- SECTION 6(m) WORKING STRESS DESIGN
- 17 WORKING STRESS DESIGN ... 82
- SECTION 6(n) DESIGN AND DETAILING FOR EARTHQUAKE LOADS
- 18 DESIGN AND DETAILING FOR EARTHQUAKE LOADS ... 85
- SECTION 6(p) FATIGUE
- 19 FATIGUE ... 89
- SECTION 6(q) DESIGN ASSISTED BY TESTING
- 20 DESIGN ASSISTED BY TESTING ... 98
- SECTION 6(r) DURABILITY
- 21 DURABILITY ...101
- SECTION 6(s) FIRE RESISTANCE
- 22 FIRE RESISTANCE ...102
- SECTION 6(t) FABRICATION AND ERECTION
- 23 FABRICATION AND ERECTION ...107
- ANNEX A ANALYSIS AND DESIGN METHODS ...114
- ANNEX B DESIGN AGAINST FLOOR VIBRATION ...115
- ANNEX C DETERMINATION OF EFFECTIVE LENGTH OF COLUMNS ...116
- ANNEX D ELASTIC LATERAL TORSIONAL BUCKLING ...121
- ANNEX E CONNECTIONS ...124
- ANNEX F GENERAL RECOMMENDATIONS FOR STEELWORK TENDERS AND CONTRACTS ...128
- ANNEX G ...131
- LIST OF STANDARDS ...132
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
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National Building Code Sectional Committee, CED 46
This Code (Part 6/Section 6) covers the structural design aspects of steel structures in buildings.
This Section covers the use of hot-rolled structural steel sections and steel tubes in buildings. It covers the design by limit state method and plastic theory, and also enables design by working stress method.
This Section is based on IS 800 : 2007 ‘Code of practice for general construction in steel (third revision)’ and IS 806 : 1968 ‘Code of practice for use of steel tubes in general building construction (first revision)’.
This Section was first published in 1970 and was subsequently revised in 1983 and 2005. In the 2005 version, the detailed design provisions were not included as IS 800 : 1984 ‘Code of practice for general construction in steel (second revision)’ was under revision at that time. This Section, therefore, then referred to the 1984 version of IS 800 with the remarks that the latest version would prevail, as and when published. It, however, covered the philosophical aspects of limit state design which was being introduced in the revision of IS 800. IS 800 was subsequently revised and published as IS 800 : 2007 and hence, became the prevailing version for use under this Section.
In the revision of IS 800 in 2007, in view of the development and production of new varieties of medium and high tensile structural steels in the country, the scope of the standard was modified permitting the use of any variety of structural steel provided the relevant provisions of the standard were satisfied. IS 800 : 2007 makes reference to the latest Indian Standards available for rivets, bolts and other fasteners. The standard is based on limit state method, reflecting the latest developments.
This revision of the Section is intended to bring it in line with the revised version of IS 800 : 2007 read with its Amendment No. 1 and to take care of further improvements and incorporations required. In this revision, therefore, the following major modifications have been made:
a) The table on tensile properties of structural steel products has been updated by including yield stress values of bolts of sizes 3.6 to 6.8 mm as per IS 1367 (Part 3) ‘Mechanical properties of fasteners made of carbon steel and alloy steel – bolts, screws and studs (fourth revision)’ and aligning the corresponding provisions of the table with those given in IS 2062 : 2011 ‘Hot rolled low, medium and high tensile structural steel (seventh revision)’.
b) Provisions relating to mechanical properties of structural steel have been elaborated.
c) Provisions related to effective wind pressure on exposed circular tube members have been included.
d) Provisions related to requirement of temperature stress analysis have been updated.
e) Provisions related to slender cross-sections have been updated, with the inclusion of two new figures on effective sections of slender members under pure and flexural compression.
f) The table on deflection limits has been suitably updated with respect to the load nomenclature of the load and their combinations.
g) The term ‘imposed load’ has been substituted for ‘live load’ in coherence with the use of terminology in other published standards.
h) Warping restraint conditions in the table on effective lengths for simply supported beams have been clarified.
j) The provision on assessment of conditions under which fatigue assessment is to be done, is covered comprehensively in 19.2.1.2 ‘Necessity for Fatigue Assessment’, which takes care of 13.5.1 of IS 800 : 2007 as well.
k) Provisions related to design of end panels in stiffened web panels have been updated.
m) Provisions related to design for earthquake loads relating to load and load combinations, response reduction factor for various types of frames, connections in the critical zones of frames, and bracing members have been updated.
n) The nominal bearing pressure between the column base plate and the support has been updated to bring
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
it line with the corresponding provisions of IS 456 : 2000 ‘Code of practice for plain and reinforced concrete (fourth revision)’.
p) Provisions related to design of single angle struts loaded through one leg have been revised.
q) Provisions related to non-ductile braced and moment resisting frames have been included.
r) Provisions on ductile designing with respect to column base connections have been updated.
s) Reference design conditions under fatigue for S-N curve have been modified.
t) Provisions on clearances related to fabrication procedures have been updated.
u) Sketches of the restraint conditions under table on effective lengths of single stepped columns have been updated.
w) Certain terminologies, symbols and figures have been updated based on the changes incorporated.
y) List of Indian Standards cross-referred in this Section has been updated.
The revised Section will enhance the confidence of designers, engineers, contractors, technical institutions, professional bodies and the industry and will open a new era in safe and economic construction in steel.
In the earlier versions of this Code, the provisions relating to the use of steel tubes in general building construction was based on IS 806 : 1968 ‘Code of practice for use of steel tubes in general building construction’, which was complementary to IS 800. The design using tubular steel in structural applications may be carried using the limit state design provisions in this Section until IS 806 is revised as per limit state design method.
The design of cold-formed steel members is based on the provisions of IS 801 : 1975 ‘Code of practice for use of cold formed light gauge steel structural members in general building construction’. The standard is currently under revision to be based on the limit state design practice. Until the revised IS 801 is published, the cold formed steel members may be designed using the current IS 801, based on the working stress method.
All standards, whether given herein above or cross-referred to in the main text of this Section, are subject to revision. The parties to agreement based on this Section are encouraged to investigate the possibility of applying the most recent editions of the standards.
For the purpose of deciding whether a particular requirement of this Section is complied with, the final value, observed or calculated, expressing the result of a test or analysis, shall be rounded off in accordance with IS 2 : 1960 ‘Rules for rounding off numerical values (revised)’. The number of significant places retained in the rounded off value should be the same as that of the specified value in this Section.
PART 6 STRUCTURAL DESIGN
Section 6 Steel
SECTION 6(a) GENERAL
1 SCOPE
1.1 This Code (Part 6/Section 6) covers the structural design aspects of steel structures in buildings. This Section applies to general construction using hot rolled steel sections and steel tubes joined using riveting, bolting and welding. Cold formed light gauge steel sections, etc, are covered in separate standards. This Section covers the design by limit state method and plastic theory, and also enables design by working stress method.
1.2 This Section gives only general guidance as regards to the various loads to be considered in design. For the actual loads, such as dead, live, snow, wind and earthquake loads and load combinations to be used, reference may be made to Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.
1.3 Fabrication and erection requirements covered in this Section are general and the minimum necessary quality of material and workmanship consistent with assumptions in the design rules. The actual requirements may be further developed as per other standards or the project specification, the type of structure and the method of construction.
1.4 For seismic design, recommendations pertaining to steel frames only are covered in this section. For more detailed information on seismic design of other structural and non-structural components, reference may be made to 5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code and other special publications on the subject.
2 TERMINOLOGY
For the purpose of this Section, the definitions given below shall apply.
2.1 Accidental Loads — Loads due to explosion, impact of vehicles, or other rare loads for which the structure is considered to be vulnerable as per the user.
2.2 Accompanying Load — Live (imposed) load acting along with leading imposed load but causing lower actions and/or deflections.
2.3 Action Effect or Load Effect — The internal force, axial, shear, bending or twisting moment, due to external actions and temperature loads.
2.4 Action — The primary cause for stress or deformations in a structure such as dead, live, wind, seismic or temperature loads.
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
2.5 Actual Length — The length between centre-to-centre of intersection points, with supporting members or the cantilever length in the case of a free standing member.
2.6 Beam — A member subjected predominantly to bending.
2.7 Bearing Type Connection — A connection made using bolts in ‘snug-tight’ condition, or rivets where the load is transferred by bearing of bolts or rivets against plate inside the bolt hole.
2.8 Braced Member — A member in which the relative transverse displacement is effectively prevented by bracing.
2.9 Brittle Cladding — Claddings, such as asbestos cement sheets which get damaged before undergoing considerable deformation.
2.10 Buckling Load — The load at which an element, a member or a structure as a whole, either collapses in service or buckles in a load test and develops excessive lateral (out of plane) deformation or instability.
2.11 Buckling Strength or Resistance — Force or moment, which a member can withstand without buckling.
2.12 Built-Up Section — A member fabricated by interconnecting more than one element to form a compound section acting as a single member.
2.13 Camber — Intentionally introduced pre-curving (usually upwards) in a system, member or any portion of a member with respect to its chord. Frequently, camber is introduced to compensate for deflections at a specific level of loads.
2.14 Characteristic Load (Action) — The value of specified load (action), above which not more than a specified percentage (usually 5 percent) of samples of corresponding load are expected to be encountered.
2.15 Characteristic Yield/Ultimate Stress — The minimum value of stress, below which not more than a specified percentage (usually 5 percent) of corresponding stresses of samples tested are expected to occur.
2.16 Column — A member in upright (vertical) position which supports a roof or floor system and predominantly subjected to compression.
2.17 Compact Section — A cross-section, which can develop plastic moment, but has inadequate plastic rotation capacity needed for formation of a plastic collapse mechanism of the member or structure.
2.18 Constant Stress Range — The amplitude between which the stress ranges under cyclic loading is constant during the life of the structure or a structural element.
2.19 Corrosion — An electrochemical process over the surface of steel, leading to oxidation of the metal.
2.20 Crane Load — Horizontal and vertical loads from cranes.
2.21 Cumulative Fatigue — Total damage due to fatigue loading of varying stress ranges.
2.22 Cut-Off Limit — The stress range, corresponding to the particular detail, below which cyclic loading need not be considered in cumulative fatigue damage evaluation (corresponds to 108 number of cycles in most cases).
2.23 Dead Loads — The self-weights of all permanent constructions and installations including the self-weight of all walls, partitions, floors, roofs, and other permanent fixtures acting on a member.
2.24 Deflection — It is the deviation from the standard position of a member or structure.
2.25 Design Life — Time period for which a structure or a structural element is required to perform its function without damage.
2.26 Design Load/Factored Load — A load value obtained by multiplying the characteristic load with the partial safety factor for loads.
2.27 Design Spectrum — Frequency distribution of the stress ranges from all the nominal loading events during the design life (stress spectrum).
2.28 Detail Category — Designation given to a particular detail to indicate the S-N curve to be used in fatigue assessment.
2.29 Discontinuity — A sudden change in cross-section of a loaded member, causing a stress concentration at the location.
2.30 Ductility — It is the property of the material or a structure indicating the extent to which it can deform beyond the limit of yield deformation before failure or fracture. The ratio of ultimate to yield deformation is usually termed as ductility.
2.31 Durability — It is the ability of a material to resist deterioration over long periods of time.
2.32 Earthquake Loads — The inertia forces produced in a structure due to the ground movement during an earthquake.
2.33 Edge Distance — Distance from the centre of a fastener hole to the nearest edge of an element measured perpendicular to the direction of load transfer.
2.34 Effective Lateral Restraint — Restraint that produces sufficient resistance to prevent deformation in the lateral direction.
2.35 Effective Length — Actual length of a member between points of effective restraints or effective restraint and free end, multiplied by a factor to take account of the end conditions in buckling strength calculations.
2.36 Elastic Cladding — Claddings, such as metal sheets that can undergo considerable deformation without damage.
2.37 Elastic Critical Moment — The elastic moment, which initiates lateral-torsional buckling of a laterally unsupported beam.
2.38 Elastic Design — Design, which assumes elastic behaviour of materials throughout the service load range.
2.39 Elastic Limit — It is the stress below which the material regains its original size and shape when the load is removed. In steel design, it is taken as the yield stress.
2.40 End Distance — Distance from the centre of a fastener hole to the edge of an element measured parallel to the direction of load transfer.
2.41 Erection Loads — The actions (loads and deformations) experienced by the structure exclusively during erection.
2.42 Erection Tolerance — Amount of deviation related to the plumbness, alignment, and level of the element as a whole in the erected position. The deviations are determined by considering the locations of the ends of the element.
2.43 Exposed Surface Area to Mass Ratio — The ratio of the surface area exposed to the fire (in mm²) to the mass of steel (in kg).
NOTE — In the case of members with fire protection material applied, the exposed surface area is to be taken as the internal surface area of the fire protection material.
2.44 Fabrication Tolerance — Amount of deviation allowed in the nominal dimensions and geometry in fabrication activities, such as cutting to length, finishing of ends, cutting of bevel angles, etc.
2.45 Factor of Safety — The factor by which the yield stress of the material of a member is divided to arrive at the permissible stress in the material.
2.46 Fatigue — Damage caused by repeated fluctuations of stress, leading to progressive cracking of a structural element.
2.47 Fatigue Loading — Set of nominal loading events, cyclic in nature, described by the distribution of the loads, their magnitudes and the numbers of applications in each nominal loading event.
2.48 Fatigue Strength — The stress range for a category of detail, depending upon the number of cycles it is required to withstand during design life.