Timber Engineering Europe
Buildings and Earthquake
WHY DO SOME BUILDING SURVIVE EARTHQUAKES WHEN OTHERS FAIL?
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.
||Killed in T Frame
||T Frames Involved
|San Fernando 1987
(Source E. Karacabeily (Forintek) 2006)
The EMS, most recently updated in 1998, is the basis for evaluation of seismic intensity in
European countries. Unlike earthquake magnitude, which indicates the energy a quake expends,
EMS98 intensity denotes how strongly an earthquake affects a specific place. The European
Macroseismic Scale has 12 divisions, as follows:
- Not felt - Not felt, even under the most favorable circumstances.
- Scarcely felt-Vibration is felt only by individual people at rest in houses, especially
on upper floors of buildings.
- Weak - The vibration is weak and is felt indoors by a few people. People at rest feel a
swaying or light trembling.
- Largely observed - The earthquake is felt indoors by many people, outdoors by very few.
A few people are awakened. The level of vibration is not frightening. Windows, doors
and dishes rattle. Hanging objects swing.
- Strong - The earthquake is felt indoors by most, outdoors by few. Many sleeping people
awake. A few run outdoors. Buildings tremble throughout. Hanging objects swing
considerably. China and glasses clatter together. The vibration is strong. Topheavy
objects topple over. Doors and windows swing open or shut.
- Slightly damaging - Felt by most indoors and by many outdoors. Many people in buildings
are frightened and run outdoors. Small objects fall. Slight damage to many ordinary
buildings; for example, fine cracks in plaster and small pieces of plaster fall.
- Damaging - Most people are frightened and run outdoors. Furniture is shifted and objects
fall from shelves in large numbers. Many ordinary buildings suffer moderate damage: small
cracks in walls; partial collapse of chimneys.
- Heavily damaging - Furniture may be overturned. Many ordinary buildings suffer damage:
chimneys fall; large cracks appear in walls and a few buildings may partially collapse.
- Destructive - Monuments and columns fall or are twisted. Many ordinary buildings partially
collapse and a few collapse completely.
- Very destructive - Many ordinary buildings collapse.
- Devastating - Most ordinary buildings collapse.
- Completely devastating - Practically all structures above and below ground are heavily
damaged or destroyed.
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
category as an engineered construction.
The following points are considered when assessing ERD and would be considered undesirable:
Timber frame structure is not affected by any of the above if built to the correct specifications.
- 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.
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.