Seismic Stabilization of Historic Adobe Structures

AbstractMany historic adobe structures in the southwestern United States and throughout the world are vulnerable to destruction or damage by earthquakes. In the recent Northridge earthquake in Los Angeles, a significant number of the remaining, unretrofitted adobe structures dating to the 18th and early 19th century were extensively damaged. Yet it has been observed that many other adobes have performed well during large seismic events. Responses differ because low-strength adobe, when used in thick walls, develops cracks during seismic events, but these cracks do not necessarily lead to catastrophic building collapse. The concept of seismic retrofitting based on improving the stability of the adobe buildings, rather than the more conventional criterion of improving the strength, seems to be a valid approach. The objective of such a concept is to prevent walls from overturning by restricting the relative displacement of blocks formed following cracking, thereby enabling the structure to dissipate energy by friction and rocking without catastrophic collapse.Shaking table tests of model adobe buildings have been carried out. Various types of simple retrofitting measures that were evaluated were designed to be relatively noninvasive and respectful of the historic fabric of the building. These measures consisted of thin, flexible horizontal or vertical straps applied to both sides of walls and small diameter steel rods inserted within the walls used in conjunction with a thin wood bond beam. The intent of these measures was to provide overall structural continuity rather than strength improvement. The results of the tests involving nine 1:5 scale model buildings clearly demonstrated the effectiveness of the stability-based techniques. Wall height-to-thickness ratios were varied from 5 to 11, and maximum table displacements of ±38 cm (prototype domain) were used. Some retrofitted models were able to withstand twice the displacement that resulted in collapse of an unretrofitted model. Additional tests carried out on two instrumented 1:2 scale model adobe buildings further demonstrated the effectiveness of the retrofitting measures.