Mastercivilengineer

Building failure occurs when a building fails to do its designated functions. Such failures can be either structural failures or serviceability failures. Failure of a building can take place in various ways. The factors affecting maybe one or more of the following viz.

• Geometrical shape and size of the building
• The structural aspects of the site
• The construction practice and
• The construction techniques adopted
• Poor/Improper workmanship
• Corrosion, improper periodic maintenance, and supervision, etc.

The failure often occurs in the form of slowly developed cracks and progress over the period of time.

Building Damage Due to Natural Causes

• Some of the major natural causes that result in the failure of a building are rainfall, temperature, wind pressure, etc. Due to these natural factors coming into force may take place slowly or suddenly and/or structurally designed to take care of these factors.
• An earthquake is the result of a sudden release of energy in the crust o the earth that generates seismic waves. At the surface of the earth, earthquakes manifest themselves by shaking and sometimes displacement of the ground. Due to the seismic effect, structural failures may take place suddenly if seismic provisions are not kept during design and construction. Earthquakes are measured using observations from seismometers. Earthquakes magnitude 3 or lower is mostly almost insensible and magnitude 7 and over potentially causes serious damage over large areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9 on the rector scale, although there is no limit to the possible magnitude.

Evaluations of many multi-storied buildings in urban areas of India have exposed the vulnerability of many RCC framed structures in high seismic zones. The experience with regard to earthquake damages to buildings has mostly been on non-engineered buildings. These buildings of low heights are the first targets of the earthquakes.

A number of defects were observed in the non-engineered buildings, such as:

• Unsymmetrical plans and elevations resulted in the development of torsional shear stresses in the walls during earthquakes.
• Large spanned walls with openings for doors and windows at improper positions became unstable during earthquakes.
• The use of poor quality building material was also one of the major causes of damage to buildings.
• Improperly designed floor slabs and lintels do not provide adequate diaphragm action for providing lateral supports o the walls.

The construction of reinforced concrete buildings with brick masonry infill walls is a very common practice in urban India. In most of the multi-storied RCC framed buildings, parking is provided at the ground floor, where only columns are there and no infill panels are provided. This resulting in the soft storey of the building which means the lateral resistance offered by the storey is much less as compared to the storey above or below it resulting in the crushing of concrete or columns, failure of steel, shear failure due to excessive torsion. In RCC framed building at many places, the failure of beam-column joints is observed after an earthquake of moderate intensity.

The degree of damage depends on the earthquake intensity and the quality of the building. stiff structures such as low rise masonry structures survive when founded on soft clay. Stone and brick masonry buildings of low height have a short fundamental time period. The seismic response for such a period is very high if the buildings are constructed on hard soil or rock.

The most common damage in buildings observed is the developed of vertical cracks resulting separation of perpendicular walls at the corner and T-junctions during earthquake conditions. In some cases, inclined cracks have developed in the walls starting from the corner of door/window openings, horizontal cracks at lintel, sill, and plinth level.

A large number of houses are collapsed and/or severely damaged due to out of plane failure and in-plane failures resulting in caving in of roofs. Buildings having wooden floors experience more damage than those having reinforced concrete floors due to diaphragm action.

Techniques of Retrofitting   

The traditional techniques such as plaster with wire mesh, cement grouting, pre-stressing, RCC and steel ties and beams, continuous lintels, the connection between walls and floors, etc. are recommended by most of the codes and research institutions. Some modern techniques which are presently emerging such as the use of masonry arch, base isolation, and composition material are adopted also to strengthen the damaged structure as well as to existing structures against the earthquakes and wind forces.

The in-plane and out of plane collapse can be prevented by providing properly designed reinforcements. It is experimentally observed that the in-plane lateral load capacity of the wall with a 10% opening reduces to 50% of the solid wall for the same vertical to lateral load ratio and it reduces to 5% with 20% percent opening. The strength can be increased by introducing arch over door and window opening. The purpose of the base isolation system is to isolate the structure from the ground motions which actually impose the forces on the structure.

Summary & Conclusion   

In a high degree earthquake-prone area, the following points must be kept in mind while designing the building:

• Multi-storied buildings should not be designed with smaller lower floor areas than the upper floor areas, because, in earthquake conditions, these buildings are affected most.
• Designing asymmetric buildings should be avoided because; these buildings have got lesser stiffness to withstand seismic forces. In case of extreme need, a proper expansion joint must be provided between the asymmetric portions.
• Large cantilever projections must be avoided beyond the column line.
• In multi-storied R.C.C framed buildings, parking space design must be done with infill brick panels suitably located at the ground floor in conjunction with the columns to avert soft storey development.
• Long brick masonry walls should be avoided. If there is no intervention of perpendicular walls in between.
• Band lintel should be provided above doors and windows between two floors to lessen the slenderness ratio, which would be seismic efficient.
• Large openings, doors, and windows must be kept as far as possible from the corner of the building.
• The plot with possibilities of having liquefaction during an earthquake must be avoided.
• Beam column junction must be properly taken care of during designing and construction/execution. Designed with smaller lower floor areas than the upper floor areas, because, in earthquake conditions, these buildings are affected most.
• Designing asymmetric buildings should be avoided because; these buildings have got lesser stiffness to withstand seismic forces. In case of extreme need, a proper expansion joint must be provided between the asymmetric portions.
• Large cantilever projections must be avoided beyond the column line.
• In multi-storied R.C.C framed buildings, parking space design must be done with infill brick panels suitably located at the ground floor in conjunction with the columns to avert soft storey development.
• Long brick masonry walls should be avoided if there is no intervention of perpendicular walls in between.
• Band lintel should be provided above doors and windows between two floors to lessen the slenderness ratio, which would be seismic efficient.
• Large openings, doors, and windows and must be kept as far as possible from the corner of the building.
• The plot with possibilities of having liquefaction during an earthquake must be avoided.
• Beam column junction must be properly taken care of during designing and construction/execution

Most of the earthquake-related damages can be avoided utilizing the current knowledge of earthquake-resistant design and detailing practice, good quality control of material, and good engineering construction practice.