Welcome to NPTEL project, building materials and construction.
This is module one and in the module one we shall be dealing with function of building,
requirements of functional requirements of building, types of loads and role of materials.
The course is on materials and construction. The first thing we got to understand is the
role of materials and to understand the role of materials, it is necessary to understand
what are the functions of buildings and structures and that is why, this module includes this
four components, okay. So, general outline of the course, we can
look into like functions of buildings and structures, as I mentioned just now, followed
by types of loads and structures, then functional requirements of buildings, because load is
one issue in case of buildings and structures. But there are functional issues, so we shall
be discussing functional requirements of buildings, and then from this it will follow automatically
the role of materials. So, starts with function of buildings. What are the functions of building? Function
of building is to provide a desired spatial environment within it continuously for a given
human activity. This space may be controlled or uncontrolled. Controlled example is an
air conditioned building where the temperature, the relative humidity, etc are controlled.
Uncontrolled is any ordinary building where no temperature control, no humidity control,
etc, etc. are actually done. So, this is what is the function of the building?
Now, we will, building must provide this environment. After all, building is supposed to provide
as a space in which human activity will go on and must provide continuously an environment,
which is as we desire. Now, this must be safe. Safety is the first requirement. It should
be safe against all kind of natural forces and also from man-made forces. So, this space,
that we are talking of, that is, the building, must be safe and comfortable. It must provide
for safe and comfortable internal environments against existing external and unwanted internal
conditions for any given human activity. Now, when you mean human activity, human activity,
you mean, for example, in a classroom it is the teaching. So, that is the human activity.
In a factory, possibly, production of certain items in a living space living is the human
activity and so on. So, human activity, we mean this.
And this must be done quite economically. We cannot go on to do any amount of expenditure
we like, to construct such a space. So, we must do it with economy. This is the difference
between a monumental structure and modern functional building. Monumental structure
like Taj Mahal was constructed by a king and he was not bothered about the monitory implication
of such a construction. But modern functional construction must be done economically and
also, within a reasonable time. So, both these issues of time and money is important in modern
functional buildings. Since we said it must be safe and safety is
most important, utmost in buildings or any structure for that matter, since we have said
it should be safe, so let us define the safety first. Safe against forces of nature that
is what I said safe against forces of nature. For example, well, anything on the earth has
the tendency to go down. Therefore, against gravity load, against, you know, that means,
for a building or a structure it must stand erect, it must stand erect, should not collapse
on its own weight or something like that. Then, it must be safe against wind, safe against
rainfall or rain forces, due to rain or snowfall and also against earthquake or such kind of
natural forces that exist. Besides there could be some load due to human
actions. There could be some loads due to human actions. For example, if there is the
factory and there is continuous impact is imparted onto the floor, then that floor must
be safe against such impact, or if you are putting lot of furniture in a space, then
it must be, the floor should be able to withstand or hold down to those furniture or human being
or and so on. So, these are the loads that are coming from human being.
So, there are, what you have seen, it must be safe against force of nature. One is from
gravity that includes all the loads that might come because of human being. An impact is
one, which is additional human actions that puts the additional load and wind and earthquake
load, etc. Two types of load, therefore, vertical load, gravity load is the vertical
load, so gravity loads usually and then horizontal loads, which might come from wind as well
as earthquake. Now, there could be other loads and certain
inclusion and so on, but generally the natural forces are like this. Natural loads are of
this kind, the forces, that you know, natural forces that actually leave load, imparts load
to the building or to the structure. So, you call them loads, okay.
And the function of the building or the structure is to withstand these loads safely. It must
withstand these loads safely during its designed service period, for example, 50 years or whatever
it is for a bridge it may be 100 years. So, it must withstand these loads for its life,
designed life, whatever you conceive as a life. So, that is how we define safety. Well the issue of safety we have already talked
about, let us see the other issue. We said it must provide for a comfortable environment.
Now, comfortable environment, internal environment against, what? Against external environment,
and what are the component of external environments? Let us see. For example, humidity is one of
them. Then, you have got air motion, air motion; noise, that is of course a human creation
as come. Then, light hitting this, again the force of nature; pollution,
a human creation; rain or snowfall, totally it is known as precipitation; then, sunlight
and air temperature; solar radiation. So, the external environment with which the building
interacts are these. Now, some of these ones are desirable. The
building must act like a filter. It should allow for some of those desirable aspect of
the environment to come in. For example, natural light, sunlight, in a school you like to provide
some amount of natural light. Similarly, natural ventilation, air movement. So, you would like
to provide some amount of air ventilation, but at the same time you like to cut off some
other things such as excess temperature, which can generate heat within the buildings.
Say, in a summer condition in a tropical place, tropical location, the building envelope should
provide with kind of a filter such that the heat radiation does not enter into the room.
But that sunlight comes in not direct, diffused sunlight, which does not create glare to the
eye, should come in, so that you can comfortably see things. So, building acts like a, like
a filter. It allows for certain good things to come in, does not allow for certain undesirable
thing to come in. So, that is how it interacts with the environment, surrounding environment. Now, comfortable environment. Therefore, comfortable
environment, it must provide this space within the building, must provide for a comfortable
environment against natural external environment, like I mentioned just now, temperature, relative
humidity, in case of moisture due to rain, condensation, etc, etc.
Condensation is the phenomena more common in western climate, not in Indian situation,
except for possibly Jammu and Kashmir or Ladakh and such areas where temperature within the
space is relatively high compared to outside. So, a temperature gradient exists across the
wall of the building or the section of an envelope. Envelope is wall roof, etc,
etc, which is cladding, which is the outer shell.
So, temperature gradient may exist between the inside air and the outside air across
the wall. And wherever temperature is lowering down, the air, that contains moisture might
certainly contain down within the wall. So, that is, the condensation is the phenomena
not very common in Indian situation, but moisture ingress is more common that may come because
of the rain penetration because of ingress of moisture through seepage or ground water
by Kepler reaction or may be due to leakage of pipe etc.,etc . So, these are
some of those external environment. Some of the man-made environment, like I showed
earlier, noise is one of them. The noise is a man creation of industrialization. Well,
if you look at it in a very, very rural area, very tranquil rural area, the noise pollution
level will be very, very low. Practically, no noise will exist. A lit bit of birds, you
know the sound or bird singing and so on. So, it is a tranquil environment in rural
area. If you look to an urban area, the noise from traffic, the noise due to industrialization
and so on. So, discomfort can arise from such noises. Therefore, the internal space mentioned
for specific activity. For example, a classroom, it should be free from external noise.
So, noise is another man-made one and the space within the building must provide for,
must provide for, must provide for a comfortable environment against noise or similar other
man-made agency, let us say, air pollution. Pollution is, again air pollution from industry
or traffic, vehicular pollution and so on. These are again man made things.
So, the space within the building must provide for comfortable environment as desired, depending
upon the activity. For example, you know, just living, you may not require high level
of comfort. But if it is an, if it is an industrial situation, for example a factory that is making
electronic chips that would require a very high amount of dust, temperature and humidity
control, etc . So, desirable environment is that how, what
kind of environment you require for that particular activity. For example, somewhere in a watch
repairing area, you might require a very high level of lighting. Visual comfort must be
very, very high and so on. So, man-made and external environmental perturbation, which
are there. The environment within the building must provide for a comfortable environment
against such both man-made as well external perturbation or disturbances.Well I must mention here something about micro-climatic changes because these are overall global climatic
changes or in an area one can look into, but locally there may be some changes in a particular
site. For example, an urban area is always at a much higher temperature than the rural
area just because in an urban area there are a lot buildings and these building have their
thermal capacity and therefore, they store it. So, the temperature in urban areas is
slightly high. Besides most of the urban areas are piped
and in piped areas there is, the moisture does not, moisture does not stick to that
place and no evaporative cooling takes place. So, temperature in piped areas, again temperature
is higher because surface water will run off. The water will not remain there for evaporation
in future and evaporation process, as you know, results in cooling.
So, micro-climatic changes of the, changes, that is caused by the local topographic change
that has been brought in by construction of or by human activity. The tall buildings usually
obstruct air movement and therefore, they would cause micro-climatic changes. Micro-climatic
changes could be there and therefore, while looking at the building, it must provide a
comfortable environment against any kind of micro-climatic changes that has been caused
by local human construction. So, this is another issue one may look into when thinking of comfortable
environment within the space. Well, I have already mentioned the examples
of man-made and natural, you know, external environmental conditions and these are, say,
external temperature, as I mentioned, relative humidity, solar radiation, air movement and
heavy storm and rain, gusts, snowfall, etc, etcetera .
Temperature, as you know, if it is high temperature, the deep body temperature should be maintain
at 37.4 for a comfortable situation. If the temperature is changed very much, let us say
small, not very large, let us say a degree or so beyond 37.4 within the body, beyond
the limit even it could be fatal, but you start feeling discomfort as the temperature
goes higher. And the inside temperature actually depends upon the surrounding temperature,
therefore comfort compute condition is the function of the temperature.
It is also a function of relative humidity that you can feel, say, in cities like Chennai,
Kolkata or even in Delhi where during July, August, the month of July, August and so on,
after the rainfall a lot of humidity outside. So, although temperature might be lower, you
still might be feeling discomfort because the relative humidity is very, very high,
especially if you are away from the fan or similar sort of places where there are some
force cooling takes place. So, humidity and temperature, together with
solar radiation, air movement, etc., etc., are the external environmental conditions
against which you must feel the building space must be, within a building, within space a
person must feel comfortable. The man-made, I already mentioned, there is
noise, fire, accidental initiation of fire; fire is a very important thing. This is not,
this is not necessarily causes discomfort, but it can be related to safety and so on,
and waste, odour, fumes, etc. These are the examples of man-made and natural environmental
conditions against which the space should be comfortable. Well, continue with the same. So, desirable
internal environment may require when provision of good lighting because lighting was not
talked in previous slide, so we just now decided to talk about lighting.
So, lighting is another, another thing, visual comfort. Now, if you are trying to write there
is particular amount of lugs or illumination level, you require. If somebody is repairing
watch, for such working in small details one requires certain amount of visual comfort
and therefore, the illumination level should be accordingly there.
Circulation is the other issue, comfortable movements. So, the space should be such, the
building space, the space in the building should be such that there is adequate space
for movement of people. If it is movement of material, then there should be adequate
space for movement of material or movement of any other equipment and so on. So, therefore
circulation is an important issue. Visual comfort is another important issue while dealing
with planning of building. We should look into more details into these issues sometime
later on in this lecture or in another lecture in the same module. Well, so far I talked mostly about the buildings,
but what about other structures like let us say, bridges. Well, function of the bridge
is to allow safe and smooth passage of traffic over it, so that is the function of a bridge.
So, it must allow for safe and smooth passage of traffic over it. Similarly, if I look at
other structure, let us say, dam. Function of a dam is to allow safe storage and distribution
of water. Now, these functions, therefore, dictates
what should be the size of the structures, what should be the overall dimension of the
structure or building for that matter. We shall look into building planning, as I mentioned
a little bit earlier, that we will be looking in details. But for any structure, the functional
issues dictate the overall dimensions. So, we get the overall dimension to functional
design, right. So, first we do functional design.
And how we get overall dimensions? For example, if it is bridge, suppose, to provide crossing
over a river 1 kilometer long, then the bridge length would be dictated by the, you know,
width of the river is 1 kilometer. Therefore, accordingly the length of the bridge would
be dictated, the width of the bridge will be dictated by the number of lanes you want.
These are the issues related to functional design.
As opposed to this, well, in case of building also space would be decided by its functional
requirements. As opposed to this, the structural design, you know, design of any buildings
and structure, we can divide into two parts, one is functional design, another is structural
design. Structural design, essentially, looks into
the size detailing because it is mainly against safety, you know, mainly for safety against
loads, forces of nature. Therefore, it looks into size detailing. The example of the same
bridge if I look into, the length of the bridge, the width of the bridge, these are decided
by the functional requirements. Whereas, what should be the size of the or depth of the
girders in the beam, that size detailing, what should be the thickness of the deck,
that will be decided by structural design. How many piers you need, it would be decided
by economic conditions, besides also whether you need navigation below it or not. If navigation
is required in case of bridge, the level of the bridge has to be higher than possibly
a bridge, which does not require navigation. So, this is the part of the functional design.
What should be the height of the bridge above the highest flood level, highest water level,
how many piers should be there because the space between the piers may be dictated by
navigational requirement besides of course, economy and soil condition and so on. Those
also dictate those. But functional design dictates these issues. The type of bridge
then would be selected based on this. Now, structural design would find out what
is the dimensions of each of the component of this bridge structure. So, bridge structure
might consist of the piers, so dimension of the piers would be decided upon how much load
it is going to carry, that is, by structural design.
The deck system and deck system would again, would depend upon the, depending upon the
type of deck systems through structural design, what load it is going to carry. Based on that
how much is the thickness of various components of the deck systems that would be designed.
So, two types of design when you talk of designs of buildings and structures.
We have two steps in design, one is the functional design and other is the structural design.
Functional design deals with overall dimension; structural design deals with size detailing,
you see overall these dimension and size detailing. So, that is what is the part of the design? Well, then we can look into the first structural
design, the safety, because we said, that safety is more important than the issue of
comfort, so you look into the issue of comfort later on. So, in this part of the second part
of this lecture let us look into what are the types of loads and safety against loads.
As I said, safety is again various kinds of loads, we call it loads. Loads are basically
the forces; loads are generated from the forces. Three type of loads you can define. One is
steady, that is, it is there all the time, it does not vary with the time and I can determine
it relatively more precisely. So, what we call some loads are steady.
We call some loads are steady, some loads are some type of loads, that is, due to self-weight;
it is self-weight. First of all, the structure must bear its own weight. First of all, if,
if the structure has to carry some other load, first of all it must be safe itself. It must
be able to carry its own load. Now, that kind of load is very much easy determinable. We
can find it out, there is no problem about it, because if I know the dimension of the
structure, we will be able to find it out. We will see that somewhat in details and that
load do not change with time. We call this load as a steady load. We call this load as
steady load because it is steady with time, it is not, it is time invariant.
Now, second type of load is, some are deterministic, but may change with time, say, load is due
to furniture occupant or machinery, etc So, in this classroom or in a class, people
come and then they go. So, this is not permanent; this is not permanent.
But supposing, you know, this is not permanent, so it is there. It varies over the day or
you know, depending upon the human activity cycle within the space it might vary within
the day and then go on changing over the, over the day itself. Suppose I change this
classroom, after 10, 15 years change it to something else, may be, may be some other
space, into a laboratory space. So, it is the loading pattern. People, they were coming,
number of people coming would change. So, therefore, its pattern on loading may change
with time. It is not steady, changes with the time.
And this load also we can determine, somewhat, somewhat relatively, with relative more uncertainty
than the earlier load, which are permanent, but we can still determine them in some manner.
There is of course difficulty, as you shall see, but these are not steady with time. They
are not same with time, therefore there is quasi steady. Once it is a classroom it will
be there. It will be there for it will be the classroom or its function as a classroom
will be there for at least 10, 15 years of time, there for a very long period of time
and therefore, it is steady for a long period of time. But after that it may change because
furniture itself may change, because lifestyle changes.
If you see earlier buildings, very old building, you will see very heavy timber furniture.
Today, you come to a classroom, many classroom, you will not find timber, timber furniture.
Instead you might find light weight plastic or polymer composite furniture. Therefore,
the weights have, because of the change in style, the loads have changed.
So, these loads may change from time to time, so they are not steady. But over a large period
of time they are quite steady. We call them quasi steady. We shall see diagrammatically
a lit bit later. A third type of load are totally uncertain.
They come suddenly and therefore, very short period of time and then after that they would
not be there. So, they act for a very short period of time and that is why, you call them
as transitory in nature. They are uncertain because most of the time I will not be able
to find out their value precisely. I can find out, make some good guess about them because
otherwise, I would not be able to design a structure, but I can make good guesses about
them. But they are, there is a lot of certainty in finding out the value precisely, so we
call them uncertain. We shall see them some, some, you know, next slide and so on diagrammatically.
For example, as mentioned here, seismic load, that is, the load due to earthquake. So, it
is very difficult to, till without current scientific knowledge it is very difficult
to predict the earthquake, load the amount, you know, its amplitude or the amount of force
that is going to impart to the structure, we can find out with certainty. So, this type
of load we call it uncertain. This is uncertain and it is transitory, acts for very, very
short period of time. Earthquake acts for a very short period of time, it is not permanent
in nature, just there for few minutes or few seconds and that is the end of it. Wind load,
heavy wind load, gusts, etc, they act for very short period of time. Therefore, these loads are dynamic. They are
not steady, they change with time. They are dynamic and in the dynamic also it is actually
transitory for a very, very short period of time it is. Diagrammatically, we try to
look this load, it will be something like this. With time if I try to see variation
of this load, a steady load remains constant. As I said, self-weight, a steady load, it
will remain constant throughout the, throughout its life.
So, if this is the design, life of the structure, which is not there in the diagram, it will
remain steady all the time. It does not change the weight of the structure, does not change.
So long the structure is there, the weight is there, the mass is there. So, therefore
this is the steady load. Quasi steady load, as I said, well there are
small perturbation. As you can see, there are small perturbation. You can see over this
period, but it is, it is by and large the pattern follows and it is more or less constant.
But if the function of the structure change or if the furniture etc . change, for
example a factory building may be converted into a bank building. There are similar examples,
plenty or certain things is not economically viable.
Functional life of a building may change because life of buildings can be defined in, you know,
two, three ways. One is physical life, that is, the building or structure, it physically
remains, but functional life is when it changes function. For example, many theaters in recent
years have converted into shopping Centre. The structure is very much there physically,
but its usage has changed, so its functional usage has changed. So, it is being converted
into, a theatre being converted into, a cinema being converted into shopping Centre. That
is the example of change of functional life. Up to certain years it was a cinema hall,
so that is the life as a, functional life as a cinema hall. After that it becomes a
shopping center. So, so its function, now it has got a new
function. Now, the furniture would change. The number of people coming in, etc,
will change and therefore, the load that is semi-temporary, you know, semi-permanent,
that would also change. But once it starts, it is therefore some number of years. That
is what it has been shown. This is constant for some number of years
and after that it may change or may not change in many cases, but in some cases it may change
and it becomes higher. It can be lower also. And for example, in this period it is being
lower, these loads are not steady, they are not fully steady, but they are quasi steady,
but remain. Therefore, certain period of time and within that period of time also there
are small perturbations. For example, people will come and go, so human load. So, that
is what these loads are quasi steady load. And then, the third variety, which I mentioned
was transitory. It is over a very, very short period of time, as you can say, and it comes
and it is there and just vanishes, and it is dynamic in nature. So, it is, it is, suddenly
it comes. It may come once or twice or may be depends upon, depends upon chance. Most
of these loads are wind load and earthquake load. This depends upon chance. It may come
once in 50 years or a particular load or it may come once in 100 years or may come even
twice in 50 years and so on. Depending upon those it is very, very uncertain
because many a times so many factors, which control them, so many factors, which causes
them and good lot of it is even unknown today, particularly one related to earthquake. So,
that is not even known to us today and therefore, they are quite uncertain. But since we have
to design the building, we have to find out the load in some manner or other. We estimate
them. So, what you do? We estimate the load Now, these loads, as you can see, the steady
load is constant, constant over time. Steady load is constant over time, as you can, as
you can see, steady load is constant over time. This is varying with time within this
period also, but the pattern remains same for a long period of time and again changes
and this is sudden. Now, there is no, there is no fixity and there
is no fixed value with time. If I take, I have to take all loads into account in some
manner. In fact, the summation I should be taking for design of structures, some sort
of summation some way although the peak of all the loads do not act together as we understand.
But I have got to take account of all these loads in some manner, so how do I do it? Well,
in some way I do it and by doing this some, some based on certain some mathematical ways
as well as some experience and so on. We take this load into account, and then we find out
what is known as design load against which the structure must be safe.
So, design load is that load, which is used in design calculations and it is safe against
such a design load. So, long the load is lower than this, it is safe. Well, what happens
if it is higher? We will also look into that. We take this value design load in such a manner,
we take this design load in such a manner, that during the design period, the life period,
the structure remains safe against such design load. So, during this design, load design
period, during that service, you know or design period, the load that is likely to come, maximum
load that is going to likely to come is design load.
Well, slightly more complex it is than what I am just now saying when you go into the
details. But design load is, we can, for the time being the very first lecture we can understand,
the design load is that load against which I design the structure, which I used in design
calculation. You know, it is used for calculating the, making all design calculations and find
out the dimensions of the elements of structure or various components of the structure based
on this design load. So, that is one must understand at this point of time. Now, as I said, precise computation of actual
load that the structure is likely to encounter in service is difficult. You cannot find out
precisely. I can find out possibly steady loads fairly precisely, but when it comes
to even quasi steady loads it is not so easy to find them out what should be the design
load because it can keep on changing. So, there has to be some way to do that, and when
comes to transitory load, extremely more difficult. So, precise computation of such load at any
point of time, it is practically not possible. It is practically not possible. So, I should
have some way to do that. So, what we do? Well, we rely on experience
and also scientific knowledge. We rely on experience as well as scientific knowledge.
It must be understood here, the civil engineering is not totally mathematics based. There is
certain amount of experience always is involved because we are dealing by and large with natural
system. You are dealing by and large with natural material like soil quite often, or
slightly processed materials, let us say, like concrete and sometime, of course, well
processed material like steel. Most quite often also we deal with steel or even sometime
polymeric materials, polymeric composites and so on.
But we, quite often we deal with natural materials such as soil or slightly processed materials
as I call it, relatively more processed, but not as processed as steel, man-made material
although it is, but processed, less processed than steel, so that is concrete. So, when
you are dealing with such material and then you want this material to perform against
natural forces, which are not known to us, not within our control, so we cannot rely
totally on scientific knowledge. The experience is an integral part of civil engineering design
or on the standing level. Experience of people accumulated over years
that is used in civil engineering design. For example, you know, whether it is Egyptian
pyramid or Qutub Minar, Taj Mahal or similar other temples of south India and so on, these
were built even before modern scientific knowledge was available. By empirical observation and
by simple experience people have built those structures. So, this experience earlier were
never recorded in systematic manner, but these days we have systematic way of recording those
experiences. And therefore, in civil engineering design and practices you will find out, that
all the time something called code of practices referred to, code of practice.
And there are organization or institutions, which actually maintains those code of practice.
Say, for example, Indian standard code of practice is developed and maintained by Board
of Indian Standard. Since there are a lot of experience is involved, therefore it has
to be written down somewhere for anybody to use. And by consensus of people come out,
come to in a large number of gather together or committees are formed. They put down the
experience of other people and their own into booklet. Those are known as code of practice.
So, loads are based on, loads are decided on the basis of experience and therefore,
loads are decided on the basis of experience and therefore, they are put down into the
code of practice. And also, currently acceptable scientific knowledge, wherever we can add
scientific knowledge we add that. Based on scientific knowledge plus the experience of
people that is codified, that is put into practice and we write it down in books called
Codes of Practice. So, how to find out design load? We can do
into codes, which are available. In India, of course, we look into Indian Standard Codes,
as they called, and it is maintained by Board of Indian Standard, the Government of India
body. And it defines different types of loads, the
classification of loads as per Indian Standard. I will just mention now one by one. Dead load
is the first one. It is one load I will define what it is. And the corresponding code is
IS 875 Part 1. So, these are, they have got number. IS stands for Indian Standard and
they have got numbers 875 Part 1 is deals with dead loads.
Then, there are some other load called imposed loads. I have mentioned it earlier there are
three types of loads, but for practical usage while finding out how much is the load I should
use in design, it is more possibly just by defining loads, their steady, quasi steady
and you know, transitory is not sufficient. Possibly, because each of the type of load
I must determine the design loads, I must determine separately.
And therefore, from that point of view the classification of the loads are: dead load,
imposed load, then wind load, snow loads, earthquake load, special load and load combinations.
And the corresponding codes are shown by the side of it. For example, IS 875 Part 1 deals
with dead load. What is dead load, I will just mention. Imposed load is IS 875 Part
2. IS 875 Part 3 deals with wind loads and IS 875 Part 4 deals with snow load.
Earthquake loads is dealt specially, because it is special in nature because of its uncertainty
and also the importance it is given to because calamities failure due to earthquake or even
fatalities due to earthquake is much more. Then, seismic load, I mean, that is same as
the earthquake load. Then, special loads, now loads like fire, temperature, etc., etc., these are encountered in special loads.
So, question is now, of course, what is, how do we define what is dead load, and so on.
We will come into come to it. Dead load is the load that is permanent to the structures;
dead load is the load that is permanent to the structure that is given in IS 875 Part
1, as I just mentioned. So, dead load is the load, that is permanent to the structure,
therefore it would include the self-load of the structure itself.
Load of all finishing, which remains there in the structure all the time, does not go
away, so it is a steady load. So, this is what is called dead load, dead, you know,
it is not alive. Alive load is, on the other end is the one, which can change, but dead
load does not change and therefore, we call it dead load. How do we determine it? Well,
we will see in the next slide how do we determine it. It is very simple because if I know the
dimension of the structure, then I can find it out very easily.
Imposed load is, on the other end, is imposed over the structure itself. It is not permanent,
but it is on those, those kind of loads, which is quasi steady. You know, it is that load,
which is quasi steady. So, imposed load is that load, which is quasi steady, which comes
into the structure. So, includes the loads from the furniture, loads from human being,
load from any other thing. So, imposed load would come from what has been imposed over
it by human being for its, for its activity and so on. So, this is what we call as imposed
loads. Wind load is very simple, it is coming from
the wind and I shall, I shall mention something more about it, how we find out in the next
slide. Snow loads are coming from the snow, as the name suggests. Earthquake loads coming
from the earthquake and special loads and load combinations. One thing is load combination.
I said, special load includes load from, let us say, temperature because of temperature
changes. And you know, you know, in tropical country
like India we have some portions, which are desert, dry desert in the western, north-western
part of the country places like Jaisalmer, Jodhpur, Bikaner and so on, those are hot
dry desert sort of climate. So, temperature variation during the day and night is very
large; over 24 hour duration cycle, the temperature variation is very, very large. And you know,
it can vary from, let us say, 48, 49 to about 25. So, about 25, 30 degree centigrade diurnal
variation, as we call it, may be there. Also, there is a large season of variation.
If I see the yearly cycle, in winter lowest temperature is fairly low in such places,
could go as low as about 2, 3 degrees or you know, of that order. So, maximum temperature
is 50, minimum is about 2, 3 degree centigrade and on top of it, the structure might receive
direct solar radiation. So, temperature fluctuations could be very large. So, when such large fluctuations
are there, the material expansion And if the material is not
allowed to expand or contract, there is a restraint to its movement. Then, this induce
some amount of loads. So, temperature load is one of the special loads.
Fire can cause some certain amount of load, so these are other ones. So, such special
load is coming from, let us say, soil or similar, you know, in a retaining wall load coming
from soil and so on. So, those are special loads, but besides that something called,
something called load combination is also an important issue.
How do you combine all these loads to calculate out the design load because all loads are
not going to act together? And especially, they will not act together at their maximum
big value, so all loads do not act together especially to the big value. Therefore, how
do I combine these loads? So, these are dealt in with a special, you know, especially in
this, this particular code IS 875 Part 5. So, IS 875 Part 5 deals with this and this
is how, you know, combine these loads. That is what we talked about in this particular
code and we will see how it is done little bit later. Well, calculation of dead load is pretty simple,
as I said, because it is deterministic, I can determine it, number one, and it is constant
with time. So, there is no much variability. Once I calculate out its value, that is my
design load. How do I calculate out since it is permanent feature of the structures,
size of the structure itself, its self-weight that contribute to the dead load? So, I got
to know its dimension. If I know its dimension, what is the sizes, length, breadth, width?
Then, I can find out its volume and volume multiplied by the density of that particular
materials that has gone in for making. For example, if it is steel, simply 7850 kg per
meter cube is the density, that is known to us, multiplied by the dimension of the structure,
dimension of the structure itself. That will give me the dead load that is supposed to
carry. I might express this load in terms of kg per meter length of the material. So,
it is the, in that case, it will be the area multiplied by 1 meter length, that gives me
the volume multiplied by the density. I am not going to detail calculation, so I
am not really going into how detail, you know, detail calculations are done, but I am trying
to give you an idea what is dead load. So, dead load and how easily it is, you know,
why we separate dead load out from the rest because it is very easily calculable.
Thickness of the finishing if I know, I have structure made of concrete, then I know its
density is 2500 kg per meter cube, the dimension is not, is known, then that dimension will
give me the volume multiplied by the density, that will give me the weight that it has to
extent from its self-load over the thickness of any finishing item I have used. For example,
the flooring in a floor. So, if I know what is the thickness of the flooring and the density
of the flooring material, I can find out easily. Now, this particular code, which I mentioned
earlier, that is 875 Part 1, it gives you density of varieties of types of materials,
which are used for construction, so that you know because steel, there is quite a bit of
certainty, density quite a bit of certainty. But concrete, its density is not as certain
as steel because it might vary a little bit, plus minus 50 kg per meter cube from 2400
or 2500 kg per meter cube. So, what value I should take? I might take 2400 and student
like you might take to be 2500 and some of my colleague might take it to be 2600. So,
then it becomes problematic thing. 100 kg per meter cube difference and multiplied by
the volume, it can lead to all sort of variations. So, codes gives you the value that should
be used. So, density of the, density of the concrete is given in the code, so the value
is specified in the code and therefore, as a practice you use that value because it has
come through wisdom of many people who have designed this code or written down this code.
So, wisdom of people are actually, actually written down and codified in codes. And therefore,
what should be the density of the particular material you should use?
Concrete is relatively more easily, more widely used, therefore it is known, but many finishing
materials, their densities, are not so well known, readily not available in anybody’s
minds. So, you can refer to such a code and get that density values and use the dimension
to find out the . Next is, of course, the imposed loads. Now,
imposed loads, I said that it is a quasi-steady load and it comes from furniture, human being,
etcetera, etcetera, therefore imposed load desired in on building depends upon the type
of the building and this type of building is classified in what is known as occupancy
classification. Occupancy classification, for example, this occupancy classification,
you know, depending upon what kind of activity would go on, depends based on that the occupancy
classification has been defined and this is given in national building code of India.
It is another code, national building code of India, NBC, and in that buildings has been
classified into let us say, residential, then industrial, institutional. It is very simply
understood, load would vary upon whether it is residential, the number of person per unit
area in a floor is the function of type of occupancy.
If it is residential, you will have possibly, may be, 1 or 2 person come. But if it is the
classroom, then, then the number of person sitting within 1 meter square area could be
relatively more or total area in a, say 20 meter square or 30 meter square area, number
of persons would be occupying the space at certain point of time would be much larger
than possibly, residential area. In an industry, of course, the machine might
come, so the flooring has to be designed for such machine loading. Therefore, depending
upon the occupancy the impose load has been given in the code for various types of room
that is possible in such a occupancy. For example, in an industry, industrial flooring,
the impact also to be taken into account. So, what should be the load for such impact?
How much percentage increase you should have? If you want to consider the impact load as
an equivalent static load, etcetera, etcetera are given.
So, as I mentioned, in a classroom, what should be the load in an institutional building?
You will have classroom, the residential occupancy will not have a classroom. So, in institutional
building what are the possible rooms that are given? And corresponding loads are given
in the code 875 Part 2. And imposed loads are based on experience of people because
you cannot, weight of mass of person or student would vary, mass of furniture would vary.
So, many variabilities are there. How much load do I take, such loads have been
actually, such loads have been actually arrived at what should be the design load by consensus,
by experience of people and then by committee sitting or large committee sitting, different
countries, and then you know, the varieties. For example, in India, there is a committee
in Indian Standard Board of Indian Standard has a committee for deciding upon the load.
They look into the other codes of other country say Canadian code or American code, American
concrete institute code or British code and so on. Their experience are also taken in
and put together with our own experience and this load is decided because I cannot find
out precisely. So, by consensus these loads have been arrived at.
Now, one more important point I would like to mention here is characteristic load. Now,
the load I should design, I should use in design it should be such, that by and large,
this load should not be exceeded during the service life of the structure. This load I
call as characteristic load. A characteristic load is that load, which
will not be exceeded more than 5 percent of the time, 95 percent of the time the load
would be lower than this load. Only 5 percent of the time load can be exceeded. You see,
it comes from statistic. The 5 percent of the chance you can take, otherwise the load
that would be taking is very, very large. So, characteristic load, as we call it, it
is that load, which will be exceeded only 5 percent of the time, 95 percent of the time
load is not exceeded. And how much this load I should take, that is codified.
And how did it come into the code? Experts sitting together, by consensus they arrived
at this load. In many countries these loads are there and for various codes, there are
worldwide different codes, there are international codes also, but various codes are there and
they are experienced of all the people, experts all over the world. They are taken into account
deciding upon the load that is to be taken in a given country. So, in our IS code also
it has been formulated similarly. And these values have been decided. 5 percent of the
risk you take, and then of course, that is not the end of it. I do not design only for
that, but I multiply this by factor called factor of safety, which I shall discuss sometime
later on. So, imposed loads are decided like that.
Then, it comes to wind load. Well, when it comes to wind load, we design mostly, most
commonly, you design it taking it as an equivalent static load, static force, horizontal force.
I mentioned horizontal loads earlier. So, horizontal equivalent horizontal force if
I consider and which is static, which is acting all the time because it is, it is, it is relatively
mathematically slightly more difficult to handle dynamic load, which is changing continuously.
But today, it is all possible and also dynamic analysis of structures are done. I will just
mention this later on, but this is not purpose of my lecture.
Anyway, I just want to tell you what are the types of load and how one arrives at design
load, a little bit introduction to it because this course is more on construction, on the
material and we are trying to look at the role of material. There are courses on design
of structures where loads, detailed estimation of loads have been taught, higher level courses
are there. So, wind load, first of all we found out equivalent
static load and that depends upon design wind speed. You see, from basic fluid mechanics
you can find out there is the force or the pressure that would come in, is a function
of the velocity square density and velocity square half rho v square, that is what, you
know, I can show that pressure is equal to this, that would come from basic fluid mechanics.
Well, I am not going into details of this. So, it depends upon the velocity, it depends
upon equivalent static load, depends upon basic wind speed.
Now, basic wind speed at a given location I got to find it out and that comes from metrological
data. Data on wind speed at particular height, usually at 10 meter height, this has been
collected all metrological stations all over the country and 50 years data, you know, last
year, many years data is taken into account. And the wind that would be coming, probability
of wind coming in given year, 0.02. Well, some basic statistical statistics are taken
in account and the basic wind speed with 50 years of return period. This 50 years of return
period is, that is, probability of finding, that wind speed once in 50 years that is taken
into account and that particular wind speed is recorded down again in the code. The code
gives you basic wind speed values. If you are studying a course on design against
wind load, then details of these would be told you at that point of time, but for the
moment we can understand, that wind load is calculated based on basic statistical that
are available to us for last so many years. And taking that in account, the wind load
that would be exceeded once in 50 years, that is, with 15 years, 50 years of return period
is taken as basic wind speed. This basic wind speed is used together with terrain topography.
If it is undulated ground, the forces would be one kind, what is the height of the structure;
that is another issue. If the height of the structure is more, the velocity would be different
and therefore, the equivalent static pressure or equivalent static load is calculated based
on basic wind speed location, terrain topography, height and risk factor of the structure, etcetera
., etcetera . More details I think is unnecessary for this course. So, this is how the wind
load is calculated. Snow loads depends upon, well just going back
a little bit and this, how you calculate the wind load that is given in code 875 Part 3.
So, the formula, equations, etc., is given there, right. And snow load, that is
given in Part 4 of the code I mentioned earlier, that depends upon how much snow is likely
to accumulate over the given surface. What is the slope of the surface? If it is
the flat surface you will have more, you know, snow load on to it compared to possibly inclined
surface. So, based upon inclination of the surface, shape of the roof that is what I
mentioned, shape of the roof and amount of snow accumulation in a particular location.
One can calculate snow load and that is what is given in the particular code, right.
Where it is do not available, where the data for a particular location is not available,
one can approach appropriate body, appropriate government organization, which keeps this
data. Code gives you the values wherever is available; wherever is not available, it also
suggests, that you can approach the appropriate body. So, snow load is calculated on this
basis. Earthquake load is more complex to calculate
and it is much more complicated. In fact, well, again it is conventional design based
on calculating an equivalent horizontal load. You know, earthquake is nothing but ground
motion, both are horizontal and there could be vertical component also. It, in, in fact
it is random motion. I am not again going into details of this, but then, one can calculate
out an equivalent horizontal load for design purposes and that was also done earlier in
course and even now this is allowed to be done.
But how much is the equivalent horizontal load? That depends upon importance of the
structure types of foundation, which is zone, whether it is earthquake prone zone or not.
For example, this country has been divided into part five earthquake zones, zone 1 to
zone 5 and zone 5 is the most seismic prone. So, based on those flexibility of the structure,
etc, you can calculate earthquake. Dynamic analysis is done for wind and earthquake
load. So, you can do dynamic analysis to find out what is the, how much is the wind and
earthquake load. That means, you can take into time variant and nature of the earthquake
motion, its down motion, exact more details you can calculate out today with the advent
of computer and so on, and modern mathematical tools those are available.
So, experts on earthquake engineering, they do this job. And if you take course on that
subject, designing a earthquake load, then you will be coming closer. But this particular
code, 1893 tells you how to take an equivalent static load and a little bit of some sort
of, you know, the process that is involved in it and the factors that are involved in
earthquake load calculation. Well, lastly in the load type, special load,
I mentioned the temperature is one of the special load because diurnal variation can
be very large, seasonal variation can be very large, that can cause expansion of the construction
of the material. And if the material is not allowed to expand and contract, then it can
result in temperature load. So, how do we take this into account that is given in type
5 of the code 875. Then, you know, material like concrete, the
string there is shrinkage. So, those shrinkage loads, moisture, fire, etcetera, hydrostatic
pressure due to water, soil pressure and those are given.
Fatigue is one thing, which I mentioned. Fatigue comes from repetitive loading. You might have
experienced, you might have seen, if you take a small wire and turn it, bend it like this,
after sometimes it breaks, although you have not applied very high load, but it has broken
in fatigue. So, that is called fatigue load. These are special loads. In the bridge there
is always a reversal of stresses because a vehicle comes, then it goes away in very quick
short succession. So, always there is reversal of stresses. The structure goes down and comes
up, so that is called fatigue. So, these are special loads.
How you take account of fatigue load, how you take load during construction or impact
of collision, you know, because of certain situations. So, those are taken into account
in this particular code. This is the special load and these are taken into account in,
in the last code, you know, how you take into account. These are given in the code 875 Part
5. Load combination, as I said, all loads do
not act together and therefore, how do I combine this load to find out design load that is
also given in this particular. So, I think, this we will conclude this lecture. In the
next lecture, additional aspects of load and load type, we will mention whatever was left,
and then we will follow up the other aspects of functional requirements of building and
role of materials, which is part of this module in the next lecture.