Building Vulnerability to Earthquake is measured using several different factors and the concept of vulnerability is fundamental to modern intensity scales. Say two groups of buildings are subjected to the same shaking and one group suffers less damage than the other, it can be said that the buildings less damaged are more Earthquake resistant. Clearly the amount of shake required to destroy a poorly built adobe hut is not the same as that required to destroy a modern office block.
In 1956 the Modified Mercalli Scale and in 1964 the MSK scale addressed the issues of assessment of earthquake damage in modern buildings. The did so using the following principles:
- Building Type
- Construction Materials used for the lateral load resisting system.
In this, building type was used as a simple analogue for vulnerability.
The MSK scale defined building classes by type of construction generally to express vulnerability. The EM has actually classified the building types into 6 catagories (A-F) in decreasing vulnerability. This is a very complicated classification and very lengthy, so I will just show the basic rules:
Classes A – B – C. represent: Abdobe 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 are buildings with ERD. These buildings may include masonry construction, re-inforced concrete or steel. Inc. Retro-fitted property.
The highest level and subsequent safest level applying the principles of MSK is
An engineered construction with ERD
Because British standards require our engineers to design a building on an elemental basis, i.e. taking into account location, wind and 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.
