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IS: 800 – Indian Code of Practice for Construction in Steel
Comparison with International Codes Arijit Guha Sr.Manager (Civil & Structural) & Manas Mohon Ghosh Institute for Steel Development & Growth
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TOPICS COVERED INTRODUCTION BRIEF DISCUSSION ON DESIGN METHODOLOGIES
BRIEF OVERVIEW OF IS 800 (2007) IS 800 (2007) & OTHER INTERNATIONAL CODES CONCLUSIONS
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INTRODUCTION Codes & Standards provides specifications and stipulations for state-of-the-art design to be put into practice. Codes and Standards pertaining to Steel Design must be Understandable Shall be based on good structural theory Shall deal with elastic instability, dynamic loads and fatigue IS Basic Code for Design of Steel Structures The code was last revised in the year 2007 prior to which it was in 1984.
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Table: 1 Countries and their Design Format
INTRODUCTION Almost all countries have adopted more efficient techniques of design based on various efficient codes. The current practice all over the world is based on Limit State Method (LSM) or Load and Resistance Factor Design (LRFD) method. Country wise practice of design procedure is given in Table 1. Table: 1 Countries and their Design Format Australia, Canada, China, Europe, U.K., Japan Limit State Method (LSM) U. S. A Load and Resistance factor Design (LRFD) Method & Allowable Stress Design (ASD) India Allowable Stress Design (ASD)
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INTRODUCTION LSM has become the design philosophy in most of the International design standards. LSM design ensures Rationality in Design Economy of Design It was also felt that this modification would render steel design novel and will facilitate accuracy of design.
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BRIEF DISCUSSION ON DESIGN METHODOLOGIES
ASD METHODS Unit stress is not allowed to exceed a pre-defined allowable stress, factual < fallowable where The allowable stress is defined by a limiting stress divided by a factor of safety fallowable = (fy / Fs) (fy = minimum yield stress and Fs = factor of Safety) factual is determined using a load factor of 1.0 for all load combinations. Factor of safety (Fs) is fixed.
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BRIEF DISCUSSION ON DESIGN METHODOLOGIES
ASD METHODS (Contd.) No matter how variable the loads are in terms of frequency or magnitude, the factor of safety is always the same. Advanced knowledge about strength of materials beyond yield point and its plastic plateau led to the development of LSM as an alternative to ASD. A better way than “Effective length” methods can also be adopted using Merchant – Rankine approach to find the limiting load of the whole structure. 1/Plimit = 1/Pfield + 1/Pcritical Where, Plimit, Pfield, and Pcritical are the factored limit load of the structure, load at plastic collapse ignoring instability, and the elastic critical load of the structure respectively.
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BRIEF DISCUSSION ON DESIGN METHODOLOGIES
B. LIMIT STATE METHOD (LSM) The Limit State Method of design was developed to address the drawbacks of the ASD. LSM makes use of the plastic range of material for the design of structural members. It incorporates Load Factors to take into account of the variability of loading configurations. A rational but variable factor of safety in different structural performance enables to use steel efficiently and economically in different structural systems to withstand tension, compression etc. LSM considers the good performance of steel in tension compared to compression and specifies variable factors.
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BRIEF DISCUSSION ON DESIGN METHODOLOGIES
B LIMIT STATE METHOD (LSM) The main advantage of the limit state method is that it takes into account this variance by defining limit states, which address strength and serviceability. This method renders a structure or part of it unfit for use when it exceeds the limit states. Beyond this limit states the structure infringes one of the criteria governing its performance. The two limit states are classified as Limit State of Strength It takes care of the structure from strength point of view Limit State of Serviceability ---- It takes care of the structure in terms of safe operation
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BRIEF DISCUSSION ON DESIGN METHODOLOGIES
B LIMIT STATE METHOD (LSM) The criteria which defines limit state of Strength are Strength (Yielding & Buckling) Stability against Overturning and Sway Fracture due to Fatigue Brittle failure Serviceability limit states takes care of the performance and behavior of the structure during its service period. The criteria which defines serviceability limit states are Deflection (including drift) Vibration Fatigue checks (including reparable damage due to fatigue) Corrosion
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BRIEF DISCUSSION ON DESIGN METHODOLOGIES
B LIMIT STATE METHOD (LSM) LSM considers the critical local buckling stress of the constituent plate elements of a beam. Thus, the resistance of plate elements can be increased by suitably reducing the slenderness ratio of the element. Thus, full flexural moment capacity of a member or the limit state in flexure may be developed. Based on slenderness ratio of constituent plate element a section may be classified as Plastic Compact Semi-compact Slender
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BRIEF DISCUSSION ON DESIGN METHODOLOGIES
B LIMIT STATE METHOD (LSM) In ASD (based on IS 800 – 1984) however, the extreme fibre stress is restricted to 0.66fy irrespective of the slenderness ratio of the constituent plate elements. In LSM, the factored loads, in different combinations, are applied to the structure to determine the load effects. These are then compared with the design strength of the elements.
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BRIEF DISCUSSION ON DESIGN METHODOLOGIES
B LIMIT STATE METHOD (LSM) Mathematical representation of strength check criteria in LSM is (Function of sy and other geometric variables) where gL = partial factor for loads. gf factor that takes account of inaccuracies in assessment of loads, stress distribution and construction. gm1 & , gm2 factors that take into account, uncertainty in material strength and quality, and manufacturing tolerances respectively. Qk specified nominal load. sy yield strength of the material.
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Level for different design methods at which calculations are conducted
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BRIEF OVERVIEW OF IS 800 (2007) A steel member subjected to external system of loading may be subjected to one of the following: Compression Tension Bending Combined effect of Bending and Tension Combined effect of Bending and Compression The basic stresses in a member are either Compressive Tensile Shear The primary forces are Compressive forces Tensile forces Bending Moments
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Comparative Study of Design Outputs between ASD and LSM
BRIEF OVERVIEW OF IS 800 (2007) Comparative Study of Design Outputs between ASD and LSM The comparisons have been made by designing various sets of members subjected to same tensile, compressive, or flexural Loads. The charts shown gives an account of the percentage of design strength of a member utilised in WSM w. r. t. percentage in LSM. It may be seen that LSM gives more economy in tension and flexure, whereas in compression WSM gives better results.
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BRIEF OVERVIEW OF IS 800 (2007) TENSION MEMBERS
Fig 1 Percentage strength utilized in Tension Members
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BRIEF OVERVIEW OF IS 800 (2007) TENSION MEMBERS
Margin of economy in LSM is considerably high comparative to WSM. LSM gives equal weightage to yielding of the gross section, as well as the rupture of net section and block shear failure which even nullifies the benefit of increase in stress by 33% under wind load condition in WSM. Shear lag effect is considered both in WSM and LSM. In WSM stress is allowed to reach up to the yield value in both connected and outstanding element of a member, whereas in LSM stress in the connected element is allowed up to the ultimate strength of the material and that of the outstanding member up to the yield strength.
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BRIEF OVERVIEW OF IS 800 (2007) COMPRESSION MEMBERS
Fig 2 Percentage strength utilized in Compression Members
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BRIEF OVERVIEW OF IS 800 (2007) COMPRESSION MEMBERS
The WSM method gives a better result than LSM. The major reason is that In LSM the column buckling curve is considered more rationally as per the particular column buckling behaviour whereas in WSM a mean curve is considered for all cases. This can be explained as follows: It is clear from the curves in Fig 3, that curve ‘a’ gives the maximum permissible stress fcd for a particular slenderness ratio and curve ‘d’ gives the minimum value of fcd with ‘b’ and ‘c’ in between (Refer: Eurocode 3) Most cases in LSM adopts either curve ‘c’ or ‘d’, with a few adopting curve ‘b’ and only in two cases it is ‘a’ (Refer Table 2). WSM adopts curve ‘b’ for all cases. Thus, it gives economy for most of the cases in compression members.
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BRIEF OVERVIEW OF IS 800 (2007) COMPRESSION MEMBERS fcd / fy
Slender Ratio fcd / fy 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.0 1.5 2.0 2.5 3.0 3.5 a b c d Fig 3 Column Buckling Curves
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BRIEF OVERVIEW OF IS 800 (2007) Cross Section Limits
Table: 2 Buckling Class of Cross Sections Cross Section Limits Buckling about axis Buckling Class Rolled I - Section h / b > 1.2: tf ≤ 40 mm 40 mm < tf ≤ 100 mm z - z y – y z – z y - y a b c h / b ≤ 1.2: tf ≤ 100 mm tf > 100 mm d Welded I - Section tf ≤ 40 mm tf > 40 mm y d h tw b z tf h y tw tf b z
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BRIEF OVERVIEW OF IS 800 (2007) Cross Section Limits
Table: 2 Buckling Class of Cross Sections (Contd.) Cross Section Limits Buckling about axis Buckling Class Hollow Section Hot Rolled Any a Cold Formed b Welded Box section Generally (Except as bellow) Thick Welds and b / tf < 30 h / tw < 30 z – z y - y c Channel, Angle, Tee and solid Sections Built-up Members y tw z b tf h y z y z
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BRIEF OVERVIEW OF IS 800 (2007) FLEXURE MEMBERS
Fig 4 Percentage strength utilized in Flexure Members
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FLEXURE MEMBERS BRIEF OVERVIEW OF IS 800 (2007)
Members subjected to Flexure in LSM give reasonably good economy over WSM. This is due to the fact that, in LSM the plastic moment of resistance is allowed for plastic and compact sections as limiting moment, whereas in WSM the elastic moment of resistance is the guiding criteria. Since almost all the sections available in India are either plastic or compact, design of members subjected to flexure will always give more economical design in LSM than in WSM.
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IS 800 (2007) & OTHER INTERNATIONAL CODES
C Comparison of IS 800 (2007) with International Codes A brief comparison of IS 800 and other international codes have been tabulated for a general comparative study. It may be worthwhile to notice that, like other codes though the basic design concept following the Limit States procedure are same for IS: 800, the limiting values of various parameters vary according to design and fabrication / erection practices existing in India . In India automation in fabrication / erection is fast developing but as still much behind the state-of-the-art methodologies. As such parameters governing the tolerances due to fabrication / erection as well as material strength are more conservative in IS: 800 compared to other international codes.
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Table: 3 Tension Members
PRAMETERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Partial Safety Factor 1.10 1.00 1.11 1.25 1.20 (In eff. Area) 1.31 - 0.90 Fabrication Factor For Punched Hole, dh dh + 2mm dh For Drilled Hole, dh Gross Section Capacity fy Ag / gmo f fy Ag (f = 0.90)
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Table: 3 Tension Members (Contd.)
PARAMETERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Net Section Capacity 0.9Anfu / gm1 fy Ae f 0.85 An fu f. Ae.fu (f = 0.75) Plates (Bolted Conn.) - do - Plates (Welded Conn.) Angles 0.9Anc fu / gm1 + b Ago fy / gmo U An fu / gm1 f 0.85 kt An fu Single Angle (Bolted) fy (Ae – 0.5a2 ) kt = 0.85 Double Angle (both side of Gusset) - bolted fy (Ae – 0.25a2 ) kt = 1.00 Double Angle (Same side of Gusset) - bolted Single Angle (Welded) fy (Ae – 0.3a2 ) Double Angle (both side of Gusset) - Welded fy (Ae – 0.15a2 ) Double Angle (Same side of Gusset) - Welded
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Table: 3 Tension Members (Contd.)
PARA- METERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Shear Lag Factor, U U (General) - kt Angle (n = 1) 2(e2-0.5do)/An 0.85 0.60 Angle (n = 2) Angle (n = 3) Angle (n = 4 or more) 0.80 Unequal angle (short leg conn.) 0.75 Other shapes (n=2) Other shapes (n=4)
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Table: 3 Tension Members (Contd.)
PARA- METERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Block Shear capacity (Case – 1) Shear Plane capacity 0.6 Avg.fy - f 0.6 Anv Fy Tension Plane Capacity 0.9 Atn.fu / gm1 0.6 Ke Atn.fy f Ubs Agt Fu Ubs = 1 for uniform tensile stress Ubs = 0.5 for uniform tensile stress Block Shear capacity (Case – 2) f 0.6 Agv Fy Atg.fy / gmo f Ubs Ant Fu
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Table: 3 Tension Members (Contd.)
where n = Number of bolts Avn Net shear plane area d Diameter of fasteners Atg Gross tension plane area dh Diameter of fastener hole Atn Net tension plane area x Connection eccentricity a2 Area of outstanding leg An Net area fu Ultimate tensile stress Ae Effective area fy Yield stress Avg Gross shear plane area L Length of connection
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Table: 4 Compression Members
PARAMETERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Effective Area of Cross Section Plastic Section Ae = Ag Ae = be.t = Ag Compact section Non-compact section Slender Section Ae = beff.t Ae = be.t Capacity of Cross Section fy.Ag / gmo fy.Ag f.kf .fy .An = f.fy .Ag (kf = 1) fc.fy.Ag f.kf .fy .An = f.fy .Ae (kf ≠ 1) kf = Ae / Ag & An = Ag
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Table: 4 Compression Members (Contd.)
PARAMETERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Eff. Slenderness Ratio, l Plastic Section Leff / r Compact section Non-compact section Slender Section Leff / r (Aeff / Ag )0.5 Section Capacity (Member Buckling) c.fy.Ag / gmo f ’y.Ag f.ac.fy.Ag fc.Fcr.Ag c.fy.Ae / gmo f ’y.Ae f.ac.fy.Ae fc.Fcr.Ae c = f (L/r) ≤1 f ‘y = f (L/r) ac = f (L/r) ≤1 Fcr = f (L/r) ≤1
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Table: 4 Compression Members (Contd.)
PARAMETERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Buckling Curve Rolled I -Section (z-z) tf ≤ 40 a - Rolled I -Section (y-y) tf ≤ 40 b Rolled I -Section (z-z) tf > 40 Rolled I -Section (y-y) tf > 40 c Rolled H -Section (z-z) tf ≤ 40 b ( tf ≤ 100 ) Rolled H -Section (y-y) tf ≤ 40 c ( tf ≤ 100 ) Rolled H -Section (z-z) tf > 40 d ( tf ≤ 100 ) Rolled H -Section (y-y) tf > 40 d Welded I -Section (z-z) tf ≤ 40 Welded I -Section (y-y) tf ≤ 40 Welded I -Section (z-z) tf > 40 Welded I -Section (y-y) tf > 40
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Table: 4 Compression Members (Contd.)
PARAMETERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Buckling Curve (Contd.) Welded Box-Section (z-z) tf ≤ 40 c b - Welded Box-Section (y-y) tf ≤ 40 Welded Box-Section (z-z) tf > 40 Welded Box-Section (y-y) tf > 40 Hollow Section (Hot Rolled) a Hollow Section (Cold Formed) Channel, angles Tees Two rolled section (Built-up) Imperfection Factor ( Curve a ) 0.21 0.21 Imperfection Factor ( Curve b ) 0.34 0.34 Imperfection Factor ( Curve c ) 0.49 0.49 Imperfection Factor ( Curve d ) 0.76 0.76
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Table: 5 Flexure Members (Compression Flange Laterally Restrained)
PARA- METERS IS 800 (2007) BS 5950 (2000) Euro code (1993) AS 4100 (1998) AISC 360 (2005) Bending Resistance under low shear [V ≤ 0.6Vd ] Plastic Section Zp.fy / gmo ≤ 1.2 Ze.fy / gmo Zp.fy f.Zp.fy ≤ 1.5 f. Ze.fy Mp = f.Zp.fy Compact Section Non-compact section Ze.fy / gmo - Slender Section Zeff.fy Zeff.fy / gmo f. Ze.fy (lsy – ls) Zp = Plastic Section Modulus Ze = Elastic section Modulus Zeff = Effective Section Modulus sp = Plastic Limit (Slenderness) sy = Yield Limit (Slenderness) s = Section Slenderness Ratio
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Table: 5 Flexure Members (Compression Flange Laterally Restrained)
PARA- METERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Bending Resistance (High shear) [V > 0.6Vd ] Plastic Section fy / gmo ( Zp - b.Zpv ) ≤ 1.2 Ze.fy / gmo fy ( Zp - b.Zpv ) - Mp = f.Zp.fy Compact Section Non-compact section Ze.fy / gmo fy ( Ze - b.Zpv / 1.5) fy / gmo ( Ze - b.Zpv ) Slender Section fy ( Zeff - b.Zpv / 1.5) fy / gmo ( Zeff - b.Zpv ) Zpv (equal Flanges) Zp - Zf Zv Zpv (unequal Flanges) Zf = Plastic modulus of effective section excluding shear area Zz = Plastic modulus of the shear area b (2 V / Vd – 1) 2
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Table: 6 Flexure Members (Compression Flange Laterally Un-restrained)
PARA- METERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Buckling Resistance Moment Plastic section cLT.Zp.fy / gmo fb.Zp am.as.f.Zp.fy ≤ 1.5am.as.f.Ze.fy Lp < Lb ≤ Lr - Lb > Lr f.Fcr.Ze Compact section
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Table: 6 Flexure Members (Compression Flange Laterally Un-restrained)
PARA- METER IS 800 (2007) BS 5950 (2000) Euro (1993) AS 4100 (1998) AISC 360 (2005) Buckling Resist. Moment Non-compact Section (cLT.Ze)fy /gmo fb.Ze cLT.Ze. fy /gmo Lp< Lb ≤ Lr - Lb > Lr f.Fcr.Ze ≤ 0.9E kc.Ze / l2 Slender Section fb.Zeff cLT.Zeff. fy /gmo Same as Non-compact Section - Do - cLt & fb = Depends on equivalent slenderness am = Moment Modification Factor as = Slenderness Reduction Factor
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Table: 6 Flexure Members (Compression Flange Laterally Un-restrained)
PARAMETERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Equivalent Slenderness Ratio - Imperfection factor (Rolled Section) 0.21 0.21 Imperfection factor (Welded Section) 0.49 0.49 Effective Length Normal Destab. Warping restraint 0.70L 0.85L Both Flanges fully Restrained 0.75L 0.90L Comp. Flange fully restrained 0.80L 0.95L Both Flanges partly Restrained 1.00L Comp. Flange partly restrained 1.20L Warping not restrained in both direction Compression Flange laterally restrained against torsion
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Table: 6 Flexure Members (Compression Flange Laterally Un-restrained)
PARAMETER IS 800 (2007) BS 5950 (2000) Euro (1993) AS 4100 (1998) AISC 360 (2005) Effective length (Contd.) Norm Destab - Partially restrained by bottom flange support connection 1.0L+ 2D 1.2L+ 2D Partially restrained by bottom flange bearing support 1.4L+ 2D Compression flange laterally restrained against torsion Permissible Shear Vd Av.fy / (3.gmo) 0.6 fy.Av 0.6 f fy.AwCv dw / tw ≤ 82 / (fy / 250) 0.5 f 0.6 fy.Av dw / tw > 82 / (fy / 250) 0.5 av.f 0.6 fy.Av = 0.9 to 1.0 Cv ≤ 1.0
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Table: 6 Flexure Members (Compression Flange Laterally Un-restrained)
PARAMETERS IS 800 (2007) BS 5950 (2000) Eurocode (1993) AS 4100 (1998) AISC 360 (2005) Shear Area Hot Rolled I & H Section (Major Axis Bending) h.tw A – 2b.tf + (tw + 2r).tf Rolled Channel Section A – 2b.tf + (tw + r).tf Welded I, H & Box Section (d.tw) Rolled & Welded I, H & Box Section (Minor Axis Bending) 2 b.tf 1.8 b.tf A - (d.tw) RHS Loaded parallel to depth ( h ) A h / (b + h) 0.9A h / (b + h) - RHS Loaded parallel to width ( b ) A b / (b + h) 0.9A b / (b + h) CHS 2 A / 0.6 A Plates and Solid Bars A 0.9 A _
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Domestic Crude Steel Availability Projection: 2012-13
Producer Existing Capacity Capacity by SAIL 12.84 21.40 RINL 2.9 6.3 Tata Steel 6.8 13.0 [3.0] Essar 4.6 10.0 JSW Steel Ltd. 6.6 11.0 Jindal Steel & Power Ltd. 2.4 7.2 Ispat Industries Ltd. 3.6 5.0 Other & Secondary steel 33.02 46.0 [6.0] Total 72.8 119.9 [9.0] With 70 percent actualisation, 90 percent conversion rate and 90 percent capacity utilisation, finished steel availability at 80 million tones by against projected demand of 74 million tones. [ ] -Greenfield expansion Source: JPC & Industry sources
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CHANGES IN STEEL PRODUCTION (Mt) 2008/2009
Country Rank 2009 2008 Change,% China 1 567.8 502.0 13.1 Japan 2 87.5 118.7 -26.3 Russia 3 59.9 91.5 -35.5 United States 4 58.1 68.5 -15.2 India 5 56.6 55.1 2.7 South Korea 6 48.5 53.5 -9.3 Germany 7 32.6 45.8 -28.8 Ukraine 8 29.7 37.1 -19.9 Brazil 9 26.5 33.7 -21.4 Italy 10 19.7 30.5 -35.4 Total 1219.7 1329.7 -8.3 Over the last five years, India has increased production from 32.6 Mt to 56.6 Mt. Has moved from 9th to 5th position. Even 2008 and 2009, saw 3.8% and 2.7% increase in India. AMONGST ONE OF THE HIGHEST IN THE WORLD.
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CHANGES IN STEEL PRODUCTION (Mt) 2009/2010
Rank Country 2010 2009 %2010/2009 1 China 626.7 573.6 9.3 2 Japan 109.6 87.5 25.2 3 US 80.6 58.2 38.5 4 Russia 67.0 60.0 11.7 5 India 66.8 62.8 6.4 6 South Korea 58.5 48.6 20.3 7 Germany 43.8 32.7 34.1 8 Ukraine 33.6 29.9 12.4 9 Brazil 32.8 26.5 23.8 10 Turkey 29.0 25.3 14.6
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Trend of Steel Consumption in India
( to ) CAGR Steel Consumption in India 7.7 percent annually in last decade against 4% annual growth in Global Steel Consumption Source: JPC
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CONCLUSIONS The present IS 800 (LSM) is mostly based on international standards. It is evident from the comparative charts shown above, with load factors and partial safety factors being introduced keeping Indian conditions in consideration. The code has been mainly modeled in line with the Eurocodes which are generally referred for design in the European Countries. Additional references have been taken from the existing British Codes also.
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CONCLUSIONS This code does not totally exclude the Allowable Stress Design (ASD) method of analysis. One chapter in this code has been totally dedicated to design concepts based on the ASD method, with certain modification from the existing Indian Standard (IS ) Code. In American code, both ASD and LRFD method of design is equally prescribed. In case of IS 800, ASD method with minor modification has been included to help in making a smooth and proper transition of design practice in India from ASD philosophy to LSM philosophy.
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Institute for Steel Development & Growth
Thank You Institute for Steel Development & Growth
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