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Since the forces of an earthquake are so powerful, knowing how to tell if the house is safe after an earthquake and knowing what to check is key in keeping the building safe and secure as both moderate and severe quakes can cause serious structural damage and injury.
Just as an airline pilot always does a walk around of his plane prior to a flight, you should do a structural walk around any current projects after an earthquake.
After any moderate to significant shaking, do a preliminary structural check for damage i.e if the house has shifted on the foundation; then, if there is a strong aftershock or a high wind event in subsequent days, there is heighted risk of further damage.
Buildings under construction which may have partially completed frames are at a heightened risk and are particularly vulnerable to potential movement and cracking arising from earthquakes.
This is even more pertinent for sites that may currently have restrictions not permitting work (Melbourne’s shutdown) or where a site has had to have work cease for an extended period due to other factors.
Therefore, if you note structural damage, you may suggest if the home is occupied that the occupants do not stay in the home until it has been thoroughly assessed/checked over, as it may be weaker and may not safely withstand another jolt, even a small one.
Be careful entering a home if it has sustained significant structural damage, as homes can be hazardous at times. Also apply caution in making any repairs as the damage could extend beyond the visible area and could lead to further collapsing.
If you need to do any cleaning up of debris from the earthquake also be mindful of any materials that may contain hazardous material such as asbestos and ensure correct PPE is worn.
The house may have suffered more damage than you think, particularly in older homes, and it’s important to look not only for obvious signs of damage but the less obvious signs of structural damage such as connections between structural members loosening which can have detrimental impacts for the structure.
What many people don’t often think about is that the earthquake may have loosened or damaged key components of the structure just enough that the house didn’t fall or collapse, but that with just a little bit more shaking, it may suffer more significant damage since it had already been weakened quite a bit.
The risk of damage and the severity of damage to a home depends on a number of factors and categories, including the magnitude of the earthquake, the closeness to the epicentre, and the age and type of construction of the home. Should an earthquake occur, be sure to check your home for structural damage or weakness right after the earthquake, as well as after any aftershocks.
Older homes not built to our current building codes and standards are more susceptible to damage, particularly those with brick or blockwork walls.
While most houses (Class 1a buildings) are not typically required to have specifically earthquake detailing in them under the National Construction Code and relevant standards, importantly though houses are considered to perform sufficiently in an earthquake in meeting current loading and structural framing standards in being designed for the local wind conditions.
Homes that are “T” shaped or “L” shaped are more susceptible to damage than ones that are square or rectangular in shape; ones that are irregular in design or split-level homes likewise are more susceptible to damage.
These types of homes are more vulnerable to torsion or twisting about their longitudinal axes because they do not distribute seismic forces equally or as well as square or rectangular types of homes.
Hillsides can also be more susceptible to damage due to movement resulting from earthquakes and when this occurs, homes on these hillsides generally suffer more damage than others, especially if the hillside has water saturated soil.
As such further attention in carrying out inspections to assess any damage is recommended.
Below is a checklist of items in checking of the building structure under construction is safe and secure:
One of the first things to check is the homes’ connection to the footings/slab/base frame.
This is critical, for if a home has shifted on the footing, it is usually considered an important issue and could even be more serious than what you might think, for once the connection of the house to foundation is weak or broken, another after shock may cause the house, or portion of it, to collapse or fall off of the foundation.
If you discover the house has shifted or is partially off the foundation, and occupants are living in the dwelling say while an extension/renovation is being undertaken, you may want to advise them to not stay in the house until an engineer has determine the seriousness and risk.
There may be various warning signs on the interior of the house that the framing or foundation has suffered structural damage. Should you see some of these warning signs be sure that you conduct a structural check as outlined.
Look for the following:
If you asked to undertaken a house inspection or a previous client calls regarding assessing any damage to the structure the following information may assist:
Over the years it is not unusual for existing buildings to develop some minor movement cracks, many attributable to settling and expansive soils. When earthquakes occur, it is not unusual for cracks to develop or existing cracks to widen.
When there are foundation cracks, there are many factors that relate to how serious the cracks are; i.e., “V” shaped cracks and offset cracks are generally considered a bigger concern than small hair line type cracks.
If a quake creates a crack that is a 10 mm inch or an inch wide, it would definitely be of concern; if only a hairline crack or 3-5 mm, then it would normally not be as much of a concern.
Similarly on plasterboard internally or some cracking or mortar or tiles, small hairline cracks or 3 mm, then it would normally not be as much of a concern.
Foundations that are leaning, bowed or out of level are likewise a concern. Naturally, the more movement of a foundation, the more the level of concern. If you’re not sure if the foundation is level and plumb, you can use a level to tell if it is leaning or use a long string line to see if it’s bowed.
For further information regarding tolerances and building movement can have regard to likes of the HIA Guide to Materials and Workmanship and state and territory guides to Standards and Tolerances.
Though it’s important to note that these are just a guide and were not written with earthquake movement in mind.
If the home has a crawl space under it and it has a pier and beam construction, stump and bearer or post and beam; then it is recommended that these be checked, for it is not uncommon for the post or beams to shift out of position or crack. Should this happen the floor may sag or slope and if enough seismic damage has occurred it may collapse.
Check the connection area of all the post and beams, as well as, the post connections to the piers of post pads (footings).
Do not enter any crawl spaces alone or if there is any noticeable damage to the house. You can do a cursory look from the crawl space access but do not put yourself in danger or an area where you can get trapped or injured if a tremor, aftershock or earthquake should occur.
Stand back from the house, where you can see the roof and look for changes or damage. Look at it from all sides.
Check for damaged to the rafters and trusses, as well as the rest of the roofs framing. Occasionally, in an earthquake, if the ridgeline is bowed, it’s because the walls are leaning outward at the top of the exterior walls on opposite sides.
Remember, if there is still seismic activity or after shock warnings, do not enter the attic areas where you may get trapped or injured if there was another earthquake or a roof failure.
Stand at each corner of the house and visually site down the wall to see if there is anything unusual.
Are there any bowed, wavy or damaged exterior walls?
If not sure, pull a string line down the length of the wall to see if it is straight. Use a level to see if it is leaning or tilted.
Should the wall have issues, you should examine closely the connection of the wall to the foundation. Also, the roof framing connection to the wall should be examined.
If opposite exterior walls lean outwardly, you may want to double check if the roof is sagging and check the collar ties and rafter ties in the attic to see if they have pulled loose or if they’re damaged. Check the rest of the roof and attic framing at the same time.
In solid brick or block walls, cracks are generally considered more serious, than cracks in a brick veneered wall; small hairline cracks in veneered walls are generally not of much concern. Cracks that are a quarter or half inch wide would be a concern and if the wall is leaning or bowed, they become even more serious.
Wood framed houses often perform better in an earthquake than masonry houses because they are less rigid and they tend to flex, absorbing and disperse energy better when exposed to sudden earthquake forces.
If you see evidence of soil movement or if there is a retaining wall that developed cracks, started to lean or fell over, then you may want to think of it as a “Yellow or Red Flag” that the house may have suffered structural damage. If block walls or brick fences and they are leaning or damaged, that is additional evidence that the house may have suffered damage.
Leaning and damaged retaining walls can be a safety hazard if they’re in danger of falling over. Also, if a retaining wall is close or high and it moves enough, it may have an effect on the foundation of the house, which may result in structural damage.
When in doubt about how safe the house is after an earthquake, be on the safe side and consult a professional.
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