GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.3 449 in case of presence in the lower story(ies). In the other case, structures suffer heavier damage but human life, even in the lower story(ies), is preserved. In RC structures the significant collapse mechanisms detected for infill walls were horizontal and vertical bending, global overturning or sliding. Masonry buildings, according to damage reports, presented a higher vulnerability to tsunami loads, mainly because of the poor construction technologies usually found in the regions affected by the considered tsunami events (Mallawaarachchi et al. , 2008). The observed collapse mechanisms were both in-plane and out-of-plane. In particular, the identified collapse mechanisms are mainly horizontal and vertical bending. Under these conditions, the global overturning is usually negligible because in many cases slabs and internal walls counteract such mechanism. Howsoever the tsunami flux could generate some depression involving the overturning of walls in the outer side of the building, hence much less constrained by internal walls and slabs. State of the art: existing design code. The modelling of the tsunami effects is made more complex by the high degree of uncertainty in the wave characterization and associated pressures on structures, and further studies are needed to improve current codes and guidelines. Nowadays, the principal available codes are: • FEMA P-646 ( Federal Emergency Management Agency: ���������� ��� ������ �� Guidelines for Design of Structures for Vertical Evacuation from Tsunamis ); • ASCE 7-16 ( American Society of Civil Engineering: Minimum Design Loads for Buildings and Other Structures) it is the most recent design code for tsunami forces; • SDRTEB ( Structural Design Requirements for Tsunami Evacuation Buildings ). The first two codes were developed in the U.S.A. and are based on the principle of splitting hydrostatic from hydrodynamic loads. In particular, the effects of tsunami are defined by several scenarios and forces, namely: hydrostatic force, drag force, buoyant force, surge force, impact force, debris impact force, etc. Every force depends on several parameters (e.g., inundation depth, flow velocity and maximum momentum flux) characterized by high variability in hazard maps, numerical simulations and simplified equations. The last one is a Japanese guideline and is based on the assumption that only one equivalent hydrostatic load, that includes the effect of both hydrostatic and hydrodynamic loads, is defined as horizontal force, while vertical forces are given by buoyant forces. In particular, the design inundation depth is assumed to be equal to three times (i.e., η=3) the expected inundation depth, if no specific tsunami energy dissipation structures, namely seawalls, are installed (Fukuyama et al. , 2011). It is assumed that the design inundation depth can be scaled down, depending on the presence of such dissipative structures. If there are tsunami energy dissipation structures, which are not common for Italian coast, the inundation depth can be reduced strongly, even halved. The design pressure distribution for tsunami (Fig. 1) acting along the structure’s height is assumed to have a triangular shape equal to: where: - q z : intensity of tsunami pressure at height z; - ρ : water density; - g : gravitational acceleration; - η : water depth coefficient; - h : expected inundation depth; - z : location of acting pressure measured from ground. The tsunami wave force, between two heights z 1 and z 2 , can be obtained by integrating the wave pressure on the surface area exposed to the tsunami. With this approach, the tsunami force depends on one parameter only, the inundation depth, which can be obtained by numerical simulations or hazard maps. Coherently with the goals of the present research project, only Japan guidelines are used, according to a large scale approach.