Welcome to our first lecture in the course on Design of Masonry Structures. This week

will be an introduction to masonry as a structural material; we will examine the use of masonry

as a structural solution from ancient times to the present. It would be useful at this

stage to look at the present-day use of masonry and the kind of standards or the codal framework

or the normative framework within which structural masonry is used in different countries. And,

the kind of normative framework that we have in our country as far the use of masonry is

concerned. And in a few days, you will start appreciating the fact that the term “masonry”

is quite loosely used. The word “masonry” can refer to a very

vast majority, a very vast spectrum of structural construction materials and systems. So, they

can vary right from sun-dried unburnt bricks all the way to stone masonry, and in today’s

context, cement blocks or modern materials such as aerated autoclaved blocks, fly ash

bricks and so on. So, you have an entire spectrum. It could be used along with mortar, of different

types, which leads to different types of masonry. It could be used even without mortar and we

refer to that as dry-stack masonry. And today, we have an entire class of masonry

called hollow-brick construction, very often reinforced, sometimes not reinforced. So,

you already see that with the combination of the structural units and the choice of

the mortar, you can get different types of systems which classify as masonry, ok.

Reinforce them, you get reinforce masonry, otherwise we refer to them as unreinforced

masonry. So, this entire spectrum exists and you will see that in our course we are going

to be dealing with the more formal, the more recent, modern masonry constructions, where

either cement blocks are used for construction, hollow concrete blocks are used for construction

or solid fired clay bricks are used for construction, ok.

So, we will also be largely focusing on reinforced masonry, where, as I would emphasize, in the

codal framework in the country today, unreinforced masonry is not seen as a structural typology

that should be used and therefore, we are moving towards having reinforcement in masonry.

So, predominantly our course will focus on how do you design masonry structures, particularly

reinforced masonry structures, for a combination of forces acting on it, ok. So, I begin today’s

lecture by giving you a perspective on how masonry is used and how has it been used in

the past, but that clearly comes from an understanding of what is the strength and what is the weakness

of masonry.

And when I talk of masonry, I am talking of the assembly now, unit plus some mortar acting

as a composite, right. So, if one were to examine historically how masonry has been

used, this is clearly understood as a material which is good in compression, ok. You use

stone blocks. Stone can have compressive strengths as high as 200 MPa. Granite has a range that

goes all the way from about 70 MPa to over 200 MPa.

So, if you use building stone to construct an entire structure, you can be guaranteed

that the compressive strength is good. If you make it monolithic, you are going to get

the granite compressive strength. If you use small blocks and have mortar, you

are going to be limiting the compressive strength and this is something you will understand

in a few days from now, how the mortar is going to be limiting the compressive strength

of masonry. However, given the fact that you are using a material which is strong in compression,

you are going to have to deal with the fact that this is a material which is strong in

compression, right. Now, evidence from history tells us that this

is widely used in arches, right and most massive constructions in the past, have been constructed

on the structural typology of arches. The use of arches is predominant/prevalent in

historic masonry constructions and that is simply because an arch is known to be good

in compression. Masonry is extensively used in the form of arches and this could be brick

masonry, it could be fired clay, it could be sun-burnt bricks, simply because of the

fact that you have good compressive strength, the structural typology of the arch ensures

that the entire cross section is in compression and you use a material which is good in compression.

So, masonry arches is something that you would see everywhere. Historical structures in and

around your city. This is formal construction in masonry (from 1890s). That is a massive

tower that you see there and a structure which is heavily relying on arches for equilibrating

gravity forces. So, arches are something that give clue to the fact that this material really

works well in compression and can be relied upon.

Bridges; extensive number of bridges in and around our country, all over the world, are

built in, are built using brick or stone masonry. These are structures, this particular example

that you see here, is a structure that is about 140-150 years old and they continue

to be in service conditions. One has to assess them structurally, but this is putting the

material to its best use, which is the strength of masonry itself. You would be surprised

to know that the Indian railways, for example, has 1,30,000 masonry arch bridges.

And 1,00,000 of them, i.e., 1 lakh out of this number, was built in the colonial period.

So, we are talking of bridges which have been in service condition for over 100 years easily.

Of course, you need a quantitative structural assessment of such structures and that is

needless to say. But it is the predominance of the typology, is due to the strength of

masonry itself.

We also see a large number of towers, a large number of towers, not recent constructions,

these are constructions that are at least 800 to 1200 years old towers and if you were

to use a material to build towers that are 100 meters or taller, the material is working

well in compression. Of course, we are not yet discussing the behavior of such structures

under lateral action. That is something we will examine and we will examine them particularly

to see the stability when you have a combination of gravity forces and lateral forces.

However, if you were to assume only gravity forces acting, which is not always the case,

you will have lateral forces. Under gravity, you have massive masonry structures constructed

in the past. You have two examples here, they are very famous examples, the one on the left

is the Brihadishwara tower; the Brihadishwara temple in Thanjavur which is not very far

from here, you can go and visit it. It is about thousand years old, it was constructed

between 1003 and 1010 AD in about 6 to 7 years. And of course, there is the geometry of this

structure, which is also responsible for its stability, which we will examine in a few

minutes. But it is a massive masonry tower, it is a stone masonry construction. And the

one on the right is a brick masonry tower. It’s one of the tallest towers in the world.

You can actually go up this tower. This is in Italy, in a town called Cremona and as

you can see it is a fairly slender structure. It is completely in brick masonry and rises

to about 120 metres in height. An instructive exercise is something that

you can do and I will ask you to work through this. If I were to take just a stack of bricks,

right. I am just stacking one brick above another, right. What would be the size of

one brick? Standard brick, we are not talking about historical brick, I am just taking a

standard brick size, what we use in India today.

19 by 9 19 by 9 by 9 units.

. Centimeters, ok. So, if I were to take a standard

nominal size of the brick and keep stacking bricks one over the other, ok. Could you make

an estimate of what height the stack of bricks can go before it crushes? I need to make some

assumptions, you can plug in some numbers and try to look at it. If I were to assume

that the density, it is only under gravity forces, I am not assuming the presence of

other actions now. Let us assume the density of brick, density

of brick to be about 1800 kg per metre cube. That is a fairly good estimate, 1800 to 1900

kg per metre cube as the density of brick unit itself. We are not talking of density

of masonry yet, brick unit. Only under the action of gravity forces. You know the area

of cross section now. It is 19 centimeters by 9 centimeters.

What is your estimate of the height to which you can construct this? You need to know the

compressive strength of the masonry because you can assume that it is going to fail by

crushing. What would be an estimate of the compressive strength of the masonry? You are

familiar with concrete. You talk of M20 concrete, M30 concrete or M40 concrete or so on.

Masonry units, you will you will see in the next week that we have a different classification

for their strengths, but 5 MPa or 5 Newton per millimeter square; or 10 Newton per millimeter

square is a fairly good estimate of the strength of bricks that are predominantly available.

You will see that if you assume a nominal strength for the brick and look at a brick

that is about 19 centimeter by 9 centimeters, the stack can easily go for a few kilometers.

Of course, there is going to be an issue of stability and that is the reason why these

towers have stopped at 100-120 meters in height. Of course, you can improve the compressive

behavior of a material by choosing the right cross-section, integrating stronger materials

in the cross section, and choosing a form that gives you better stability. And that

is the reason why the figure on the left, the Brihadeeswarar temple with the more stable

form, would have a better performance than something that is most slender and uniform

in cross section all along the height. Of course, the tower on the on the right,

in Cremona, also would have a wider cross section of masonry at the base than at the

top, although the tower in itself in geometry, is uniform from bottom to top overall geometry.

So, masonry is good in compression, but it is always important to do a SWOT analysis

on all structural systems. What we do understand is, this is not a material that is meant for

tension.

Masonry has never been put to use in situations of direct tension. It is rarely, you would

never find masonry be put to use in direct tension unless you have reinforcement built

in and that applies to reinforced concrete as well. Because concrete as a material is

weak in tension and you reinforced, you reinforce concrete so, that you have tensile resistance

in this system itself. The same applies to brick masonry. In fact,

it is notorious as a zero tensile strength material. If you are working with existing

masonry, particularly with the use of lime mortar or mud mortar. So, historical masonry

is notorious as a zero tensile strength material. It might have some residual, it might have

some finite non-zero tensile strength, but it is so, non-uniform/variable that you cannot

depend on it as a tensile strength of the material.

So, it is very often we assume that masonry is a zero tensile strength material. Modern

masonry will have, particularly the masonry with cement mortar, will have some tensile

resistance and this tensile resistance can be measured and can be used in your design;

however, it is very small in comparison to the compressive strength, typically of the

order of 10 percent of the compressive strength or lesser, but again so, variable that you

cannot depend on the tensile strength of masonry. So, in this particular slide there are two

things that we can look at. Tensile stresses when we exclude the possibility of direct

tension, what leads to the formation of tensile stresses? In structural systems under the

combination of gravity and lateral forces, you will have situations where tensile stresses

can be generated. Particularly in the form of flexural tension or in the form of shear

tension. That is where you get principal tension situation in masonry.

So, when you have that, the x crack that you see in the figure on your right, is the effect

of an earthquake on a 2 or 3 storied unreinforced masonry structure. It could, it is in stone

masonry. You can see the formation of these cracks which are called as classical legs

cracks and those cracks are actually forming along the lines of principal tension. This

demonstrates that masonry behaves as a very brittle structural system, as a very brittle

structural material and under the combination of gravity and lateral forces, you do not

have the necessary ductility, particularly under actions like earthquakes.

The figure on the left, has another story to tell you. And this is not so much about

masonry having low tensile strength, but an important aspect of existing masonry structures,

which is poor connections between the structural load bearing walls. Unless specifically designed,

these walls are not tied together or held together. In regions where earthquakes are

prevalent, the structure over several hundred years evolves in a manner that tensile resisting

elements or ties are introduced such that the structure works together.

We will examine this more closely when we start looking at detailing for earthquake

resistance, but historically, structures were given timber bands and sometimes steel ties

to ensure that all peripheral walls and all load bearing walls act together. However,

in the absence of any such tying material, in steel or in timber, you would have a very

low resistance to separation between orthogonal walls and that is exactly what you see here.

The only resistance that is available against the separation of orthogonal walls is really

what is referred to as the toothing between the toothing between the two walls.

Have you seen how a mason constructs a masonry building where does he begin?

. Yes.

. Yes, a mason would start constructing a masonry

wall or an or an assembly of walls from the corner, and you would see that the brick courses

are laid such that the alternate to form a toothing pattern what you see here.

However, when lateral forces are significant that toothing pattern is not sufficient to

hold the two words together. The interlocking provided by the mere toothing is not necessarily

sufficient, in some cases that may be completely absent. So, the only resistance that is there

between separation, between the two orthogonal walls from separating is the shear interlocking

available between the two orthogonal walls. Again, we are talking of a material which

is not strong in tension. So, this separation action causes tension and you can get cracks

very easily formed at the junction of orthogonal walls, orthogonally juxtaposed walls. So,

be it within a wall. So, the difference between the two slides,

the two pictures that you see here, the one on the right tells you that crack formation

within a wall happens rather easily; the picture on the left on the other hand, tells you that

separation between orthogonal walls, simply because you do not have a tensile resisting

material there can happen rather easily, when you have significant seismic forces. In fact,

even moderate seismic forces. So, this is definitely the limitation of masonry

and this limitation understood at the component level, a single wall, is the basis with which

many of our historical structures have been constructed and we will examine that in a

few minutes. Similarly, in regions of significant earthquake activity or heavy wind forces,

in regions where heavy wind forces are present, you have ties that basically hold all the

masonry walls together. So, you need something else, you need something

else to keep the structure together, ok.

In that context it becomes interesting to examine how historical masonry constructions

were conceived and how do they survive if they have to survive several centuries. They

are definitely countering different types of actions constantly.

So, I take up this example of the Ctesiphon palace in present day Iraq. In the past, it

was the part of the Mesopotamian civilization. And we will examine the equilibrium in this

mud brick wall. So, it is a sun-dried brick wall that we are examining. As you can see

with respect to the scale there, the human scale with respect to the wall, you will appreciate

that it is a multistoried construction and we are talking of a construction that is at

least 3000 years old. And this sits in the middle of the desert. We are not examining

the vault here, that is another interesting example and we will keep it for another day,

we will examine the wall.

Now this wall, if you were to take the cross section of the wall, you will understand that

we are talking of a 34 meter high wall. A 34 metre high wall would be how many storeys?

. It is 10 plus, right. Assume 3 meters per

storey and you have a 10 storey construction. Now this is the palace wall, it is the remaining

part of the palace wall and you see that the cross section is definitely thicker at the

base and tapers to the top, that some optimization that has happened there. But it is interesting

to examine why 5 meters, why 5 meters? Could it have been 3? could it have been 4? could

it have been 2? So, that is an important question. If it has

survived several thousand years in the desert, where apart from the role of carrying gravity

forces, it also has to counteract. Wind.

Wind, Right? Desert storms are common. So, you are talking of significant wind forces.

So, you have a situation where gravity forces are acting along with lateral forces. So,

this is a situation where the masonry wall is not going to be only in compression, this

will definitely be subject to some amount of tension. Question is, how did it survive?

What is the story behind the stability of this construction?

It’s going to be examined now under two conditions; one, let us assume that the wall

is not subjected to any wind and the second case we look at equilibrium under combination

of both wind and gravity forces. I will examine a section A-A from the top, at a height of

‘y’ and we will do our equilibrium calculations on that. What is true for that, is then true

for the rest of the structure and let us see if we can deduce something out of this exercise,

ok. So, I am looking at the first case which is

the no wind case, it is purely under gravity forces. Now for that section that is being

considered, the gravity force W acting about its centre of gravity at section A-A, we have

the cross section of the masonry wall, the resultant of the gravity forces acts at the

centroid of the cross section there. And since the resultant R acts at the centroid

of the cross section defined at a distance ‘a’ from the edge, You have uniform compressive

stresses in this cross section, yes? At the inner edge and at the outer edge the cross

section is going to experience uniform compression simply because the resultant is acting at

the centre of the cross section. Now, if I were to examine the equilibrium

under a combination of gravity and lateral forces, we will make little assumptions which

will not jeopardize our estimations here. I assume that the wind forces are acting uniformly

from the top to the bottom of the structure. That is, the wind pressure is uniform from

the top to the bottom of the structure. So, if I look at this height ‘y’ that

is under examination, the resultant of the wind force ‘V’ is acting at the at mid

height y by 2 and you have the gravity force W acting at the centre of gravity of this

block. And now if you examine section A-A and R, cross section at that point because

of the combination of the lateral force and the gravity force there is a tendency of the

resultant now, moving towards what we refer to as the leeward side.

So, the wall is standing against wind, you have the windward side and leeward side. Point

O is on the windward side. Due to a combination of the wind force V and the gravity force

W the resultant now, is shifting towards the leeward side. Of course, now there is an eccentricity

of the resultant with respect to the cross section, where eccentricity is e with respect

to the centroid of the cross section and therefore, the point through which the resultant is acting

geometrically is defined as a plus e which is r with respect to this point O on the windward

end of the wall itself, which is the gravity force into a and the wind force V into y by

2. From which, it is possible to get an expression

for this geometrical quantity r which is nothing but the point on the cross section through

which the resultant is acting, right. So, I rewrite this expression in terms of r and

we have a simple expression to estimate the point through which the resultant is acting

at section A-A in terms of the centroid of the cross section from the edge O which is

a plus the wind force into y by 2 divided by the gravity force W.

So, this little expression here is going to help us write down what is r at different

cross sections. If were to take ten different cross sections A-A, B-B, C-C, D-D …. I would

get r1, r2, r3 and all r can be written down, right? Of course, you need an estimate of

the wind force and you need an estimate of the gravity forces acting here. Once I do

that, I have different points estimated r1, r2, r3 and so on are and if I were to connect

all these points you are basically doing what is called the thrust line analysis.

A combination, under the combination of gravity forces and lateral forces, you are estimating

the thrust line of the system, which is the line of action of the resultant of the structure

under a combination of lateral and gravity forces. So, what we are basically doing is

a simple hand calculation referred to as thrust line analysis for stability check, ok.

So, with this background if we now examine, what might be happening to the wall under

the effect of wind and gravity. In the first situation, so, what you see here is the thrust

line that is running from the top to the bottom and the thrust line in different situation.

So, the first situation: no wind case, the stresses at the base of the structure are

fully in compression and uniformly in compression that is the compression on the windward side

and the compression on the leeward side are equal and you have uniform compression on

the cross section. But as you start having some amount of wind

force acting on the wall cross section, under the combination of lateral and gravity forces,

the resultant starts shifting slowly towards the leeward edge and in situation of light

wind under a combination of these forces, the cross section is still fully in compression,

but you have one edge which is in lower compressive stresses the other edge which is now in higher

compressive stresses. As you move forward there is a limiting case

that you will reach and that limiting case is a very important situation where the cross

section is still fully in compression, yes, but this edge which we have designated earlier

as O is now reaching a state of zero stress which is, it is no more in compression and

it has been decompressed at this point in time. Beyond that it will going into tension.

Hence that point where it has been decompressed, that the edge fibre has been decompressed,

is a limiting case for us why? Because we are looking at a material which is weak in

tension, we really do not want tension to come into this cross section.

So, that is a limiting case for us. Under the assumption that the material is behaving

in a linear elastic manner, we can assume that the cross section is under a triangular

distribution of compressive stresses, which simply implies that the resultants now is

acting at the centroid of the triangular distribution, which is at the edge of the middle one third

of the cross section, because the centroid is at two thirds. Hence the middle, the edge

of the middle one third is where the resultant of the lateral gravity plus lateral forces

is acting. If were to assume that the wind force is acting

from the other side, that the windward and the leeward sides were flipped, it means that

the resultant in this limiting case basically moves from this the right edge of middle third

to the left edge of the middle third, but always needs to remain within the middle one

third, right. If it is within the middle one third of the cross-section then there is no

tension in the cross section. If it goes beyond the middle third of the

cross section, then if the material has some tensile strength, then I can assume that it

will take some tension. But if this material does not have tensile resistance, which is

typically the case with masonry, then any tension would imply formation of a crack,

right? And that is what is written there as uplift. And that is what you would expect

that a crack is formed and slowly there is uplift or the length of contact and length

across which compressive stresses are still being transferred, starts reducing. So, the

section starts becoming partial. If you now continue increasing the wind forces,

the area under compression keeps reducing, it will reach a stage where the compressive

stresses are so high, that you have reached the crushing strength of the material and

the material crushes. When it crushes, the overturning of the system and that is how

the loss of stability is going to actually progress.

Now, I come back to the central limiting case and I said that under a combination of lateral

forces and gravity forces, the resultant has to lie within the middle one third for us

to be in the limiting case of no tension in the cross section. If it so happens that there

is going to be tension in the cross section, as a builder what could you do? as the structural

designer of that wall what would you do? But now my calculation show that resultant is

outside the middle one third what can you do?

If I were to increase the dimension the width of the cross section, width of the base itself,

the middle one third increases right. We were only talking of t by 3. If t is the thickness

or the width of the cross section, t by 3 is the middle one third. The game is bringing

this resultant within the middle third for the combination of gravity and lateral forces.

So, if one we are able to do that, the easiest way to do that would be to increase the width

of the wall cross section and that is possibly the rationale behind most historical masonry

constructions, to keep tension out, because they knew that this material does not work

in tension it is good in compression. So, ensure your cross section is fully in

compression. This is today understood as a mason’s middle third rule. So, mason would

make an empirical calculation of what could be the lateral forces coming on to the structural

wall, what is the gravity force and then dimension the cross section such that you do not have

tension in the cross section. Now, just think back about the Ctesiphon palace

wall, if it has to survive several millennia, under a combination of gravity forces and

lateral forces, there is no tension occurring in the cross section and that is why it is

5 meters at the base. So, this gives you an idea of how historically masonry was constructed.

It is conceived as a lateral force resisting element, but predominantly under gravity forces

dimensioned to keep tension out. Today you and I are able to put reinforcement

into masonry or have other elements that can give tensile resistance. So, we start reducing

the cross sections. Here if you reduce the cross section it will simply overturn because

of zero tensile strength available in the masonry, ok.

Having said that here we looked at the stability under gravity and lateral forces. Within the

resistance of a wall to lateral forces, there is a certain difference that we have to examine

and this is another aspect of weakness which becomes essential to keep in mind because

it keeps coming back to us in earthquake resistant design.

So, if you were to examine a masonry structure as composed of four load bearing walls with

a roof and we have seen a picture where separation between the orthogonal walls happens under

the effect of lateral forces like earthquakes, right. So, we are talking of this sort of

a connection between the orthogonal walls. If you were to assume that the connections

are very poor, like we saw in the previous photograph, that under the earthquake it just

ripped apart like a piece of paper. If the connections between walls is poor and

if the connection between the roof slab and the wall; so, we are talking of vertical to

horizontal system connection and connections in the horizontal system between elements.

So, if we were to examine these two types of connections and say both these connections

are poor, then if you take a masonry structure and subject it to lateral forces it will easily

separate and then the walls are all left by themselves to defend the lateral forces. It

is no longer the whole box, it is no longer the totality of the structure, but its individual

walls which will have to fend of the earthquake forces.

So, if you consider that sort of a situation where the walls are not connected to each

other and the roof slab is not connected well to the walls, right; two poor connections,

two types of poor connections exist in the structure and examine what happens to the

wall when subjected to lateral forces, in two different situations. When the lateral

force is acting in-plane of the wall, right; in the same plane of wall and when the lateral

forces are acting out of plane, are perpendicular to the wall that is the second case.

So, this is referred to as in-plane action and that is referred to as the out of plane

action and we are basically examining a wall component with respect to in-plane action

and out of plane action. You will agree with the me that both are looking at bending action

in-plane, bending action out of plane, right. It is subjected to shear, but there is bending

in that direction, predominant direction in plane and out of plane.

The second moment of area or moment of inertia is a good geometrical parameter, that captures

the bending strength, the bending resistance right. So, if I were to look at. So, what

would you expect between the masonry wall subjected to in-plane lateral forces and out

of plane which do you think is more resistant, intuitively?

In plane. In plane, right. The cartoon there says you

can even push it with your own hand in the out of plane direction, which of course, depends

on the stability in the out of plane direction, but it is very instructive to simply plug

in some numbers to the strong axis and weak axis bending strength, bending resistances

and look at in-plane versus out of plane, is their significance difference between the

resistance available in masonry? So, if I were to take the wall, put some numbers

there, we will some numbers there, let the length of the wall is L, the thickness of

the wall is t and we are looking at the major axis bending of this wall about X-X, Ixx the

second moment of area would work out to be L cube t by 12, rectangular cross section

simple. If I were to look at out of plane bending and estimate the second moment of

area Iyy same L length and t thickness would be L t cube by 12.

It is useful to plug in some numbers, let us say the wall is about 3 meters long and

about 230 mm thick, which is a standard masonry wall thickness for a one brick thick wall.

So, if I were to then plug in some numbers and take the ratio of these bending resistances,

I take I x x by I y y , knowing that I x x is going to be higher, you will see that the

ratio works out to something like 170 which means if you have two walls- you have a wall

which is in the direction of the earthquake, the in-plane wall and the other wall you have

the in plane wall and you have the out of plane wall, under shaking. Let us say this

is the direction of shaking, what do you think will happen if the structure is not behaving

as one entity. It is now simply a pack of walls, some in the in-plane direction, some

in the out of plane direction and the roof that is sitting without connections on it.

And if there is shaking in this direction what would you expect given the bending resistances

that you see. You would simply expect walls that are in the out of plane direction to

fall way before the in plane walls really start resisting and holding the structure

together because the connections are completely not there. So, even under low to moderate

earthquakes, this sort of a failure mechanism, which is referred to as an out of plane failure

mechanism is extremely predominant in masonry constructions.

Now, if tensile resisting elements were put in place, if ties were put in place to hold

all these walls together and in the cartoon its very nicely shown as some stitching that

is happening between the roof and the walls and some stitching is happening between the

walls. So, if you have good interlocking along the walls, between the walls orthogonally

and if you have good connections it could be in the form of a concrete band, it could

be the form of j bolts or whatever, if the connections positive connections are good,

then under a similar action of lateral forces acting on this wall what would you think will

happen? The out of plane wall is still the out of

plane wall the in-plane wall is still the in-plane wall, only the connections have been

improved. The out of plane walls still has lower bending resistance, but it would try

to fail, but when it tries to fail, it is held at its ends to the in-plane wall. So,

it starts transferring those lateral forces to the in-plane wall; the in-plane wall has

100 times more bending resistance and starts protecting the structure.

So, that is an important lesson in the way masonry structures will respond to lateral

forces and that is the reason why the emphasis is always on connections. So, if you improve

connections, you get what is referred loosely as ‘box action’, which is good for earthquake

resistance and this is really nothing, but the out of plane behavior is the weakest link

in the chain and the strength of a system is regulated by the strength of the weakest

link and if you fix the weakest link which is the out of plane behavior, you get better

behavior of the overall structure.

With that I would like to conclude today’s lecture looking at masonry statistics in our

country, ok. We have this interesting numbers that we can examine. I am looking at the housing

census from 2001 and the housing census from 2011, ok. And this is information that is

been pulled out of the building census, from the census of India in which the building

census is a component. If you were to look at total number of households, you can examine

the percentages, you do not have to get awed by the numbers we are running to into millions

of built residential units. So, total number of households by predominant

wall material and the predominant wall materials examined here are mud or unburnt bricks, which

is not fired, sun burnt bricks, stone and finally, burnt clay bricks, the three categories

that I am examining. You have total, you have rural and you have urban. The rural plus urban

will give you the total, let us examine the percentages.

Now, if you were to examine the percentages out of a 100 percent of these, if you add

up the percentages of unburnt brick stone and burnt brick for the 2001 census, we get

a total of 84.7 percent; meaning 84.7 percent of the bill stock of this country running

to millions, we are talking of the total of 249 million houses, these are, these are houses

housing families of 4 to 5 people, that is how the calculation is.

Now, if you were to look at the 2011 census, 10 years later, look at those numbers they

add to 85.3. Essentially we have not changed, we have we continued to build almost in the

same pattern; however, if you examine the individual numbers from 29.6 mud brick it

has come down to 23, which is good news, we are building less kuccha constructions.

But it has simply moved into the burnt clay brick or stone. So, from 10 percent it goes

up to 12 to 14 percent and 44.9 percent of burnt brick increases toward 47.5 percent.

So, essentially if you look at the build stock in this country, we are still talking of predominantly

masonry constructions. No engineer has built all these structures, mostly they are mason

constructed constructions. Majority of these constructions, if you look at a country like

India, 60 to 70 percent of our land mass is sitting in seismic zones III, IV and V- moderate

to high seismic zones. If you look at these structures, most of them

will not have any seismic resistant feature in terms of what we are talking of these ties,

these connections which have to be good and so on. So, in a country like ours, earthquake

protection or risk reduction in earthquakes, of predominant majority of the population

living in such constructions is a big challenge. So, we stop here and we continue our lecture

on the introductory aspects to masonry construction in our next lecture.