|Click a category for information about effects of ash and how to lessen their impacts||Search|
Roofs & Gutters || Loading & collapse || Corrosion || Gutters || Ash removal ||
A primary concern during ash fall is the potential collapse of buildings from the accumulation of ash on roofs, which can lead to widespread injuries and deaths. For example, the collapse of roofs from falling ash during the explosive eruption of Mount Pinatubo on June 15, 1991, killed about 300. Historical examples of the effects of ash accumulating on roofs are provided below.
If ash fall is expected, a survey should be made of the strength of roofs in the area and of the maximum thickness of ash that they will bear without danger of collapse, especially for critical facilities and buildings which are expected to provide refuge for people during ash fall. Such surveys must take into account the density of both dry and wet ash.
The effects of volcanic ash on roofs depend primarily on:
Ash density and thickness
The specific weight of dry ash can vary from 400 to 700 kg/m3, and rainwater can increase this by 50-100 percent or more if the ash becomes saturated by rain, sometimes reaching more than 2,000 kg/m3. The problems of loading by ash are similar to those from loading by snow, but the effects of ash accumulation are much more severethe load due to ash is typically much greater (see table below), ash doesn't melt, and the ash can clog gutters and cause them to collapse, especially after rainfall. In areas that have snow-loading codes, some protection against ash may result but his is highly dependent on the location of structures because snow load levels vary with altitude and location.
Building design & construction || Historical case studies ||
The effects of ash loads on buildings vary greatly depending of their design and construction, including roof slope, construction materials, roof span and support system, and age and maintenance of the building. In general, flat roofs are more susceptible to damage and collapse than steeply pitched roofs, and roofs made of smooth materials like sheet metal and glass are more likely shed volcanic ash than roofs made of rough materials like thatch and asphalt or wood shingles.
Buildings designed to withstand a heavy load of winter snow will clearly support thicker accumulations of ash than buildings not engineered for any type of load or shear stress. Surveys of buildings damaged from the accumulation of ash during the eruptions of Mount Pinatubo in the Philippines and Rabaul Caldera in Papua New Guinea indicate that roofs with wide spans (for example, warehouses) are more vulnerable to collapse than buildings with short spans typical of small homes.
Complex roof profiles or geometry and obstructions on roofs such as chimneys, parapets, roof tanks or solar panels may lead to a greater accumulation of ash next to these features if the ash is drifting with the wind. Such uneven accumulation of ash on roofs can lead to an unbalanced load on the roof, increasing the potential of roof failure (Blong, 1984, p. 206-208). Also, ash loads against these obstructions may lead to their failure and, indirectly, to failure of the roof.
Historical eruptions: effects of ash loads on buildings
1994 Eruption of Rabaul, Papua New Guinea
The effects of various ash loads on buildings in
Mount Pinatubo, Philippines, 1991 (Spence and others, 1996)
A survey was conducted of 51 buildings damaged in Castillejos, a town with a population of less than 50,000 located 27 km 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; and (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 15-20 cm of water-saturated ash on roofs would have exerted a force of 3kN/m2.
In a summary of the damage, the authors identified the following:
- 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.
- 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.
- 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; or (d) non-residential rather than residential.
- 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; and (b) large unsupported roof overhangs.
According to the authors "to protect lives, roofs of buildings exposed to possible ash fall should be designed for a superimposed load related to the probable level of ash fall, in a manner analogous to design for now loading in cold climates."
Ash can cause corrosion and may be electrically conductive. To minimize effects, tape plastics (garbage bags, plastic wrap) over external building electronics and metal surfaces, for example, security system displays, swipe card door locks, alarms, and electrical panels.
Metallic roof surfaces, particularly older galvanized roofs which are pitted, and lower gauge galvanized roofs are most susceptible to increased deterioration from the properties of ash. To prevent or reduce the accelerated deterioration of roof coatings by mildly acidic property of ash, clean and/or protect the roof surfaces accordingly.
Because gutters and drains are designed to collect water from roofs, they are perfect "ash traps" and one of the most susceptible parts of a building to damage from ash fall. A gutter that fills with ash, especially if the ash is wet, can easily pull apart or collapse from a roof.
The many downspouts and drains attached to gutters may also become clogged with ash, especially when it rains or if water is used to remove ash from a roof. If the drain pipes deliver water to a dry well, the ash can seal the well, making it inoperative.
For these reasons, it is important to keep roof drains and gutters clear of ash as much as possible.
If a building's down-spouts and drains are designed to deliver water to a community's wastewater delivery system and water-treatment facilities, effort should be made to disconnect or block a building's roof drains before or immediately after an ash fall to prevent ash from entering the drains and waste-water systems.
The most obvious action to take for ensuring the safety of a building is the removal of ash from the roof. Before beginning a cleanup operation, it may be advisable to check insurance policies to see if any actions undertaken, or inaction in some circumstances, may void the policy with respect to damage due to the ash or cleanup process.
When should ash be removed from roofs?
The range of ash densities, roof design, and construction techniques make it difficult to determine when during an ash fall that ash should be removed from a particular building's roof. Clearing ash from a roof may prevent collapse but such a decision must be weighed against the risk of personal injury working in a dark, ash-rich environmentpeople easily slip from roofs, fall from ladders, and fall through weak roofs while clearing and removing ash. Also, if an ash fall is accompanied by rain, the roof may become slippery and the wet ash could be difficult if not impossible to shovel or sweep.
It may be advisable to remove ash before it exceeds a thickness of 10-15 cm, but only if the roof is easily accessible and the ash can be removed safely. Because it is often dark or "pitch black" during an ash fall, it may not be possible to safely remove ash until a later time. After an ash fall, buildings which have received more than 10-30 cm of ash and that have not collapsed still run a high risk of load damage (for example, with the addition of more weight during cleanup operations). The ash should be removed, however, as soon as it can be done safely.
| Things to consider before removing ash from roofs:
Recommendations for removing ash from roofs:
Modified from, FEMA, 1984
The abrasive and mildly corrosive nature of ash can damage mechanical and electrical systems. Air-handling systems and air conditioners are vulnerable to ash damage and air-filter blockage, especially if air intakes are horizontal surfaces. Damage can be prevented by turning off such systems before an ash fall begins or immediately at first signs of ash fall. In many cases damage can be avoided by taking steps to avoid use and contamination during ashy conditions and thorough clea
ning of equipment.
In buildings where air quality is critical, for examples hospitals, any action should be taken under the advice of qualified personnel and engineers.
|Recommended steps for air-handling systems (FEMA,
To restart air-handling systems:
Electronics || Electric motors || Computers || Appliances ||
The abrasive and mildly corrosive nature of ash can damage computer and electronic systems. When possible, the best damage-preventive strategy is to shut down all computer, electric motors, and electronic systems until the ash has been completely removed from the equipment and the surrounding area, including air-supply and ventilation systems. Electrical panels can short out because of the high level of electical conductivity of wet ash. For electronical systems and components, the goals are to keep ash out of electronics, controlling what gets in, and cleaning and disposing of the ash.
Ash can increase the wear on brushings, brushes, thrust bearings and commutators on electric pump motors and other drive motors. Small motors blanketed with ash can generate heat and could become fire hazards (FEMA, 1984).
|Recommendations for cleaning motors (FEMA, 1984):
Computer heads and discs, and any high-voltage circuits, are especially vulnerable to ash upset and damage. Ash on digital circuits will not cause much of a problem becasue of the low voltages involved. High-voltage or high-impedance circuits are very vulnerable to leakage caused by semiconductive ash. Ash that is acidic is conductive as well as corrosive. Continual cleaning and aggressive protection of computer systems should allow for continued operation in all but the heaviest ash fallout (Labadie, J.R., 1994).
|Techniques for cleaning include (from Labadie, J.R.,
Removing Ash || Inside buildings & households || Disposal || Roofs ||
Inside buildings and households
In general surfaces should be vacuumed to remove as much ash as possible from carpets, furniture, office equipment, appliances, and other items. Portable vacuum systems equipped with high-efficiency particulate filtering systems are recommended whenever possible. The severity of ash intrusion depends on the integrity of windows and entrances, the air intake features, and the care exercised to control the transport of ash into a building or home via shoes and clothing (see recommendations for keeping ash out). Care should also be taken to avoid further contamination during the emptying, cleaning, and maintenance of vacuum equipment.
|Suggested ash removal from buildings and households
Ash removed from inside buildings and homes should be disposed in accordance to community plans and directions (for example, preparing it for pick up by clean-up crews as part of neighborhood cleanup activities). It may be advisable to request that people separate volcanic ash from normal garbage for collection or disposal at a designated locationmixing ash with normal garbage can result in damage to collection vehicles and take up space in landfills. Small amounts of ash from vacuum cleaners have been disposed successively in household gardens and lawns.
See full discussion of ash disposal issues.
Keeping Ash Out || Air-handling systems ||
During and after ash fall, keeping ash out of buildings and homes will significantly reduce cleanup costs and prevent damage to surfaces, electronics, appliances, floors, and other equipment.
Blong, R., and McKee, C., 1995, The Rabaul eruption 1994: destruction of a town: Natural Hazards Research Centre, Macqauarie University, Australia, 52 p.
Labadie, J.R., 1994, Mitigation of volcanic ash effects on aircraft operating and support systems, in Casadevall, T.J., ed., 1994, Volcanic ash and aviation safety: Proceedings of the first international symposium on volcanic ash and aviation safety, Seattle, Washington, July, 1991: U.S. Geological Survey Bulletin 2047, p. 125-128.
Federal Emergency Management Agency (FEMA), Region X, 1984, The mitigation of ashfall damage to public facilities: lessons learned from the 1980 eruption of Mount St. Helens, Washington: [Seattle, Wash.], FEMA, 70 p.