Earthquake is a natural phenomenon occuring with all uncertanities. Engineering design aims to link economics, social, environmental and safety factor to produce the best solution. India is a large country. Nearly two thirds of its area is earthquake prone. A large part of rural and urban buildings are low-rise buildings of one two three stroyes. Many of them may not be adequately designed from engineers trained in earthquake engineering. Most loss of life and property due to earthquakes occur due to collapse of buildings. The number of dwelling units and other related small-scale constructions might double in the next two decades in India and other developing countries of the world. This amplifies the need for a simple engineering approach to make such buildings earthquake resistant at a reasonably low cost.
The behaviour of a building during earthquakes depend critically on its overall shape, size and geometry, in addition to how the earthquake forces are carried to the ground. Hence, at the planning stage itself, architects and structural engineers must work together to ensure that the unfavourable features are avoided and a good building configuration is chosen. The main objective of seismic resistant construction is that the structure does not collapse during mild earthquakes. This also helps in preventing catastrophic failure of the suructure giving sufficient warning during severe earthquakes thereby saving precious lives.
In this presentation emphasis will be given to the performance of unengineered buildings during earthquake and some methods to reduce the damages during earthquake.
EARTHQUAKE RESISTANT CONSTRUCTION
Earthquake is a natural phenomenon occurring with all uncertainties. Among all the natural calamities, the most devastating one is earthquake. During the earthquake, ground motions occur in a random fashion, both horizontally and vertically, in all directions radiating from epicenter. These ground motions cause structures to vibrate and induce inertia forces on them. Hence structures in such locations need to be suitably designed and detailed to ensure stability, strength and serviceability with acceptable levels of safety under seismic effects.
The interest of an engineer in earthquakes is mainly from design point of view. He studies them so that the structure he builds can safely withstand the earthquake shocks and the associated erratic ground motion.
At present, the principle of earthquake-resistant design of building has two aims:
1. The building shall withstand with almost no damage to moderate earthquake which have probability of occurring several times during life of a building.
2. The building shall not collapse or harm human lives during severe earthquake motions which have a probability of occuring less than once during the life of the building.
In the former case deformation of the structures remain within the elastic range.
In the latter case, they may exceed the elastic limit and the building should be designed with sufficient ductility to survive collapse.
In order to satisfy these aims, building design should conform following rules:
(a) The configuration of the building (Plan and elevation) should be as simple as possible.
(b) The formation should generally be based on hard and uniform ground.
(c) The members resisting horizontal forces should be arranged so that torsional deformation is not produced.
(d) The structure of the building should be dynamically simple and definite.
(e) The frame of the building structure should have adequate ductility in addition to required strength.
(f) Deformations produced in a building should be held to values, which will not provide obstacles to safety use of building.
Intensity of an earthquake is measured by an instrument called Richter Scale. Classifications of earthquakes are as follows:
Slight: Magnitude up to 4.9 on the Richter Scale
Moderate: Magnitude 5.0 to 6.9
Great: Magnitude 7.0 to 7.9
Very Great: Magnitude 8.0 and above
- An earthquake of magnitude below 2.0 on the Richter Scale usually can’t be felt .
- An earthquake of magnitude below 4.0 on the Richter Scale don’t cause any damage.
- An earthquake of magnitude over 5.0 on the Richter Scale usually can cause minor damage.
- An earthquake of magnitude 6.0 and above is considered strong and cause
- An earthquake of magnitude 7.0 and above is a major earthquake and renders worst possible damage.
Seismic Design Philosophy for Buildings:
Severity of ground shaking at a given location during an earthquake can be minor, moderate and strong. Relatively speaking, minor shaking occurs frequently, moderate shaking occurs occasionally and strong shaking rarely. For instance, on average annually about 800 earthquakes of magnitude 5.0-5.9 occur in the world, while the number is only about 18 for magnitude range 7.0-7.9, and the rare earthquake may occur only once in 500 years or once in 2000 years. As we know that the life of the building itself may be only 50 or 100 years, a conflict arises: whether to design the building to be “earthquake proof” where in there is no damage during the strong but rare earthquake shaking or should we do away with the design to building. Clearly, the former approach is too expensive and the second approach can lead to a major disaster. Hence, the design philosophy should lie somewhere in between these two extremes.
Seismic Risk to Buildings in India:
The construction may generally be classified into two types:
1. Non-Engineered Building Construction
2. Engineered Construction including building and infrastructure
Non-Engineered buildings are those which are spontaneously and informally constructed in various countries in the traditional manner without any or little intervention by qualified architects and engineers in their design. Such buildings involve field stone, fired brick, concrete blocks, adobe or rammed earth, a combination of wood with these traditional locally available materials in their construction . Cement and lime are sometimes used as mortar. Reinforced concrete lintels, floor, roof slabs and beams are also being increasingly used. In some cases, use of reinforced columns and beams is also made particularly for shopping centers and school buildings, but here also a post beam type simple concept is frequently adopted in a non-engineered manner without taking into consideration the stability of the system under horizontal seismic forces. Masonry buildings of all types, except those constructed with earthquake resisting elements, are at the greatest risk of heavy damage in seismic zoneIII and of destruction to collapse in zones IV and V.
Classification of Seismic Zones in India:
The earthquake resisting features specified to be incorporated while constructing any new building depend on the seismic intensity, zone in which the building is located, the base soil and the functional use of the building, whether considered important or ordinary. The extra cost of these resisting features will vary accordingly.
India is divided into 5 seismic zones in ascending order of magnitude of earthquake. The map was taken up for further revision after the Lathur earthquake of 1993. the resulting revised map published in IS:1893-2002(part I) where in the number of zones has been reduced to 4 i.e. II to V only, zone I being merged in zone II , and zone III now further expanded in the peninsular area.
The seismic zone map shows that of the total land area of the country, seismic zone V covers 12%, zone IV 18% and zone III about 27%, thus 57% could be subjected to damaging e earthquake intensity, masonry building in particular.
Indian Seismic Codes:
The Indian Standard Code to be followed for earthquake resistant structures are as follows:
IS 1893-2002, 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 Low Strength Masonry Buildings
IS 13920-1993, Indian Standard Code of Practice for Ductile Detaling of Reinforced Concrete Structures Subjected to Seismic Forces
IS 13935-1993, Indian Standard Guidelines for Repair and Seismic Strengthening of Buildings
Seismic Effects on Structures:
Inertia forces in structures:
Earthquake causes shaking of the ground. So the building resting on it will experience motion at its base. From Newton’s I Law of Motion, even though the base of the building moves with the ground, the roof has a tendency to stay in its original position. But since the walls and columns are connected to it, they drag the roof along with them. This tendency of the roof to continue to remain its previous position is known as inertia. In the building, since the walls or columns are flexible, the motion of the roof is different from that of the ground.
Horizontal and Vertical Shaking:
Earthquake causes shaking of the ground in all three directions- along two horizontal directions (x &y) and the vertical direction (z). During the earthquake, the ground shakes randomly back and forth along each of these directions. All structures are primarily designed to carry the gravity loads in the vertical direction. Hence, most structures tend to be adequate against vertical shaking. However, horizontal shaking along x and y directions remains a concern. Structures designed for gravity loads, in general, may not be able to safely sustain the effects of horizontal earthquake shaking. Hence it is necessary to ensure adequacy of the structures against horizontal earthquake effects.
Causes of Earthquake Damage:
The conventional masonry, particularly in un reinforced and non- engineered structures, being very weak in resisting tensile and shear stresses, leads to disastrous collapse of the entire building/ structure, causing heavy damage to property and loss of lives.
The main deficiencies in the conventional non- engineered/ un-reinforced masonry construction and other reasons for the extensive damage in such buildings are:
1. Heavy dead weight and very stiff buildings, attracting large seismic inertia forces.
2. Very low tensile strength, particularly with poor mortars.
3. Low shear strength, particularly with poor mortars.
4. Brittle behavior in tension as well as compression.
5. Weak connection between wall and wall.
6. Weak connection between roof and wall.
7. Stress concentration at corners of doors and windows.
8. Overall un symmetry in plan and elevation of the building
9. Un symmetry due to imbalance in the sizes and positions of openings in the wall.
10. Defects in construction, such as use of sub standard materials, unfilled joints between bricks.
3.0 Behavior of Brick Masonry Wall
Masonry buildings are brittle structures and one of the most vulnerable of the entire building stock under strong earthquake shaking. Thus, it is very important to improve the seismic behavior of masonry buildings. A number of earthquake-resistant features can be introduced to achieve this objective.
Ground vibrations during earthquakes causes inertia forces at locations of mass in the building. These forces travel through the roof and walls to the foundation. The main emphasis is on ensuring that these forces reach the ground without causing major damage or collapse. Of the three components of a masonry building (roof, wall and foundation, Fig1 (a)), the walls are most vulnerable to damage caused by horizontal forces due to earthquake. A wall topples down easily if pushed horizontally at the top in the direction perpendicular to the plane (termed weak direction), but offers much greater resistance if pushed along its length (termed strong direction, Fig 1 (b)).
Horizontal inertia forces developed at the roof transfers to the wall acting either in the weak or in the strong direction. If all the walls are not tied together like a box, the walls loaded in their weak direction tend to topple. (Fig 2(a))
To ensure good seismic performance, all walls must be joined properly to the adjacent walls. In this way, walls loaded in the weak direction can take advantage of the good lateral resistance offered by walls loaded in strong direction (Fig 2(b)). Further, walls also need to be tied to the roof and foundation to preserve their overall integrity.
Improving Behavior of Masonry Walls
Masonry walls are slender because of their small thickness compared to their height and length. A simple way of making these walls behave well during earthquake shaking is by making them act together as a box along with the roof at the top and foundation at the bottom. A number of construction aspects are required to ensure this box action.
Firstly, connections between the walls should be good. This can be achieved by
(a) Ensuring good interlocking of the masonry courses at the junction.
(b) Employing horizontal bands at various levels, particularly at the lintel level.
Secondly, the size of the doors and window opening need to be kept small. The smaller the opening, larger is the resistance offered by the wall.
Thirdly, the tendency of wall to topple when pushed in the weak direction can be reduced by limiting its length-to-thickness and height-to-thickness ratios. Design codes specify limits to these ratios. A wall that is too tall or too long in comparison to its thickness, is particularly vulnerable to shaking in its weak direction. (Fig (3))
4.0 Importance of Reinforcements in Masonry Building
The walls, if constructed with plain masonry would be incapable of resisting the magnitude of horizontal shear and bending forces imposed on them during earthquakes. For this reason, in the modern reinforced masonry systems, reinforcing steel is incorporated to resist the shear and tensile stresses, so developed. When these walls are subjected to lateral forces acting on them, they behave as flexural members spanning vertically between floors and horizontally between pilasters/ lateral walls. Therefore reinforcement in both vertical and horizontal directions is required to be provided to develop resistance against torsion.
Role of Horizontal Bands
Horizontal bands are the most important earthquake-resistant feature in masonry buildings. The bands are provided to hold a masonry building as a single unit by tying all the walls together. There are four types in a typical masonry building named after their locations in the building. They are:
(a) Plinth band: This should be provided in those cases where the soil is soft or uneven in their properties, as it usually happens in hilly areas. This band is not too critical.
(b) Lintel band: This is the most important band and covers all door and window lintel.
(c) Roof band: In buildings with flat reinforced concrete or reinforced brick roofs, the roof band is not required because the roof slab itself plays the role of a band. However, in buildings with flat timber or CGI sheet roof, a roof band needs to be provided. In buildings with pitched or sloped roof, the roof band is very important.
(d) Gable band: It is employed only in buildings with pitched or sloped roofs.
Design of Lintel Bands
During earthquake shaking, the lintel band undergoes bending and pulling actions. To resist these actions, the construction of lintel band requires special attention. Bands can be made of wood including bamboo strips) or of reinforced concrete (RC) (Fig.8); the RC bands are the best. The straight lengths of the bands must be properly connected at the wall corners. This will allow the band to support walls loaded in their weak directions by the walls loaded in their strong direction. Small lengths of wood spacers (in wooden band) or steel links (in RC bands) are used to make the straight lengths of wood runners or steel bars act together. In wooden bands, proper nailing of straight lengths with spacers is important. Likewise, in RC bands, adequate anchoring of steel links with steel bars is necessary.
The Indian Standards IS:4326-1993 and IS:13828-1993 provide sizes and details of the bands. When wooden bands are used, the cross-section of runners is to be at least 75mmx38mm and the spacers at least 50mmx30mm. When RC bands are used the
minimum thickness is 75mm, and at least two bars of 8mm diameter are required, tied across with steel links of at least 6mm diameter at a spacing of 150mm centers.
Role of Vertical Reinforcements in Walls:
Even if horizontal bands are provided, masonry buildings are weakened by the openings in their walls (Fig (9)). During earthquake shaking, the masonry walls get grouped into 3 sub-units, namely Spandrel masonry, Wall Pier masonry and Sill masonry.
When the ground shakes, the inertia force causes the small-sized masonry wall piers to disconnect from the masonry above and below. These masonry sub-units rock back and forth, developing contact only at the opposite diagonals (Fig. 10(a)). The rocking of a masonry pier can crush the masonry the corners. Rocking is possible when masonry piers are slender, and when weight of the structure above is small. Otherwise, the piers are more likely to develop diagonal (X-type) shear cracking (Fig. 10(b)); this is the most common failure type in masonry buildings.
During strong earthquake shaking, the building may slide just under the roof, below the lintel band or at the sill level. Sometimes, the building may also slide at the plinth level.
How Vertical Reinforcement Helps?
Embedding vertical reinforcement bars in the edges of the wall piers and anchoring them in the foundation at the bottom and in the roof band at the top (Fig 11), forces the slender masonry piers to undergo bending instead of rocking. In wider wall piers, the vertical bars enhance their capability to resist horizontal earthquake forces and delay the X-cracking. Adequate cross-sectional area of these vertical bars prevents the bar from yielding in tension. Further, the vertical bars also help protect the wall from sliding as well as from collapsing in the weak direction.
Protection of Openings in Walls:
The most common damage, observed after an earthquake, is diagonal X-cracking of wall piers, and also inclined cracks at the corners of door and window opening. When a wall with an opening deforms distorts and becomes more like a rhombus. Steel bars provided in the wall masonry all around the openings restrict these cracks at the corners
(Fig. 12). In summary, lintel and sill bands above and below openings and vertical edges, provide protection against this type of damage.
Three important aspects to be considered in the design of earthquake resistant structures are given below:
1. The structure should be ductile, like the use of steel in concrete buildings. For these ductile materials to have an effect, they should be placed where they undergo tension and thus are able to yield.
2. Apart from ductility, deformability of structures is also essential. Deformability of structures is also essential. Deformability refers to the ability of a structure to dispel or deform to a significant degree without collapsing. For this to happen, the structure should be well- proportioned, regular and tied together in such a way that there are no area of excessive stress concentration and forces can be transmitted from one section to another despite large deformations. For this to happen, components must be linked to resisting elements
3. Damageability is another aspect to be taken into consideration. This means the ability of a structure to withstand substantial damage without collapsing. To achieve this objective “minimum area which shall be damaged in case a member of the structure is collapsed” is to be kept in view while planning. Columns shall be stronger than beams for that purpose and it is known as strong column and weak beam concept.
Tips for Earthquake-Resistant Design:
- The building plan should be in a regular shape such as square or rectangular.
- No wall in a room should exceed 6.0m in length. Use pilasters or cross walls for longer walls. In hilly terrain, it should not exceed 3.5m in length.
- The height of each storey should be kept below 3.2m.
- Don’t use bricks of crushing strength less than 35kg/cm2 for single storeyed building and of 50kg/cm2 for 2-3 storeyed building. Only solid and sound bricks/ concrete blocks should be used
- Provide a R.C.C band of 4” thickness throughout the run along wall at lintel level passing over doors and windows.
- The thickness of load bearing wall should be at least 200mm
- The clear width between a door and nearest window should not be less than 600mm.
- Location of a door or window from edge of a wall shall be 600mm minimum.