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Safety in Construction
Prof. Uma Maheshwari
Department of Civil Engineering
Indian Institute of Technology, Delhi
Module No # 02
Lecture No # 10
Full Hazard in Demolition Works
The next topic wherein fall hazard is very high ok that topic is primarily demolition.
(Refer Slide Time: 00:28)
So demolition also works in the same way like construction but in the reverse process. The only
difference between actual construction and demolition is exactly in the reverse like a
construction in terms of the sequence of operations. But demolition has the same hazard as with
the original construction. And also demolition has additional hazards also in apart from
construction that should be you know understood.
Now what is demolition? Now we will start discussing about demolition and the hazards here
demolition is the dismantling destroying or wrecking of any building or structure or any part.
The reasons can be any age structure that has lost its equilibrium then you and it shows early
signs of collapse. Then has to be demolished or a structure is partially collapsed due to any
disaster or acts of god then it has to be demolished. Or may be for want of space which is the last reason a structure may sometimes be demolished.
Sometimes a wrong foundation design or failure in excessive settlement of the foundation and a
structure is not suitable for its lifecycle. Then the structure has to be demolished overloading of
structure excessive corrosion other issues which makes the structure unserviceable for all these
reasons a structure has to be demolished.
(Refer Slide Time: 01:54)
Now how do you start with the demolition? So safe planning on the demolition work so
whenever you want to start the demolition work it is not that you go to the site and start no
collapsing the whole structure. It is not done in that way first the area has to be studied ok. The
adjacent structure all in and around building which has to be collapsed has to be first analyzed so
when you talk about the adjoining structure.
So before the starting with the actual demolition so all the adjoining structure has to be first seen
their foundations their stability and how far this structure to be demolished? Is in line with the
other structure? Everything has to be analyzed and then because the method of demolition also
should not be affecting the adjacent structures. So all those should be first cross checked. And
also suppose if there is a hospital close by and so on.
Then the nuisance effect of the demolition also should not be affecting the neighboring structure.
And all these should be kept as low as possible so these all should be kept in mind. So the
adjacent structure and the area around the structure to be demolished first as to be; thoroughly studied. Then the actual structure to be demolished also has to be thoroughly studied why?
Because that can be hazardous substances inside the structure or the structure can be partially
collapsed.
So the stability of the structure the way with which the structure is in place and so all these has to
be first thoroughly analyzed. Because the; method of demolition all depends on the actual
structure. So indefinite plan of procedure for the demolition work depending on the manner in
which the loads of the various structural parts are supported all has to be studied. To access the
possibility of an unplanned collapse of any portion of the structure when the demolition happens
all should be ascertained.
And wherever possible if there is a side wall of the adjoining structure then you should be
thinking of the way to remove the member. So all these should be thought of and permanent
protection is really provided. When in case of an unexpected collapses in case any dangerous
anticipated to the adjoining structure then you have to vacate the structure. In order to you know
protect the life of the people who are occupying inside.
And also you have to determine the types of hazardous chemicals any gases explosives of
flammable materials inside the structure before you think of demolition. So those all has to be
assessed first safe precautions before the demolition work. And as usual danger signs should be
conspicuously posted all around the structure. The area should be barricaded and no entry of
children public should happen.
And also any worker except the actual worker required for demolition work should be allowed
inside the barricaded area.
(Refer Slide Time: 05:10)And provisions should be made for at least 2 independent exists even though you are fully
barricading the duct area. At least 2 independent exist should be there in case of an emergency
for the work to leave the particular area. And even in the nights there should be red lights placed
on all the barricades and on the structure to know that there is a demolition which has happened.
Sometimes you collapse a structure and after you know sometimes it can be partial collapse
continuing to happen.
So if the area has to be warned of to the public that the demolition has happened and there can be
some unexpected evenings which may happen. No unauthorized person shall enter the site of
demolition outside the office hours. And even authorized people are not supposed to you know
go unnecessarily into the site. There should be watchman posted at tall exit points to prevent the
entry of public and other unauthorized people.
PPE should be supplied to all workers and their use also should be enforced and they should
know when to use what PPE? And everything should be known to the workers very clearly based
on the type of demolition and based on the structure with which they are going to be, demolish it
should be known to them. Now before you start with the demolition again some technical issues
what you have to do is? Number 1 is service lines electrical wires, telephone lines, water pipes,
gas etc.Which is actually entering into the particular structure which; is to be demolished it should be
switched off or relocated or rerouted and when the actual demolition is in process. So that
unnecessary catastrophes should happen in while the demolition is going on. And prior to
altering or cutting of the lines the necessary approval should be obtained from the concerned
authority and then this should happen.
If a structure to be demolished, has been partially collapsed as a result of fire or explosion or
other natural disasters. Then the walls are damaged roofs should be properly shored or braced
suitably especially when you are going for a manual means of demolition. And the walkways and
the passage ways should be provided for use of workman. Who should also be instructed how to
use the walkways and passage ways and these walkways and passage ways should be well build.
Ok there should be proper elimination they should be inspected and they should always be free
from debris for the quick moment of material and on workers. All nails in any form of lumber
should be withdrawn hammered and they should be removed from the site. If there are loose
planks and something and they should be placed or piled up for further cleaning and burning.
(Refer Slide Time: 08:05)
The again some more safe precautions if there are glaze, doors or windows or any sashes. So
they should be first removed because all these fragile and loose fixtures they may be very great
hazard especially when you are doing the demolition. That can be a great hazard even to the public ok. And loose plasters also should be stripped off through the entire building. So that you
can actually reduce the quantity of dust which starts coming out as a result of demolition.
So advantageous to remove the glasses and plastering is to have glass breakage and other hazards
safety hazards and also to eliminate lot of dust which comes as a result of demolition. So all floor
openings and shafts which are not used for material chutes use should be enclosed guard rails
and toe boards as mentioned in the earlier class on what is guard rails.
Refer Slide Time: 09:03)
Public workers protection so wherever possible the public should be avoided from into the site
and workers should have a proper way if entry into the structure and exit. So accordingly there
should be a side walk shed which should be constructed. So if the structure to be demolished is
more than 2 storeyed or 7.5 meters height. Then the adjacent road should be closed and the side
walk should be constructed. So there are clear dimensions given on how the side walk sheds
should be ok as given in OSHA IS codes.
So the side walk shed should be at least having a minimum clearance of 2.4 meters for the height
of the sidewalk shed. And the toe board should be 1 meter high above the roof of the shed as
shown here. And the toe board should be actually facing outward or it can be vertical and the
when it is facing outward should not be more than 45 degrees. In order to cash the debris which
falls onto the sidewalk shed. The roof of the sidewalk shed should be very strong and capable of sustaining a load of at least
73 Newton per mm square. And the roof should be designed taking into account impact of falling
debris that also should be taken care. So this is a particular roof the decking of the support it
should be having closed panels of not less than 50 mm thickness and they should also be water
tight ok in order to have no leaking into the sidewalk sheds.
Whenever there is debris falling onto the sidewalk shed frequently the load has to be removed so
that the maximum permissible limit on the load above the sidewalk shed is maintained. So these
are all some of the tips on the side walk sheds. Sometimes a sidewalk shed may be a little far
away from the site to enter the actual site to be demolished. Then accordingly a canopy structure
can be installed to connect the sidewalk shed and your actual structure to be demolished.
So the canopy should be at least 8 feet or 2 and a half meter in width and it should be at least 2
feet wider than the entrance with which wherein the workers has to enter. So that whatever
falling debris are not falling onto the workers And the workers are ready safe on entering the
structure to be demolished.
(refer Slide Time:11:29)
So the actual demolition operations and as I said more number of precautions should be taken.
Especially in terms of safety for a demolition work as against a regular construction. Because a
demolition operation also; has the same hazard as equal to a normal construction work. And the only difference i said is the demolition generally happens in the reverse form as construction. In
construction you go from foundation the walls column floors and so on.
But in demolition you go from top floor to floor by floor down and then you go to the ground
floor and then the basement. So the demolition shall always proceed systematically storey by
story from top floor to the bottom floor and basement. And the demolition work should be
proceed in such a way that it causes least damage and nuisance to the adjoining building and
members of the public. And also it should satisfy all safety requirements to avoid any accidents
.
Now there are 3 types of demolition which you should know manual number 1 is manual means
mechanical demolition and the third one is induced collapse. And again the all 3 have extremely
opposite characteristics for example manual means is very slow in order to bring down a
building. Because you; use actually a concrete breaker or a pneumatic breaker driller for drilling
and then breaking the whole structure.
It is a very slow process but in terms of hazard levels it is not too much hazardous in terms of
dust pollution and so on. So in that form a manual means is a good way of demolition. It is still
applicable for smaller projects pneumatic drillers, concrete breakers are generally used for
breaking down a structure. The next extreme case is induced collapse wherein you put explosives
and through blasting you bring down the building in no second.
But if you see the noise the dust pollution and so on is too high in case of an induced collapse.
Then this method is generally used for high rise structures or where a structure is you know
creating lot of warning signals no failure or sudden collapse. It is better to use explosives and
bring down the structure in no second using a detonator. The feasible option among these 2
extreme cases is mechanical demolition.
And as the name implies there is a crane which has a ball steel ball and which starts moving and
forcing onto the wall and onto the building and as a result the building starts collapsing. So this
is not that slow like a manual means not that very quick like a induced collapse but it takes
moderate time for the demolition to happen. It is most widely used and it uses tools for crushing
concrete and also for sheering steel if you have an RCC structure also so mechanical demolition
is the most commonly used(Refer Slide Time: 14:34)
And we will discuss about safety precautions in mechanical demolition and as shown in the
picture. There is a crane with a swirl the swing motion with the steel ball and with that it acts
onto the building and the building starts to collapse safety precautions. No workers shall be
permitted in any area when using a crane headache ball or your clam shell is used for pricking
out all your debris.
So no worker is allowed to enter that premises and workers necessary to perform work only they
will be permitted to work in that particular area. And the area is supposed to be completely
barricaded to a minimum distance of 1.5 times the height of the wall. Accordingly the area has to
be barricaded because this may have falling debris on quite you know wide area. Mechanical
device should be so located that the falling debris is not actually falling onto the equipment and
the equipment is giving a failure.
The equipment shall not also cause any damage to either power lines overhead lines or any lines
which are above. And also to the adjacent structures which are close by and the workers engaged
in the demolishing job should always stand on a firm base. And the free ends of cut members can
also be used as a work platform. But they must be properly showed and in fastened when the
workers are using it as a base to stand.The ball must be attached to the load line with the swivel type connection only so that it is not
actually twisted when in action ok. The ball should go very freely onto the wall and coming back
and on to the wall so it should be a swivel type connection. So that it is not twisting and turning
off. During demolition continuous inspection by a competent person should be made onto the
structure whether the weakened portions are still stable or not. And what about the collapse on
the whole structure so that should be ensured so on.
(Refer Slide Time: 16:37)
Now the demolition ball should not exceed 50% of the cranes rated load and also it is based on
the boom length and the angle of operation ok. There should not be exceeding 25% of the line
strength and also this boom and load line should be operating as close as possible. And also you
should keep the falling debris is not affecting the crane and the equipment.
(Refer Slide Time: 17:05)Now removal of materials are chutes there are so many ways to remove the debris ok. That is
what is a term, we generally use on the broken pieces of a structure. This debris can be removed
from the structure through 2 means the 2 common means are one is chute and the other one is
your floor or roof openings ok. Chutes if provided has to be at an angle of not of more than 45
degrees.
This is a building this is a load from where you are actually going to put and this is your floor
opening. So this angle is what iam talking about ok this is a place for connecting all your debris.
So if a structure is if a chute is having more than 45 degrees if this is more than 45 degrees what
happens is? The slope is very steep ok and as such it should be closed on all on the upper side
also or all 4 sides preferably in this case it is a vertical chute.
Ok that is why it is fully closed ok its fully closed and the area is also put with a; debris net. So
that there is no fault hazard outside the particular area it is barricaded completely and if the chute
is at an angle of less than 45 degrees then primarily it is close to horizontal. So it can come with
a slow speed in that case you can have an upper side open chute also that is also possible.
Suppose if you are having a very steep slope in the sense more than 45 degrees then what
happens is?
The debris can come at a very dangerous speed ok and if you are having a broken chute with
broken lines. May be for example if you see this picture i brought to show the chute connections only. These are the different chutes here ok if the lines are broken what happens is it may also try
to damage the chute itself ok. So the chute should be such a way arranged that it is having a
continuous line.
But if you start giving a continuous line then it can come with a very dangerous speed at points
ok there should be a stops given to control the movement of debris and to fall onto the collector
place. In this case this is the collector place this is a place where in the first time no debris is
there and the debris is collected here and then removed frequently. So that should be a gate or a
stop should be provided at certain places and before the closing of the bottom also in order to
control the movement of debris coming out.
And then it should be removed from the chute and collected accordingly. Opening for the chutes
shall not exceed 1.2 meters in height. For example this is the lay this is the level at which the
chute is there and this is the floor level. So this height should not be more than 1.2 meters and
along the wall of the chute. And the openings in the lower floors below should be kept closed
when not in use.
This is not regular demolition work so it is open but it is generally closed when it is not in use
and every time it has to be dumped. And opening into which; the workman dumped the debris ok
which is this opening so this is a primarily a parapet opening. So this opening should have a
proper guardrail so that the work man, are not having a fall hazard. Whenever this particular
opening is open and debris are coming in all other opening should be accordingly closed onto the
levels below.
(Refer Slide Time: 21:01)Then removal of materials through floor opening and as I, said sometimes most often the
demolition material are generally collected into the building only. They are not collected and
stored outside of the building. So there can be floor openings into which the materials can be
dumped onto the lower floor by throwing in into the lower floor. But there are some safety
precautions for throwing of the material into the floor openings and collecting it onto the ground
floor.
The total area of the whole cut ok sometimes it can be a natural cut sometimes it can be cut
specifically for throwing in and collecting all the debris. So this area whatever it is cut for
throwing in the material it should not be more than 25% of this complete wall area ok that should
be the number one safety precaution. Because the slab will lose; its strength and as such it may
start getting failed. So all the floors above should be accordingly cut when you are actually you
know trying to throw in the material from in the in terms of floor or roof openings.
It should be ensured storage floor is adequate adequately strong enough. So this is the storage
floor it is adequately strong enough to withstand the falling material because it comes with a very
great impact load onto the floor the openings in all the floors below the floor. So this is the floor
from where you are throwing the debris all the floors below should also be properly barricaded.
So there should be barricades left with the proper danger signs return saying that hazard falling
debris ok. So that people are warned of the falling debris and there should be a proper guard rails also at all
the floors below and also onto the collector point. Openings in all the floors below the floors
from which the materials are being; removed protected by standard rails barricades and
guardrails. No barricades or guardrails should be removed until the storey immediately below the
floor has been demolished down to the floor line.
So unless this floor is completely demolished this barricade should not be removed ok it should
be there in place only. And all the debris are cleared off from the particular floor when the
cutting of a floor when you are cutting a floor primarily in order to create a opening for throwing
on the debris. And in the planks are not sufficiently strong enough to support then accordingly
bracing and showing has to be done in order to make the floor strong enough in order to do the
cut portion.
Signs or warning of the falling hazards should be posted at all levels ok every level should have a
sign of falling debris.
(Refer Slide Time: 23:54)
Demolition of walls again there are lot of safety precautions given on each component of the
structure to be demolished but walls are very critical so we will discuss about walls. Walls are
not allowed to fall as a single mass ok. In the sense if you see here the workers are trying to push
the one at a very great speed and make it to fall as a single mass what happens is? Most of the road coming onto the structure is partially onto the walls and if the walls are collapsed the
structure may automatically collapse and fall hazard of the workers may normally take place.
And the walls should be removed only part by part. So the staging should be provide for the men
to work on the walls are very thin ok and dangerous to work by standing over them. Then there
should be another staging and the workers can stand over there and start among the walls. The
temporary structure should be also be used to support your centering or shuttering there should
be adequate precautions for maintaining the structure and for clearing off your walls.
No section of a wall whose height is more than 15 times of the thickness shall be permitted to
stand without a lateral bracing ok. You have to safeguard the other sides of the wall and then
only start demolishing a wall. As seen in this picture if you when the workers are trying to
collapse this particular wall a portion of a particular wall and this wall starts automatically to
collapse. Now can accordingly lateral brazing has to be there unless the wall height is more than
15 times the thickness of the wall.
It should not be allowed to stand on its own structural or lower supporting members on any floor
shall not be cut or removed until all the stories above that floor has been demolished or removed.
A simple thumb rule is, do not touch any floor down below when the work is not completed on
the floor above. Before demolishing any wall within 3 meter of the opening on the floor the
opening should be planned or it should be safe guarded with proper bracings and then only the
wall has to be demolished.
At the completion of each days work all walls should be looked for stability and inspection has
to be for sure done. Foundation walls which serves, as retaining walls to support their pressure or
adjoining structures should not be demolished, until an adjoining structure has been underpinned
or braced and then only this wall has to be removed and taken off.
(Refer Slide Time: 26:35)Catch platforms sometimes there are lot of catch platforms in order when especially when you
are a doing a multi storage building. And these catch platforms are actually outside the structure
in order to collect all your debris. So demolition of exterior wall especially in multi storeyed
structure it is advisable to provide small catch platforms of heavy planking to prevent injuries to
the worker working below and to the public when the external walls are more than 20 meters in
height.
So these catch platforms shall not be less than one and a half meters in width measured in a
horizontal direction from the phase or the structure And shall consist of outriggers and planks
and the planks shall be laid tight together without any openings between them and also between
the walls where in loose pieces do not try to fall off to the ground level. So you should be able to
catch all the debris.
The catch platforms should be provided with a continuous solid parapet along its outer edge at
least 1 meter height so that even the bigger debris are all collected inside the catch platform and
not falling off. So 1 meter height is just like a guard rail protection only it shall be capable of
sustaining a live load of not less than 6100 Newton per meter square and they shall be not used
for the storage of materials.So as and when the catch platform has reached sufficient load it should be removed and started
to reuse. So after the debris are there it should be transferred through the chutes and the catch
platform is again you know ready collecting all your debris.
(Refer Slide Time: 28:18)
Removal of debris and malba so there are actually 2 terms which comes together. Debris is
actually the serviceable component and malba is a component which has to be trashed or thrown
away. So it as soon as the demolition work keeps going on and after it is complete. The released
parts, is generally classified into 2 categories 1 is serviceable part and the other portion is
unserviceable portion.
The unserviceable portion is generally also stored and stocked at suitable time intervals for
clearing and disposal. In any case the height of the malba heap should be limited in order to
avoid toppling over or falling onto people who are passing by and so on. And the malbas should
be stored in the demolition site only and should be removed to a location as requested by the
local civil authority. And depends on the space available in the site has to be stored and stacked.
The materials which are likely to cause dust or any undue environmental pollution should be,
remove from the site very quickly and till then they should be covered. So during transportation
also these dangerous substances are malba should be again closed and covered well
transportation too. Unauthorized use of debris or malba in any work should not be permitted and even the serviceable debris which is used for recycling also should be inspected by a competent person before it is taken for use.
Now how do we classify the debris? Generally after the debris, are taken off there are so many types of debris which comes in. Which includes primarily earth, plaster, motor waste, bricks, blocks, concrete, glass, steel, wood or wall paper.