Why do some buildings survive earthquakes when others fail?
Read the original news post from Timber Engineering Europe Ltd.
Excerpt posted below for your convenience and discussion
Earthquakes can happen anywhere any time, so the idea that “We are not on an Earthquake Zone” is incorrect. In fact EVERY YEAR there are an estimated 2 million Earthquakes worldwide and 500,000 detectable Earthquakes globally of which 100,000 can be felt. Only around 100 each year cause damage.
It is a known and accepted fact that buildings and earthquakes do not go hand in hand and terrible damage is caused in many instances. Building damage avoidance is quite a simple concept, it is the materials used that cause the problems.
Earthquake resistance can and is, in many cases, built into the design of the building. It is the materials used that cause the problems.
Any building MUST be able to withstand the horizontal forces inflicted on its foundations by an Earthquake. The tremors inflict sideways loadings to the foundation and this transfers to the rest of the building in a shaking motion. Traditional foundations are designed to take the imposed loads of weight, being roof, floors and walls, so the weight pressing down on the foundation will always be constant and the foundation will have been designed to take the weight of the respective structure. In older buildings, problems occur when extra floors are added or heavier roof structures are installed and the foundations designed for the original building are not reinforced to accommodate the extra imposed loads.
To construct a house or building under static conditions, the materials need only to be stacked up, attached to each other, and balanced. These kinds of buildings are not designed to accelerate rapidly and change directions like cars or airplanes. Buildings in seismically active areas, however, must be designed and built to withstand the dynamic acceleration that can occur during an earthquake. Large buildings and structures such as bridges, in particular, must be designed so that vibrations arising from earthquakes are damped and not amplified.
Because noticeable earthquakes are rare in most areas, people may not recognize that the objects and buildings around them represent potential hazards. It is not movement of the ground surface alone that kills people. Instead, deaths from earthquakes result from the collapse of buildings and falling objects in them, fires, and tsunamis. The type of construction that causes the most fatal injuries in earthquakes is unreinforced brick, stone, or concrete buildings that tend not to be flexible and to collapse when shaken.
The most earthquake-resistant type of home is a low wooden structure that is anchored to its foundation and sheathed with thick plywood. Some of the traditional architecture of Japan approximates this shock-resistant design, including wooden buildings that are more than a thousand years old. Strangely, both unreinforced masonry and shock-resistant wood houses are used by different cultures in areas of high earthquake risk.
Timber Frame is without doubt the safest and most durable form of construction in Earthquake conditions. It is lightweight and can stand the horizontal forces imposed during an Earthquake because it has lateral bracing built in as part of its earthquake resistant design. Timber will flex and return to its original shape, unlike concrete and masonry buildings. Joints are also a major fail zone in traditional buildings as the reinforcing and/or joints will loosen during an Earthquake and cause the building to fail. This does not happen with timber frame and it is rare for a timber frame building to collapse if Engineered and erected correctly. The figures shown below speak for themselves, but there is no question that Timber Frame buildings are far safer and more desirable than any other form of building.
|Earthquake||People Killed||Killed in T Frame||T Frames Involved|
|Alaska 1964||130||10||Not recorded|
|San Fernando 1987||63||4||15,000|
Source E. Karacabeily (Forintek) 2006
|Classes A ; B ; C. represent:||Adobe House, Brick Buildings and Reinforced Concrete constructions.|
|Classes D ; E. represent:||Buildings with an element of Earthquake Resistant Design (ERD) and also for
WELL BUILT TIMBER, reinforced or confined masonry, and steel buildings.
|Class F represents:||Resistant design. i.e. a structure of the highest earthquake resistance, due to
the incorporated design principles.
The lowest level and therefore the type of building which would suffer most damaged would be buildings without ERD both engineered and non-engineered constructions.
The second level is buildings with ERD. These buildings may include masonry construction, reenforced concrete or steel. Inc. Retrofitted property.
The highest level and subsequent safest level applying the principles of MSK is An engineered construction with ERD
Because EU standards require our engineers to design a building on an elemental basis, i.e. taking into account location, wind and snow loadings, historic weather patterns and many other conditions our products do fit into the ERD category as an engineered construction.
The following points are considered when assessing ERD and would be considered undesirable:
- Heavy Roof systems
- No flexibility of materials during earthquake
- Unable to return to original shape after shaking
- Large window/door openings in Stone/Masonry/R.C.
- Weak non-mechanical joints in materials
- Weak horizontal elements.
Timber frame structure is not affected by any of the above if built to the correct specifications. This is a very brief explanation of a very complex subject but you are safe to advise clients that on information known Engineered Timber Frame construction with ERD is acknowledged as the safest form of building in an Earthquake.