Unsafe ceilings and storage facilities can be deadly
By Patricia Thompson
Commercial property owners must not ignore the potential dangers of the failure of non-structural components of buildings - such as ceilings, parapets, facades, chimneys, partition walls and storage racks - during an earthquake, says Dr Rajesh Dhakal of the University of Canterbury.
Dr Dhakal is an associate professor in the Department of Civil and Natural Resources Engineering and a management committee member of the New Zealand Society for Earthquake Engineering (NZSEE).
He was among those responsible for inspecting buildings following the 4 September and 22 February Canterbury earthquakes.
Dr Dhakal says that non-structural and content damages were widely reported following the 4 September earthquake and contributed a major share to the overall loss.
“However, the extensive structural damage requiring demolition of a large number of buildings after the 22 February earthquake has overshadowed the significant risks posed by non-structural components,” he says.
While there was considerable focus on the loss of life in buildings that collapsed on 22 February, Dr Dhakal says that a number of fatalities were due to the failure of non-structural components, such as falling parapets and facades.
“It is important that building owners are aware of this risk. We have to change the public perception that people are safe if a building avoids structural collapse,” he says.
Dr Dhakal says that in commercial buildings, ceilings usually consist of panels resting on the flanges of a grid of lightweight inverted T-shaped beams hung from metal wires anchored to the floor above.
“Lightweight ceiling panels are now available but many buildings still have heavy panels, weighing up to 15kg. If those panels fall from more than five meters height on to people underneath, there is a reasonable chance of someone being injured or killed,” he says.
Dr Dhakal warns that we should not be fooled by the lack of ceiling-induced casualties in the two earthquakes in Christchurch; collapsed ceilings killed many people in the March earthquake in Japan.
“In many earthquake-prone countries, including the USA and Japan, seismic braces are frequently used in ceilings. However, assessments following the 4 September earthquake revealed that this wasn’t the case in Christchurch. A number of ceilings were found to have suffered dislodging or breaking of panels and failure of the grid members, perimeter angles and connections,” he says.
“Following the 22 February earthquake, the immediate need was to inspect buildings for structural damage. However, while doing so, I observed that in most buildings that were damaged structurally, the non-structural damage was even worse. In fact, I saw very few commercial/office buildings in the CBD where the ceiling was intact.
“We need to send a very strong message to building owners that they should have ceilings of their buildings inspected. Unbraced ceilings should preferably be braced and heavy ceiling panels replaced with modern lightweight panels.”
Dr Dhakal visited several commercial and industrial buildings following the two earthquakes and he found racking and shelving systems to be another area of concern. He stresses the need to change our existing installation and storage practices to achieve better seismic performance from these systems in future.
“Racks and shelves should be properly anchored to walls wherever possible, as a properly designed and adequately anchored racking system will provide the best seismic performance in relation to collapse prevention and protection of contents,” he says.
“Ideally, racks and shelves should be tied together to make the system stiffer, which considerably reduces the risk of failure and also the probability of contents falling down in an earthquake.
“However, merely providing a tie without any consideration to the likely demand will not be enough; I observed that in libraries even shelves which were tied together collapsed because ties were not strong enough to resist the tilting tendency of the shelves. However, shelves anchored to the wall or interconnected using strong ties remained intact. Strictly speaking, all members used to support/brace any non-structural component should be properly designed to resist the effect of seismic actions.”
Dr Dhakal also notes that wooden shelves in offices are often divided by timber planks which are free to slide and may fly out in an earthquake, which could easily be avoided by using stoppers at the front face.
Dr Dhakal says that damage to racks and contents is also affected by the distribution of weight. He notes that racks perform better when the weight of the contents is uniformly distributed along its length. Similarly, a rack with heavier contents in the bottom tier is significantly more stable than the same rack with heavier contents on the top.
Dr Dhakal recommends that legislation may be necessary to ensure that racks and shelves are adequately anchored and that insurers can encourage these preventive measures by offering rebates for seismically stiffened shelving and racking systems.
Further details of Dr Dhakal’s findings on the effect of non-structural damage from the September earthquake can be found in the December 2010 Bulletin of the New Zealand Society for Earthquake Engineering. Copies can be ordered through the NZSEE website: www.nzsee.org.nz