Pinatubo 1991

Ashfall from the 15 June 1991 eruption of Mt. Pinatubo resulted in the accumulation of 50-100 mm (2-4 in) of wet ash in the area of former U.S. Clark Air Base, located 20 km (12 mi) northeast of the volcano. Densities of ash samples collected here ranged from 1,200 to 1,600 kg/m3 (dry) and 1,500 to 2,000 kg/m3 (wet).

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.

Survey of buildings

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:

    (1) principal constructional materials used
    (2) number of stories
    (3) roof structure, shape, and pitch
    (4) building's usage (residential or nonresidential)

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:

  1. Although many roofs had been cleared by the time of the survey, there was evidence from the uncollapsed roofs that the wet ash was able to accumulate to depths of at least 15 cm on metal sheet roofs of pitch up to 25 degrees, without slipping from the roof.
  2. Out of the total sample of 51 buildings in Castillejos, 17 suffered partial or complete roof damage, while 18 suffered no damage or only light damage.
  3. Buildings tended to suffer worse damage if they were
      (a) constructed with long-span roofs (greater than 5 m clear span), rather than with short-span domestic scale construction
      (b) of timber frame rather than reinforced concrete frame construction
      (c) of higher rather than lower roof pitch
      (d) non-residential rather than residential
  4. Some other factors that seem to have contributed to damage, though statistical evidence is inadequate to demonstrate their significance are:
      (a) unbraced supporting walls or columns
      (b) large unsupported roof overhangs

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."

See Also:
Buildings > Roof Loading > Design & Construction