Transcript PRACTICAL TIPS - Structural Engineering Forum of India

PRACTICAL TIPS
Presented by Er.T.Rangarjan,
B.E,M.Sc(Struc.Engg), F.I.E, CEng
INTRODUCTION
Even though we learn many Theories,
Studies and Text books it is difficult to get
many practical tips that are known through
practical experience
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The following PRACTICAL TIPS are from my long
experience with the construction industry both in
India and abroad.
The tips are only on Concrete and its design
principle.
1)
ABRASIVE RESISTANCE of concrete
increases with compressive
strength and use of aggregate
having low abrasion.
2)SULPHATE RESISTING CEMENT is considered
INEFFECTIVE in an environment where both
Sulphates and Chlorides are present.
Reason: SRC has alow content of C3A to reduce the
influence of Sulphate
attack. But in environment with both sulphates and
chlorides, the C3A in
the cement reacts preferentially with the Sulphates
and enough C3A is left to bind the chlorides.
3)The basic mechanical properties for
“Structural design” for steel reinforcement
are:
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a)
b)
c)
The characteristic yield strength
Ultimate tensile strength
Elongation
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4) Why Fe500 and above grade of steel
reinforcing bars are not allowed for members
subject to SEISMIC forces?
Reason: The bars having yield strength higher
than 500N/mm2 tend to possess lower
percentage elongation which is not acceptable
for Seismic prone structures since plastic hinge
formation is not possible.
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5)Do you know that:
For steel bars to loose one mm diameter due to
corrosion, it takes about 12.5 years. But due to
practical reasons the number of years reduces
due to hostile corrosive environment.
For 6mm dia. To corrode completely it takes
about 75 years.
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6)Cracking levels depend on,
d)
tensile strength of concrete.
e)
The cover thickness.
f)
The diameter of rebar &
g)
Rate of corrosion.
7)Corrosion takes place only in the presence
of MOISTURE & OXYGEN.
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8)The relation between the cube strength
& cylinder strength is
f ’c =0.8 fck where f ’c= cylinder strength,
fck= cube strength.
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9)The static Modulus Ec(Mpa) in terms of
characteristic cube strength fck(Mpa) ,
Ec=5000√fck N/mm2,(IS code),
Ec=0.0427√β3f’c (ACI code),
=4500√fck where β =2400Kg/m3.
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10) Poisson’s ratio:
A value of about 0.2 is usually considered
for design.
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11) Direct tensile strength of concrete is
equal to about 7 to 15% of the
compressive strength.
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12) For normal density concrete the
splitting strength is about 2/3 of the
modulus of rupture.
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13) Modulus of rupture:
fcr=0.7 √fck --IS CODE fck in N/mm2.
Fcr=0.623√f’c—ACI Code .
Use of lower value of fcr will result in
more conservative (lower) estimate of
cracking moment.
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14) Sherar strength: The strength of
concrete in PURE SHEAR has been
reported to be in the range of 10 to 20%
of its compressive strength.
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15) Temperature and shrinkage causes
tensile forces in concrete.
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16) CREEP: When concrete is subject to
sustained compressive loading , its
deformation keeps increasing with time
and this time dependent component(excluding strains introduced by shrinkage
and temperature variations) of the total
strain is termed as CREEP.
17) Factors influencing creep:
Creep increases when,
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a) cement content is high,
b) w/c ratio is high,
c) aggregate content is low,
d) air entertainment is high,
e) relative humidity is low,
f) temperature (causing moisture loss) is high,
g) size / thickness of the member is small,
h) loading occurs at an early age &
i)loading is sustained over a long period.
18)Effect of creep:
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Determental results in RC structures due
to creep:
a) increased deflection of beams and slabs.
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b) Increased deflection of slender columns(
possibly leading to buckling)
c) Gradual transfer of load from concrete to
reinforcing steel in compression members.
d) Loss of prestress in prestressed concrete.
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19) In order to reduce the effect of creepdeflection it is advisable to use 0.2% of
cross sectional area at the compression
face.
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20) Ultimate creep co-efficient ө,
ө = 2.2 for age of loading at 7 days,
= 1.6 –do- at 28 days,
= 1.1 –do- at one year.
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21)Effective modulus of elasticity for
concrete, Ece=Ec/1+ ө.
22) Alternating drying and wet conditions
will cause alternating volume changes in
concrete. So, curing by this method is not
recommended.
Continuous curing is mandatory to get the
best results.
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23) Symmeterical arrangements of
reinforcement will aid to avoid the
differential restraint.
24)IS code recommends the co-efficient of
thermal expansion of concrete from 6x10^-6
mm/mm per degree c. to 12x10^-6 mm/mm
per degree c.
25)The co-efficient of thermal expansion of steel is
12x10^-6mm/mm per degree c
26)The water content normally ranges from
180 to 200 lit per m3in concrete mix.
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27)The ratio of Fine aggregate to Coarse
aggregate is 1:2 or 0.35:65.
28)Es modulus of elasticity of steel
Es=2x10^5Mpa(n/mm^2)(200kn/mm^2)
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29)Hanger bars of nominal diameter used for the
purpose of holding stirrups DO NOT normally qualify
as Compression reinforcement –unless the area of
such bars is greater than 0.2% of sectional area of
the member.
30)Shall we use Fe500 grade of steel for stirrups to
resist the shear forces?
No. Under clause c1.39.4, the IS code IS 456 limits
the value of Fe 415Mpa as high strength
reinforcement may be rendered brittle at sharp bends
of the WEB reinforcement, also a shear compression
failure could procede the yielding of the high strength
steel.
31)The shear capacity of concrete shall not be
considered effective for members subject to
SEISMIC forces and hence whole value shall
be resisted by closed vertical stirrups.
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32)Shear Modulus G =0.4Ec
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33)Torsional reinforcement is provided in the
form of closed stirrups and Longitudinal
bars which are distributed around the cross
section, close to the periphery.
34)To achieve economy and importantly to get
the ductility requirements the members
always should be designed as “ UNDER
REINFORCED “ section by limiting the Pt(the
ration of reinforcement steel area ) to 75% of
Pt limit(Balalnced section).
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35)IS456-2000 allows only 30 %( max)
moment redistribution in General for beams
and slabs( NOT FOR COLUMNS)-10%(max)
for structures subject to vertical gravity loads
only.
36)Reduction of moments on account of
moment redistribution is generally NOT
APPLIED TO COLUMNS.
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37)To find the weight of the steel bars per
meter, mutiply by 0.006162times
dia^2.(Kg/m)
Example : to find weight of 25 mm
bar/m=0.006162*25^2=3.85Kg/m
38)Is it good to apply epoxies on WET or
DAMP surfaces? No. Because the epoxies do
not bond on wet or damp surfaces.
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39)For cantilever beams: Where
the main bars shall be placed? Top
or bottom. If bottom, the member
will collapse immediately after
removing the centering. It should
be placed on top as the tension
is on the top surface.
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40)For cantilever beams: How will
be positioned the hooks of the
stirrups in a cantilever beam? Is it
at the top as usually done for beam
or at the bottom? It should be at
bottom only since the hooks if
placed at the top will open up when
it bends while it is tension state.
41) In frame analysis, centre line dimensions of beams
and columns are generally used to define the
geometry of frame “line diagram”. The BM
obtained is on Centre line which has to be reduced
by Vb/3 . ie Ms-Vb/3 where Ms is the moment at
centre line and V is the shear at the centre line and b
is the width of the column or beam. This enables to
get lesser steel area which aids in avoiding
congestion of reinforcement at the beam column
joint to some extent. (vide page 309 –RC DESIGN
By S.Unnikrishna Pillai and Devadas Menon.)
42) The shear also should be taken at a distance
of d the effective depth from the face of the
column or beam .
43) For all buildings which are more than 3
storeys in height, the min. grade of concrete
shall be M20. (clause 5.2 of IS 13920:1993)
44) ….It may be clarified that REDIRTIBUTION
of MOMENTS permitted in IS 456:2000 will be
used only for VERTICAL LOAD MOMENTS
AND NOT FOR LATERAL LOAD MOMENTS.
(clause 6.2.4 of IS 13920:1993)
45) The contribution of bent up bars & inclined
hoops to shear resistance of the section shall
not be considered while designing against the
SEISMIC FORCES. (clause 6.3.4 of IS
13920:1993)
46) The mix proportion 1:2:4 or 1:1.5:3 is by
weight or by volume?
47) 1000 litre of water weighs to 1000Kg. Is it
right?
48) One litre is equal 1cu.m. Is it correct?
49) A first class brick should not absorb water
more than ….of its own dry weight after 24
hours immersion in cold water.
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10% b)15% c) 20% d) 25%.
50) A first class brick should have a minimum
crushing strength of
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70Kg/cm^2
105Kg/cm^2
125 kg/cm^2
140 Kg/cm^2.
51) Excess of Alumina and Silica in the clay:
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makes the brick brittle and weak,
makes the brick crack and wrap on drying.
Changes colour of the brick from red to yellow,
Improves the impermeability and durability of
the brick
Leaves high powder deposit on the brick.
52) The shrinkage of ordinary concrete is 0.3
to 0.6 mm/m.
53) The permissible limit for solids in water used for
concrete mix as per IS456:2000 are:
1.Organic
200mg/l
2.Inorganic
3000mg/l
3.Sulphates(as So3)
400mg/l
4.Chlorides(as cl)
2000mg/l for concrete not
containing embedded steel
and 500mg/l for RCC
works.
5.Suspended matter
2000mg/l.
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54) Cracks are of live and dead . That is
moving and non moving cracks.
For moving cracks use always a material that
will accommodate the movement. Ex. Joint
sealant.
For rigid –structural cracks, use a materila that
will add strength by bonding with the parent
member. Ex. Epoxies or cementitious
proprietary material
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55) To find the depth of RCC member from
the moment for M20 & Fe 415,
d= 670.82√M/b where M is Knm,b=breath of
the member in mm & d is in mm.
This is for the balanced reinforced section.
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56) to find the steel for a singly reinforced
section of M20 and Fe415,
Ast =3077.44*M/d where M in Knm,
d in mm & Ast in mm^2.
When M/bd^2 is less than 1.27 the steel area
should be calculated using the lever arm
z= 0.95d.
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57) The lap lenth of bars shall be in tension for
Seismic forces and it can be safely taken as
Ld= 50*dia of bar.
58)Strength of concrete for various period are:
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Days/months
7 days
2/3 of 28day strength(CP114)
28 days
2 months
1.0
1.1 (Table 5.1-p298-Properties of Concrete
by Adam Neville.)
1.16
1.2
1.24
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strength
3 months
6 months
12 months
59) The relation between the 28-day strength and
7 day strength which lies between as given in
Germany is
 fc28 =1.4fc7 +1.0 &
 fc28 =1.7fc7 +5.9 where fc being expressed in Mpa.
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(page 300 –Properties of concrete by Adam Neville)
ACI RECOMMENDS
fcm(t) =f28{t/(4+0.85t)}
For 7 days the value comes to 0.71% of 28 days strength.
For 3 days the value comes to 0.458% of 28 dyas strength.
60) For rough estimation of reinf. Steel in construction projects
following thumb rules may be adopted:
 SLAB
50 TO 80Kg/m^3 of concrete.
 Sunshade
50 Kg/m^3 of concrete.
 Lintels
80Kg/m^3 of concrete.
 Beams
100TO 150 Kg/m^3 of concrete.
 Columns
150 to 225 Kg/m^3 of concrete.
 Footing slab
80Kg/m^3 of concrete.