seismic resistant structures due to its ability of large deformation recovery, high
ultimate tensile strength and dissipation of seismic energy without residual
deformations. The phenomenon of pseudo-elasticity effect results from the
transformation phase above austenite. When the applied stress reaches a critical value,
the austenite material begins to transform into the martensite phase. Once the loading is
released, the stress inducing the martensite returns to the austenite phase without
residual strain (Pan and Cho, 2007). Austenite and martensite phases are the initial and
the deformed molecular arrangements, respectively, which characterize the behavior of
shape-memory alloy (Wikipedia, nd). As a result, one hysteretic loop can be formed by