INSDAG GUIDE FOR
THE STRUCTURAL USE OF
STEELWORK IN BUILDINGS
Compiled by:
Dr. Rangachari Narayanan
Dr. V. Kalyanraman
Published by:
Institute for Steel Development And Growth
Ispat Niketan', First Floor 52/1ABallygunge Circular Road Kolkata - 700 019
Phone: (033) 2461 4045/47/66/76, Fax: (033) 2461 4048 E-mail: [email protected]; insdag@caj2^nLneLui
March 2003
INSDAG 7
REVISED PRICE
Copyright reserved 1 0 0 0 / -
52/1 A , Bally gunge CUcuUi Road -Kolkata-700019
Although care has been taken to ensure, to the best of our knowledge that all the data and information contained herein are correct to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, Institute for Steel Development And Growth (INSDAG) assumes no responsibility for any errors in or mis-interpretations of such data and/or information or any loss or damage arising from or related to their use. __________________________________ U S E M O R E S T E E L - T H E P R E F E R R E D M A T E R I A L O F T H E N E W
FOREWARD
INSDAG has played a pivotal role over the last few years in propagating the awareness amongst students, faculties of various engineering institutes and experts and professionals from various industries, about the advantages and benefits of usage of steel in the construction sector.
It is now being accepted by most engineering professionals both academic and industrial, that the main stumbling block in the development of the steel construction industry in India is the primitiveness of the methods of design adopted by the Indian codes as against the international codes which allow higher flexibility in design approach. The relevant Indian codes of practice (IS: 800-1984 and IS: 801-1975) applicable for hot-rolled and cold-formed steel are based on the "Allowable Stress Design" approach as against the more internationally popular "Limit State Method" approach which has been proved to be technically sound and its use results in optimum economy of the structure.
With the technical contributions from leading academics and professionals, INSDAG has already brought out various publications on the design methodology of steel structures using the Limit State Method of Design (LSM), which have been beneficial to the engineering fraternity in learning the most intricate facets in LSM design.
On request from INSDAG, this publication in the form of a Guide book has been written and compiled by Dr. Rangachari Narayanan and Dr. V. Kalyanraman for the benefit of not only the student community both under-graduate and post graduate level, but also other engineering professionals across the country, since most of the engineering institutions have started including the LSM design in their curriculum and also the engineering professionals need to update themselves with the latest technological advancements. The publication is very timely as it coincides with the revision of IS: 800- 1984, which is at its advanced stage.
The entire book has been reviewed by^Dr. T. K. Bandyopadhyay, Deputy Director General and Mr. Arijit Guha, Manager (Civil & Structural). Comments and suggestions received from a large number of faculty member*, have been incorporated. INSDAG expresses its indebtedness to Dr. R. Narayanan and Dr. V. Kalyanraman, academics and researchers of international experience for agreeing to bring'out this publication.
Special Note
The entire document has been written considering Limit State Method of design following stipulations laid down in the relevant British code, BS: 5950 Part -1, 3 & 5 and Eurocode - 3 & 4. Since IS: 800 (Code of Practice for General Construction in Steel) is presently being revised to Limit State version, this guide book may undergo certain modifications in some chapters after the publication of revised IS: 800 (LSM version) to accommodate the possible variation in stipulations that are likely to be considered in the revised code.
However, this document will be extremely useful to the students of Civil I Structural Engineering to understand the theoretical background associated with advancement in structural steel design based on Limit State Method. ______________________________
CONTENTS
Pages
1. General 2 - 3
2. Material 4 - 4
3. General Design Requirements 5 - 1 5
4. Tension Members 16 - 18
5. Classification of Cross Sections 19 - 21
6. Axially Loaded Columns 2 2 - 3 1
7. Design of Members Subjected to Bending 32 - 58
8. Elements Subjected to Axial force and Bending 59 - 64
9. Beams of Hot Rolled Sections Subjectedto Torsion and
65 - 65
10. Portal Frames 66 - 72
11. Multi - Storey Buildings 7 3 - 8 8
12. Connection Design 89 - 109
13. Cold Formed Steel Sections 110 - 130
14. Basic Concepts of Composite Construction 131 - 139
15. Composite Beams and Slabs 140 - 153
16. Steel - Concrete Composite Columns 154 - 167
LIST OF APPENDICES
A. Appendix - A: Terminology 168 - 169
B. Appendix- B : Symbols 1 7 0 - 1 7 2
C. Appendix - C: Relevant Indian Standards 173 - 174
D. Appendix - D: An Approximate Method of Torsion Analysis 175 - 180
1 PREFACE
The low usage of structural steel in India is attributable in part to the prevailing out-of-date design practices, which result in uneconomic designs. The relevant Indian Codes of Practice (IS: 800 - 1984, IS: 801 - 1975) applicable to the structural use of hot-rolled and cold-rolled steel are largely based on "Working Stress Method". The more modern "Limit State Design Approach" developed in the 1970's in the West, is technologically sound and results in significant economies in completed structures. This is of particular advantage, as steel is reusable and environment friendly. Compared with competing materials of construction, steel framed buildings have significantly better blast and earthquake resistance and take less than half the time to build. In passing, it may be noted that the Indian Codes of Practice applicable to concrete structures have been revised to conform to Limit State Methodology. This makes the choice of steel in construction an uneconomic proposition. It is also noted that the Code of Practice for steel-concrete-composite buildings (IS: 11384 - 1985) is based on the Limit State approach but is very limited in its coverage, besides being inconsistent with IS: 800 and IS: 801 written in Working Stress format.
This situation posed a challenge, when the Government of India, Ministry of Steel initiated steps to rectify the skills shortage in Steel Construction in 1998. The newly started Institute for Steel Development and Growth (INSDAG) was entrusted with the tasks of (a) improving the teaching standards of Structural Steel Design in Indian Universities, (b) organising in-career courses for enhancing the level of competence of practising engineers (c) publishing design guidance documents for disseminating latest Steel Design Technology (d) organising design competitions for encouraging state-of-the art Structural Steel Designs. As a part of that initiative, an up-to-date Resource Material for disseminating the latest Steel Design Technology has been compiled and published in the web site of INSDAG (www.steel-insdag.org). This Design Guide has been complied, as a complementary document and has been drafted after studying the background research work carried out largely in the Western World, which led to the latest British, American, Canadian, Australian and European Codes. Many of the design specifications contained herein have been adopted from these Western Codes and will hopefully serve as a Draft document, when the Bureau of Indian Standards eventually decides to revise the Steel Codes, relevant to Construction.
The technical support provided by two young engineers, Mr. S. Sambasiva Rao and Miss P. Usha in compiling this document is gratefully acknowledged.
Dr. T.K. Bandhyopadhyay of INSDAG and Professor A. R. Santhakumar of Anna University had reviewed the document before its publication as a draft. Suggestions and comments aimed at improving this document are welcome. We are also grateful to the many engineers - too numerous to mention - who suggested improvements in the drafting stage.
Rangachari Narayanan V. Kalyanaraman
SECTION 1: GENERAL
2 1.1 Scope
This Guide provides general recommendations for the design of structural steel work in buildings and allied structures. In the absence of an Indian Standard written in the modern Limit State Format for steel construction, this guide generally follows the provisions contained in British Standard, BS: 5950 (various parts). INSDAG has a Memorandum of Understanding with the British Steel Construction Institute and several supporting documents are available from INSDAG at largely discounted prices for the use of steel designers in India. It will not apply to bridges, chimneys, cranes, tanks, transmission line towers, storage structures, tubular structures, however, general .principles discussed in this guide could be adopted in the design of such structures appropriately..
This guide is in three parts and covers the design of building structures using (i) Hot Rolled Steel section (ii) Cold Rolled Steel sections and (iii) Steel Concrete Composite sections.
The guide provides only general advice regarding the various loads to be considered in design. For actual loads to be used reference may be made to IS: 875-1987.
This document is NOT a statutory document and intended as a guide for students and practicing engineers. It is not intended to replace Codes of Practice.
1.2 Terminology - For the purpose of this Guide, the definitions of various terms are given in Appendix A.
1.3 Symbols - Symbols used in this Guide are defined in Appendix B.
1.4 Reference to other Standards - All the standards referred to in this Guide are listed in Appendix C and their latest version shall be applicable:
1.5 Units and Conversion Factors - The SI system of units is applicable to this Guide. For conversion of system of units to another system, IS: 786-1967 (supplement) may be referred.
1.6 Standard Dimensions, Form and Weight
The dimensions, form, weight, tolerances of all rolled shapes and other members used in any steel structure shall, wherever available, conform to the appropriate Indian Standards.
The dimensions, form, weight, tolerances of all rivets, bolts, nuts, studs, etc. shall conform to the requirements of appropriate Indian Standards, wherever available.
3 1.7 Plans and Drawings
Plans, drawings and stress sheet shall be prepared according to IS: 696-1972 and IS: 962-1967.
Plans - The plans (design drawings) shall show the complete design with sizes, sections, and the relative locations of the various members. Floor levels, column centres, and offsets shall be dimensioned. Plans shall be drawn to a scale large enough to convey the information adequately. Plans shall indicate the type of construction to be employed; and shall be supplemented by such data on the assumed loads, shears, moments and axial forces to be resisted by all members and their connections, as may be required for the proper preparation of shop drawings. Any special precaution to be taken in the erection of structure from the design consideration shall also be indicated in the drawing.
Shop drawings - Shop drawings, giving complete information necessary for the fabrication of the component parts of the structure including the location, type, size, length and detail of all welds, shall be prepared in advance of the actual fabrication. They shall clearly distinguish between shop and field rivets, bolts and welds. For additional information to be included on drawings for designs based on the use of welding, reference shall be made to appropriate Indian Standards. Shop drawings shall be made in conformity with IS: 696-1972 and IS: 962-1967. A marking diagram allotting distinct identification marks to each separate part of steelwork shall be prepared. The diagram shall be sufficient to ensure convenient assembly and erection at site.
It is essential that Steel Designers familiarize themselves with protection methods for structural steelwork, with regard to fire and corrosion. For a great majority of steel buildings which are not subject to alternate wetting and drying, corrosion is NOT a problem. Authentic guidance on protection methods is available from INSDAG.
SECTION 2: MATERIALS
4
Structural Steel - Ail structural steels used in general construction coming under the purview of this Guide shall, before fabrication conform to IS: 2062-1984, IS: 8500-1977 and IS: 1977-1975, as appropriate.
Any structural steel other than that specified in 2.1 may also be used provided that the characteristic yield stresses and other design provisions are suitably modified and the steel is also suitable for the type of fabrication adopted.
Other Material - All other materials including manufactured products, welding consumables, steel castings, bolts and nuts and cement concrete shall confirm to the requirements of the appropriate Indian Standards.*
SECTION 3: GENERAL DESIGN REQUIREMENTS 3.1 Aims of Structural Design
The aim of structural design is to provide, with due regard to economy, a structure which is fit for its intended purpose, i.e., it should be capable of fulfilling its intended function and sustaining the design loads for its intended life. The design should facilitate fabrication, erection and future maintenance.
The structure should behave as a three-dimensional entity. The layout of its constituent parts, such as foundations, steelwork, connections and other structural components should constitute a robust and stable structure under normal loading to ensure that in the event of misuse or accident, damage will not be disproportionate to the cause. To achieve this, it is necessary to define clearly the basic structural anatomy by which the loads are transmitted to the foundations. Any features of the structure, which have a critical influence on its overall stability, can then be identified and taken account of in design.
Each part of the structure should be sufficiently robust and insensitive to the effects of minor incidental loads applied during service that the safety of other parts is not prejudiced.
3.2 Overall Stability
The designer responsible for the overall stability of the structure should ensure the compatibility of design and details of parts and components. There should be no doubt of this responsibility for overall stability when some or all of the design and details are not made by the same designer.
3.3 General Principles of Limit State Design
Structure should be designed considering the Limit States at which they would become unfit for their intended purpose. For verifying the adequacy of the structure, appropriate partial safety factors, based on semi-probabilistic methods described below shall be used. Two partial safety factors, one applied to forces due to loading and another to the material strength shall be employed.
allows for;
(a) the possible deviation of the actual behaviour of the structure from that of the analysis and design model,
(b) the deviation of loads from their specified values and
(c) the reduced probability that the various loads acting together will simultaneously reach the characteristic value.
(e) the possible deviation of the material in the structure from that assumed in design (f) the possible reduction in the strength of the material from its characteristic value and (g) manufacturing tolerances.
(h) Mode of failure (ductile/brittle).
3.3.1 Partial safety factors
In general, calculations take the form of verifying that
where is the calculated factored load effect on the element (like bending moment, shear force etc) and is the calculated factored resistance of the element being checked, and is a function of the nominal value of the material yield strength.
is a function of the combined effects of factored dead, live and wind loads. (Other loads - if applicable, are also considered)
In accordance with the above concepts, the safety format used in this guide is based on probable maximum load and probable minimum strengths, so that a consistent level of safety is achieved. Thus, the design requirements are expressed as follows:
where = Design value of internal forces and moments caused by the design Loads, Characteristic Loads. (From IS: 875 - 1987)
a load factor which is determined on probabilistic basis
where = a material factor, which is also determined on a 'probabilistic basis'
when considering yield stress and 1.25 when considering fracture ultimate stress). It should be noted that IS: 11384 - 1985 (Code of Practice for Composite Construction) has prescribed for Structural Steel when considering yield stress. The value
suggested is therefore consistent with that.
3.3.2 Limit states
(1) A limit state is a state beyond which the structure no longer satisfies the design requirements.
7
(2) Ultimate limit states are limit states of collapse or other structural failure, which might endanger the safety of people, including:
• Excessive deformation / formation of mechanism. • Rupture
• Loss of stability • Loss of equilibrium
(3) Serviceability limit states are limit states beyond which specified service criteria are no longer met, including those for:
• Deflection • Durability • Ponding • Vibration
Thus the following limit states may be identified for design purposes and are provided for in terms of partial factors reflecting the severity of the risks.
• Ultimate Limit State is related to the maximum design load capacity under extreme conditions. The partial load factors are chosen to reflect the probability of extreme conditions, when loads act alone or in combination.
• Serviceability Limit State is related to the criteria governing normal use. Unfactored loads are used to check the adequacy of the structure.
• Fatigue Limit State is important where distress to the structure by repeated loading is a possibility.
An illustration of partial safety factors suggested for ultimate load conditions is given in Table 3.1. These values are based on recommendations adopted by Eurocodes. (The Committee formed to review BIS standards have adopted these values). Reference to the Code of Practice for Earthquake Resistant Design should be made, where appropriate. (At the present time, this Code is being revised).
Loading Yf
DL LL WL
Dead Load (unfavourable effects) 1.35 - -
Dead load restraining uplift or overturning 1.0 - -
Dead Load + Imposed Load 1.35 1.5 -
Dead Load + Wind Load 1.35 - 1.5
Dead Load + Imposed Load + wind Load (Major Load)* 1.35 1.05 1.5 Dead Load + Imposed Load (Major Load) + wind Load* 1.35 1.5 1.05 Crane Load effects (from BS 5950, Parti)
Vertical load 1.6
Horizontal load 1.6
Vertical load acting with horizontal load 1.4
(Crabbing or Surge)
Crane load acting with Wind load
1.2 1.2 1.2
*If in doubt, calculations for both conditions are needed
8 3.4 Loading
3.4.1 Types of loads - For the purpose of computing the maximum stresses in any structure or member of a structure, the following loads and secondary effects shall be taken into account, where applicable:
a) Dead Loads, Imposed loads and Wind loads (as per IS: 875 - 1987) b) Earthquake loads (as per IS: 1893 - 1991)
c) Erection loads; and
d) Secondary effects due to contraction or expansion resulting from temperature changes, creep in steel, shrinkage and creep in contiguous concrete members, differential settlements of the structure as a whole and its components.
e) For fire resistant design and fire rating, reference may be made to appropriate specialist publications [For example, Design guide on Structural Fire Safety C1B-W14)
3.4.2 Erection loads - All loads required to be carried by the structure or any part of it due to storage or positioning of construction material and erection equipment including all loads due to operation of such equipment shall be considered as 'erection loads'. Proper provision shall be made, including temporary bracings to take care of all stresses due to erection loads. The structure as a whole and all parts of the structure in conjunction with the temporary bracings shall be capable of sustaining these erection loads. Dead load, wind load and also such parts of the live load as would be imposed on the structure during the period of erection shall be taken as acting together with the erection loads.
3.4.3 Temperature effects
(a) Expansion and contraction due to changes in temperature of the materials of a structure shall be considered and adequate provision made for the effects produced.
(b) The temperature range varies for different localities and under different diurnal and seasonal conditions. Published data should be consulted in assessing the maximum variations of temperature for which provision for expansion and contraction has to be allowed in the structure.
(c) The co-efficient of expansion for steel shall be taken as 0.000012 per degree centigrade per unit length.
3.5 Robustness Requirements
The requirements for all buildings to maintain Structural integrity (as prescribed by BS: 5950, Part 1 following the Ronan Point Collapse) are given below:
Structures should remain as complete integral units even when (due to an accident such as explosion) one of the members fail or become inoperative. This requirement provides a
significant measure of safety for the occupants and is termed "Structural integrity requirement" or "Robustness requirement".
All building frames should be effectively tied together at each principal floor and roof level, in both directions. Either the beams or tie members should be designed so that they provide for the anchorage. Ties may be steel members or steel reinforcement, which are properly anchored to the steel frame work.
Each section between expansion joints should be treated as a separate building. These requirements are aimed at ensuring that the collapse of one element of a structure does not trigger the failure of the structure as a whole. By tying the structure together, it is possible to ensure that there is an alternative load path that would help to avoid progressive collapse.
Suggested requirements for integrity of buildings of five storeys or more are given below:
• For sway resistance, no portion of structures should be dependent on only one bracing system.
• The minimum tie strengths (in respect of the ties referred above) should be
internally and externally (but not less than 75 kN for floors and 40 kN at roof level), where
- total factored load /. unit area - tie spacing
- distance between columns in the direction
• At the edge of the structure, columns should be restrained by horizontal ties resisting 1% of column load.
• Columns should be continuous vertically through the floors, as far as possible.
• Column splices should be capable of resisting a tensile force of two - thirds of the factored vertical compressive load on the column below the splice.
• Collapse must not be disproportionate and the role of key elements should be identified. • Precast floors must be anchored at both ends against sliding of supporting members. • At each storey in turn any single column or beam carrying a column should be capable of
being removed without causing collapse beyond a limited portion of the building in the vicinity of the member; in this event substantial permanent deformation may be accepted. This is termed as " Localisation of damage".
• If the removal of one of these members would cause substantial damage, the member should be designed as a "key element" so that it has a very low probability of failure. Any member or other structural component, which provides lateral restraint vital to the "key element", as well as the "key elements" themselves should be checked for safety and stability, (using appropriate load factors and including the likely accidental loads) in the appropriate directions.
10
General Principles and Design Methods 3.6
3.6.1 Methods of design - The design of any structure or its parts may be carried out by one of the methods given in (a) to (d). In all cases, the details of members and connections should be such as to realise the assumptions made in design without adversely affecting any other parts of the structure.
(a) Simple design - The connections between members are assumed not to develop moments adversely affecting either the members or the structure as a whole.
The distribution of forces may be determined assuming that members intersecting at a joint are pin connected. The necessary flexibility in connections may result in some non-elastic deformation of the materials, other than the fasteners.
It is necessary to maintain stability against sway and this is ensured complying with provisions of 3.6.2.2 (c).
(b) Rigid design - The connections are assumed to be capable to developing the strength and / or stiffness required by an analysis assuming full continuity. Such analysis may be made using either elastic or plastic methods.
(c) Semi-rigid design - Some degree of connection stiffness is assumed, but it would be insufficient to develop full continuity.
(i) The moment and rotation capacity of the joints should be based on experimental evidence, which may permit some limited plasticity. On this basis, the design should satisfy the strength, stability and stiffness requirement of all parts of the structure when partial continuity at the joints is to be taken into account in assessing moments and forces in the members.
(ii) As an alternative, in simple beam and column structures an allowance may be made for the inter-restraint of the connections between a beam and a column by an end restraint moment not exceeding 10% of the free moment applied to the beam, assuming this to be simply supported, provided that the frame is braced against side sway in both directions.
(d) Design based on experiments - Where structure is of non-conventional or complex in nature, the design may be based on full scale or model tests subject to the following conditions:
(i) A full-scale test of prototype structure may be done. The prototype shall be accurately measured before testing to determine the dimensional tolerance in all relevant parts of the structure; the tolerances then specified on the drawing shall be such that all successive structures shall be in practical conformity with the prototype. Where the design is based on failure loads, a load factor of not less than 1.5 on the loads or load
combinations given in Table 3.1 should be used. Loading devices shall be previously calibrated and care shall be exercised to ensure that no artificial restraints are applied to the prototype by the loading systems. The distribution and duration of forces applied in the test shall be representative of those to which the structure is deemed to be subjected.
(ii) In the case where design is based on the testing of a small-scale model structure, the model shall be constructed with due regard for the principles of dimensional similarity. The thrusts, moments and deformations under working loads shall be determined by physical measurements made when the loadings are applied to simulate the conditions assumed in the deign of the actual structure.
3.6.2 Ultimate Limit States 3.6.2.1 Limit state of strength
(a) General - In checking the strength and stability of the structure the loads should be multiplied by the relevant ^factors given in table 3.1. The factored loads should be applied in the most unfavorable realistic combination for the part or effect under consideration.
The load capacity of each member and its connections, as determined by the relevant provisions of this Guide, should be such that the factored loads would not cause failure.
3.6.2.2 Stability limit state
(a) General - In considering the overall stability of any structure or part, the loads should be increased by the relevant factors given in table 3.1. The designer should consider overall frame stability, which embraces stability against overturning, and sway stability as given below.
(b) Stability against overturning - The factored loads should not cause the structure or any part of the structure (including the foundations) to overturn or lift off its seating. The combination of imposed and dead loads should be such as to have the most severe effect on overall stability.
Account should be taken of probable variations in dead load during construction or other temporary conditions.
(c) Sway stability - All structures, including portions between expansion joints, should be adequately stiff against sway. To ensure this, in addition to designing for applied horizontal loads, a separate check should be carried out for notional horizontal forces.
These notional forces may arise from practical imperfections such as lack of vertically and should be taken as the greater of:
1% of factored dead load from that level, applied horizontally;
0.5% of factored total gravity load (dead plus vertical imposed) from that level, applied horizontally.
The notional forces should be assumed to act on all structures in any one orthogonal direction at a time and should be applied at each roof and floor level or their equivalent. They should be taken as acting simultaneously with vertical loads.
The notional force should not be:
• applied when considering overturning; • combined with horizontal loads; • combined with temperature effects;
• taken to contribute to net shear on the foundations.
Sway stability may be provided for example by braced frames, joint rigidity or by utilising staircase, lift cores and shear walls. Whatever system is used, reversal of loading should be accommodated. The cladding, floors and roof should have adequate strength and be so secured to the structural framework as to transmit all horizontal forces to the points of sway resistance. Where such sway stability is provided by construction other than the steel framework, the steelwork designer should state clearly the need for such construction and the forces acting upon it.
3.6.2.3 Foundation design - The design of foundations should accommodate all the forces imposed on them. The stiffness (deformation) of the foundation should reflect the boundary condition assumed in the analysis model of the structural system. Attention should be given to the method of connecting the steel superstructure to the foundations and the anchorage of any holding down bolts. Where it is necessary to quote the foundation-reactions it should be clearly stated whether the forces and moments result from factored or unfactored loads. Where they result from factored loads the relevant factors for each load in each combination should be stated.
3.6.2.4 Fatigue - Fatigue need not be considered unless a structure or element is subjected to numerous significant fluctuations of stress. Stress changes due to fluctuations in wind loading need not be considered but account should be taken of wind-induced oscillations.
3.6.2.5 Earthquake Resistant Design - The standards appropriate for earthquake resistance of buildings in various parts of the country should be carefully considered and suitable provisions should be made taking into account the Capacity design and requisite ductility.
13 3.6.3 Serviceability Limit State
3.6.3.1 Deflection - The deflection under serviceability loads of a building or building component should not impair the strength of the structure/components or cause damage to the finishing. When checking for deflections the most adverse and realistic combination of service loads and their arrangement should be checked by elastic analysis.
Table 3.2 gives recommended limitations for certain structural members. Circumstances may arise where greater or lesser values would be more appropriate. (Where the deflection due to Dead + Live load combination is likely to be excessive, consideration should be given to pre-camber the beams)
Table 3.2: Deflection limits other than for pitched roof portal frame ( a ) Deflection on beams due to unfactored imposed loads
Cantilevers Length / 180
Beams carrying plaster or other brittle Span / 325 finish
All other beams Span / 325
( b ) Horizontal deflection of columns other than portal frames due to unfactored imposed and wind loads
Tops of columns in single-storey Height / 325 buildings
In each storey of a building with more Height of storey under consideration / 325 than one storey
( c ) Crane gantry girders Refer to IS: 800 - 1984
NOTE 1. On low-pitched and flat roofs the possibility of ponding needs consideration for Composite Construction using metal decking.
3.6.3.2 Durability - Several factors affecting the durability of the buildings under conditions relevant to their intended life, are listed below:
(a) the environment; (b) the degree of exposure;
(c) the shape of the members and the structural detailing (d) the protective measure if any;
(e) whether maintenance is possible.
Detailed advice on protection of steel for various environmental/exposure conditions is contained in an INSDAG publication titled "Corrosion Protection for Structural Steel".
3.6.3.3 Ponding
a) All roofs with a slope of less than 5% must be checked to ensure that rainwater cannot collect in pools. Allowance must be made for possible construction inaccuracies, settlements of foundations, deflections of roofing
materials and structural members and the effects of pre-camber. This also applies to floors of car parks and other open-sided structures.
b) Pre-cambering of beams can be used to reduce the likelihood of rainwater collecting in pools, provided that rainwater outlets are appropriately located.
c) Where the roof slope is less than 3%, it must be checked that collapse cannot occur due to the weight of water (or snow- if applicable) collected in pools, which might be formed due to the deflection of structural members or roofing material.
3.6.3.4 Dynamic effects
a) The design must make suitable provision for the effects of imposed loads, which can induce impact, vibration, etc.
b) Vibration caused by machines and oscillation caused by harmonic resonance must be considered, and provided for.
c) To avoid resonance, the natural frequencies of structures or parts of structures must be sufficiently different from those of the excitation source.
d) Table 3.3 gives limiting values for the natural frequency or the alternative total deflection to avoid resonance.
Fig 3.1 Vertical deflections to be considered
sagging in the final state relative to the straight line joining the supports, pre-camber (hogging) of the beam in the unloaded state, (state 0)
variation of the deflection of the beam due to permanent loads immediately afterloading, (state 1)
variation of the deflection of the beam due to the variable loading plus any time dependant deformations due to the permanent load, (state 2)
SECTION 4: TENSION MEMBERS 4.1
Limiting Load on Plates in Tension
In the design of tension members, the load-causing yield across the section is taken as one of the limiting loads. The corresponding design strength for the member under axial tension is given by
where, is the yield stress of the material (in M P a), is the gross area of cross section in and Ym is the partial safety factor for failure in tension by yielding. (The suggested value of
The design strength in tension as governed by net cross-section at the hole, is given by
where, is the ultimate stress of the material, is the net area of the cross section after deductions for the hole and is the partial safety factor against ultimate tension failure by rupture (The suggested value of . Similarly threaded rods subjected to
tension could fail by rupture at the root of the threaded region and hence net area, is the root area of the threaded section.
The lower value of the design tension capacities, as calculated by Eqn. 4.1 and 4.2, will govern the tensile design strength of a plate with holes.
Fig. 4.1 Plates with Bolt Holes under Tension 16
When multiple holes are arranged in a staggered fashion in a plate (Fig 4.1), the net area corresponding to the staggered section will be given by
(4.3)
where, n is the number of bolt holes in the staggered section and the summation over is carried over all inclined legs of the section. The design strength in tension will be obtained by substituting the value of in Eqn. 4.2
4.2 Limiting Load on Angles under Tension
When a connection is made through one leg of an angle, the stress in the outstanding leg at the ultimate stage will be closer to the yield stress (due to shear lag) while the net section of the connected leg will often reach the ultimate stress The tensile strength of angles connected by one leg, is evaluated accounting for this phenomenon by
1. limiting the stress in the outstanding leg to (the yield stress) 2. and the connected leg having holes to (the ultimate stress).
In addition, the potential for "block shear failure" should also be assessed. The design tensile strength, will be the minimum value obtained from (4.4), (4.5), and (4.6) below:
(i) Strenfith as governed by the yielding of gross section:
where, is the gross area of the angle section.
(ii) Strength as governed by tearing at net section:
where, and are the yield and ultimate stress of the material, respectively. and are the net area of the connected leg and the gross area of the outstanding leg, respectively, accounts for the end fastener restraint effect.
when the number of fasteners
when the number of fasteners is 3 if the number of fasteners is 1 or 2 and if the connection is adequately welded
(iii) Strength as governed by block shear failure:
A tension member may fail along end connection due to block shear as shown in Fig. 4.2. If the centroid of bolt pattern is not located between the heel of the angle and the centerline of the connected leg, the connection shall be checked for block shear strength. The corresponding design strength in tension shall be evaluated as the lower of the value obtained from the following equations.
Fig. 4.2 Block Shear Failure
where, and = minimum gross and net area in shear along a line of transmitted force, respectively, and = minimum gross and net area in tension from the hole
to the toe of the angle, perpendicular to the line of force, respectively.
4.3 Maximum Slenderness Ratio
The maximum slenderness ratio (length/least radius of gyration of the cross section) of a tension member is limited to 400 (This will provide a margin of safety for members normally acting as ties but subject to reversal of stresses due to wind and earthquake. It will also provide a margin for avoiding excessive self-weight deflection).
SECTION 5: CLASSIFICATION OF CROSS SECTIONS 5.1 Basis
The proposed classification of cross sections is illustrated by considering the idealised moment-rotation characteristics of a symmetrical beam subjected to incremental flexural loading continued till its collapse. A beam capable of developing full plasticity would exhibit an idealised elastic/plastic moment-rotation curve as shown in Fig. 5.1. At failure, the stress distribution across the section will consist of two rectangles and a significant rotation will take place. Such a stocky section is termed as a 'plastic' section, and it exhibits considerable "ductility" is the rotation at the onset of plasticity; is the lower limit of rotation for treatment as a plastic section)
Fig. 5.1 Elastic/plastic moment-rotation curve.
On the other hand, a cross section may develop fully plastic stress distribution across the entire cross section but may not have adequate ductility The horizontal part of the moment-rotation diagram will be limited. Such a cross-section is termed 'compact' section.
If the section were to be even more slender (higher ratios of it may only be able to sustain an elastic moment up to the attainment of yield strength in the extreme fibres, with a triangular stress distribution. This section is termed as 'semi-compact'.
If the section were to be further more slender still (i.e. yet higher values of local buckling would occur before the attainment of yield stress in the extreme fibres, i.e. before attaining the theoretical elastic moment capacity. Such a section is termed as 'slender'.
Assuming that the flange plate or the web does not buckle locally, these four different modes of behaviour can be expressed graphically on a plot of stress against strain at the
extreme fibres (Fig. 5.2). These different modes of behaviour can also be shown by the stress patterns, as in Fig. 5.3.
Fig. 5.2 Stress/strain relation of extreme fibres for different classes of sections
Fig. 5.3 Bending stress distribution for different classes of sections
The class of a section is determined by the lowest class of all its constituent elements, i.e. flange plates and web plate. The class of section determines its resistance (e.g. Moment resistance, shear resistance etc.).
Only plastic sections can be used in forming plastic collapse mechanisms. Compact sections can generally be used in simply supported beams failing soon after reaching at one section. In elastic design, semi-compact sections are to be used with the understanding that they will fail at The slender section design is discussed in the section on Cold-Form Steel member design.
Table 5.1. Limits on Width to Thickness Ratio of Plate Elements* Type of Element Type of
Section Outstand element of compression flange Welded Rolled Internal element of compression flange Welded Rolled Web with neutral axis
at mid depth
All Web under uniform compression Welded Rolled Single/double angle T-stems Rolled Circular tube with
outer diameter D where
are the limits for b/t
width of the flange overhang depth of the web
outer diameter of the circular tubular section thickness of the plate
* This table is derived from BS 5950: Part 1.
SECTION 6: AXIALLY LOADED COLUMNS
6.1 Axial Compression Resistance of Columns
The axial load resistance of steel columns is governed by the type of cross section and the axis of buckling. Axially loaded columns having a slenderness ratio values
below are "stocky" and will fail by yielding across the entire cross section. For columns having values in excess of the following
computations are necessary. The choice of axis of buckling to obtain the design strength is not always clear, so calculations have to be canned out in respect of both principal axes and the lower value of load resistance chosen.
The design axial load resistance for a member subjected to axial compression is given
(Note that no calculations for is needed when as the column would fail by squashing at The compressive strength curves obtained for the various types of sections are shown in Fig 6.1.
Fig. 6.1 Compressive strength curves for struts for different values'of [For = 250 Mpa; Based on BS 5950: Part 1J
Table 6.1: Choice of appropriate values of
Welded Sections: for cross sections fabricated by welding of plates 20 N/mm2 should reduce the value of
Table 6.2 gives the ultimate compressive stress values in compression members corresponding to various values of and for Graphs (similar to Fig. 6.1) and Table 6.2 may be constructed for different values of using equations 6.1 to 6.6.
Table 6.2: Ultimate Compressive stress i
6.2 Effective Length of Columns
Designs of columns have to be checked using the appropriate effective length for buckling about both the strong and weak axes. Effective length, may be regarded as the equivalent length of a pin-ended column having the same cross section, which would be expected to have the same strength and stiffness as the column being designed. The recommended effective lengths for design purposes are given below
6.3 Cross Sectional Shapes for Compression Members and Built - Up Columns
When compression members are required for large structures like bridges, built-up sections will be used. Cross section shapes of rolled steel compression members and built-up or fabricated compression members are shown in Fig. 6.2 and Fig. 6.3. For preliminary calculations, approximate values of radii of gyration given in Fig. 6.4 for various built-up sections may be employed.
Fig 6.2: Cross Section Shapes for Rolled Steel Compression Members
( d ) Plated I Section (e) Built - up I Section
Fig 6.4: Approximate radii of gyration (Continued in next page)
Fig 6.4: Approximate radii of gyration
6.4 General Guidance for Connection Requirements
When compression members consist of different components, which are in contact with each other and are bearing on base plates or milled surfaces, they should be connected at their ends with welds or bolts. When welds are used, the weld length must be not less than the maximum width of the member. If bolts are used they should be spaced longitudinally at less than 4 times the bolt diameter and the connection should extend to at least times the width of the member.
When single angle discontinuous struts connected by a single bolt are employed, it may be designed for 1.25 times the factored axial load and the effective length taken as the centre-to-centre distance of the intersection at each end. Single angle discontinuous struts connected by two or more bolts in line along the member at each end may be designed for the factored axial load, assuming the effective length to be
0.85 times the centre to centre distance of the intersection at each end.
For double angle discontinuous struts connected back to back to both sides of a gusset or section by not less than two bolts or by welding, the factored axial load is used in design,with an effective length conservatively chosen. (A value between is chosen depending upon the degree of restraint provided at the ends).
All double angle struts must be tack bolted or welded. The spacing'of connectors must be such that the largest slenderness ratio of each component member is neither greater than 60 nor less than 40. Spacing of tack bolts or welds should be less than 600 mm. A minimum of two bolts at each end and a minimum of two additional connectors spaced equidistant in between will be required. Solid washers or packing plates should be used in-between.
For member thickness up to 10 mm, M16 bolts may be used unless otherwise noted. For members of large thickness M20 bolts may be used.
The following guide values are suggested for initial choice of members: (i) Single angle size: 1/30 of the length of the strut
(ii) Double angle size: 1/35 of the length of strut (iii) Circular hollow sections diameter = 1/40 length
6.5 Design Considerations for Laced and Battened Columns
The two channel constituents of a laced column, shown in Fig. 6.5(a) and 6.5 ( b ) have a tendency to buckle independently. The load that these tying forces cause may be assumed to cause a shearing force equal to 2.5% of axial load on the column. (Additionally if the columns are subjected to moments or lateral loading the lacing should be designed for the additional bending moment and shear). To prevent local buckling of unsupported lengths between the two constituent lattice points (or between two battens), the slenderness ratio of individual components should be less than 50 or 70% of the slenderness ratio of the built up column (whichever is less). In laced columns, the lacing should be symmetrical in any two opposing faces to avoid torsion. Lacings and battens are not combined in the same column. The inclination of lacing bars from the axis of the column should not be less than 40° nor more than 70°. The slenderness ratio of the lacing bars should not exceed 145. The effective length of lacing bars is the length between bolts for single lacing and 0.7 of this length for double lacing. The width of the lacing bar should be at least 3 times the diameter of the bolt. Thickness of lacing bars should be at least l/40th of the
length between bolts for single lacing and 1/60 of this length for double lacing (both for welded and bolted connections).
The slenderness ratio of battened columns shall be calculated using the following formula:
(6.7)
where, is lower value of slenderness of the individual vertical members between centre to centre of batten intervals and is slenderness of the overall column, using the radius of gyration of the whole built up section.
The imperfection factor is calculated from
(6.8)
The strength of the battened column is evaluated from
= effective slenderness with computed as given in Eqn. (6.8) = calculated using values given in Eqn. (6.7)
6.6 Base Plates for Concentrically Loaded Columns
For a purely axial load, a plain square steel plate or a slab attached to the column is adequate. If uplift or overturning forces are present, a more positive attachment is necessary. These base plates can be welded directly to the columns or they can be fastened by means of bolted or welded lug angles. These connection methods are illustrated in Fig. 6.6.
Fig. 6.6 Column base plates
A base plate welded directly to the columns is shown in Fig. 6.6 ( a ) . For small columns these plates will be shop-welded to the columns, but for larger columns, it may be necessary to ship the plates separately and set them to the correct elevations. For this second case the columns are connected to the footing with anchor bolts that pass through the lug angles, which have been shop-welded to the columns. This type of arrangement is shown in Fig. 6.6
When there is a large moment in relation to the vertically applied load a gusseted base may be used. If column base plates are insufficient to develop the applied bending moment or if thinner plates are used, some form of stiffening must be provided.
Concrete support area should be significantly larger than the base plate area so that the applied load can disperse satisfactorily on to the foundation. To spread the column loads uniformly over the base plates, and to ensure there is good contact between the two, it is customary not to grind or machine the underside of the base plate, but grout it in place.
Columns supporting predominantly axial loads are designed as being pin-ended at the base. The design steps for a base plate attached to an axially loaded column with pinned base are explained below.
Procedure for empirical design of a slab base plate for axial load only (pinned connection)
1. Determine the factored axial load and shear at the column base.
2. Decide on the number and type of holding down bolts to resist shear and tension. The chosen number of bolts is to be arranged symmetrically near corners of base plate or next to column web, similar to the arrangement sketched in Fig. 6.6.
3. Maximum allowable bearing strength = 0.4 (where = cube strength
ofconcrete) Actual bearing pressure to be less than or equal to 0.4 4. Determine base plate thickness
For channel, box or columns
but not less than the thickness of the flange of the supported column.
= pressure in on underside of plate, assuming a uniform distribution. = larger plate projection from column [See Fig.
6.7]
= smaller plate projection from column = design strength of mild steel plate, but not
greater than divided by
Fig. 6.7 Base plates subjected to concentric force
5. Check for adequacy of weld. Calculate the total length of weld to resist axial load.
6. Select weld size.
7. Check shear stress on weld.
8. Vector sum of all the stresses carried by the weld must not exceed the design strength, of the weld.
9. Check for bolt. Check maximum co-existent factored shear and tension, if any, on the holding down bolts.
SECTION 7: DESIGN OF MEMBERS SUBJECTED TO BENDING 7.1 General
The main failure modes of hot rolled beams of compact or plastic cross section are as follows:
• If the beam is prevented from buckling laterally, and the component elements are compact or plastic, then the failure will be triggered by excessive flexure and the collapse will follow the formation of plastic hinges. Such a beam is termed restrained beam".
• "Long beams" which are not suitably braced in the lateral direction will fail by a combination of lateral deflection and twist. These are termed "unrestrained beams".
• Fabricated plate girders may fail by web shear buckling or local buckling of a flange. This type of failure is unlikely to be encountered in hot rolled sections. • Local failure by (a) shear yield of the web. (b) local crushing of the web or (c)
buckling of thin flanges may sometimes be encountered. These are to be eliminated by provision of web stiffeners for (a) and (b) and the welding of additional flange plates to reduce the plate ratio, in the case of (c).
7.1.1 Laterally restrained beams
"Laterally Restrained Beams" are those, which will not fail by lateral instability. Lateral Instability or Lateral Torsional Buckling of beams can be prevented by providing full restraint to the compression flange of member. Adequate restraint may be regarded as being available if there is a positive connection of a floor or other construction fixed to the compression flange capable of resisting a lateral force of not less than 2.5% of the maximum factored force in the compression flange of the member.
The design adequacy of a laterally restrained beam is verified using the following criteria:
• lateral restraint force
• bending resistance of the cross section • shear resistance of the cross section
• combined bending and shear at locations where there are
(a) combinations of maximum factored bending moment and co-existent shear and
(b) combinations of maximum factored shear force and the co-existent bending moment.
7.1.2. The influence of local buckling of flanges and webs
In section 5, all rolled steel sections used as beams are classified in four ways in order to reflect the effect of local buckling of the beam elements.
• Slender - the elastic moment capacity of the cross section can NOT be attained
• Semi-compact - The elastic moment capacity of the cross section can be attained, but NOT the plastic moment capacity
• Compact - The plastic moment capacity can be attained, but the cross section has little rotation capacity.
• Plastic - as for compact, but there is sufficient rotation capacity in the cross section, so that the frame can be designed by plastic methods. Hot rolled sections used as beams are generally of the "plastic" or "compact" cross sections.
For the plastic or compact sections, the design bending resistance of the cross section is given by
Slender cross sections will not be able to resist a moment equal to the elastic moment resistance, as the maximum fibre stress at failure will be less than The design bending resistance in these sections is given by
7.1.3 Span of beams: The span of a beam should be taken between the effective points
of support.
7.1.4 Length of cantilevers: The length of a cantilever should be taken as the distance from the effective point of the support to the tip of the cantilever. 7.1.5 General conditions: All members in bending should meet the following
(a) At critical points the combination of maximum moment and co-existent shear,and the combination of maximum shear and co-co-existent moment should be checked at the ultimate limit state
(b) The deflection limits prescribed under "serviceability Limits" (Table 3.2) should be adhered to.
(c) Unless the compression flange has full lateral restraint, the resistance of the member to lateral torsional buckling should be checked in accordance with specifications detailed in 7.3 section
(d) Local buckling should be considered as given in Table 5.1.
(e) When loads or reactions are applied through the flange to the web, the conditions of 7.2.5 and 7.2.6 for web buckling and web bearing should be
met. 7.2 Shear
7.2.1 Plastic and compact sections
The design shear resistance, of a plastic or compact cross section is taken as
Where = shear area given by the following for the three cases: (a) Rolled and channel sections, load parallel to web
(b) Built-up sections and boxes, load parallel to webs (c) Solid bars and plates
Where
= thickness of the web
= Total depth of the section = depth of the web
= area of the plate or bar. 7.2.2 Elastic shear stress
In sections where webs vary in thickness or have holes significantly larger than those required for fasteners, the shear stress should be calculated from first principles assuming elastic behaviour.
7.2.3 Moment resistance with low shear load
Where the design shear force is less than 0.6 times the design shear resistance of the cross section the design moment resistance,should be taken as the value
obtained from
• Equation (7.1) for plastic and compact sections • Equation (7.2) for semi-compact sections and
• Equation (7.3) for slender sections
When the depth to thickness ratio, of a web exceeds where then it
should be checked for shear buckling in accordance with the requirements set out under Section 7.4.
7.2.4 Moment resistance with high shear load
Where the design shear force exceeds 0.6 times the design shear resistance, (defined in equation 7.4) the moment resistance, should be taken as follows. (a) For plastic or compact sections:
and is taken as follows:
For sections with equal flanges: the plastic modulus of the shear area,
For sections with unequal flanges: the plastic modulus of the gross section less the plastic modulus of that part of the section remaining after deduction of the shear area.
7.2.5 Web buckling
To prevent the web buckling under point loads or reactions (applied through the compression flange) the following check is required to be carried out on all beams
Fig. 7.1 Effective width for web buckling
If the applied load or reaction (as the case may be) exceeds suitable stiffness should be provided.
7.2.6 Web Bearing
For all beams, the web crippling resistance should also be checked at its junction with the flange to the flange-to-web connection at a slope of 1:2.5 of the plane of the flange. The buckling resistance in crippling, is given by
where = crippling resistance of the webin buckling
=design yield stress of the web
= length obtained by dispersion through the flange-to-web connection at a slope of 1:2.5 to the plane of the flange.
Fig. 7.2 Effective width of web bearing
If the applied load or the reaction exceeds the crippling resistance of the web, suitably designed bearing stiffeners should be provided.
7.2.7 Plastic and compact beams with web openings
Beams with web openings are frequently required for passing service ducts. Beams having (a) an isolated hole (b) a series of web openings at regular intervals are included in this guide.
When designing holes in webs, the following aspects should be kept in view: • The effect of bending
• The possible need to provide stiffening around the hole
• The effect of openings on slender webs (covered in the section 7.4) • The effect of opening on the stiffness of the section and deflections.
Unreinforced circular openings having a diameter not exceeding 10% of the web depth may be located within the web of compact beams without considering the net section properties, provided that
• the holes are located within the middle third of the depth and middle half of the span of the member.
• the load on the member is substantially uniform and no point loads are situated within a distance from the edges of the hole, equal to the depth of the girder.
• the spacing between the centres of any two adjacent openings measured parallel to the axis of the member is at least 2.5 times the diameter of the larger opening.
• the factored maximum shear at the support does not exceed 60% of the shear resistance of the section.
When the hole diameter exceeds 10% of the depth of the girder, or if any of the above conditions are not satisfied, the net section properties should be computed and the adequacy of the design should be verified.
If web reinforcement is provided, it may be either around the hole or as a flat reinforcement carried past the opening for such a distance that the local shear stress due
to the load being transferred from the reinforcement does not exceed 7.3 Laterally Unrestrained Beams of Plastic and Compact Sections 7.3.1 Lateral torsional buckling of symmetric sections
The elastic critical moment resistance of a symmetrical I beam subjected to equal end moments undergoing lateral torsional buckling between points of lateral support is obtained as
Comparing the two cases covered by Eqns. (7.6) and (7.7) the ratio of the tw constants
is often termed "the equivalent uniform moment factor" Its value is a
direct measure of the severity of a particular pattern of moments relative to the basic case. This is clear from Fig.7.3. Several factors affect the lateral stability of beams and these are outlined below:
( a ) Support conditions
Lateral buckling involves three kinds of deformations, namely lateral bending,
twisting and warping. Various types of end conditions are consequently possible but
the supports should either completely prevent or offer no resistance to each type of deformation (Solutions for partial restraint conditions are complicated). The effect of various support conditions is taken into account by way of a parameter called effective
length. For a beam with simply supported end conditions and no intermediate lateral
restraint, the effective length is equal to the actual length between the supports. The effective length factor would indirectly account for the increased lateral and torsional rigidities provided by the restraints. As an illustration, the effective lengths appropriate for different end restraints according to BS 5950 are given in Tables 7.1 and 7.2.
( b ) Level of application of transverse loads (Stabilising and destabilising loads)
The lateral stability of a transversely loaded beam is dependent on the arrangement of theloads as well as the position of application of the loads with respect to the centroid of thecross section. A load applied above the centroid of the cross section causes an additional overturning moment and becomes more de-stabilising than when the same load is applied at the centroid. On the other hand, if the load is applied below the centroid, it produces astabilising effect.
Table 7.1 Effective length of beams of Compact Plastic Cross section between supports
(c) Influence of the type of loading
So far, only the basic case of beams loaded with equal and opposite end moments has been considered. But, in reality, loading patterns would vary widely from the basic case. Cases of moment gradient, where the end moments are unequal, are less prone to insiability and this beneficial effect is taken into account by the use of "equivalent uniform moments". In this case, the basic design procedure is modified by comparing the elastic critical moment for the actual case with the elastic critical moment for the
basic case. The equivalent uniform moment is defined as
where m = equivalent uniform moment factor and bending moment.
Fig. 7.3 Equivalent uniform moment
( d ) Slenderness correction factor ( n )
For situations, where the maximum moment occurs away from a braced point, e.g. when the beam is uniformly loaded in the span, a modification to the slenderness, may be used. The allowable critical stress is determined for an effective slenderness, where
n is the slenderness correction factor, as illustrated in Fig.7.4 for a few cases of loading.