Contributor : Tamal kanti Das, Assistant Professor, Department of Civil Engineering, SOET, Adamas University
Generally any type of building structures whether it is Reinforced Concrete (RC) or Steel buildings are subjected to different type of loads during its design life. Normally these loads can be categorised by gravitational loads and lateral loads. Loads coming under the gravitational loads are – dead load (DL), imposed load (IL), snow load (SL), rain load (RL) etc. SL and RL are considered occasionally in special cases. Whereas, earthquake or seismic load (EL) and wind load (WL) are under the group of lateral or horizontal load. According to the latest design practice of building structures gravitational loads are considered as service loads under which buildings must perform (carry and transmit loads with acceptable deformation) in normal situation. But as per design it is also considered that during the design life of building structures , it will be under earthquake or seismic action at least once. In that situation building will be under the action of combined gravitational and lateral load. Therefore various Indian codal (Ref IS 456:2000, IS 1893:2002-part-1 and IS 875 etc) guidelines are available considering number of set or arrangement of loads by which load estimation can be done followed by analysis and design of any type of buildings. For RC, pre stressed concrete structures and in case of plastic design of steel structures different types of set of loads generally taken into consideration according to the limit state design (LSD) in association with partial factor of safety which is provided in IS 1893:2002-(part-1).
This section of discussion is focusing on RC buildings as majority of the buildings in our country are based on RC. Since the topic of discussion is based on earthquake thus first load combination is of paramount importance in this context. To built any building structures which is earthquake resistant IS 1893:2002 (part -1) guidelines need to be followed. Those are explained in the following sections.
General design concepts
Earthquake resistant design of building can be done by considering arbitrary and random ground movement at the base of building. As a result of this inertia forces generates in building structures which causes different degree of stresses based on intensity of ground motion at the base of structures. The inertia load develops in building structures governs by mass of building in a proportional manner. In this context and as per earthquake design philosophy, creation of building shall be done in such a way that structure can be able to resist any three types of shaking when buildings are subjected to seismic forces those are –
Minor, moderate and severe shaking. In case of minor shaking building subjected to earthquake does not damage any components of building whether it is structural as well as non-structural. Under moderate shaking structural and non-structural components of building will damage. When building subjected to severe shaking it may experience structural damage. But during severe shaking of building entire collapse of building will not occur. This type of severe design is quite unsatisfactory from the
Any type of earthquake resistant building can be designed and built based on only any one type of shaking concept. No building can be designed considering combination of three type of shaking. Also consideration of severe shaking design concept for all type of buildings irrespective of its importance is quite unsatisfactory from the economical aspect. Though this concept is satisfactory from the performance point of view.
Virtues of earthquake resistant building
There are four virtues or important and desirable characteristics which need to be incorporated in earthquake resistant building design so that it can perform under seismic action satisfactorily. Those four virtues are –
- Adequate configuration – It ensure non detrimental effect on satisfactory performance of building under seismic action. Only it can be obtained by avoiding choices or preferences of architectural arrangements over functional requirements.
- Stiffness – In each plan directions of a building structure must have minimum lateral stiffness. As we know that ground motion may occur in any direction. Thus to avoid discomfort to users of any building minimum stiffness must be provided in either direction of building plan.
- Strength – Adoption of minimum strength parallel to the direction of ground base level (both plan directions) in building structures ensure satisfactory resistance against ground motion with lower intensity with no damage of building components.
- Ductility – Materials which are to be used to built earthquake resistant building from the bottom to the roof level must have adequate ductile behaviour. So that under the ultimate stage or condition components undergo large extent of inelastic deformation before its failure.
Any building structures having these four characteristics perform well under seismic action. And building can be designated as earthquake resistant structure.
So far the discussion has been done in a qualitative manner which are no doubt need to be followed judiciously. But along with that quantitative approach also need to be taken into consideration which are discussed below.
Estimation of earthquake load
Before conducting design of any building it is mandatory to estimate all possible loads that are about to come on the structures during its design life. Thus as per IS code (IS 1893:2002 part-1) final effect of ground motion may be expressed in the form of “Design Base Shear”. This force depends on various factors those are – extent of hazard in site location which is termed as earthquake zone factor, adoption of importance factor that in turn drag down extent of damage under seismic action, response reduction factor depending on various building systems may subjected to lateral load, acceleration coefficient which depends on type of soil and natural period (obtained from height of building structures) as well as damping of structures and total weight of building by summing up seismic weight of all the building floors.
The obtained base shear shall be vertically distributed to various floor levels of building proportionately as mentioned in IS 1893:2002, cl-7.7.1.
Earthquake zone map of India
As per IS1893:1984, location of building was identified by seismic zone map which classified our entire country into five different zones i.e. I,II,III,IV and V with ascending order of seismic intensity respectively. But after tremendous earthquake disaster in Bhuj (Gujrat) in 2001 further revision has been done and Indian code came up with modified 2002 version of IS1893:2002. In this code zone I is straight away omitted and finally four zones (II,III,IV and V) are now indicated in the sketch of earthquake zone map of India.
When earthquake occurs, it comes suddenly and release energy through ground motion. Thus seismic load induced in building structures. As a result of this building collapse. Due to this property damages, considerable amount of casualties, landslides, liquefaction etc.
Conventional earthquake design process permit certain amount of deformation as well as fixed amount of damage in building structures. But now a days in advanced building structures designed and constructed with more satisfactory level of performance along with considerably higher safety compared to traditional concept.
In this section some important some of the techniques or control systems will be mentioned. Incorporation of such systems or techniques enhance the performance and safety levels of building structures when it will be under the action of environmental loads i.e. earthquake load. Following are the techniques which are found to be effective in building structures under seismic load :
Vibration control systems including base isolation techniques, dampers, active, semi-active and if required hybrid systems etc.
While constructing new or retrofitting structures expected to be affected or already affected by seismic force incorporation of base isolation found to be quite effective tools by providing sufficient integrity of superstructure through the reduction of story displacement. Use of different shapes of dampers in building structures now a days also enhance the performance level of structures in all aspects by means of absorbing or dissipating considerable amount of earthquake energy though plastic deformation of metal elements used as construction materials as per earthquake resistant design.
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