1. INTRODUCTION
1.1 OBJECTIVES OF EARTHQUAKE RESISTANT DESIGN:
Ø Earthquake resistant design of building structures is carried out with following
1. Preserve the functionality of the structure in the earthquake
2. Minimize damage the structure during the earthquake and
.1.2 Importance of Seismic Design Codes
Countries around the world have procedures outlined in seismic codes to help design engineers in the planning, designing, detailing and constructing of structures. An earthquake-resistant building has four virtues in it, namely:
(a) Good Structural Configuration
(b) Lateral Strength
(c) Adequate Stiffness
(d) Good Ductility
1.3 Indian Seismic Codes Seismic codes are unique to a particular region or country. They take into account the local seismology, accepted level of seismic risk, building typologies, and materials and methods used in construction. Further, they are indicative of the level of progress a country has made in the field of earthquake engineering.
The first formal seismic code in India, namely IS 1893, was published in 1962. Today, the Bureau of Indian Standards (BIS) has the following seismic codes: IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision)
IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd Revision)
IS 13827, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings
IS 13828, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Low strength masonry buildings
Is 13920, 1993, Indian standard code of practice for ductile detailing of reinforced concrete structures subjected to seismic forces
Is 13935, 1993, Indian standard guidelines for repair and seismic strengthening of buildings
The regulations in these standards do not ensure that structures suffer no damage during earthquake of all magnitudes. But, to the extent possible, they ensure that structures are able to respond to earthquake shakings of moderate intensities without structural damage and of heavy intensities without total collapse.
1.4 SEISMIC ZONES OF INDIA
A seismic zone map is required to identify these regions. Based on the levels of intensities sustained during damaging past earthquakes, the 1970 version of the zone map subdivided India into five zones – I, II, III, IV and V. The maximum Modified Mercalli (MM) intensity of seismic shaking expected in these zones were V or less, VI, VII, VIII, and IX and higher, respectively. The map has been revised again in 2002 (Fig2), and it now has only four seismic zones – II, III, IV and V. The areas falling in seismic zone I in the 1970 version of the map are merged with those of seismic zone II. Madras now comes in seismic zone III as against in zone II in the 1970 version of the map. This 2002 seismic zone map is not the final word on the seismic hazard of the country, and hence there can be no sense of complacency in this regard.
3.DUCTILITY REQUIREMENT IN RCC STRUCTURE
3.1REINFORCED CONCRETE STRUCTURES
In recent times, reinforced concrete buildings have become common in India, particularly in towns and cities. A typical RC building is made of horizontal members (beams and slabs) and vertical members (columns and walls), and supported by foundations that rest on ground. The system comprising of RC columns and connecting beams is called a RC Frame. The RC frame participates in resisting the earthquake forces. Earthquake shaking generates inertia forces in the building, which are proportional to the building mass. Since most of the building mass is present at floor levels, earthquake-induced inertia forces primarily develop at the floor levels. These forces travel downwards -through slab and beams to columns and walls, and then to the foundations from where they are dispersed to the ground. As inertia forces accumulate downwards from the top of the building, the columns and walls at lower storey experience higher earthquake-induced forces (F) and are therefore designed to be stronger than those in storey above.
Ø 3.1.2 Roles of floor slabs and masonry walls:
1.Floor slabs are horizontal plate like elements, which facilitates functional use of buildings..in residential multi-storey buildings, thickness of slabs is only about 110-150mm .when beams bend in the vertical direction during the earthquakes, these thin slabs bend along with them and, when beams move with columns in the horizontal directions, the slab usually forces the beam to move together with it.
In most buildings, the geometric distortion of the slab is negligible in the horizontal plane; this behavior is known as the rigid diaphragm action .
After columns and floor in RC building are cast and the concrete hardens, vertical spaces between columns and floors are usually filled-in with masonry walls to demarcate a floor area into functional spaces (rooms). Normally, these masonry walls, also called infill walls, are not connected to surrounding RC columns and beams. When columns receive horizontal forces at floor levels, they try to move in the horizontal direction, but masonry walls tend to resist this movement. Due to their heavy weight and thickness, these walls attract rather large horizontal forces .However, since masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus, infill walls act like sacrificial fuses in buildings ;they develop cracks under severe ground shaking but helps share the load of beams and columns until cracking .earthquake performance of infilled walls is enhanced by mortars of good strength, making proper masonry courses ,and proper packing of gaps between RC frame and masonry infill walls .however, an infill wall that is unduly tall or long in comparison to its thickness can fall out-of-plane (i.e..along its thin direction),which can be threatening .also ,placing infills irregularly in the building causes ill effects like short column effect and torsion.
Ø 3.1.3 Horizontal Earthquake Effects are Different
Gravity loading (due to self weight and contents) on buildings causes RC frames to bend resulting in stretching and shortening at various locations. Tension is generated at surfaces that stretch and compression at those that shorten. Under gravity loads, tension in the beams is at the bottom surface of the beam in the central location and is at the top surface at the ends. On the other hand, earthquake loading causes tension on beam and column faces at locations different from those under gravity loading; the relative levels of this tension (i.e. bending moment) generated in members. The level of bending moment due to earthquake loading depends on severity of shaking and can exceed that due to gravity loading. Thus under strong earthquake shaking, the beam-ends can develop tension on either on top or bottom faces. Since concrete can not carry this tension steel bars required on both faces of beams to resist reversals of bending moment similarly steel bars are required on all faces of columns too.
3.2 BEAMS IN RC BUILDINGS
Ø 3.2.1 Reinforcement in seismic damage:
In RC buildings the beams and columns are built integrally with each other. Thus, under the action of loads, they act together as a frame transferring forces from one to another.
Beams in RC buildings have two sets of steel reinforcement, namely: (a) longitudinal bars placed along its length, and (b) stirrups placed vertically at regular intervals along its full length.
Ø 3.2.2 Beams Sustains Two Basic Types Failures:
a) Flexural (or bending ) failure : As the beam sags under increased loading, it can fail in two possible ways. If relatively more steel is present on the tension face, concrete crushes in compression; this is a brittle failure. If relatively less steel is present on the tension face, the steel yields first. Redistribution occurs in the, until eventually, the concrete crushes in compression. This is a ductile failure and hence desirable.
b) Shear Failure: A beam may also fail due to shearing action. A shear crack is inclined at 45° to the horizontal; it develops at mid-depth near the support and grows towards the top and bottom faces. Closed loop stirrups are provided to avoid such shearing action. Shear damage occurs when the area of these stirrups is insufficient.
Shear failure is brittle, and therefore, shear failure must be avoided in the design of RC beams.
Ø 3.2.3 design strategy
Designing a beam involves the selection of its material properties and shape and size; these are usually selected as a part of an overall design strategy of the whole building. And, the amount and distribution of steel to be provided in the beam must be determined by performing design calculations as per IS: 456-2000 and IS: 13920-1993.
The Indian Ductile Detailing Code IS: -13920-1993 prescribes that:
(a) At least two bars go through the full length of the beam at the top as well as the bottom of the beam.
(b) At the ends of beams, the amount of steel provided at the bottom is at least half that at top
Ø Stirrups in the RC beams help in three ways,
(i)They carry the vertical shear force and thereby resist diagonal shear crack
(ii) They protect the concrete from bulging outwards due to flexure.
(iii) They prevent the buckling of the compressed longitudinal bars due to flexure.In moderate to severe seismic zones, the Indian Standard IS13920-1993 prescribes the following requirements related to stirrups in reinforced concrete beams:
(a) The diameter of stirrup must be at least 6mm; in beams more than 5m long, it must be at least 8mm.
(b) Both ends of the vertical stirrups should be bent into a 135° hook and extended sufficiently beyond this hook to ensure that the stirrup does not open out in an earthquake.
(c) The spacing of vertical stirrups in any portion of the beam should be determined.
(d) The maximum spacing of stirrups is less than half the depth of the beam.
(e) For a length of twice the depth of the beam from the face of the column, an even more stringent spacing of stirrups is specified, namely half the spacing mentioned in (c).
Steel reinforcement bars are available usually in lengths of 12-14m.thus, it becomes necessary to overlap bars when beams of longer lengths are to be made .at the location of the lap, the bars transfer large forces from one to another. Thus, the Indian standard IS:13920-1993 prescribes that such lap of longitudinal bars are (a) made away from the face the column, and (b) Not made at locations where they are likely to stretch by large amounts and yield (e.g., bottom bars at mid-length of the beam). Moreover, at the locations of laps, vertical stirrups should be provided at a closer spacing .
3.3 COLUMNS IN RC BUILDINGS
Ø 3.3.1 Design Strategy:
Designing a column involves selection of materials to be used (i.e, grades of concrete and steel bars), choosing shape and size of the cross-section, and calculating amount and distribution of steel reinforcement. The first two aspects are part of the overall design strategy of the whole building. The Indian Ductile Detailing Code IS: 13920-1993 requires columns to be at least 300mm wide. A column width of up to 200mm is allowed if unsupported length is less than 4m and beam length is less than 5m. Columns that are required to resist earthquake forces must be designed to prevent shear failure by a skillful selection of reinforcement.
Ø 3.3.2 Vertical Bars tied together with Closed Ties:
Closely spaced horizontal closed ties help in three ways, namely (i) they carry the horizontal shear forces induced by earthquakes, and thereby resist diagonal shear cracks, (ii) they hold together the vertical bars and prevent them from excessively bending outwards (i.e. buckling), and (iii) they contain the concrete in the column within the closed loops. The ends of the ties must be bent as 135° hooks. Such hook ends prevent opening of loops and consequently buckling of concrete and buckling of vertical bars.
The Indian Standard IS13920-1993 prescribes following details for earthquake-resistant columns:
(a) Closely spaced ties must be provided at the two ends of the column over a length not less than larger dimension of the column, one-sixth the column height or 450mm.
(b)Over the distance specified in item (a) above and below a beam-column junction, the vertical spacing of ties in columns should not exceed D/4 for where D is the smallest dimension of the column (e.g., in a rectangular column, D is the length of the small side). This spacing need not be less than 75mm or more than 100mm. At other locations, ties are spaced as per calculations but not more than D/2.
(c) The length of tie beyond the 135° bends must be at least 10 times diameter of steel bar used to make the closed tie; this extension beyond the bend should not be less than 75mm.
Ø 3.3.3 lapping vertical bars
In the construction of RC buildings, due to the limitations in available length of bars and due to constraints in construction. A simple way of achieving this is by overlapping the two bars over at least a minimum specified length, called lap length. For ordinary situations, it is about 50 times bar diameter. Further, is: 13920-1993 prescribes that the lap length be provided only in the middle half of column and not near its top or bottom ends. Also, only half the vertical bars in the column are to be lapped at a time in any storey. Further, when laps are provided, ties must be provided along the length of the lap at spacing not more than 150mm.
However following clauses has to be satisfied to incorporate provision of IS 13920.
(i) Minimum grade of concrete shall preferably be m20 (clause 5.2).
(ii) Steel reinforcement of grade fe 415 or less only shall be used (clause 5.3)
(iii) The minimum dimension of column member shall not be less than 200mm .for columns whose unsupported length exceeds 4m; the shortest dimension of column shall not be less than 300mm. (clause 7.1.2).
(iv) The spacing of hoops shall not exceed half the least lateral dimension of the column, except where special confining reinforcement is provided (clause 7.3.3).
3.4 COLUMN-BEAM JOINTS IN RC BUILDINGS
Ø 3.4.1 Earthquake behaviors of joints:
Under earthquake shaking, the beams adjoining a joint are subjected to moments in the same direction. Under these moments, the top bars in the beam-column joint are pulled in one direction and the bottom ones in the opposite direction. These forces are balanced by bond stress developed between concrete and steel in the joint region. If the column is not wide enough or if the strength of concrete in the joint is low, there is insufficient grip of concrete on the steel bars. In such circumstances, the bar slips inside the joint region, and beams lose their capacity to carry load.
Further, under the action of the above pull-push forces at top and bottom ends, joints undergo geometric distortion; one diagonal length of the joint elongates and the other compresses. If the column cross-sectional size is insufficient, the concrete in the joint develops diagonal cracks.
Ø 3.4.3 Reinforcing the Beam-Column Joint
Problems of diagonal cracking and crushing of concrete in the joint region can be controlled by two means, namely providing large column sizes and providing closely spaced closed-loop steel ties around column bars in the joint region. The ties hold together the concrete in the joint and also resist shear force, thereby reducing the cracking and crushing of concrete. Indian Standard IS: 13920-1993 recommends continuing the transverse loops around the column bars through the joint region. In practice, this is achieved by preparing the cage of the reinforcement (both longitudinal bars and stirrups) of all beams at a floor level to be prepared on top of the beam formwork of that level and lowered into the cage.
3.5. REQUIREMENTS FOR RCC STRUCTURE
Codal provision gives reasonable minimum protection against earthquake disaster.
The different elements of the whole structure and super structure system should be tied together so that they can work as a single unit.
The super structure and the non-structural components should be light and should not have unnecessary masses.
The structure should preferably not have large height/width ratio.
The super structure of a structure should preferably have uniform and continuous distribution of the mass, stiffness, strength and ductility avoiding formation of soft storey and /or weak storey….
The superstructure should preferably have relatively shorter spans than non-seismic resistant structure and should not have long cantilever.
The structure should be carefully detailed with due attention to the joints, and the shear reinforcement rings to achieve adequate confinement of concrete.
The design of shear reinforcement stirrups at the beam and column ends in moment resisting frames should be carried out, on the basis of moment capacity of the beam.
Fe 500 should not be used in earthquake resistant structures. High strength steel is less ductile therefore it should be used only with care.
Two different grades of steel should not be used in the same columns as longitudinal reinforcement to resist axial and bending forces.
Bent bars are not of much use in resisting shears produced due to earthquake forces.
In short, strength stiffness and in elastic deformation capacity to resist inertia forces. Approximate structural configuration. Careful detailing of structural members such as beams, columns, & shear walls and the connection between them .the adequate foundation for the structure form the essence of a good earthquake resistant structure.
4. SHEAR WALLS
Ø 4.1 WHAT IS A SHEAR WALL BUILDING
Reinforced concrete (RC) buildings often have vertical plate-like RC walls called Shear Walls in addition to slabs, beams and columns. These walls generally start at foundation level and are continuous throughout the building height. Their thickness can be as low as 150mm, or as high as 400mm in high rise buildings. Shear walls are usually provided along both length and width of buildings. Shear walls are like vertically oriented wide beams that carry earthquake loads downwards to the foundation.
Ø 4.2 Advantages of Shear Walls in RC Buildings
Properly designed and detailed buildings with shear walls have shown very good performance in past earthquakes. However, in past earthquakes, even buildings with sufficient amount of walls that were not specially detailed for seismic performance (but had enough well-distributed reinforcement) were saved from collapse.. Shear walls are easy to construct, because reinforcement detailing of walls is relatively straight-forward and therefore easily implemented at site. Shear walls are efficient, both in terms of construction cost and effectiveness in minimizing earthquake damage in structural and nonstructural elements.
Ø 4.4 Ductile Design of Shear Walls
Just like reinforced concrete (RC) beams and columns, RC shear walls also perform much better if designed to be ductile. Overall geometric proportions of the wall, types and amount of reinforcement, and connection with remaining elements in the building help in improving the ductility of walls. The Indian Standard Ductile Detailing Code for RC members (IS: 13920-1993) provides special design guidelines for ductile detailing of shear walls.
Ø 4.4.1 Reinforcement Bars in RC Walls: Steel reinforcing bars are to be provided in walls in regularly spaced vertical and horizontal grids. The vertical and horizontal reinforcement in the wall can be placed in one or two parallel layers called curtains. Horizontal reinforcement needs to be anchored at the ends of walls. The minimum area of reinforcing steel to be provided is 0.0025 times the cross-sectional area, along each of the horizontal and vertical directions. This vertical reinforcement should be distributed uniformly across the wall cross-section
Ø 4.4.2 Boundary Elements: Under the large overturning effects caused by horizontal earthquake forces, edges of shear walls experience high compressive and tensile stresses. To ensure that shear walls behave in a ductile way, concrete in the wall end regions must be reinforced in a special manner to sustain these load reversals without loosing strength. End regions of a wall with increased confinement are called boundary elements. This special confining transverse reinforcement in boundary elements is similar to that provided in columns of RC frames. Sometimes, the thickness of the shear wall in these boundary elements is also increased. RC walls with boundary elements have substantially higher bending strength and horizontal shear force carrying capacity, and are therefore less susceptible to earthquake damage than walls without boundary elements.
5. CASE STUDY
GUJARAT EARTHQUAKE (Bhuj):
In the history of the earthquakes disasters in the Indian subcontinent, the Gujarat earthquakes of January 26, 2001 has not only devastated the major towns and rural areas in the state of Gujarat but also stricken the metropolitan city Ahmedabad causing major damages and collapse of engineered multistoried buildings.
This earthquake has brought several cases of negligence on the part of building professionals, flaws in the building regulations and their implementations and also the ignorance of the occupants of the buildings. This tragedy has given an opportunity to analyse the technical managerial and legal aspects of construction practices and learn certain lessons for the building professionals. Hopefully a greater awareness has been created for earthquake resistant safer buildings although such awareness needs to be sustained on a long-term basis through appropriate procedures and practices.
COMMON OBSERVATIONS ON THE COLLAPSE
The following are the common observations on selected collapsed buildings studied;
Ø Buckling and collapse of the open ground storey columns under large sway (lateral deflection) conditions causing the crushing of the concrete core and buckling of the longitudinal reinforcement bas of the columns. The lateral reinforcement of the columns were widely spaced single loop of stirrups around the periphery of column core. The ends of the stirrups were provided only with right angle bend without effective anchorage for developing its full strength. These stirrups were only 6mm dia untested mild steel coils although the column main reinforcement were high strength cold twisted deformed bars. Some of the columns had excessive cover exceeding 75mm thus reducing the effective core area of the column.
Ø The floor beams were not of adequate design and detailing. Even some ground floor beams of the ten storeyed blocks in the span range of 3 to 4 m.had grossly inadequate bottom steel (3 bars of 10mm dia) and widely spaced shear stirrups. some of the collapsed floor beams also revealed the absence of adequate compression reinforcement and also inadequate shear reinforcement.
Ø These were no confining stirrups in the beam-column joints of the frames and ductility detailing of the beams and columns were absent.
6. CONCLUSION
Ø Any building design should be stable to withstand all the forces acting on it. Particularly multistoried buildings should be designed for lateral forces such as seismic force, though it is not frequent but may cause severe disasters depending on intensity, as even single storey building fell like cards. “As we know earthquake don’t kill the people, building do”.
Ø Every building should be designed for seismic forces .the design of new buildings or any structure to withstand ground movement is strictly the responsibility of design engineers.
Ø IS CODES (codal provision) related to earthquake are giving us guidelines (for the construction or) seismic resisting structures. Strict adherence to there design guidelines especially for ductility, soft storey, and masonry buildings is the responsibility of the design engineer.
REFERENCES
Ø Dr.Thiruvengadam V. “Earthquakes Resistant Building Design & Construction Practices In India”, Indian Buildings Congress, (2001), Vol (8) P.P.142-149.
Ø Dr.Pal S.C. & Vinaykumar C.H. “Earthquake Resistant Structures In The Wake Of Gujarat Earthquake”. Indian Buildings Congress, (2001), Vol (8) P.P.150-157.
Ø Madan Alok & Raj Mahendra.C “Briding The Gap Between Theory & Practice Of Earthquake Engineering In India”, Indian Buildings Congress, (2001), Vol (8) P.P.158-167.
Ø Is 456-2000 Code of Practice For Plain & Reinforced Concrete.
Ø Is 13920-1993 (BIS) Ductile Detailing Of Reinforced Concrete Structure Subjected To Seismic Forces-Code Of Practice.
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