Basic concepts of earthquake resistant structures, Biology

The aim of earthquake-resistant structure designing

The main objectives of earthquake resistant structures are as follows:

(i)     To avoid loss of lives due to the destruction of a building in the event of a major earthquake.

(ii)   To minimize injuries and damages to building if there is a moderate earthquake.

(iii) To minimize damages and inconvenience during moderate and minor earthquakes.

(iv) The best strategy to protect loss of lives is to ensure that the building does not collapse.

Guidelines for earthquake-resistant construction practices (conventional approach)

To ensure the building's resistance to potential earthquakes it should be configured, detailed and constructed accordingly. Any weakness in this regard may affect the building's resistance against earthquakes.

1.      Building must be capable of resisting loads from any direction :

(i)     Earthquakes cause ground shaking.

(ii)   Ground shaking produces inertial loads in the elements of the building. Stronger ground shaking and heavier building elements increase the amount of loads.

(iii) Mostly earthquake load are in the horizontal direction, but they may strike from any direction.

(iv) Earthquake loads are cycle.

(v)   Imagining a building on the back of a truck, traditional masonry building with weak mortar will collapse during acceleration or deceleration of the truck. A tall and slender building will topple over but if a building is light weighted, modestly proportioned, with good connections that are properly attached the foundation, shall remain intact on the truck.

To be able to resist loads from any directions, the building must be ready to resist loads from two orthogonal directions. An earthquake load from any direction can be divided into x and y components which can be resisted by the structure in these two directions.

(i)     Loads can be resisted by moment resisting frames, braced frames, shear walls or frames filled with masonry.

(ii)   At the designing stage it should be kept in mind that the building elements are capable of resisting lateral loads in two orthogonal directions.

(iii) Due to the ground shaking as a result of an earthquake, the ground beneath a building is displaced laterally. The inertial effects of this displacement generate loads in the upper part of the building the resulting shear forces are generally maximum just above the level of foundation.

(iv) The earthquake loads are dependent upon the building mass, period of vibration of the building, foundation materials etc.

(v)   Stiff buildings and stiff elements are more prone to greater loads than flexible elements.

(vi) Heavy roof elements such as tiles sustain greater loads than lighter elements.

(vii)  Combination of different types of elements to resist loads in the same direction must be avoided.

2.      Horizontal forces must be transferred to the ground:

(i)     Inertial forces are generated in a building during an earthquake.

(ii)   The path of the load should be defined clearly so that the forces from all the elements are transferred to the ground.

(iii) To facilitate transfer of all the forces to the foundation, the load paths should be in continuity.

(iv) As the roofs are a major load, the structure of the roof should be designed in such a manner that it is able to distribute loads to the walls. This loads is then carried by side walls to the foundations.

(v)   Walls are also a major load. Face loaded walls or the walls which are perpendicular to the earthquake load transfer their loads to the eaves and down to the foundation.

(vi) All loads must be taken to the foundation. The foundation must be able to transfer the horizontal loads to the sub-grade (soil, rock etc.).

3.      Other alternates for lateral load resistance:

(i)     The primary systems to resist loads consist of shear walls, moment resisting frames, braced frames etc. a building can be equipped with different systems in different directions

(ii)   Reinforced concrete walls on a well-designed foundation are the best system to resist seismic load in low to medium rise constructions. However, these are expensive, need expensive foundations and limit the internal layout.

(iii) It has been shown that columns are vulnerable to failure during earthquakes, so it is very important to ensure stronger columns and beams and an adequate number of ties between them.

4.      Proper configuration of buildings to resist loads :

(i)     In order to respond to a moderate to strong earthquake a building must be well configured.

(ii)   The seismic resisting elements of the building should be arranged in a symmetric manner. The importance of this fact increases with the height of the buildings.

(iii) The main elements that resist seismic loads should be distributed symmetrically and evenly in the plan area of the building. Walls and frames should preferably be on the perimeter of the building. These elements should not be concentrated in one area of the building because it will cause torsion which can lead to failure.

(iv) In the "L" or "U" shaped building the length to width ratio of the wings should be less than 3. Otherwise, it is better to separate the wings and design them as separate structures.

(v)   In the buildings of more than one story, vertical asymmetry should be avoided.

5.      Building elements should be tied together:

(i)     All the building elements must be tied together to make sure that the loads are transferred as expected.

(ii)   Loads should be transferred in a cycle manner.

The path of load transfer should be from slab to wall and wall and column to the foundation.

(iii) Brittle failure of connections should be avoided.

(iv) In the reinforced concrete as steel is a ductile material and it can hold the structure together over many cycles.

(v)   Roofs should be of light weight.

6.      Structural weaknesses should be avoided in the building construction:

(i)     Structural weaknesses like symmetry in plan and elevation should be avoided as much as possible.

(ii)   The load paths should be continuous from roof to foundation. Columns, beams and walls that are very slender should not be used.

(iii) There should be good separation between the adjacent buildings to prevent pounding.

(iv) The roof and other elements in the upper portion of the building should be of light weight.

(v)   Any large openings like door and windows should be adjacent to the corners of the building.

(vi) Any heavy, weak and brittle material should not be used for construction.

(vii)  Drainpipes should not be placed within slender columns.

(viii)Poor connections between to earthquakes should be taken into consideration from the beginning of the design process as it is very difficult and unsatisfactory to achieve it afterwards.

(ix)  The resistivity of building to earthquakes should be taken into consideration from the beginning of the design process as it is very difficult and unsatisfactory to achieve it afterwards.

(x)    All the exterior walls must be designed to carry face loads (wind load).

(xi)  Lintels over the doors and windows must be properly designed.

7.      Brittle materials should be avoided :

(i)     All brittle materials like unreinforced brickwork, unreinforced concrete block work and unreinforced concrete should be avoided in the primary vertical load and lateral load resisting elements.

(ii)   In moderates, brittle materials crack and their resistance to future lateral loads is reduced.

(iii) In earthquakes of greater magnitude and intensity the brittle materials fail suddenly without any warning signal. After failure, brittle elements are unable to sustain gravity loads and the building collapses.

(iv) On the other hand ductile building materials have the ability to sustain gravity load without collapse as they are more flexible. Strengthening connections and columns with steel gives a structure a high degree of ductility and avoids collapse.

(v)   If unreinforced masonry is unavoidable it should be made sure that the mortar used is able to accommodate movement. A mortar made from cement, lime and sand can accommodate greater movement than pure cement of sand mortar.

8.      Foundation and building site :

(i)     Construction of building directly above steep slopes should be avoided as they may become unstable.

(ii)   There should not be any construction below slopes also as they are susceptible to landslides and rockfalls.

(iii) No construction should be done close to river banks.

(iv) Building must be constructed in a good ground. The foundations are the most important part of the building.

(v)   The sub-grade should sound with a good bearing pressure.

(vi) Any unstable ground should be avoided.

(vii)  Poor soil conditions may result in an increase in seismic forces.

(viii)The ground should be free from water at foundation level.

(ix)  If any surface water is present, suitable drainage should be installed. The drain invert level should be deeper than the foundation level.

(x)    No construction should be done directly over fault lines.

(xi)  The soil types, changes in layers, depth of rock and depth of water level should be taken into consideration as they affect the actual loads.

(xii)   Foundations should be stronger than the building element above to avoid any foundation failure.

(xiii) One type of foundation should be used throughout if it is possible.

(xiv) Individual foundations must be tied together in both directions.

(xv)   The ground beams should be as deep as columns.

(xvi) Liquefaction and consolidation underlying soil should be taken into consideration.

9.      Reinforced concrete shear walls :

(i)     Reinforced concrete shear walls on a well designed foundation are considered the best approach to resist seismic loads in low to medium rise constructions.

(ii)   They are easier to control in terms of quality than brickwork or block work.

(iii) The walls should be distributed symmetrically and evenly in both directions.

(iv) The footing must be sized to limit bearing pressure and prevent overturning.

(v)   Large openings in primary walls should be avoided.

(vi) The reinforcing steel should be in two orthogonal directions and be throughout length and height of the wall.

(vii)  Additional wall reinforcement may be done at the wall ends.

Posted Date: 7/12/2012 3:55:58 AM | Location : United States







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