Pre Stressed Beams
Transcript of Pre Stressed Beams
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Pre-stressed Beams
From Reinforced Concrete to
Pre-stressed Applications
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The Disadvantages of
R/C
Reinforced Concrete does
have shortcomings…
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Concrete Beams require a large
amount of formwork Timber is expensive
The shuttering is
afterwards mostlythrown away
A lot of material is
wasted for props andsupports
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R/C Formwork is Time
Consuming
Fabrication takes a long time Craftsmen are required for formwork
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Concrete requires long curing
The props cannot
be removed for 3
weeks andprevent any
progress
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Large Amount of Steel
for Reinforcement Mild Steel is used for
Reinforcement bars
The maximum tensilestress mild steel can
take is 3000kg/cm2
This is quite low and
requires relatively
high amounts of steel
in R/C
3 Tons
One
cm2
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Bar cutting,
bending andtieing is time
consuming
As a result, R/Cbeams are rather
expensive
and therefore not
much used for
roof construction
Stirrups
Reinforcement
Bars
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Pre stressed beams reduce the
cost of concrete slabsdramatically
You can make a concrete slab for
the cost of an entire roof andhave no trouble with leaks
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On top of it, you get a
roof top garden free
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Advantages of Pre-stressed
Concrete
High Tensile steel wire is used instead
of mild steel bars
The cross section area is reduced since
the steel wire can take a much greaterload
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RC Beam
Pre-stressed
Beam
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To increase the load capacity, the steelwire is kept under tension of more than2000 kg during casting and curing
This results in a reduction of thenumber of bars and a substantialreduction of concrete material andcost
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What is more,
The beams can be pre cast on ground
due to their small size which enables
the constructor to lift them on the brickwall to build a slab in a short period of
time.
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And
there are no shuttering and props
required. This again results in huge
savings
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What does a concrete slabwith pre-stressed beamslook like ?
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Pre cast concrete planks
Pre-stressed beams
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Plan
In the following section we willshow you how to make
pre cast beams
and how to stress them
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Plan
Then we will show you
how to make a slab
and a slab roof.
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Steel Formwork
Design,Dimensions,Details
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Formwork Elements
l
These elements of 2.40m lengthare joined serially. As a result, a
number of beams can be cast and
pre-stressed in one stretch.
The wooden blocks
5x5cm are removed after
casting
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Align the Formwork•Level the floor exactly and alignthe wooden blocks horizontallywith the help of guide lines to
bring them in to one level.
•Align the formwork with guidelines.
The wooden blocks
5x5cm are removed after
casting
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Dimensions
Steel Formwork for Beams
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The Metal Sheet Forms –
3m BeamGauge
Side – s 10.5 cmWidth-w 10.0 cm
w
s L= 2.4 m
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Metal Sheet Form -3.6m Beam
Gauge
Side-s 13.25Width-w 12.7
w
s L= 2.4 m
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Metal Sheet Form -4.2m Beam
Gauge
Side-s 16 cmWidth-w 15 cm
w
s L= 2.4 m
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Metal Sheet Form -5.2m Beam
Gauge
Side-s 19 cmWidth-w 18 cm
w
s L= 2.4 m
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Frame and End Plate
37mm L
angle
The Forms
are
supported
by welded
frames
Each beam is
separated by a
endplate that
determines the
length of the
beam
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Plates separate
each beamwithin the Formwork
They
determine theLength of each
Beam
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Apply Formwork Oil
Mix Oil and Grease 1:2
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Tricks
Formwork oil prevents the formwork
from sticking to the concrete. It eases
dismantling
Formwork oil gives the concrete a finefinish
Never use burnt engine oil. It will blackenthe concrete and spoil the formwork
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Stressing
Pre-stressing yard
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Setting up a Pre-stressing yard
Select a flat land in excess of 600m2(25
perches)
Level and compact an area of 10mX30m
Put a 5cm thick lean concrete layer andlevel it perfectly
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The Yard
Consists of
a number of rows of steel form work
Two concrete abutment blocks on either
end of the formwork to apply tension
onto the wires
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Build two Abutments with suitable
foundations able to withstand the force
(F) induced by stressed bars
Consider the soil condition of the ground !
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Concrete
Anchoring
Formwork,
Wires
Jacks
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The Hydraulic Unit
To apply tension on the steel wires, a
hydraulic unit is required
It consists of 2 hydraulic jacks mounted on
a base plate, a pressure plate with wedgingcones to lock the wires
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Base
Plate
5cm Opening
Tension is applied on the wires
with two 10T jacks
welding
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A Pressure Plate is added
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A Pressure Plate is added
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And mounted on a Abutment
Cast Beam
Abutment
1m
50
cm
Jack
Steel
wire
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The Wires are Passed Through
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Two Wedges are inserted and the
Wires Passed trough them as well
Wires
Wedging Cones
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And tighten the wedge to fix the
Wires
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Pump up the Jacks equally and apply
tension on the wires
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Pump up the Jacks equally and apply
tension on the wires
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Mark the Wire to determine the
Tension
Pre-stressing means putting the wire
under a tension of at least 2000 kg
In the process the wire extends andgets longer
This can be observed by marking and
measuring the extension which is calledelongation
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The elongation is
of the entire length of the wire
0.353 %
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The Wire Length
The wire length is determined by your yard:the entire distance from anchoring block tothe wedge of the hydraulic unit.
length
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Elongation Table
Length of Wire Elongation
10m 3.5cm
20m 7.0cm
30m 10.6cm
50m 17.0cm
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Concreting
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Concrete Mixture
Cement : Sand : Coarse aggregate
Mixture Grade Beam length1:1½:3 40 < 4m
1:1:2 50 4m< <5m
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Without Interruption
Complete Casting within the shortest
time possible Vibrate while concreting using Form
Vibrators
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Vibrate until air
bubbles popping
stops Vibrate evenly
along the
formwork
About Vibration……..
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Over vibration
Over vibration is dangerous to the concrete
Aggregates settle to sediment at the bottom
and only cement and sand would collect attop
As a result aggregate would not bond and
remain weak and break
See that the aggregates remain visible on the
top!
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Under Vibration
If the concrete is not sufficiently vibratedair bubbles remain between the
aggregates and become later visible! And honeycombs
These honeycombs are specially unacceptable for fair face concrete
Honeycombing is also the cause forwater leakage
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Leveling
Use the upper edge of the form work as
the guide to level the concrete
Use a large heavy trowel and floated onthe surface exactly down to formwork
edges until the beams are absolute
straight and even
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Make yourself a wooden Float
This wooden plank with the handle is ideallysuited for floating, plastering and leveling
(60X15X2cm plank)
The thickness of the handle should be 4cm
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Removal
The formwork can be removed after onefull day
Do not remove the hydraulic unit.The beams need to be cured for 4-5days under tension
Remove the blocks and drop theformwork on the floor
Set the formwork up for the next cast
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Curing
Keep the beams covered with bags toprevent them from heating up. Heat isharmful to the bonding process of the
cement. Water the beams every day. Cement
cannot harden without water.
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Making a Floor Slab
Cast a slab with pre-cast
beams
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Set The Pre-stressed Beams
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Add the pre-cast concrete planks
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Pre-cast Slab Planks !
Pre-cast the slab planks to be ready
when they are needed for the floor slab
Reinforce them with 10X10cm mesh Cast them on the ground in a matrix of
60X60cm
The thickness should be about 5 ..7cm
C i C Pl k f
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Casting Concrete Planks for a
Floor Slab
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Curing
Remove the planks carefully on the next
day and keep them under cover over a
waterbed The humidity is sufficient to cure the
cement
W t b d ith Pl ti Sh t
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Waterbed with Plastic Sheet
Cover Makes a Curing Chamber
Reapers
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Method
L l th ll ith 10
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Level the walls with a 10 cm
Screed Concrete
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Lift the Beams onto the Walls
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Place them in a 60cm Pitch
Fill U th S ith
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Fill Up the Spaces with
Brickwork
Pl th t t
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Place the pre-cast concrete
Planks evenly
Add a brick frame as form ork
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Add a brick frame as formwork
for the rendering
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Add a wire mesh on spacers
Do the rendering section by
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Do the rendering section by
section
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Roof Slab
The pre-cast slab is a very
good roof
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Concrete does not leak
Usually people think that flat roofs leak.
This is not necessarily true.
For one, concrete does not leak if there isno honeycombing.
and if the concrete is not exposed to the
sun.
I i h
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It is the exposure
to sun that makesthe concrete slab
crack,
not the water !
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The best heat insulation
Compacted clay is the perfect insulation
against heat and rain. This property hasbeen used for roof insulation by the
ancient and even now in Pakistan and
India.
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The best heat insulation
If the concrete slab is insulated by a
layer of 30cm clay the slab does notcrack and never leaks.
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A cool house
The clay layer
absorbs heat and
water
This ensures very
good cooling of the
rooms
Instead of cement
plaster clay or
cement tiles can be
laid in cement as
well
A balustrade and
foliage makes it arooftop garden
After all, you get a roof
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y g
top garden free
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A pre-cast roof slab
does not cost more !
Compare the cost of a roof
construction with a pre-castslab
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Comparison of Rates /sq.m
Roof
construc-
tion with
tiles cover
Roof
constructi
on with
asbestos
sheet
cover
Asbestos
Ceiling
Pre-
stressed
beams
with floor
slab
Rs1280
Rs1024
Rs500
Rs1382
Dimensions of Pre-stressed
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Dimensions of Pre-stressed
Beams
There are standard lengths available for
house construction !
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Dimensions
Length- l
Height-h
Bottom width – 5.04 cm
Top width-w
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The cross sectional area of beams
depend on their length
The table shows dimensions and loads
permitted
Dimensions in Relation to Length
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g
& Load
Length
-l-
Height
-h-
Top
Width
-w-
UDL concentrated
> 3m 10 cm 10 cm 153kg/m2
200 kg
>3.65m 13 cm 13 cm Do Do
>4.25m 15.5cm 15.5cm Do Do
>5.25m 18 cm 18 cm Do Do
Load
Introduction to Load
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Introduction to Load
Parameters
In this section we show you
to differentiate betweenload parameters
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Dead Load
The dead load is the weight of the slab
including beams and any other layer like
rendering, tiles etc
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Concentrated Load
This term denotes the maximum load
allowed on a spot of a slab.
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The load parameters of slabs
The rated live
load is usually= 153 kg/m2
The rated
concentratedload is usually
= 215 kg
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Live Load on a Slab=
Area of the slab x rated live load
1.0 X 0.6 X 15391.8 kg
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Calculation of Dead Load
In the next exercises we learn how tocalculate the dead load of a beam, this
is the weight which we must know for
transport and lifting
Calculate the Dead Load of
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Calculate the Dead Load of
Beams:
The Weight is
= Volume x weight of concrete
Th C S i l A
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The Cross Sectional Area
A = c x ½ (a + b) c
b
a
Th V l f B
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The Volume of a Beam
A
l
V = A x
Th W i ht f B
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The Weight of a Beam
W = Volume x
2300kg
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The Weight of Pre-stressed
concrete is 2300kg/m3
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Weight of beam =
Sectional Area x length xwt of concrete
0.0075 X 1 X 2300
17.25 kg
Calculate the Weight of a
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C cu e e We g o
Concrete Slab
Length x Width x Thickness x Wt of Mass concrete
1
w
t
x w x t x 2200
Th W i ht f M C t
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The Weight of Mass Concrete
2200 kg/m3
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Weight of Slab =Length X Width X Thickness
X Wt of Mass Concrete
1.0 X 0.6 X 0.08 X 2200
105kg
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P t d B i P h
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Pre-stressed Beams in a Porch
Brick
border
Plaster Clay
Prestressed
Beam
ColumnHouse
Slab
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House roof slabPrestressed
beams
Over lapping
Main
wall
Column
Provide
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Insert the
anchor through
the slots.
slots for the
anchorwhile
casting by
inserting
rigid foam
T32
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Add a
formwork in
the size of
the column.
And cast
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And cast
the entirecorner on
top of the
column
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An Alternative
Commercially available
Pre-cast beams with soffitmasonry blocks
Pre stressed Beam
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Pre-stressed Beam
3.5 cm
3.96m - 4.87m
4.5cm
110cm
3.5cm
Soffit Masonry Block
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Soffit Masonry Block
52.7cm
10.0cm
20.3cm
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Construction Method
Place the Beams on the Wall
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(single beam for housing)
57.2cm
Insert soffit masonry blocks
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Insert soffit masonry blocks
Double beam (for shop & Office)
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Double beam (for shop & Office)
57.2cm11cm11cm
Lay a 10 x10 mesh on spacer
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blocks
Lay a 10 x10 mesh on spacer
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blocks
Add the Formwork
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Add the Formwork
Lay a 5 cm concrete Layer
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Lay a 5 cm concrete Layer
Lay a 5 cm concrete Layer
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(Grade 20)