Contrasting effects of ash loading on buildings were experienced as a result of the 1991 eruption of Mt. Pinatubo, both based on type of building, but also proximity to the eruptive source.
A survey of damaged buildings in Castillejos, 27 km (17 mi) southwest of Mt. Pinatubo identified the roof structure as the most significant indicator of damage in a comparison of residential and nonresidential buildings. Sixteen percent of surveyed buildings with short-span roofs suffered major damage, with 43% having no significant damage. 75% of buildings with long-span roofs (greater than 5 m clear span) were severely damaged, and only 17% were without significant damage (Spence et al., 1996, p. 1059). See the online report See online report, Building damage caused by the Mount Pinatubo Eruption of June 15, 1991.
A survey was conducted of 51 buildings damaged in Castillejos, a town with a population of less than 50,000 located 27 kilometers southwest of Mount Pinatubo. The thickness of ash in the town was about 20 cm. For the survey, the building analysis included identification of:
The principle cause of damage to the sample of buildings was that the load of ash on the roof exceeded the strength either of the roof sheets or of the roof supporting structure, or both. The load of 150-200 mm (6-8 in) of water-saturated ash on roofs would have exerted a force of 2kN/m2 (~400 kg/m2).
In a summary of the damage, the authors identified the following:
According to the authors "to protect lives, roofs of buildings exposed to possible ashfall should be designed for a superimposed load related to the probable level of ashfall, in a manner analogous to design for now loading in cold climates."