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Engineering Aspects of Significant Increscent of Seismic Resistance Of Buildings
Категория: Стр. тех. и арх. | Новость от: Admin | 27-12-2005

According to UNESCO data the earthquake occupies one of the first places among natural disasters. The earthquake is inevitable. However it is necessary to minimize fatal consequences and damage of buildings.

In contemporary seismological science it is still impossible precisely to forecast the time, place and intensity of the possible disaster therefore the first condition of the struggle against the earthquake is a qualitative performance of seismic resistant buildings.

After tragic Spitak earthquake of December 7, 1988 the people began to realize high vulnerability of buildings. Destructive earthquakes are known from ancient times at the territory of Armenia. But the Spitak earthquake was the largest one in the North Armenia and the most severe in Caucasus by instrumental records. Estimates say the quake killed up to 25 00 people:; 7,5 thousand houses, 83 schools, 88 nursery schools, a number of installations, architectural monuments, engineering structures and others were fully collapsed. (The intensity of the earthquake was estimated in Spitak with magnitude 10,5 hit, Gyumri 9,0 – 9,5 on the MSK scale.

Proceeding from the lessons of destructive earthquakes occurring in the world the objective of the given report is to develop scientific and practical proposals to eliminate the failure of buildings and to save people’s life.

From this point of view we think that in comparison with large-panel and framed buildings monolithic reinforced concrete ones are appeared to be reliable enough.

Recommendations of the given report refer to design and erection of new types of buildings with high seismicity (1-2; 4-5; 9 storied and higher). They are made of reinforced concrete on the basis of lightweight non-ore materials (natural and bloating pumices, slags and pearlites, the wastes of tuff quarry and others). Besides it is expected to introduce new bearing enclosure structural decisions and up-to-date design premises and to develop seismic protection methods. The testing of the new erected experimental building is provided to reveal dynamic (seismic) effects by vibration machine and by underground explosions. The tests of buildings and constructions are performed on the models according to the similarly theory of solid mechanic bodies for building structures. Seismic effects are also simulated.

Now let’s introduce everything in details:

1. Building Codes on Earthquake Engineering (SNIP – II. 2.02.94)

During the Spitak earthquake such large damages were caused by low degree of seismicity of the Armenian territory provided by the old Codes of the Former USSR. New national Codes (first being developed in independent Armenia) meet strict requirements for increscent of the level of seismic security of the people taking into account the analysis of Spitak earthquake. They are the following:

- The development of new seismic zoning maps of the Armenian territory according to which the whole territory of Armenia was referred to the zone with magnitude of 9 hit. Besides the values of expected maximum accelerations of the ground oscillations are assumed within 0, 2 – 0,4g. Another main feature of the Codes is the influence of soil conditions (geological and hydro geological) of the site on seismic effects, that is the definition of the level of seismic effect on buildings during the earthquakes, as the behaviour of buildings depends on the nature foundations. New approach for the definition of the value of seismic forces by the dynamic coefficientb.

2. Increscent of design accuracy for seismic effects:

- Structural analysis by accelerograms (records of the ground accelerations) during the earthquakes. It is available to record the acceleration of the given building site, as accelerograms more precisely registrate high-frequency oscillations. For analysis it is necessary to have catalogues of severe earthquake accelerograms of the world. They are selected according to the groups, depending on magnitude and epicentre distance. Several accelograms (3-4) will be included into the set. Then each building is analysed separately by selection of the most adverse condition of the building. For this method the choice of the structure and design scheme of the building is to eliminate the resonance phenomenon between the building and soil. It is very important. Saving of materials is provided by the precision design method.

- New design methods have been developed in Armenia taking into account upper and lower “flexible” storeys; joining elements and unloading frames being located separately from the building; instrumental observations of the behaviour of full-scale buildings by installation of engineering and seism-metric stations; absorbers made of rubber and metal, supporting pads and others.

3. Application of Contemporary Methods of Seismic Protection

Engineering analysis of the Spitak earthquake consequences revealed the necessity of the development and introduction of new, more reliable system of seismic protection both of recently designed buildings and existing ones without moving the dwellers.

- Method of “flexible” upper story is used as a damper of the oscillations. For the first time this method was proposed by Armenian scientists. The experiments performed for 9-storeyed carcass building in Vanadzor (Armenia) revealed double forces in comparison with the building free of damper.

- Method of seismic protection system

- Increscent of seismic resistance of buildings by using sandwich structure of rubber and metal supports of seismic isolation.

The supports were made at chemical plant of synthetic rubber “Nairit” in Yerevan by special composition.

The performance of above-mentioned methods of seismic protection has been tested in Armenia with great properties.

4. The use of Lightweight enclosure structures of buildings.

Cellular concretes: the reduction of materials and energy consumptions is very important in national economy, particularly in the field of construction.

- Cellular, free of cement concretes of autoclave hardening; fine grining volcanic rocks are used as adhesion, being activated by lime and gypsum. In the end cellular concrete is obtained with volume mass of 4 00 – 1200 kg/m3 ; compressive strength 2,5 – 20 MPa. It is used in wall structures, partitions and others.

- Free of cement, dense silicate concretes of autoclave hardening. Grinded volcanic rocks are used for the mixture, being activated by lime and ----- gypsum. The volume, mass is 1400-1600 kg/m2: durability is 20-40 MPa. The field of application: panels, reinforced concrete structures (beams, columns and others).

- Free of cement cohesions and concrete on their base, grinding tuff wastes, slags and pumices are used in natural form. Free of cement non-autoclave concretes were obtained with volume mass- 1000 – 1200 kg/m2, durability is 30 MPa. By solution of this problem the mass of the building is reduced and its seismic resistance is severely increased. At th same time the ecological problem is also solved – recultivation of the lands for agriculture.

- Fibrous concrete. One of the worth-while works of Armenian scientist is a technological elaboration on manufacturing of glass-fibrous concrete. Its composition consists of fine-grained concrete reinforced by glass and basalt fibres. Three-layered wall panels, flat slabs are made of fibrous concrete glass (basalt). It can also be used as a retained formwork for monolithic houses.

Summarizing the above-mentioned it should be stressed, that the objective of the work is to obtain seismic resistant housing of higher degree, that is, seismic resistant building must be erected to prevent its structural damages during severe earthquakes (the level of design is of the I order) and to protect people’s life the level of design is of II order. The acceleration of the ground oscillation of 200-25- cm/sec2 is accepted for the I level and 450-500 cm/sec2 for the II level.

Conclusion: Our recommendations can be used both in neighboring regions of Armenia of and in the USA, where the earthquakes occur very often (particularly in Californian). We are also worried about the community safety. Special attention should be given to children. It is necessary to organize training courses on children’s behavior before, during and after earthquakes. Finally we must act in accordance with the principles; “Our city has been designed to provide people’s safety.

Author: Suren G. Shahinyan

Academician of IEA RA
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