The repair and retrofitting scheme presented in this document is based on good ... You will learn the answers to many of the listed questions. ... It is now a known fact that continents have been moving a little towards .... The windows are few and very small. ..... using clay mud mortar and two storeys when using cement sand.
A GUIDE BVOOK TO UNDERSTAND THE RELATIONSHIP BETWEEN THE TWO
Anand S. Ayya Aromar Revi Pawan Jain
TARU FOR DEVELOPMENT AND YUVA September, 1994
Copy Rights TARU AND YUVA, 1994,
Portions of this document may be freely reproduced only with the source acknowledged.
DISCLAIMER: The repair and retrofitting scheme presented in this document is based on good engineering practice. When executed properly with proper specifications and workmanship, it is believed to achieve collapse-proof (but not damage-proof) houses in earthquake intensity upto MM VIII. The authors will however, not be responsible for any adverse consequences occurring in the field because of the breach of these guideline.
Dear Reader,
This book is an attempt to inform you about those questions which we have been asked on our various trips to the earthquake affected area, such as the following: 1.
What is an Earthquake ? Can it be prevented ?
2.
How does it happen ?
3.
Where does and can it happen ?
4.
Will it happen again and again at the same place ?
5.
Is it man made ? Is it because of some dam being built ?
6.
When will the next earthquake occur ?
7.
What do the Engineers know ? What is Earthquake Engineering ?
8.
What can we do to protect ourselves from the damaging earthquake effects?
9.
Is there an earthquake proof house ? What is it ?
10.
If my house is not affected in this earthquake, will it get affected in the next earthquake?
11.
Are rumours about a hot line running under the villages true?
12.
Have our houses collapsed because they were on black cotton soil?
13.
Will building on rock be more stable?
2
14.
What materials should we use for our new houses and what should the houses be like ?
15.
Should we build everything in concrete ?
16.
What should we do in the event of an earthquake ? Are there any precautions ?
17.
What's wrong with our earlier houses ?
18.
If we build in stone, will it collapse in the next earthquake ?
3
No single person has all the answers. So, we collected the answers and now present them in an order. But the gist of the entire book is to understand at least four basic things, so that you should be in a position to take good decisions about your own houses. 1.
About the earthquake phenomenon : its occurrence, effects, magnitude and intensity.
2.
The earthquake resistance of various types of buildings.
3.
Understanding your existing building : its good and bad aspects, strong and weak points.
4.
Suitable building materials and technologies.
IF YOU UNDERSTAND THESE FOUR THINGS THEN
1.
You can understand the purpose of each part or element of the building including their relation with one another.
2.
You can make small changes in your existing building to make it earthquake resistant.
3.
You can carry out repairs in buildings affected by the earthquake.
4.
You can make major changes in your existing building, if required.
5.
You can make informed choices about building materials and construction method to achieve earthquake resistance (decide about shape, size and looks) for your house.
6.
By making the maximum use of the dismantled material you can reconstruct your house to be earthquake resistant and fulfil your needs as much as your earlier house. Therefore, you should educate yourself, your children and others about what to do in the event of an earthquake. 4
CONTENTS IN THE FIRST PART You will see how an Earthquake occurs. You will also understand some other
facts related to earthquakes in this region. IN THE SECOND PART You will learn how damage happened to various building types; what kind of houses got damaged more and why, based on an engineer's assessment of the buildings. THE THIRD PART You will learn the answers to many of the listed questions. Is there an earthquake proof building and what makes a building resistant to earthquakes. You will also see a collection of peoples' ideas and aspirations for their future houses. THE FOURTH PART You will learn how to repair damaged buildings, and how to strengthen existing buildings to face earthquakes in the future. THE FIFTH PART You will learn about new materials that are available in the market. You will see in this part, the advantages and disadvantages of these materials and methods of use and precautions to be taken in using these materials. IN THE SIXTH PART You will examine a large drawing which gives a detailed picture of what should be done to strengthen existing buildings. THE SEVENTH PART You will learn about new techniques which can be used to improve the comfort of your house. This part discusses some simple rules of buildings, as well as Do's and Don'ts.
5
PART 1: EARTHQUAKES The whole Earth is so big that an affected area of 14,600 sq km in Latur and Osmanabad looks like a dot.
WORLD It is now a known fact that continents have been moving a little towards one another for millions of years. The resistance to this sudden movement at the boundaries between continents strains the rocks. The sudden release of this energy (stored over many years) results in the unlocking of these rocks causing sudden displacements along faults. The propagation of this energy in the form of waves shakes the earth all around, which we call Earthquakes. v
Earthquakes occur due to sudden movements many kilometers below the ground. The causes are natural, beyond human control.
v
Every earthquake starts from a central area where the effect is maximum or the intensity of shaking of the ground is most severe. This is termed the Epicentral area.
6
•
•
Even if there are no instruments on site, this Epicentre can be found from sensitive instruments located thousands of kilometers away from the earthquake site. Other things which are observed about an earthquake from its instrumental record are : the frequency of ground wave (how many shakes or pushes in a second), and amplitude (the size of every push).
This data helps us in determining the energy of the quakes. Quakes can have an energy equivalent to many thousands to millions of tonnes of explosives. • Earthquake prone zones are basically determined by an earthquake occurrence record. Where earthquakes have occurred before, more may occur in future.
•
Another indicator of earthquake zones is the presence of active geological faults. More earthquakes happen along fault lines at the contact of continents and the seismic belts. But damaging earthquakes do some times occur on inner fault planes, such as the Koyna earthquake of Dec, 1967 and the Killari earthquake of Sept., 30th 1993.
A geological fault is a fracture (crack) or a zone of fractures in rock (land mass) along which the two sides have been displaced. In one sense it is a thin Jeep crack in the land. Earthquakes occur on such faults if they are still active. These faults have been wrongly called as "Hot Line" by some misinformed people. Some people have been misled that Earthquakes are man-made. This is not I rue * No human can stop or control the shifting of continents * No human can stop them or control them * No human can even predict them properly, as yet. In some cases, small quakes do come before the occurrence of a major earthquake (termed foreshocks) which can be called indicators for prediction, but that is not always true. The only protection we have against earthquakes is to * Build houses that are earthquake resistant * Locate villages on low risk sites * Take simple precautions in everyday life
7
THE MARATHWADA EARTHQUAKE The total area in which the 30th, Sept. '93 quake caused damage was 5,200 sq. km but the main area which had severe damage and death was around 500 sq. km.
The Marathwada earthquake focus was just about 4 km below the ground, that is where the rocks slipped and sheared. Earthquakes happen much deeper many times. Because of the greater depth, they shake a much larger area, sometimes thousands of sq. km. but there maximum epicentral intensity may be relatively less. The picture on the next page shows that the focus or vhypocentre' of an Earthquake is very deep as compared with the height of buildings or even dams.
OTHER EARTHQUAKES The map below shows earthquake occurrences in recorded history. It shows the major zones of seismic activity around the Pacific Ocean and other areas like the Himalayas. However, earthqaukes do occur within continental masses like the Killari earthquake. 8
9
PART 2: HOUSE TYPES AND DAMAGE We shall see in this chapter, which types of houses got damaged, how they were damaged and the difference between various kinds of damage in the Marathwada earthquake. There are broadly four types of houses in Marathwada as shown in the following section.
1. MUD AND TIMBER ROOF ON STONE WALLS
v
v
v v v
The roof is made with mud and timber. Mud is placed over planks and timber beams spaced at a distance of approximately 1.2 m (4'-0"). These beams are supported on timber columns. Each 1.2 m segment is called a Khan. Sizes of houses are often measured by the number of Khans since the width of a Khan is nearly the same everywhere. The walls of this house type are usually 750 mm (2’-6”) or more thick and the room inside is about 2.1-2.4 m (7'-0" to 8'-0") wide. The windows are few and very small. There is often a hole in the roof for ventilation. A lot of these houses, have a verandah attached to them, either made with a similar roof as the rooms or with a CGI sheet roof. About 84% of houses in Marathwada are of this type. Damage in such houses has been very large.
10
2.
THE THATCH ROOF HOUSE
v
v v
v v
Usually the smaller houses in the region have thatch roofs with walls made of mud or stone-mud matrix. The walls range from 450 mm (1 '-6") to 750 mm (2'-6") in the thickness. A lot of thatch roofs, are placed on the walls without being tied down. The weight of thatch roofs is relatively low when dry but increases considerably on wetting. They are also flexible because of the nature of the material. About 8% of the houses in Marathwada are of this type. Damage to such houses has been small.
11
3. THE CGI SHEET ROOF HOUSE
v v
v v v
The CGI Sheet is a very light material, easy and fast to build with. It is the choice of some people, for the roof of the entire house even though a lot of people use it to cover the verandah. The inside of a room with a CGI sheet roof heats up quickly. But it also cools very fast. The walls of these houses are made of mud (in most cases) but are sometimes constructed in stone or brick masonary. Walls vary
from 450 mm (l'-6") to 750 mm (2'-6") in thickness. About 6% of the houses in the region are like this. Damage in such houses has been slight.
12
4. HOUSES WITH THE REINFORCED CONCRETE (RCC) ROOF ON STONE WALLS
v
v v
v v
CEMENT
A few wealthy house-owners had earlier changed the material of their roof and replaced it with RCC slabs. RCC is a relatively an expensive material to use. The replacement of existing mud-on wood roof with an RCC slab did not an improve help the stability of houses during the earthquake. RCC is a material which requires a lot of information and skill to make properly. In case of damage, repair of concrete is quite difficult. A lot of concrete used in buildings in Marathwada is of bad quality and was wrong in both composition and technique. About 2% of the houses are of this type. There was a lot of damage seen in such houses including the complete collapse of houses with stone walls and RCC roofs.
13
DAMAGE Damage to buildings has been of many types and different degrees. Some houses cannot be repaired and have to be reconstructed. An international scientific classification has been made in order to understand the degree of damage in various types of houses. Houses are classified in (1) Slightly damaged, (2) Heavily damaged, (3) Destroyed and (4) Collapsed categories. Out of the four categories, those in the undamaged and slightly damaged category can be repaired and strengthened at relatively low cost without evacuating the inmates; those in heavily damaged category will require evacuation of the residents and larger repair and strengthening cost, while others in ‘destroyed’ and ‘collapsed’ categories have to be dismantled and rebuilt. The pattern of damage to the houses in Marathwada are shown below:
14
15
Twenty of the worst hit villages had, on an average, 47 per cent houses collapsed, 19 per cent destroyed, 20 per cent had heavy damage and 14 per cent were slightly damaged. You shall now see how the damage occurred.
CAUSE AND LOCATION OF DAMAGE
(1) The bad construction quality of stone walls made them split due to earthquake vibration and crumble. The entire structure came down along with the way and killed many of the occupants.
(2) The corners of the walls were weak, (i.e. one wall was not bonded with the perpendicular wall) there fore the corners opened up. Absence of long stones at the corners was as the cause of lack of bonding. Good slightly dressed stones were used on the wall faces, but small stones with mud were filled within the stone sides (whythes). There were no 'through' stones (called 'headers' by local masons) used in the wall because of which the two faces of the wall acted independently, bulged and crumbled. (3)
16
(4) The roof came down with the walls, where there was no wooden 'Khan1 framing inside.
(5) The roof fell down because the structural timber was rotten or eaten by termites/borers (some timber was installed wet and without seasoning which made it an easy prey for white ants).
(6) The stone walls were too thick, built in weak mortar. When the ground shook slightly, cracks appeared. Under stronger shaking they widened, and caused the falling of stones.
(7) The walls had only low or no tensile resistance to a shake wave coming along their length. They develop diagonal cracks. Reason for this is mortar and bad bonding between bricks and stone used for the construction.
17
8) The roof on wooden columns acted like an inverted pendulum (like a heavy pole hinged at the bottom), hitting the stone walls and causing them to fall.
9) The columns of the khans sometimes moved from their place. Even though they were not displaced from their stone pedestals, in most cases.
10) Cracks appeared in stone joints in masonary walls. The existing masonry (bonding) has been compared with the correct bonding in the illustration.
18
SOME IMPORTANT POINTS 1) In the recent Marathwada earthquake the single biggest reason for the damage to buildings and the loss of such a large number of lives is bad workmanship of stone masonry and more specifically the absence of 'through' stones, also called 'Bond stones' or 'Header stone' in the walls. These are long stones placed across the thickness of the wall i.e. perpendicular to the length of the wall covering both the inside and outside layers (or wythes) of the stones. These stones are crucial to the strength of the wall. Since 'through' stones of adequate length for thick walls (600 to 1200 mm) are hard to find and handle, thinner walls, only about 450 mm in thickness, should be used. Without through stones, a wall behaves like a pile of stones which falls apart easily in an earthquake.
19
2) Wherever such stones have been used like in most gateways, walls were shaken but did not fall. Such sights are very common through out the region.
3) The frames remained standing even in tilted condition and survived even after the walls had totally crumbled. Such sights are also very common in the region.
20
Now that you can understand the damage that has happened to your house, the places/points in the house that have been damaged *
the extent of damage, and
*
the reasons for the damage
You will be able to examine the possibility of repair in Part 4. You will also see what kind of damage can be repaired and what damage requires reconstruction of the entire house (or a part of it).
21
PART 3: UNDERSTANDING THE KNOWLEDGE HIDDEN IN YOUR EXISTING HOUSES Knowledge of the fundamental physical aspects of a problem is Science finding solutions to these problems with the use of science and our skills is Engineering and Technology. Existing houses in Marathwada may look very ordinary and commonplace to us, but there is actually good science and understanding that has been used in them. The present house with timber 'Khans', Stone walls and Mud on the roof represents a good mix of scientific understanding and technological solutions evolved over centuries to local problems. It is comfortable and suitable for all socio-cultural and occupational needs of the occupants. The only problem is that it is not strong enough to resist even for even a moderate intensity earthquake (MSK VII or VIII). THICK STONE WALLS In old times there was plenty of clay, stone and wood locally available along with cheap human labour, which are all inputs in constructing walls. One reason why a lot of thick walls were built, was to be able to accommodate niches and almirahs within the wall. At that time, people did not have banks for safekeeping of money and jewellery and depended on thick walls for safe keeping. The other reason for building thick walls was safety. It is not easy for a thief, or a robber to enter such houses either by making a hole in the wall or by climbing over it. Thick walls also make a building cool in summer and warm in winter. The fact that an earthquake of damaging intersity could occur in the area and that thick walls are unsuitable for Earthquake resistance was naturally in unknown then. The modern engineering of buildings for earthquakes is not more than 100 years old. There is almost no wall in which it has not cracked at the Almirah. In many cases, it has been the starting point of the crack which brought the wall down. But a large number of almirahs can be tilted inside the space saved if strong thin walls are built instead of weak thick walls as shown in the illustration. 22
THICKNESS OF ROOFS Modern houses built with thin walls and thin CGI sheet or RCC roofs even in cool places like Mussoorie & Nainital are fitted with fans now. In summer afternoons it gets very hot in these structures. On the other hand in the traditional houses in Maharashtra, people do not have fans even with temperature outside reaching 40-45° C. The reason for this is that in the peak of summer 55% of the heat falls on the roof, 20% on the west wall, 15% on the South/North wall, 10% through windows/doors. Marathwada house builders perhaps understood this very well and used thick mud on the roof to stop/reduce/cut the penetration of heat into the house. Another reason for the addition of clayee earth year after year was to prevent rain water leakage, which increased the roof thickness to underable levels. The fact that this will make the houses vulnerable in an earthquake was not known then. After the Earthquake people have realized that heavy roofs are dangerous and we will not pile 500-700 mm (1 '-6" to 3'-0") of mud on the roof any more. But the recurrent problem of heat has not gone away. We have to find a long term solution to this problem. If we do not, we will have to suffer from heat, and spend money every day to remain comfortable in the summer both during the day and the night and depend on the unreliable electric supply. Clay is also an excellent waterproofing material as it is expand in size when it comes in contact with water. Whenever, it rains the Mud larger expand and all the holes, gaps and cracks get blocked. Our anscestors understood this property of mud and never felt the need for any other water proofing materials.
23
PART 4: MAKING HOUSES EARTHQUAKE RESISTANT There are three ways in which we can make buildings not fail in an earthquake. 1.
2.
3.
To make them Flexible like a cardboard box.
To make them rigid like a strong tin box (Sandooq).
To take a path in the middle of 1 and 2 above.
Often the middle path or No. 3 is what we have to take because we have a limited choice in building materials for walls and roofs which perform all the functions we want and yet are cheap and widely available. Buildings which are flexible to a certain point and rigid beyond that are the best in earthquake. We shall now see how we can use this knowledge to our advantage. 24
Every structure can bend a bit but when one push takes the walls beyond their capacity, they crack. Nothing will happen to a tent or pavilion made with bamboo and plastic sheet because it will shake and bend and come back to its usual position. But its not possible to make permanent structures so flexible. Earthquakes, shakes the structures in all directions forward-backward, leftright, upward-downward as shown in the next page. It helps if houses are flexible in all directions and strong to absorb deformations without cracking. If we replace the plastic sheet with a piece of Shahbad stone then the pavilion looks unsafe. The push comes from the ground in an earthquake. The legs of the pavilion are suddenly taken to one side by the push and it may collapse because it is heavy on the top. Walls shake like this in an earthquake because they do not have enough strength against being pushed sideways. An earthquake pushes walls sideways too. The heavier the roof, the higher the possibility of damage. 25
Effect of Ground Shaking on a structure.
26
Stone wails behave like an unjointed stack of stones in an earthquake and we have to devise methods to strengthen them to prevent this behaviour. A reinforced cement concrete band in the middle of a wall over the door, this binds the structure and keeps it from failing in an earthquake. A similar band at the floor level of the building, which prevents one part of the floor to shake independently from the other part of the floor. If we bind the horizontal bands with a vertical 'splint1 at the corners, then the structure becomes even more secure.
The first band at the door lintel level is essential for all small-single storeyed buildings built with stone in earthquake risk zones. The additional second band is essential for black cotton soil areas. The vertical splint is required for houses with two or more storeys and are built in severe earthquake risk zones.
27
Just as a straight piece of paper does not stand but a folded piece does, a wall corner has more stability and therefore more earthquake resistance than a flat wall. But it should be seen in this case that the corner is strong. We will have the walls falling down independently after separating from one another if this is not so. A band at lintel level in all walls binds the walls together and prevents corner separation.
A roof ties all the walls together. We have to make sure that all the walls are well tied to the roof. This will impart a rigidity to the structure. Walls will have another binding at the corners.
28
Windows completely change the behaviour of a wall from a flat big surface sometimes even to a set of thin columns (piers). We have to prevent this, because too many windows make a wall weak.
Most houses in Marathwada have small windows and therefore are exposed to less risk. Modern buildings have too many large windows. We will have to give very special attention to walls, both to the design as well as the quality of workmanship. To do this we have to understand the difference between a load bearing and a frame structure.
29
A bed made by keeping a large plank over some bricks is like an ordinary/load bearing wall structure.
A charpoi (cot) is a frame structure because its members are joint firmly with one another.
If we bury the legs of the charpoi in the ground for some depth then it becomes like a frame structure with foundation.
Such frame structures in materials like timber steel or RCC are more earthquake resistant than simple load bearing wall structures, because they are capable of resisting bending without breaking. Frame structures are very expensive to construct. We should try to bind the elements of a house such as the roof, walls and the foundation together to get a structure which is integrated like strong tin box, so as to resist earthquake forces as effectively as the framed structures.
30
FUTURE HOUSE - PEOPLES1 OPINIONS We talked to a large number of people across the earthquake affected region in order to find out what kind of houses people wanted. We found that most people now want houses which perform the same functions as their earlier house. *
is as comfortable as their earlier house.
*
are earthquake resistant.
*
have thinner walls but not in stone. Some people realise the errors in stone masonry and are willing to consider it if quality is assured atleast up to sill level.
*
are single storeyed but some people do not mind a first floor if earth quake safety is assured.
*
have a light roof.
*
have a bathroom.
*
need not have a latrine.
*
have more light and ventilation.
*
have enough space to house their annual stock of food grains.
31
EARTHQUAKE RESISTING FEATURES There is no building which is earthquake proof. All building will be affected by major earthquake to a lesser or greater extent. The major consideration in an earthquake resistant building, is that the building remains together, even if there are extensive cracks in its: should not kill or injure people. An earthquake resistant buildings is one which: 1. 2.
may crack but does not collapse. has a safe place inside where people can remain unharmed, (for instance the corners.) 3. has a roof that does not collapse suddenly but caves so that the people inside have time to escape. For this we have to tie the various components of the building together: Wall to wall; walls to foundation, wall to roof and one storey to the other storeys.
in slowly
Just as large strong looking trees get uprooted in high velocity winds and tin roofs fly off, walls behave very differently in an earthquake than they normally do. The more they are bound together with the roof, the stronger the house resists earthquake damage.
32
Even if someone took a strong rope and tied up a pucca stone/concrete building up like a bundle it would become earthquake resistant.
(This is just to understand the principle. Do not try to make buildings earthquake resistant like this.) The other methods which are used to tie buildings, achieve this effect. In the case of new buildings, these tying mechanisms may be introduced while building a new structure. In old buildings, it is possible to add these later and achieve almost the same earthquake resistance as a new earthquake resistant structure. This is called seismic retrofitting. The relative cost will be more in the latter case.
People with old surviving buildings need not worry. Most buildings that have not been severely damaged can be repaired and strengthened to become safe against a future earthquake. Let us see how:
33
Once we have identified the elements which make a building earthquake resistant, we have to make sure that the resistance giving elements are present in these houses in the right place. It is possible to add these elements to existing buildings without much problem and expenses of not more than 15 to 20 per cent of the cost of rebuilding the house. The following are the requirements of seismic strengthening of an existing stone house: * There should be at least one through stone in about one sq. metre of wall. * The wall is not thicker than 900 mm (3' - 0"). It will be more difficult to provide 'through' stones in thicker walls. * The mud filling on the top should not be more than 200 mm (8"). Where more, it has to be reduced along with introduction of our waterproofing membrane. * There has to be a lintel level band (to be provided externally in existing houses). * There should be at least six stones or 'dowel' in every storey at the corner which are 150 mm longer than width of the wall (otherwise an arrangement for preventing corner separation has to be made). * If the construction is in the ^Khan' system, then there has to be a rigid connection between the columns and the beams. This could be j
achieved by knee-braces between the beams and columns in both directions of the building. * Lintels on the top of the doors and windows, have to be at least 450 mm (18") longer than the openings resting at least 225 mm (9") on each side on the wall. * Structural timber should not be rotten or eaten by wood borers or termites. If they are, then they should be replaced. * Drainage of the roof should be proper with slopes and spouts . * The height of the structure should not be more than one storey when using clay mud mortar and two storeys when using cement sand mortar 1:6. You must check the above list and decide what steps are necessary in your house to make it earthquake resistant. Details of carrying out repair and retrofitting are presented in the following pages:
34
REPAIR - CRACK STITCHING If there is a crack in your house, then you have to do as follows:
(1)
Clean the crack
(2) Rake the joints across the crack in a length of 600 mm (2'-0") as deep as conveniently and safely possible without disturbing the stones.
(3) Clean the joints at least 300 mm (I'-O") on each side of the crack.
(4) Clean everything with wire brush, and remove dust.
35
(5) Fill the cracks and raked joints with (1:6) cement-sand or (1:3) lime-sand mortar.
(6) Sprinkle water on the cement mortar for a minimum of seven days. (7) The wall will then become strong again and this repaired area will not act as a weak point in the wall for future earthquakes.
Remember if the wall has moved out of plumb or there is a bulge along with the crack, then the wall cannot be repaired. The bulged wythe has to be reconstructed, ensuring bonding with the vertical wythe.
36
RETROFITTING - (A) CORNER STRENGTHENING If the stone wail built in mud mortar does not have enough long stones at the corners, provide a horizontal band at each, outside corner as follows:
(1)
Remove plaster if any in a depth of 400 mm at 800 mm above plinth level and a length of 300 mm more than the cross wall thickness on each wall meeting at the corner.
(2)
Rake the exposed joints to a depth of 20 mm. Clean the joints with a wire brush and remove dust.
(3) Take welded wire mesh of the25x50mm(l"x2 f t ) mesh with 8 gauge wire.
37
(4) The width of mesh should be 350mm. The length should be 600 mm (2'-0") more than twice the thickness of the walls.
(5)
Hang the mesh on the wail but 15 mm (5/8") away from the wall with the help of long nails with flat heads.
(6)
Plaster with a cement sand plaster (1:4).
(7)
The plaster should be 35-38 mm (1 3/8" - 1 1/2") thick so that the mesh is covered by a 15 mm (5/8") thick cover.
(8) Cure for at least 14 days, twice a day.
38
(9) If mesh is not available in the required width, use strips. The joint of strips should not be a vertical one. It should be horizontal.
Corner Strengthening
39
RETROFITTING - (B) REDUCING LOAD ON ROOF If the thickness of mud on your roof is more than 200 mm (8") then you need to reduce it to 200 mm (8fi). You may have to add a black plastic sheet within the soil to prevent rain water leakage. (1)
Dig and remove all the earth on top of your roof and expose all the planks.
(2)
Paint the planks with coal tar or creosote oil if possible.
(3)
Lay back a layer of mud (clay. mud with straw mixture after wetting and rotting in a pit for seven days) of about 100 mm (4") thickness with adequate slope for drainage. Let it dry for at least 2 days.
(4)
Lay a black polythene sheet which is available in 3600 mm (12'-0") width over the mudlayer. Make sure not to tear the sheet.
(5)
Gently lay the mud straw mix in another 100 mm layer, making the full thickness of 200 mm (8M).
(6)
You may have to lower the water spout to a level just below the polythene sheet. 40
RETROFITTING - (C) ADDING A BAND To tie the walls together and prevent wall separation at the corners, we must provide a band just below the ceiling and above the door level in the following way. (1) Clear the plaster if any in a strip of 450 - 600 mm (1'- 6" to 2'-0") width just below the wood beam of the roof or floor. (2) Hang a strip of welded wire mesh 25 x 50 mm (I11 x 2") made with 8 gauge wire 15 mm (5/8") away from the wall. (3)
Provide 300 mm (I'-O11) overlap between meshes at the corners.
(4)
Use fat 150 mm (6") long nails to hang the mesh.
(5)
Use (1:4) cement sand plaster embedding the mesh around the building. The total thick ness of plaster should become 38mm (1.5").
(6)
Cure the beam thrice a day for 14 days starting two days after casting.
41
(7) The band should be cast just below the ceiling level. The top of the band should be the same level or lower than the c e i l i n g . I f c a s t h i g h e r than the roof, it is not useful.
(8) The fixing of the mesh and plastering over is shown in the following
illustration: This is the most important earthquake protection measure in all masonry houses. 42
RETROFITTING - (D) PROVIDING THROUGH STONES If you find that the walls of your house do not have through stones (headers), you will have to insert an alternative which will prevent splitting of the walls. (1)
Make through holes in your wall by removing single stones from the wall as shown here. There should be one hole in one square meter. Removal of stone should be done gently so that surrounding stones are not disturbed.
(2)
Remove the infill material between the two removed stones like a thin tunnel.
(3)
Insert a hooked piece of steel from both sides and fill the holes with concrete. Both ends of the steel rod should get embedded in the concrete.
(4) Cure the ends regularly for at least 7 days.
43
RETROFITTING - (E) INSTALLATION OF KNEE BRACING A typical timber frame of buildings in Marathwada is such that its top can sway on each side without the base of columns shifting from their place. It is advisable to restrict this sway. In order to do that, we can use specially made corner braces like this, or similar to this.
These knee braces should be fixed with atleast four screws or long nails. Wood should be predrilled to prevent splitting. The braces have to be used to join the columns to the primary beams and to the secondary beams. Make holes with a drill before using nail or screw. The use of these braces is also shown in the large drawing in Part 6.
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RETROFITTING - (F) TYING THE WALLS If you have a situation where a lintel level band can not be cast around the house,
or if the building is very long (room longer than four khans) then we have to tie the two opposite long walls with a steel tie rod at the places indicated in dotted line, passing through the bands on opposite walls.
a) For installing such tie rods, you will need two pieces of steel plate 6 mm thick and 150 x 150 mm (6" x 6") with a 12 mm (1/2") dia hole in the middle for each bar
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b)
You will also require the tie rod of 10 mm (3/8") dia which should be 100 mm (4") longer than the width of the room plus the thickness of both walls. The length is indicated with an arrow.
Make threads at the end of the rod and get four nuts of the same thread for each rod.
d)
e)
Make holes in the opposite walls and Insert the rod along the perpendicular wall as shown. If the hole has become big, you may have to fill it with concrete. Use the 6 mm thick plate pieces as washers and tighten the nuts, two on each side. This rod will hold the lintel level band on the opposite walls together. The ends of the tie rod with nuts may be covered with the plaster of the lintel band.
BY FOLLOWING RETROFITTING STEPS (a) TO (e) YOU WILL ACHIEVE PROTECTION AGAINST ALL FUTURE EARTHQUAKES IN MARATHWADA. 46
PART 5: TECHNOLOGY CHOICE We shall see in this chapter, the various options available to us in materials for different parts of the building. The purpose of explaining these smaller details is to help you to take your own decisions. If you find out the prices of building materials in your village, you can make an accurate estimate of the expenditure you are going to incur on your proposed house with those prices. This section is divided into 4 parts. *
Walls - Functional requirement of a wall, list of material available, properties of materials.
*
Roofs - Functional requirement, material and their properties.
*
Foundations - Functional requirement, material and their properties.
*
Methods of use of timber.
The correct choice of materials is not the key to an earthquake resistant or a comfortable house. It is the system of use of these materials and the quality of workmanship in the use of material. Safety and comfort depend totally on the effective use of Earthquake resistance designs shown in the preceding parts. Safe and comfortable houses can be built with stone when done properly and houses made with concrete if not done properly can be dangerous
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WALLS The wall should be 1. 2. 3. 4. 5. 6. 7. 8.
well bonded together with the vertical joints in stones, staggered strong so that it can vibrate in an earthquake without breaking. not too high if it is a compound wall. made of locally available material. amenable to repair and construction by local craftsmen. not too thin to let the inside of the room get heated up, nor too thick. such that it can be tied to the roof. economical.
The wall should also serve the usual functions of a wall: * * * * * *
It should not be possible to hear voices from across the wall Be possible to put nails in it for hanging things. Be weather proof Prevent termites and rats from making homes in it. It should have impact resistance (it should be able to take some banging) Possible to fix doors and windows easily.
Walls may be made of the following materials * * * * * * * *
Stone Fired clay Bricks Concrete Blocks Hollow Concrete Blocks Sponge concrete Unfired clay bricks (Adobe) Compressed Soil Blocks, Stabilized soil block Cast-in situ concrete
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MATERIAL PROPERTIES Stone * *
*
Free for most households from old houses. Permanent water proofing Good thermal performance Improved masonry possible with same stones. Craftsmen widely available for stone work Plaster not required.
* Looks good. Bricks * * * * * *
Good quality bricks not available in the area Expensive 9" walls heat up insides in the summers Plaster essential for water proofing Easy maintenance Skills for use are easily available
Concrete Blocks * * * * * * *
Even 8" thick walls are more expensive than bricks Not widely available Difficult to use (each block is heavy) Has to be plastered Difficult to repair, skills not available. House heats up in summer Have to use cement mortar which can be more dangerous than mud morlar if not adequately cured. 49
Hollow Concrete Blocks * * *
* * * *
Cost the same as concrete blocks Thermal performance slightly better than concrete Bad quality blocks can cause loss during an earthquake.
Expensive Very much more difficult to maintain, can't put nails etc in walls Quality varies, need to inspect each batch of blocks Move to use cement mortar which can be more dangerous than mud mortar if not adequately cured.
Sponge Concrete * * * *
Same advantage as Concrete but Weight of blocks lower (better for Earthquake) Thermal performance better than concrete Has to be plastered Easy to use and repair Very expensive
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ROOFS Functional requirements If we look at different roofing materials available to us then we find that some have no insulation properties while others have no earthquake resistance. Some have too much weight and some are non-permanent. Some are fire proof, some are not. Some are smooth, some are not. It is a tough task to decide which material to use where, if we don't know them well enough. Some of the different options available to us are
1. 2. 3. 4. 5. 6. 7. 8.
Mud and timber ‘Khan’ system. CGI roof on timber beams. Thatch on timber poles. In-situ (made on site) Concrete. Precast Concrete panels on precast beams. Shahbad Stone on timber beams. Shahbad Stone on steel beams. Double (composits) roof.
We shall compare the roof with Khan and mud for insulation, CGI sheet for cost, CGI for weight and for ease in repair and maintenance with the Khanmud system. If we say the cost is high, it means higher than CGI and if we say it is difficult to repair, it means difficult to repair as compared with the Khan-mud system.
MATERIAL PROPERTIES Concrete * * *
Cost about twice that of CGI Considerable infrastructure, skill and equipment required Very heavy weight - about 20 times a CGI Roof
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* * *
Not waterproof, needs more waterproofing treatment absorbs heat, and releases it inside at night. Life is 30 years, if not cast properly with machines (vibrator) and could be less. Earthquake resistance depends on the strength of walls. A con crete roof on weak walls can causal more disaster. A RCC roof on a frame structure can be strong.
CGI * * * *
Cheap Light Very bad thermal performance. Additional insulation has to be added below or on top of the sheets. Life of over 30 years.
Shahbad Stone * Good alternative * Bad thermal performance but we can put mud phuska or inverted Kulhars (earthen cups) on top. * Expensive, since good straight timber or steel has to be used and both of them are expensive. * About six times the weight of CGI sheets
Precast Concrete/ Brick Panels * * * * *
Expensive Not widely available Difficult to procure, cast, install and repair About ten times the weight of CGI sheet roof Requires expensive waterproofing
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Double Roof * * * * *
Expensive, two times the CGI cost Good thermal performance Good water proofing About six times the weight of CGI sheet roof Good earthquake resistance
Thatch Roof * * * * * *
Very good for insulation Inexpensive Does not last long, but can be fairly permanent if done properly Fire hazard, requires care Very good for earthquake resistance Easily available material and skills
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FOUNDATIONS Functional Requirements *
Foundations provide a firm base for the house to be built on. Foundations are wider than walls, so that the wall doesn't cut into the ground. They distribute the weight of the building over a large area.
We can build foundations with stone or with brick, but common foundations in the area are built with stone. There is not much reason to change this. However, we have to improve the technique vastly. * * * * * *
*
Cut stone or angular should be used and certainly not round boulders. Long 'through' stones have to be used. Good mortar using cutl sand is necessary. Good anti-termite treatment is a also a must. All precautions which have to be taken in walls have to be taken in foundation masonry. In black cotton soil areas, the space left after foundation masonry should be filled with sand. If sand is not available, it should be filled with any mini which is not clay. Such filling of sand is also advisable for all houses as it is good to prevent entry of termites into the house.
There is no value addition for a foundation if it is made with a more expensive material like concrete block etc. The only exception is that of a RCC frame structure in which case the foundation is made of RCC
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METHODS OF USE OF TIMBER In bad timber work, pieces of timber can bend or warp and untreated timber is easily eaten by white ants and wood borers. This happens because of the presence of excessive moisture in the timber. To prevent this we can season the timber and treat the timber. For seasoning, sawn timber should be kept in shade in such a way that there is enough air passing through the stack but each piece is weighed down by the piece above. You should keep some weight on top of the stack to weigh down the upper pieces. Few pieces on the top may even be nailed together to prevent them from warping. Water should be sprinkled on the stack once a day during the dry season. This is required, so that the outer surface of the timber does not dry very fast which causes cracks. One month later, the timber may be used after treating.
CORRECT WAY TO STACK TIMBER
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TREATMENT OF TIMBER The simplest way to treat timber is to paint it with a anti-termite solution or wood preservative. This is available in two colours, black and transparent. We can dilute this with some kerosene oil and apply it with a brush or a piece of cloth. If no solution is available, then you can mix some DDT with kerosene or diesel and put a coat on the timber. All the above solutions are poisonous and care should be taken in their use. A simple set of surgical gloves should be used if applying this mixture with a piece of cloth. It is better to plane the timber before applying the preservative because less solution will be used and the spreading will be even.
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PART 6: SUMMARY OF EARTHQUAKE RESISTANT FEATURES The following drawing will help you in visualizing the points in your house which need strengthening. It will also give you enough information coupled with information of the preceding chapters to carry out repairs in your house. This drawing should be saved for future use and reference.
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PART 7: IMPROVING HOUSE DESIGNS In order to make your house more comfortable, you have to improve certain features which are listed below:
Light and Ventilation In order to improve the light and ventilation, you can take the following steps. * * * *
Make slightly larger windows Use glass in the shutters of windows. Use smokeless wood stoves (Chulhas) with a chimney. Build the house a little away from the plot boundary to make windows possible in outer walls.
Sanitation In order to improve the sanitation of the house *
* *
Make pucca drains which can be cleaned. Channelise the used water to a soakage pit or a vegetable patch/tree. You can use brick or stone or a half cut stoneware pipe to make a drain. Make pucca floor with a slope for the cattle so no dung or urine is scattered around. If there is a latrine in the house, then make sure that at least 5 litres water is used after every use.
Thermal Comfort A new problem which is going to arise after the new kind of houses are built after the earthquake, is that of protection from intense heat in the summer. The remedy is to put a layer of some material between direct sunlight and your roof or wall. Some methods to prevent the entry of heat through the roof are to * Cover the roof with empty inverted earthern cups (Kulhars). This will effectively form another layer on top of the roof. * Place a layer of crop residues such as stalks or a groundnut shells on the roof. 58
* *
Put a layer of some cloth or old bags inside the roof to act as a double ceiling. Paint the top of a CGI Sheet roof white or silver so that it reflects most of the heat that it receives.
To prevent the entry of heat through the walls. * * * *
Grow shady trees near the south and west facing walls which shade the walls during the summer. Train a creeper or flower bush on the south and west wall so that direct sunlight does not fall on the wall. Make a lean-to sloping roof along the south & west walls for storage/cattle. Open your windows or doors as far as possible on north and east side.
Some important do's & don'ts for earthquake resistant construction and for a comfortable house are shown in the following pages.
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DO'S AND DON'TS
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DO'S AND DON'TS
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DO'S AND DON'TS
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DO'S AND DONTS
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ABOUT THE AUTHORS Prof. AnandS. Arya, the doyen of Earthquake engineering in India is Professor Emeritus at the University of Roorkee. He is the author of numerous books, recipient of many awards and is a veteran who has studied many earthquakes around the world since the 1960's. He is a Fellow of the Indian National Science Academy and a Fellow of Indian National Academy of Engineering. He was a member of the high level technical committee appointed by the Government of India to review the reconstruction programme in Marathwada. Aromar Revi is a Civil Engineer by training but works mostly on housing and human settlement policy after having worked in applied research on building materials and improved traditional construction. He is a member of various national and international bodies working in the field of housing. Aromar Revi is a founder member of TARU and lives in Delhi. He has done extensive work in Uttarkashi after the earthquake. He was the leader of the TARU team that prepared the earthquake reconstruction plan for Marathwada for the Government of India. Pawan Jain is an architect who lives and works in Dehradun. He has been associated with rural architecture and engineering for the last ten years. He studied the Uttarkashi Earthquake closely, co-authored a booklet on reconstruction and was associated with the making of a short film on reconstruction. He was a member of the TARU team that prepared the reconstruction plan for Marathwada for the Government of India. He has made all the illustrations for; this booklet.
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