The Earthquake Engineering Field Investigation Team (EEFIT), which has carried out the first comprehensive study into the tragic events of December 26, 2004, has also come up with a number of key recommendations on how to design in high-danger zones.
The report says that unless structures were of well-designed, modern construction, when wave heights exceeded 2.5m they generally suffered complete collapse. In addition when buildings that were made of reinforced concrete (RC) frame construction suffered extensive structural damage it was usually due to poor detailing, such as inadequate beam-column connections.
The report goes on to say that houses did not provide a safe location from the severe and rapidly rising floods; even well-built RC frame structures that survived without major structural damage often did not provide adequate safety to their occupants. The collapse of windows, doors and infill panels allowed water to rapidly flood the buildings.
The following recommendations for structural-design criteria in areas of tsunami risk were made by the report:
structural designers need to better understand the nature of tsunami and establish minimum specifications for building and infrastructure in the larger context of other tsunami protection measures, such as evacuation strategies;
single-storey family dwellings, often of masonry construction, should be designed to support larger horizontal loads. Though not offering life safety under large inundations, buildings whose structure has survived the tsunami can be reoccupied sooner, hence reducing the strain on the reconstruction effort. Cost-effective improvements to traditional construction techniques should be developed and disseminated;
construction of RC-framed buildings, whose upper storeys might offer protection from the inundation, should be designed to withstand the impact of water and debris, without causing incremental collapse of the structure. This requires a review of the detailing of beam to column connections and additional stiffening of the framing system, with either reinforced concrete shear walls or well detailed masonry infill panels placed perpendicular to the coast. It also requires that a minimum specification for masonry infill panel be developed for walls parallel to the coast;
for engineered structures, in areas of high hazard, a minimum dimension of 300mm for columns be specified, and the placement of reinforced concrete shear walls perpendicular to the beach also be specified;
piles are recommended for use in areas of high tsunami hazard, especially for structures founded on loose sand;
in regions of high tsunami hazard, the scour protection of the abutments of near coastal bridges should be considered;
in regions of high tsunami hazard, the protection to services crossing near coastal rivers should be considered;
the resistance of toughened glass to water pressure and debris damage is investigated, with a view to determining if specifying toughened glass for seaward-facing windows would increase life-safety; and
that all buildings within the expected inundation zone have landward-facing egress and easy access to higher floors or roofs.
by Will Hunter