Steel Details -Jamal

8/11/2019 Steel Details -Jamal

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Structural

PRACTICAL TIPS

Presented by Er.T.Rangarjan,

B.E,M.Sc(Struc.Engg), F.I.E, CEng


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Even though we learn many Theories,

Studies and Text books it is difficult to getmany practical tips that are known throughpractical experience

INTRODUCTION


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The following PRACTICAL TIPS are from my longexperience with the construction industry both inIndia and abroad.

The tips are only on Concrete and its designprinciple.


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1) ABRASIVE RESISTANCE of concreteincreases with compressivestrength and use of aggregatehaving low abrasion.


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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 theinfluence of Sulphate

attack. But in environment with both sulphates andchlorides, the C3A in

the cement reacts preferentially with the Sulphatesand enough C3A is left to bind the chlorides.


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3)The basic mechanical properties for

Structural design for steel reinforcementare:

a) The characteristic yield strength

b) Ultimate tensile strength

c) Elongation


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4) Why Fe500 and above grade of steel

reinforcing bars are not allowed for memberssubject to SEISMIC forces?

Reason: The bars having yield strength higherthan 500N/mm2tend to possess lowerpercentage elongation which is not acceptablefor Seismic prone structures since plastic hingeformation is not possible.


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5)Do you know that:

For steel bars to loose one mm diameter due tocorrosion, it takes about 12.5 years. But due to

practical reasons the number of years reducesdue 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

fc=0.8 fck where fc= cylinder strength,

fck= cube strength.


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9)The static Modulus Ec(Mpa) in terms of

characteristic cube strength fck(Mpa) ,

Ec=5000fck N/mm2,(IS code), Ec=0.04273fc (ACI code),

=4500fck where =2400Kg/m3.


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10) Poissons ratio:

A value of about 0.2 is usually consideredfor design.


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12) For normal density concrete thesplitting strength is about 2/3 of themodulus of rupture.


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13)Modulus of rupture:

fcr=0.7 fck --IS CODE fck in N/mm2.

Fcr=0.623fcACI 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 beenreported 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 , itsdeformation keeps increasing with time

and this time dependent component-(excluding strains introduced by shrinkageand temperature variations) of the total

strain is termed as CREEP.


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17) Factors influencing creep:Creep increases when,

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.


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18)Effect of creep:

Determental results in RC structures dueto creep:

a) increased deflection of beams and slabs.

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 creep-deflection it is advisable to use 0.2% ofcross sectional area at the compression

face.


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20) Ultimate creep co-efficient ,

= 2.2 for age of loading at 7 days, = 1.6do- at 28 days,

= 1.1do- at one year.


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21)Effective modulus of elasticity for

concrete, Ece=Ec/1+ . 22) Alternating drying and wet conditions

will cause alternating volume changes in

concrete. So, curing by this method is notrecommended.

Continuous curing is mandatory to get thebest results.


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23) Symmeterical arrangements ofreinforcement will aid to avoid thedifferential restraint.


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24)IS code recommends the co-efficient of

thermal expansion of concrete from 6x10^-6mm/mm per degree c. to 12x10^-6 mm/mmper degree c.


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25)The co-efficient of thermal expansion of steel is12x10^-6mm/mm per degree c


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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 reinforcementunless the area ofsuch bars is greater than 0.2% of sectional area ofthe member.

30)Shall we use Fe500 grade of steel for stirrups toresist the shear forces?

No.Under clause c1.39.4, the IS code IS 456 limitsthe value of Fe 415Mpa as high strength

reinforcement may be rendered brittle at sharp bendsof the WEB reinforcement, also a shear compressionfailure could procede the yielding of the high strengthsteel.


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31)The shear capacity of concrete shall not be

considered effective for members subject toSEISMIC forces and hence whole value shall

be resisted by closed vertical stirrups.

32)Shear Modulus G =0.4Ec


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33)Torsional reinforcement is provided in the

form of closed stirrupsand Longitudinal

bars which are distributed around the crosssection, close to the periphery.

34)To achieve economy and importantly toget the ductility requirements the membersalways should be designed as UNDERREINFORCED section by limiting the Pt(the

ration of reinforcement steel area ) to 75% ofPt limit(Balalnced section).


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37)To find the weight of the steel bars per

meter, mutiply by 0.006162timesdia^2.(Kg/m)

Example : to find weight of 25 mmbar/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? Topor bottom. If bottom, the memberwill collapse immediately after

removing the centering. It shouldbe placed on top as the tensionis on the top surface.


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40)For cantilever beams: How will

be positioned the hooks of thestirrups in a cantilever beam? Is itat the top as usually done for beam

or at the bottom? It should be atbottom only since the hooks ifplaced at the top will open up whenit bends while it is tension state.


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41) In frame analysis, centre line dimensions of beams

and columns are generally used to define the

geometry of frame line diagram. The BMobtained 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 bis 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 309RC DESIGNBy S.Unnikrishna Pillai and Devadas Menon.)


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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 concreteshall be M20. (clause 5.2 of IS 13920:1993)


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44) .It may be clarified that REDIRTIBUTIONof MOMENTSpermitted in IS 456:2000 will be

used only for VERTICAL LOADMOMENTS

AND NOTFOR LATERAL LOADMOMENTS.

(clause 6.2.4 of IS 13920:1993)

45) The contribution of bent up bars & inclinedhoops to shear resistance of the section shall

not beconsidered while designing against the

SEISMIC FORCES. (clause 6.3.4 of IS13920:1993)


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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?


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49) A first class brick should not absorb water

more than .of its own dry weight after 24hours immersion in cold water.

10% b)15% c) 20% d) 25%.

50) A first class brick should have a minimumcrushing strength of

70Kg/cm^2

105Kg/cm^ 2 125 kg/cm^2

140 Kg/cm^2.


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51) Excess of Alumina and Silica in the clay: 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 ofthe brick

Leaves high powder deposit on the brick.

52) The shrinkage of ordinary concrete is 0.3to 0.6 mm/m.


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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 rigidstructural cracks, use a materila thatwill 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= 670.82M/b where M is Knm,b=breath ofthe 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.


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58)Strength of concrete for various period are:

Days/months strength

7 days 2/3 of 28day strength(CP114) 28 days 1.0

2 months 1.1 (Table 5.1-p298-Properties of Concrete

by Adam Neville.)

3 months 1.16

6 months 1.2

12 months 1.24


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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.

(page 300Properties 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.


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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.

Steel Details -Jamal

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