13 Earthquakes
In Germany, earthquakes pose a risk to cultural property in certain regions only. If an institution is located in an earthquake-risk zone – mainly in southwestern Germany and in the area to the west of Cologne – or in a (former) mining region, preventive measures such as earthquake-proofing buildings or enhanced earthquake protection for movable cultural assets should be taken.
After every major earthquake, the question arises as to why they are so dangerous for buildings. Earthquakes cause major damage, up to and including the collapse of buildings, because they exert enormous horizontal pressure on the supporting structures of a building. Normally buildings are only exposed to vertical stresses (weight, loads etc.). They therefore often have structural weakness with respect to horizontal resistance (see Fig. 1).
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| Fig. 1: Earthquakes are dangerous for buildings because they exert great pressure in the horizontal direction. (Lestuzzi, P.: Séismes et Construction. Eléments pour non-spécialistes. Lausanne, 2008) |
1. Typical earthquake damage to buildings
Following every earthquake, similar damage to structures is repeatedly seen. In most cases this damage can be attributed to errors and deficiencies in structural design. Damage occurs mainly as a result of the following deficiencies:
- Inadequate horizontal stiffening
- Soft story
- Non-load bearing elements
- Collision with adjacent buildings
- Soil liquefaction
Inadequate horizontal stiffening
Horizontal stiffening is crucial to withstand the horizontal forces that are generated during an earthquake. This requires vertical building elements (e.g. walls, frames, half-timbering) which are capable of absorbing horizontal forces without deforming too much – they require a certain rigidity.
A large number of buildings with inadequate horizontal stiffening can still be found even in regions where there is a high risk of earthquakes (Figs. 2a and 2b). The impact of such weaknesses is catastrophic, as it can often lead to the complete collapse of a building.
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| Fig. 2a: Izmit 1999. Photo: P. Lestuzzi, Swiss Society for Earthquake Engineering | Fig. 2b: Kobe 1995. Photo: Architectural Institute of Japan |
Soft story
A soft story is one of the most commonly found weaknesses which can lead to a large part of a building collapsing following an earthquake. In many countries, and unfortunately often in major earthquake zones, the stiffening elements present in the upper storeys are omitted on the ground floor and instead supporting pillars are used in order to create space for shops. This arrangement is extremely unfavorable with respect to the impact of earthquakes, as often the supports cannot withstand the seismic shaking and fail.
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| Figs. 3a and 3b: Soft story, Izmit 1999. Photos: P. Lestuzzi, Swiss Society for Earthquake Engineering |
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Non-load bearing elements
Non-load bearing elements can also cause a lot of damage. In fact, even with relatively moderate tremors, the damage to non-load bearing elements is responsible for the majority of financial losses. It is therefore essential to consider these building elements during the design phase, especially as they can lead to the collapse of the supporting structure (e.g. blocking in a frame structure can lead to a shear fracture in a brace).
Falling facade elements can cause serious damage during an earthquake and they can block access for the emergency services and are a danger to anyone on the street below. The anchoring for the facade elements must therefore be designed not only for vertical loads but also to withstand horizontal stresses.
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| Figs. 4a and 4b: Fallen facade elements, Kobe 1995. Photo 4a: Architectural Institute of Japan, Photo 4b: T. Wenk, Swiss Society for Earthquake Engineering |
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Collision with adjacent buildings
Earthquakes result in major deformations and can lead to buildings colliding with adjacent ones. This is particularly dangerous if the ceilings are at different heights and fall against supports.
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| Figs. 5a and 5b: Collision of adjacent buildings, Izmit, 1999. Photos: P. Lestuzzi, Swiss Society for Earthquake Engineering |
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Soil liquefaction
In response to heavy shaking, fine sandy and loam soils can behave like a liquid if combined with a high water content. This phenomenon is referred to as soil liquefaction. Buildings can then sink or tip over.
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| Figs. 6a and 6b: Soil liquefaction, Izmit 1999. Photos: P. Lestuzzi, Swiss Society for Earthquake Engineering |
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2. Earthquake risk
Earthquake risk refers to the likelihood of an earthquake occurring. It is usually stated for a periodicity of 475 years. This corresponds to a 10% probability of exceedance in 50 years. 50 years is assumed to be the normal life of a building. The risk is usually displayed on earthquake risk maps. These maps can indicate the macroseismic intensity for example (Fig. 7).
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| Fig. 7: Earthquake risk for Germany, Austria and Switzerland (H. Bachmann: Erdbebengerechter Entwurf von Hochbauten – Grundsätze für Ingenieure, Architekten, Bauherren und Behörden. Guidelines of the Swiss Federal Office for Water and Geology (BWG), Bern, 2002) |
Standards tend to specify the maximum horizontal ground acceleration in the various earthquake zones (Fig. 8).
DIN standard 4149 (2005) divides Germany into four earthquake zones (zone 0, 1, 2 and 3, Fig. 8). The following values are prescribed for horizontal ground acceleration in the various zones:
– Earthquake zone 1 : 0.4 m/s2
– Earthquake zone 2 : 0.6 m/s2
– Earthquake zone 3 : 0.8 m/s2
No acceleration value is specified for earthquake zone 0.
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| Fig. 8: Earthquake risk for Germany (DIN 4149 (standard): Bauten in deutschen Erdbebengebieten – Lastannahmen, Bemessung und Ausführung üblicher Hochbauten. Berlin 2005) |
Most of Germany is not considered at risk of earthquakes. The areas most at risk in zone 3 are around Basel and Aachen, as well as in the Swabian Jura and in the vicinity of Chemnitz.
3. Preventive measures
Earthquake-resistant construction
There are methods of construction which are beneficial in the event of an earthquake in that they allow considerable deformation in response to horizontal loads. If executed correctly, the following types of construction can be classed as earthquake-resistant:
- Steel structures
- Reinforced concrete structures with in-situ concrete preparation (i.e. the liquid concrete is not prepared before it arrives on the construction site)
- Steel and reinforced concrete composite structures
- Timber frame structures
- Half-timbered structures
The following construction principles which provide greater resistance to earthquakes should be adopted:
- Redundant building elements (additional, normally unnecessary building elements which take over the function of an element if it fails)
- Symmetrical ground plans (floor plan design: compact and irregular floor plans)
- Massive core structures in the vertical plane (regular and irregular elevations)
- Horizontal stiffening, e.g. by means of bulkhead walls
- Ductile (i.e. not brittle) materials and joints
Protecting movable cultural assets from earthquakes
Protecting a building from earthquakes is the single most important way of protecting the collection it houses. There are however measures that can be taken within a building to protect movable cultural assets.
First ensure there is an inventory of all the objects so that if damage occurs, it can be established what has been destroyed.
In regions at risk of earthquakes, movable cultural assets must be stored correctly. Exhibits must be securely hung or fastened. In libraries, archives and museum storage areas, shelves and cabinets must be fixed to the wall or to the ceiling in order to prevent them tipping over. Objects on shelves should be secured so they cannot fall off. This can be accomplished by providing glass panes or bars. In regions where the earthquake risk is high, it is possible to minimize damage to objects that fall over by providing soft shelf mats. Drawers should be constructed in such a way that they cannot open by themselves.
In Switzerland, special rooms for storing movable cultural artifacts have been build in recent decades. These rooms are used as storage facilities for important movable cultural assets. Although they are designed to protect such assets during an armed conflict, they are also suitable in the event of an earthquake.