ОРГАНИЗАЦИИ

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    The latest events in El Salvador, India and Afghanistan have clarified again the importance of building design that is suitable for earthquakes. We will all remember the dramatic pictures of the region Izmit in Turkey that could be seen all over the world in August 1999. The evaluations and damage analyses especially of this event supplied European earthquake experts with valuable knowledge and new starting points for research.
    It is not only the region of Izmit that counts as one of the most endangered areas - the following map shows all earthquake events in Europe since 1975.
    According to investigations, a decisive cause for the damage observed was on one hand the ignorance of elementary construction rules, on the other hand the incorrect choice of building materials. Designers are therefore obliged to take a lot of care with the earthquake resistant design of buildings and with the choice of building materials.

   Damage on masonry buildings
    In order to determine favourable construction forms and suitable materials, it is especially important to evaluate the damage occurred at masonry buildings subject to earthquake actions.

    The following pictures show typical damage cases and their causes:
    Picture 1 shows a typical shear crack, caused by the brittle material behaviour of the brick masonry used. The occurring shearing forces resulting from seismic stress can no longer be absorbed.
    The following picture shows a frequent damage case with double leaf wall constructions. The external shell chips off as a result of deformations.
    This print shows the complete separation of the masonry out of the bearing structure, caused by the different deformation behaviours of the bearing structure of reinforced concrete and of the infilling masonry.
    These damage pictures explain the demand for constructions with the following characteristics:
    - homogeneous
    The chosen constructions shall contain as little change in material as possible. This means that the masonry for smaller residential and office buildings shall be loadbearing. The additionally requested confining elements - reinforced concrete elements, dowels - are to be arranged in adequate distances, i.e. according to the requested standard; its quantity shall nevertheless be kept as low as possible.
    - high-value
When choosing the building materials, a high quality standard and the corresponding material parameters such as sufficient shear and tensile strength shall be observed.
    - optimized thermal insulation
    With regard to thermal insulation, a better solution are one-leaf wall constructions out of highly insulating building materials such as AAC.
In spite of all these requirements, the economic efficiency of the constructions shall not be disregarded.

   Material properties of AAC masonry
    The Eurocode 8 "Design provisions for earthquake resistance of structures" part 3 describes requirements on masonry buildings. As an example, it is requested to use bricks with a perforation volume 50% and a minimum thickness of the external webs of 15 mm in order to avoid local brittle failure. Moreover, the compressive strength of blocks of Fb = 2.5 N/mm2 vertically to the longitudinal joint and Fbh = 2,0 N/mm2 parallel to the longitudinal joint is to be guaranteed.
    AAC is a homogeneous, isotropic material with equal characteristics in all load directions. The minimum compressive strength fb requested by EC 8 is already achieved for the lowest strength class. A high degree of shear and tensile strength can be taken as a basis even for a comparably low block strength. Due to its low weight, the acceleration mass is clearly reduced in case of earthquakes.
    A further important property of AAC is its ductility and dissipation of energy (refer also to q-value). Due to the permanent quality control during production, YTONG material can be classified as category I according to EC 6. For this reason the partial safety factors for design can be reduced.
    The same applies to the category of construction - category A . Due to the high quality level as a result of the thin layer mortar technique, the requested safety factors can also be reduced here.
    Finally, another important point of view is to be mentioned - YTONG is not combustible. Experience has shown that the quantity of victims is not a result of the direct effects of the earthquake but of the consequences of fires.

   Type of construction and ductility
    The behaviour under earthquake actions is influenced by the type of construction and its ductility. Masonry buildings are differentiated into two types of masonry
- plain masonry
- confined masonry
    The meaningfulness of the behaviour factor on the "earthquake suitability" of a construction becomes especially clear with the following equation. It describes the resulting horizontal design force, which is the total seismic force acting on a building.

Fb-Sd(T)W

whereas:
Fb = total horizontal design force
Sd(T) = Ordinate of design acceleration spectrum

     f(a(S,T,po)
 = --------------------
      q

W = Total vertical weight of the structure
 

   Earthquake Resistant Design ofLoadbearing Masonry Structures
    According to the type of the structure, the seismic parameters and the structural layout, the following methods of analysis are applicable:
- Multi modal response spectrum analysis
Buildings shall be analyzed using a spatial model.
- Simplified modal response spectrum analysis
The simplified modal response spectrum can be applied to buildings that can be analyzed by two planar models. Their response is not significantly affected by contributions from higher modes of vibration. The requirements will be explained at a later stage.
- "Simple masonry buildings"
An explicit safety verification is not mandatory for smaller residential and office buildings provided that certain criteria such as structural layout, number of storeys above ground, minimum cross section of bearing walls are considered.
 It is often very difficult - and often only possible in approximate figures - to determine exactly the seismic stress to be expected. There are many influencing factors e.g. the ground quality or the situation of the building as well as seismic parameters like type and period of the ground vibrations. If for this reason a costly - and often only partly applicable - spatial analysis of the building (see item one) is not carried out, a number of requirements must be kept to. At first, there are general construction rules of earthquake resistant design. Moreover specific requirements on masonry buildings exist. If additionally the requirements on "simple masonry buildings" as described in EC 8 are taken into account, an explicit safety verification of the building may be dropped.
 Some requirements, depending on the respective method of analysis, are listed in excerpts as follows:

   1. Simplified modal response spectrum analysis
     - General construction rules (excerpts)
     An important criteria is the distribution of the building mass. The following graphic chart shows examples of building types with favourable and unfavourable mass distribution.
 

    Requirements on masonry buildings  (excerpts)
- Masonry building shall be composed of floors and walls. Each type of floor may be used, providing that the general requirements of continuity and effective diaphragm action are satisfied.
- Shear walls shall be arranged at least in two orthogonal directions.
They must fulfill the following geometric requirements:
The thickness t of the wall must be

 - for plain masonry
 t 300 mm    in high/medium seismic zones
 t 175 mm    in low seismic zones

 - for confined masonry
 t 240 mm

- Confined masonry
The horizontal and vertical confining elements shall be bonded together and anchored to the elements of the main structural system.

    2. "Simple masonry buildings"  (excerpts)
    According to EC 8, part 3, the following requirements for simple masonry buildings shall be considered:
- The allowed number of storeys above the ground level must not exceed the following values, with respect to the design ground acceleration:
- A minimum of two parallel walls has to placed in two orthogonal directions. At every floor the total cross section of all shear walls , as a percentage of the total floor area, must be at least:
- When arranging wall openings, certain limit values are to be considered for the measures d1, d2, ds and d4.

    As a practical help for the designer, YTONG has drawn up the above recommendations and the knowledge gained from research activities in a brochure: the Guideline "Earthquake Resistant Design of YTONG Loadbearing Masonry
Structures".
    The behaviour of a construction in case of an earthquake is generally described by one factor: the
behaviour factor q. This factor mainly depends on
- the material quality
- the type of construction
- the type of bonds.

The behaviour factor is expressed by the ratio

     Fu
 q = -----------------
      Fo

Fu = ultimate seismic force in the elastic range of behaviour
Fo = seismic force at the yield point.

    The following graphic chart shows the different properties of plain and confined masonry.
    The more favourable behaviour of confined masonry under earthquake actions is expressed by an increased behaviour factor.
 In order to determine the behaviour factor ofAAC masonry, test walls were exposed to dynamical loads by the NISI-Institute of the university in Sofia under the direction of Prof. Dr. Mincho Dimitrov. The test walls were composed of both masonry types- plain and confined - as well as with the different arrangement of openings.
    The investigations confirmed the ductile behaviour of AAC, reflected in the q-values determined.