IS: 800-2007--- Indian Code of Practice for Construction in Steel Dr. T. K. Bandyopadhyay Joint Director general Institute for Steel Development & Growth

TOPICS COVERED INTRODUCTION BRIEF DISCUSSION ON DESIGN METHODOLOGIES BRIEF OVERVIEW OF IS 800 (2007) IS 800 (2007) & OTHER INTERNATIONAL CODES CONCLUSIONS

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 800 --- Basic Code for Design of Steel Structures The code was revised in the year 2007 and available since Feb’08 i.e. after release of the document. Earlier version of the code was much outdated compared to the recent developments in steel design all over the world.

INTRODUCTION Earlier IS: 800 was based on Allowable Stress design (ASD) methodology. Methodology of Design of Steel Structures has undergone major changes during the last two decades due to research all over the world. Revision of many other steel related codes in India are also dependent on revision of IS 800. An out-dated code is detrimental to the very purpose of the code of practice itself. Thus, revision of IS 800 was essential to include design stipulations as are prevalent all over the world and to ensure availability of efficient sections.

Table: 1 Countries and their Design Format INTRODUCTION Almost all countries are adopting 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)

INTRODUCTION LSM has become the design philosophy in most of the International design standards. LSM design ensures Rationality in Design Economy of Design In India it was felt that IS: 800 should be modified to LSM keeping ASD as a transition alternative. It was also felt that this modification would render steel design novel and will facilitate accuracy of design. However, it is important that the basic philosophy of both the design methods is understood by all.

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) Factor of safety (Fs) is fixed.

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.

BRIEF DISCUSSION ON DESIGN METHODOLOGIES B. LIMIT STATE METHOD (LSM) 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.

BRIEF DISCUSSION ON DESIGN METHODOLOGIES B LIMIT STATE METHOD (LSM) 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 Ultimate Limit States ---- It takes care of the structure from strength point of view Serviceability Limit States ---- It takes care of the structure in terms of safe operation

BRIEF DISCUSSION ON DESIGN METHODOLOGIES B LIMIT STATE METHOD (LSM) The criteria which defines ultimate limit states 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

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. Based on slenderness ratio of constituent plate element a section may be classified as Plastic Compact Semi-compact Slender

BRIEF DISCUSSION ON DESIGN METHODOLOGIES B LIMIT STATE METHOD (LSM) 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.

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.

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

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 give 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.

BRIEF OVERVIEW OF IS 800 (2007) TENSION MEMBERS Fig 1 Percentage strength utilized in Tension Members

BRIEF OVERVIEW OF IS 800 (2007) COMPRESSION MEMBERS Fig 2 Percentage strength utilized in Compression Members

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

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

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

BRIEF OVERVIEW OF IS 800 (2007) FLEXURE MEMBERS Fig 4 Percentage strength utilized in Flexure Members

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)

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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 _

CONCLUSIONS It is evident from the comparative charts shown above, with load factors and partial safety factors being proposed 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.

CONCLUSIONS Contd. An important aspect of this latest code is that this code does not totally exclude the existing 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 Indian Standard (IS 800- 1984) 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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