Author: Jayanta Nath Chowdhury

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.

Earthquake hazards

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.

Mitigation techniques

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.

In the civil engineering field concrete is a construction materials is extensively used in construction work. Generally it is mixture of cement, fine aggregate (normally termed as sand) coarse aggregate (termed as stone chips or broken stones), water and some times  admixture . It is used  to develop or construct different type of structures or systems. These  structures are generally two types , they are P.C.C and R.C.C. P.C.C stands for Plain Cement Concrete and R.C.C stands for Reinforced Cement Concrete. Now P.C.C structures are developed by mixing cement, sand, stone chips and water where admixtures are optional. In case of R.C.C along with P.C.C  ingredients  steel bars embedded into it. Performance of concrete in P.C.C and R.C.C is quite satisfactory from the strength point of view as well as many other aspects. One of the important performance of concrete in R.C.C is prevention of steel bars from external hazards. Both of the type of systems  at early stage offers considerable resistance but as the time passes  the degree of resistance of systems reduces.  Thus resulting unsatisfactory performance of structural systems. Structural systems those are experiencing such type of adverse situations if not provided selective or necessary treatments. Most of the concrete structures or systems  deteriorate through four different ways. Those are classified as –

1. Chemical deterioration

This type of deterioration takes place due to chemical reaction at the concrete covers of R.C.C members. This reaction may take place in several ways –

3. Carbonation: Generally reinforced steel bars are surrounded by a particular grade of concrete. Harden concrete made up of calcium silicate gel and calcium hydroxide (Ca [OH]₂). Ca [OH]₂ ensure high alkaline nature of concrete to fight against corrosion and save steel from rust by creating a passive film around the steel bars. Due to this film,  O₂ and H₂O fail to reach the main steel bars. Thus corrosion avoided.  But if environment surrounding  concrete covers, contains carbon dioxide( CO₂) in air it may be vulnerable to RC members. CO₂ in moisture enter in concrete cover  through concrete pores. It reacts with Ca [OH]₂ and produces calcium carbonate (CaCO₃) and water (H₂O). Thus reduces pH value of concrete and formation of acidic substance i.e, CaCO₃. Due to this acidic substance passive film decays and open up the entry  of O₂ and H₂O to main steel bars. This initiate the corrosion of steel and in tern resulting damage of RC structures.

Remedy – Application of different types of polymer based coating, stabilising primers on concrete surfaces offers protective barrier against concrete carbonation and slow down this adverse process. Adoption of epoxy coating on  steel bars, safe guard the passive layer of steel bars also controls the carbonation process.

1. Alkali-Aggregate Reaction : If aggregates containing reactive silica are used in concrete making, it is observed that alkaline solution reacts with silica present in aggregates and produces alkali-silica gel. The product volume is quite higher and bring out all the undesirable properties in concrete. Due to higher volume internal stresses generates and cracks develop in concrete structures or systems.

Remedy – Avoid use of  reactive silica based aggregates in concrete making to control the Alkali-Aggregate Reaction.

1. Sulphate attack : It is also one of the important cause of concrete based structure’s deterioration. Sulphate may exists in nature e.g, in ocean water, ground water and industrial pollution etc in different soluble forms like calcium, sodium, magnesium etc. Sulphate attach may takes place by two reactions. When concrete structures come in contact with water contaminated by sulphates, then Ca [OH]₂ , sodium based sulphate salt and water. Therefore produces gypsum thus reduces pH value due to loss of Ca [OH]₂ from concrete. In the other way sulphate ion react with tri-calcium aluminate (C₃A) and produces calcium sulphoaluminate hydrate (termed as ettringite) as major product. Both gypsum and ettringite posses higher volume expansion resulting internal stresses in concrete body. Finally concrete surfaces spall of from the actual concrete.

Remedy – Use of sulphur resisting cement, fly ash based cement, adequate compaction of concrete etc prevent this type of attack in concrete systems.

1. Chlorine attack :  Another adverse situation that happens to concrete when RC element’s constituents or environment contaminated by chlorine. Resulting corrosion of steel in RC structures or systems. If concrete come in contact of sea water or sea water used to produce concrete, higher chloride content in aggregate so on and so forth. Chlorides reached to passive layer surrounding steel bars and destroy it and then the bars are exposed fully to the environment. So corrosion initiates and reduces the cross sectional area of bars. Therefore tension carrying capacity of systems or structures reduces. While rusting occurs in bars, its volume immensely increases causing internal thrust in whole system and cracks generates that further accelerates corrosion process. Finally failure of structures or systems takes place.

Remedy – The best possible and easiest way to slow down this attack is to increase concrete cover, epoxy coating on steel bars and surface coating on concrete systems.

2. Mechanical deterioration

Mechanical decay of concrete systems may occur through several ways like erosion, abrasion and cavitation. Concrete used to construct hydraulic structures are often subjected to the action of abrasion by fluid flow containing solid substances. Wearing of concrete surfaces due to this flow of fluid results erosion of systems termed as erosion. Sometimes movement of solid substances over concrete surfaces develops friction between each other. Some of the examples related to this type of friction are – acceleration and de-acceleration of moving  vehicle’s  wheels, movement of heavy machine over the industrial floors etc made up of concrete.  Therefore considerable amount of material loss from system will occur. In the other way  loss of materials from surface of concrete systems  may occur as a result of irregular flow of fluid/water , change in the direction of flow creating vapour bubbles. This in turn show up as decay of concrete surfaces.

Remedy – Ensuring elimination of solid materials in fluid flow, treatment of  concrete surfaces, use of higher grade of concrete can avoid and prevent this type of deterioration.

3. Physical deterioration

This is a type of deterioration takes place mainly due to freezing and thawing. Locations where cold and hot weather conditions occurs frequently, in cold situation freshly produced concrete having adequate quantity of water free zed and converted into ice. Therefore volume of water increases and creates internal stresses into the system. While during in hot weather concrete thaws i.e, ice converted to water. Again freezing occurs and followed by thawing in repeated cycles. Thus resulting disruption of system and reduces the strength.

Remedy – Application of air entraining agents during concrete making dissipate internal stresses in concrete systems.

4. Deterioration due to constructional errors

Constructional errors may occur through different ways those are given below :

1. During production of concrete if water-cement ratio maintained is higher than specified value and adequate compaction is not achieved in concrete then it will show up in the form of tiny holes, honey combing etc over the concrete surfaces. This will not only interrupt the performance of concrete systems but the appearance of surfaces as well.
   Remedy – To avoid this error adequate care should be ensured during concrete making from the point of view of compaction and proportions of ingredients.
2. Inappropriate placing of reinforcing steel bars in form work of RC members may also cause errors which finally become the reason of deterioration.
   Remedy – Placing of bars according to the codal guidelines help to avoid this type of error.
3. Dislocation of form work and insufficient strength of temporary supports to form work during concrete placing resulting variation of dimensions of concrete structural members as well as violate design stipulations. As a result of which stress distribution throughout the members become non uniform and finally causes deterioration of systems.
   Remedy – Continuous supervision throughout the construction  work ensure avoidance of this errors and deterioration.
4. Also improper finishing and curing of concrete systems whether it is P.C.C or R.C.C, results deterioration of systems. Because if curing is not ensured properly, hydration reaction and production of calcium- silicate-hydrate (C-S-H)  gel will not be adequate. Thus directly affects the performance of structures from the strength as well as durability point of view.
   Remedy – Based on the situation  of construction  sites (i.e, whether member is vertically or horizontally placed, temperature-atmospheric conditions and availability of resources etc.) finishing and curing (periodically) techniques should be adopted.

Further Studies :

• https://www.fprimec.com/deterioration-of-concrete-structures/
• https://www.nbmcw.com/tech-articles/concrete/28072-causes-for-accelerated-structural-deterioration-of-reinforced-concrete.html