Ground Motions and Structures - Earthquake Engineering for Structural Design - page 328

328 Earthquake Engineering for Structural Design

exceeded in practice and, therefore, this loading condition can be considered as
exceptional (Mazzolani, 2002). Earthquakes can occur in non-seismic areas, where the
structures are not prepared to withstand these events. The goal of robustness is to limit
or mitigate the consequences of the unforeseen seismic conditions (acceleration,
duration, directivity, pulse effect, etc.).
There are two main factors influencing the residual resistance of structures:
constructional materials and structural configurations.
A ductile or brittle collapse is strongly dependent on the used structural material .
Three materials, masonry, concrete and steel, those mostly used for structures, are
discussed in the following.
Unreinforced masonry is typically non-elastic, non-homogeneous and anisotropic
material composed of two components of quite different properties: stiff brick and
relatively soft mortar. Masonry is normally designed to resist compression actions
only, being very weak in tension and shear (Kaushik and Rai, 2007). Figure 8.29
shows the stress-strain curves for masonry prisms for different grades of mortar. One
can see that the maximum residual compressive stress is very low: 0.2 f m, where f m is
the compressive prism strength of masonry. Due to this fact, a structural load-
displacement curve must neglect the residual resistance. In fact, the observed collapse
of masonry during the strong earthquakes shows that this failure is of brittle type and
there is not any residual resistance. Therefore, the masonry structures are not robust
structures. The use of FRP (Fiber Reinforced Polimers) as reinforcement can improve
the robustness of masonry structures (Nagy-Gyorgy, 2007)

Figure 8.29 (continues)