Structural Design In Steel (introduction)

CHAPTER 1.1

1

INTRODUCTION

GENERAL

Of all structural building materials in use today, steel is perhaps the most universally acceptable as a versatile material for engineering construction. This, of course, is the result of its many fine qualities so eminently suited to modem engineering structures. Other materials may also possess some of these qualities in varying measures. However, their undesirable qualities often outweigh their better characteristics. Timber, for example, is easily workable, pleasing in appearance and fairly strong when properly laminated by moden techniques. Nevertheless, it is far inferior to steel in strength, toughness, durability and versatility, needs special—and therefore expensive— connections, is extremely difficult to connect to other structural materials such as steel and reinforced concrete and is prone to attacks by the elements and termites unless suitably protected with expensive chemical treatments. Concrete, on the other hand, is very strong in compression, lends itself well to diverse and demanding architectural expressions, can be readily moulded to desired shapes and sizes and is frequently more easily available — and at a lesser cost — than steel. However, it is extremely weak in tension and, therefore, has to be reinforced with steel for most structural purposes. Moreover, it has to be poured in forms which need to be adequately, though temporarily, supported until it hardens and acquires adequate strength. Steel does not suffer from many of the disadvantages of timber and concrete enumerated above. Instead, it is characterized by the following desirable qualities: ength: Steel is equally strong in tension and compression as long as its tendency 1. High Str Strength: to buckle under compressive loads is adequately controlled. Shapes strong in torsion can be easily fabricated—both in the fabrication shop as well as in the field—from basic steel shapes such as angles, channels, plates, etc. Obviously the high strength of steel means less dead weight of the structure since smaller and lighter sections may be used to satisfy the structural design criteria. This, in turn, means economy in steel construction and ease of handling and fabrication. Since material in typical steel construction accounts for only about 30% to 40% of the total cost of the project whereas fabrication and installation account for the rest, the high strength of steel can, more often than not, lead to substantial savings in the project cost. 2. Elasticity: Steel is elastic in its behaviour and follows Hooke’s law upto its limit of proportionality. It satisfies the assumptions usually made in structural analysis and design more closely than other materials and, therefore, its behaviour under applied loads can be predicted and accounted for with a very good degree of precision and certainty. 3. Ductility: A material which can withstand large deformations under applied forces without failure is said to be ductile. Steel is highly ductile so that steel structures have a high 1

2

STRUCTURAL DESIGN IN STEEL

reserve of strength beyond their normal working loads. Their behaviour under over-loads is characterized by large and highly visible deformations, absence of abrupt and brittle failures and a redistribution of stresses from areas of overstress to those with lesser stress. In other words, even badly designed steel structures give sufficient and clearly visible warning before they fail unlike, for example, similar concrete structures which are prone to sudden and catastrophic failure. This aspect of the behaviour of steel under loads is more fully explained in Chapter 12 (Plastic Analysis and Design). 4. W Workability: orkability: Steel can be sheared, punched, drilled, ground and welded, etc., with ease and a high degree of precision. Its versatility and adaptability to the demands of modern structural engineering practice is a direct result of its workability. Steel structures are often fabricated in shops and are then transported to the job site for installation where modern means of field erection e.g., high strength bolting, welding etc., contribute to the overall economy of time and money and guarantee good quality of the structure. 5. Permanence: When properly painted, galvanized, or otherwise maintained, steel structures last for a long time. Some of the newer steels developed abroad do not require any painting or maintenance at all. They develop a thin coating of rust over a period of time and cease to rust thereafter with the initial coating of rust acting as a protective barrier against further corrosion and deterioration. 6. Adaptability: Perhaps one of the strongest advantages of steel as a structural material is the relatively small labour and expense involved in connecting it to existing structures of steel as well as of other materials. This quality—so notably absent in other materials—has been extensively demonstrated for steel in modifications and expansions of buildings, bridges and even aircraft. 7. Scrap V Value: alue: Steel has considerable scrap value. Whereas a demolished concrete structure is literally a total waste, a demolished steel structure can be sold as scrap and thus translates into monetary savings. The advantages of steel described above far outweigh the few disadvantages it may seem to possess, e.g., the need to paint and maintain steel structures—particularly in corrosive environments—and its decreased strength when exposed to fire. Naturally, therefore, steel finds innumerable structural uses ranging from buildings to bridges, transmission towers to power plants, airplanes to ships, etc. Some of the structures in daily use simply cannot be built of any other material. The diversity and complexity of steel structures in use today makes it impossible for all of them to be covered in one volume. The principles of structural design in steel are, perhaps, best explained with the help of the most common of all steel construction, i.e., buildings. This book would, therefore, initially present the analysis and design of steel members and systems commonly used in modern building construction. It would, thereafter, extend this process to the analysis and practical design of other common steel structures.

1.2

ANALYSIS AND DESIGN

The function of all structures is to withstand stresses due to applied loads, wind, earthquake, temperature changes, shrinkage, etc., without failure or undue distress such as excessive deflections, dangerous vibrations, etc. Buildings, for instance, support loads from people, furniture, machinery and wind, bridges carry loads from wind and traffic in addition to their own weight;

INTRODUCTION

3

transmission towers are subjected to such diverse forces as weight of cables, wind on cables, stringing—pull during erection, etc., and aircraft have to withstand live loads, dynamic forces of wind and acceleration. All of them must necessarily support their respective imposed loads safely and economically. The task of the structural engineer is to propose a suitable system for the specified purpose and loads, to examine its overall stability and, finally, to assess its structural viability for the applied loads. The term Structural Design, therefore, signifies a process by which a structural engineer puts together a functionally efficient, economically affordable and structurally safe system for a set of given applied loads. This process ordinarily involves several steps: 1. Planning: As a first step, the engineer has to plan a functional, buildable, economical and aesthetically pleasing layout of the structural systems (floor plans, stairs and elevators, wind bracing, etc.) that go into the making of the total project. He has, of necessity, to take into account such factors as the available physical space, dimensional clearances required around and between the component systems, openings and penetrations (e.g., ducts, conduits, doors, windows, stair wells, etc.), types of materials, fabrication and erection processes and shapes most suitable for the system and last, but not least, the overall relationship of individual systems to the parent structure. 2. Loads: A decision as to the applicable loads (nature, magnitude, disposition, etc.) and their various combinations which the structure may be reasonably expected to experience during its service life has to be made next. The designer may choose to use such load combinations as are most likely to cause high stresses in the structure. He may, similarly, discard loads which are statistically not likely to occur during the life of the structure. For example, a 200 km per hour wind likely to occur once every fifty years may not be used to design a temporary warehouse but may have to be used in the design of a high-rise apartment complex in the same geographical area. 3. Analysis and Design: Whereas structural analysis involves the calculation of forces, moments, deformations, etc., in the structure produced by the applied loads, design involves the selection and proportioning of its various structural components (i.e., beams, columns, trusses, bracing, foundations, etc.) to satisfy requirements not only of structural strength and soundness, but also of public safety, economy, ease of fabrication and erection. Thus, it would simply be bad design if the structure was too difficult or too costly to build. It would, of course, be entirely unacceptable if it was structurally inadequate and thus jeopardized human life or property. Whereas analysis involves familiarity with the various methods of analysis and their underlying assumptions and limitations, the process of design involves translating the results of analytical computations into a practical structure. It may be seen from the above discussion that structural design does not involve merely the selection of suitable shapes and sizes of steel members from a handbook but represents a complex process. Inasmuch as literally thousands of components may go into the final making of a complete structure and since all of them have to be put together properly to function effectively as intended by the designer, important considerations other than mere shape selection inevitably become a vital part of this process. A few of these factors are discussed below by way of clarification and emphasis: (i) The designer needs to be closely familiar with the availability and cost of steel shapes in the market. He should avoid specifying shapes and sizes which may not be readily available or may be too costly to buy. A special rolling of specified shapes not otherwise available or their transportation from distant locations may add unnecessarily to the overall cost of the project. Other things being equal, commonly available shapes and sizes are economical and should be used.

4

STRUCTURAL DESIGN IN STEEL

(ii) Since steel is bought by weight, it may be thought that the lightest sections which fulfill the functional and structural design criteria would also prove to be the most economical. However, this may not always be true because structural members need to be connected together in the field to form a complete structure. The labour and cost of physically connecting a very large number of widely differing member sizes at hundreds of places may become prohibitively large. A good designer would, therefore, select such sizes as would facilitate efficient and simple detailing and fabrication in the shop and speedy connections in the field even though some of the members may have to be somewhat oversized. (iii) A sound knowledge of shop and field fabrication practices, tolerances, erection machinery and procedures as also their limitations is also highly desirable. The transport facilities available, the maximum lengths of members that can be fabricated and transported to the site, the maximum clearances under the bridges, power lines, etc., enroute from the shop to the field need careful consideration in designing structures of any size and complexity. Obviously, fabricating a massive component in the shop cheaply serves no useful purpose at all if it cannot be shipped to the site on available transports or if it can not be erected due to the absence of suitable machines or skilled labour. Similarly, designing a system at a low cost but having to fabricate it at an unduly large price is nothing more than an exercise in futility. (iv) The designer needs to be conscious of the future maintenance and upkeep of the structure. Areas and pockets likely to collect water and dust should be avoided. Simple details and connections designed for easy accessibility for cleaning and painting and adequate clearances contribute towards a sound design and a better performance as well as a long life of the structure. A good design, therefore, is a judicious combination of numerous factors not the least important of which is the so-called ‘engineering judgement’ which consists of a sound theoretical background, long and diverse practical experience, and a fair measure of common sense. A complex design, thus, is not necessarily a good design. In fact, a simple concept may often lend itself to a better overall design. It needs to be emphasized at this time that structural design is not an exact science and necessarily involves a fair amount of trial and error as well as lessons learnt from past experience on the part of the designer. There are simply no unique solutions to design problems so that the same structure designed by two engineers independently may differ radically in essential as well as secondary details. Since, for example, the theoretical assumptions forming the basis of analytical procedures are more often than not less than true in practical structures, design assumptions made by two engineers may differ sufficiently to affect their final designs substantially. To cite an example: whereas one designer may choose to consider a member as fully fixed at its supports the other may assume it to be either simply supported or only partially fixed. This difference in approach may change the selected member size and, more importantly, the end connection details. Perhaps the most important reason for this state of affairs is the fact that actual loads on structures are, at best, only approximately known; at times they may indeed be no better than mere guesses. Whereas, the dead weight of a structure may possibly be estimated to within 10 to 15% of its actual value, the determination of the live load, the wind pressure, and the seismic forces and particularly their directions and disposition at any given time (or even their maximum values ever) with any accuracy is an almost impossible task. It always involves such factors as an educated guess based on past history or statistical data, engineering judgement, the importance of the structure and the nature of the possible loss and damage in the event of its partial or total

INTRODUCTION

5

collapse. The difficulty inherent in any effort to quantify such imponderables is obvious. In the final analysis, a well designed structure should satisfy the following general criteria: (a) Functionally it must fulfill the specific needs of the structure efficiently. A hospital, for instance, must function as a hospital in terms of the practical layout of its various components, facilities (i.e., administrative offices, patient wards, diagnostic laboratories, etc.), efficient movement of men and material, noise control and ease of maintaining a sanitary environment. (b) Structurally it must be strong, safe, sound, and durable in order that it may give a fair return for the investment of time and money in it over its life span. Moreover, it must have built in flexibility and allowances for future expansions and modifications. (c) Economically it must be affordable not only in its first cost but also in its ongoing maintenance during its service life. This may be affected by a simple but efficient layout, selection of suitable building materials, proper fabrication and by tailoring the construction to the available means and facilities. (d) Aesthetically it must be pleasing to the eye both externally as well as internally. The engineer and the architect must, therefore, work together closely in every phase of design development.

1.3

LOADS

As has already been discussed briefly in Sec. 1.2, the determination of design loads for a structure is a difficult problem and calls for a high degree of judgement on the part of the engineer. He must decide the types and magnitudes of loads and their likely dispositions, assign relative importance to them and then apply them judiciously to the structure for the worst possible effect. He may, for instance, choose to use a light wind pressure or a light snow load for a temporary structure since the statistical probability of a high wind or a severe snow during its short service life may not be significant enough to warrant assuming larger loads. Similarly, he may decide to use increased allowable stresses for certain structural members subjected to abnormally high loads for a short time such as occur during the removal of heavy equipment for servicing in power plants. On the other hand, he may opt to design certain other members for large fictitious loads on the assumption—probably based on past experience with similar structures—that service crews may use them for lifting and moving heavy objects during maintenance or in order to provide for similar unforeseen loads in the future. Needless to say, no guidelines can possibly be given for such unorthodox, though entirely practical, situations. There is simply no substitute for sound engineering experience. The following is a brief discussion of some of the loads commonly encountered in structural design: 1. Dead Loads: As the name implies, these are gravity loads and consist not only of the self weight of the structure itself but also of the weights of such immovable objects and attachments as do not change their positions with time. Floor and roof systems, electrical conduits, air conditioning ducts, piping, plumbing and lighting fixtures, fire proofing, etc., qualify as dead loads. Some of these loads may be known with reasonable accuracy (e.g., weights of various fixtures from the manufacturers’ catalogues) while others may have to be estimated on the basis of some empirical thumb rules or, more commonly, past experience. All of them may be calculated quite accurately after the structure has been completely designed, at which time these weights should be compared with the originally assumed values and the design revised to the extent that the differences between the two sets of weights may demand.

6

STRUCTURAL DESIGN IN STEEL

Table A-l in Appendix A lists the dead weights of common building materials and may be used to estimate design dead loads. 2. Live Loads: Gravity loads that are time dependent, i.e., that can vary in magnitude and location with time are termed live loads. These include occupants, cranes, furniture, stored goods, etc. It is very difficult to estimate/design live loads and their critical dispositions for various types of buildings to any degree of precision. In order to standardize structural design everywhere in the country and also to minimize legitimate public concern for safety and structural adequacy of buildings, such loads are usually prescribed through standards and guidelines by competent government agencies. Bureau of Indian Standards formerly known as The Indian Standards Institution (I.S.I.) recommends minimum design live loads for different types of buildings as listed in Table A-2 of Appendix A. These loads reflect the opinion of a large number of engineering experts, are conservative and are based on their collective experience with similar structures all over the country. It should be remembered, however, that these loads do not represent the results of any calculations. Partitions in office buildings are considered live loads since they can be relocated at any time as required. In office buildings, the floor and its supporting members shall be designed for an additional uniformly distributed load per square meter of not less than 33% of the weight per meter of finished partitions subject to a minimum of 100 kg per square meter, provided that the total weight of the partition walls per square meter of the wall area does not exceed 150 kg per square meter and the total weight per meter length is not greater than 400 kg. Since the live loads are movable by their nature, the designer may place them in such locations and configurations on the structure (for instance alternate bay loading, partial loading, etc.) as may cause the worst desired effects (bending moments, shear forces, deflections, etc.) in it. Thus, for example, the disposition of loads required for maximum bending moment in a beam may not necessarily produce the maximum shear force also. Similarly, since the probability of an entire building floor being fully loaded with its design load over its entire area—and specially of all floors in a multi-storey building being similarly loaded simultaneously—is small, live loads are allowed to be reduced in certain types of buildings by the specifications as shown in Table A-3 of Appendix A. This provision may often translate into a considerable reduction in loads with a resultant economy in the design of columns and foundations which support large floor areas in such buildings. 3. Impact Loads: Certain types of live loads subject structures to vibratory or dynamic effects called Impact. Traffic on bridges, cranes on craneways in industrial buildings and machines in power plants are examples of such loads. They have two types of effects on supporting structures. Firstly, they impart a vertical impact to them by virtue of their rapid and sometimes abrupt movement. This is more or less equal in its effects on the structure to a suddenly applied vertical load and, consequently, causes greater stresses in supporting members than would be produced through a static application of the same load. This effect is taken into account in the design process by increasing the moving load by a so-called Impact Factor and calculating the stress accordingly in lieu of complex and time consuming dynamic analysis. Secondly, the moving loads subject the supporting structure to either transverse or longitudinal forces because of their sudden starts, braking, etc. Although such forces are small in magnitude their effects on structures can be significant in that they may cause supporting members to bend about their weak axes. This effect of moving loads is usually accounted for by specifying a certain percentage (usually 10%) of these loads to be applied to the steel structure in specific locations and directions. Impact loads for crane and gantry girders shall be discussed at appropriate places as and when necessary. Table 11.5 lists the Impact Factors to be used in steel design as a percentage of the applied vertical loads.

INTRODUCTION

7

ind Loads: All exposed structures are subjected to wind loads of varying magnitudes. 4. W Wind However, wind effects become structurally significant only for tall structures and multistorey buildings. The determination of the design wind pressure for a structure is a complex problem depending, as it does, on such factors as the geographical location of the structure, the meteorological history of the area, the shape and height of the structure the size and disposition of its openings, if any, (i.e., doors, windows, etc.) and many other variables. Wind pressures are usually linked to wind velocities measured over a statistically acceptable period of time through empirical formulas. Where extensive and reliable records of wind velocity in the area are not available, estimating wind loads becomes pure guess work on the part of the designer. Wind exerts a positive pressure on the windward side of a building and a negative pressure or suction on its leeward side. It may subject the roof also to either positive or negative pressures. Moreover, if its openings are primarily on the windward side with the leeward side relatively closed to the wind, the building may be subjected to an additional internal pressure trying to “puff’’ the building outwards. If, on the other hand, the openings are on the leeward side an internal suction may be produced. Again, at times, certain parts of the building or the entire building may be shielded from the effects of wind by adjacent taller structures. All of these factors complicate the problem of wind pressure evaluation greatly. The designer should, therefore, use the design wind loads prescribed by the Bureau of Indian Standards. Further details of wind loads shall be given at appropriate places as necessary. 5. Seismic Loads: Although earthquakes can occur anywhere anytime, certain regions lying within the so-called seismic belts are more prone to them than others. Structures in such regions are required to resist seismic forces. There is, of course, no such thing as an earthquake proof structure. Seismic design is only an earthquake resistant design. Earthquakes subject structures to horizontal and vertical forces which depend on the location of the structure, its dead weight, its stiffness, soil characteristics, etc. These effects are allowed for by applying to the structures seismic forces as a specific percentage of their dead weights along their main axes. The designer should, therefore, use seismic forces prescribed by the Bureau of Indian Standards. Further details of seismic loads shall be given at appropriate places as necessary. 6. Miscellaneous Loads: Other loads which are caused by temperature changes, shrinkage, handling, etc., may sometimes be high enough to warrant careful consideration. Structures are sometimes subjected to loads during their erection which may far exceed the normal design loads for them. The contractor may, for instance, store huge quantities of steel, bricks and other materials on floors for easy access and speedy construction. Such practice should be guarded against and, if considered unavoidable, should be adequately provided for in the design. Floors and beams may also undergo extreme distress e.g., cracking and buckling and even failure if high temperature and shrinkage stresses are not dissipated through proper and acceptable means such as expansion joints.

1.4 FACTOR OF SAFETY Steel structures are usually so proportioned that the maximum stress produced by the design loads anywhere does not exceed a prescribed value called the Design Stress (also called Allowable Stress), which is well below the yield point of steel. The difference between the design loads (i.e., the loads that cause the maximum stress anywhere in a structure to equal the allowable stress) and the yield loads (i.e., the loads that produce yield stress at the point of maximum stress

8

STRUCTURAL DESIGN IN STEEL

in the structure) represents a reserve of strength for the structure. Simply stated it means that the structure can withstand loads considerably in excess of its actual design or service loads without failure. The ratio of the actual capacity of the structure to its maximum calculated stress under working loads is defined as its Factor of Safety. Since the yield-stress determines the capacity of a steel structure for all practical purposes, the factor of safety is given by FS =

Py Pw

...(1.1)

where

FS = Factor of safety Py = Yield stress of steel Pw = Allowable stress in steel. It should be emphasized at this point that a structure does not necessarily collapse if it develops the yield stress at one or two points. A redistribution of stresses occurs within the body of the structure with the less stressed regions picking up progressively increasing loads as the overstressed regions continue to yield. In structural parlance a system is said to have failed when it ceases to perform its intended function satisfactorily. Thus an excessive deflection of a roof system causing the ceiling plaster to crack or water to collect on it, buckling of compression members in a bracing system, excessive wind induced vibrations of latticed structures may all signify failure of these structures. A well designed structure has a large amount of reserve strength even beyond its elastic limit and gives sufficient and clear warning through large deflections, cracking, buckling, component failure, etc., before its total collapse. It may well be asked as to why a structure should be designed to have extra strength beyond its working loads. The fact of the matter is that the factor of safety merely reflects the realities of life for a structure and is essential for a number of reasons: 1. The physical properties of steel change with time (e.g., due to rusting and corrosion), temperature variations, shop and field work (e.g., welding, drilling, grinding, etc.) and fatigue. 2. The actual field conditions of loads, end supports, etc., are never the same as are assumed in theoretical analysis and design. 3. The design loads are not known to any appreciable degree of accuracy and some of them are mere guesses on the part of the designer. 4. Poor or careless workmanship, accidental overloads, acts of nature (storms, high winds), stress concentrations etc., are extremely hard to anticipate and evaluate. 5. Erection loads cannot always be anticipated and provided for in the design. Unknown erection or other loads can possibly be taken care of by the factor of safety.

1.5 ELASTIC DESIGN AND PLASTIC DESIGN 1.5.1 General Currently there are two accepted methods of structural steel design in common use, i.e., the Elastic Design (or Working Stress Design) Method and the Plastic Design (or Limit Design) Method. The elastic design method is based on the premise that the maximum stress anywhere in a structure subjected to its working loads should not exceed a pre-designated allowable stress

INTRODUCTION

9

value. This value is obtained by dividing the steel yield stress with a factor of safety {see Eqn. (1.1)}. Specifications list values of different types of allowable stress (i.e., bending, shear, bearing, etc.) without indicating the inherent factors of safety (See Table 1.2). Elastic design is by far the more common of the two methods and forms the subject matter of this book. In the plastic design method the service loads for the structure are estimated and then multiplied by a set of so-called Load Factors to obtain Ultimate Loads. The structure is then analysed and designed on the premise that its maximum stress under the application of the ultimate loads should not exceed the steel yield stress. This method would be briefly presented in chapter 12 on Plastic Analysis and Design.

1.5.2 Methods of Design IS-800 (Code of Practice for General Construction of Steel) recognizes the following as acceptable methods of structural steel design: (1) Simple Design: This method assumes the end-connections of structural members to be simply supported so that they do not exert any restraining moment on the members. This method is based on the following assumptions: (a) All members are simply supported at their ends. (b) All end-connections are virtually flexible with no restraining moments and are to be designed for applicable shear forces only. (c) Compression members are subjected to forces applied at applicable eccentricities and with applicable effective lengths. (d) Tension members are subjected to longitudinal forces applied on their net area of cross sections. (2) Semi-Rigid Design: This method assumes partial fixity at the ends of the structural members in an amount somewhere between a pin (or simply supported) connection and full fixity (i.e., zero relative rotation between members at the joint). An experimental investigation is required in order to show that the stress in the structure remains within the allowable stresses prescribed by the codes. This method is, therefore, not very practical for obvious reasons. (3) Fully Rigid Design: This design method is based on the premise that the end connections are rigid enough to hold the original angles between the connecting members, or the members and the end-supports, virtually unchanged. This translates into a restraining moment at such connections with a resultant decrease in the moment in members which, in turn, leads to economy of material in the structure. Since nothing in the real world conforms to the ideal conditions assumed in design, virtually all connections are semi-rigid in that their behaviour lies somewhere between pin-connections and fixed-joints. Even the so-called simply supported connections offer some moment restraint to the members and the so-called fixed joints do allow a certain amount of joint rotation. In all methods of design, the stresses are required to conform to the limits prescribed by the Indian Standard Specifications.

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1.6

STRUCTURAL DESIGN IN STEEL

STRUCTURAL STEEL SECTIONS

Steel members are hot rolled in steel mills under strictly controlled conditions in different shapes, e.g., I, channel, tee, angles, etc. These shapes—either singly or in suitable combinations—are selected for use as structural members. Table 1.1 lists the various shapes and also gives their standard designations. Appendix B gives a systematic listing of such properties of structural steel members as are commonly used in design and detailing of structures. These Steel Tables would be used extensively as aids to design in this book. TABLE 1.1 Shape

Designation

Remarks

ISJB ISLB ISMB ISWB ISHB

Beam Sections: Referred to as, for example, ISWB 350 @ 56.9 kgm. The figure 350 signifies the depth in mm.

ISJC ISLC ISMC

Channel Sections: Referred to as, for example, ISMC 200 @ 22.1 kgm. The figure 200 signifies the depth in mm.

ISA

Angles (equal leg or unequal leg) referred to as ISA 65 × 65 × 8 where 65 mm is the leg dimension and 8 mm its thickness.

ISNT ISHT ISST ISLT ISJT

Tee-Sections: Referred to as, for example, ISNT 60 @ 5.4 kgm. The figure 60 signifies the depth in mm.

a mm sq bar

aφ bar

Note:

Square Bars: Referred to as a mm square bar.

Round Bars: Referred to as aφ bar where a signifies its diameter (φ) in mm.

A square bar may also be referred to as ISSQ e.g., ISSQ 80 will mean a square bar of 80 mm size. Similarly, a round bar may also be referred to as ISRO e.g., ISRO 100 will mean a round bar of 100 mm diameter.

INTRODUCTION

1.7

11

SPECIFICATIONS

Design specifications are a set of guidelines and recommendations for safe structural design. They are put together by committees of experts with vast and varied experience and represent the best available information on the state of the art of design. Inasmuch as each designer works differently and, since he cannot be expected to have encountered all possible structural problems in his career, the specifications standardize the design process as far as possible by prescribing design loads, allowable stress, acceptable methods of analysis, tolerances, shop and field practices, etc. In effect, they make many of the key decisions for the designer and help him produce a safe and conservative design. Another important objective of the specifications is to alleviate legitimate public concern and ensure safety by requiring strict adherence to safe and accepted practices. It is important that the designer understands and uses these specifications intelligently. The following is a partial list of Indian Standards and Specifications used in steel design. They are originated by the Government of India’s organization: Bureau of Indian Standards. IS IS IS IS IS IS IS IS IS

: : : : : : : : :

226 961 812 813 800 806 875 277 807

Structural Steel. High Tensile Structural Steel. Glossary of Terms Relating to Welding and Cutting of Metals. Scheme of Symbols for Welding. Code of Practice for General Construction in Steel. Use of Steel Tubes in General Building Construction. Code of Practice for Structural Safety of Buildings, Loading Standards. Galvanized (Plain and Corrugated) Steel Sheets. Design, Manufacture, Erection and Testing of Cranes.

1.8 PROPERTIES OF STRUCTURAL STEEL As has already been mentioned in Sec. 1.1, steel is widely used as a structural material because of its numerous desirable qualities. Two of the more important characteristics, i.e., elasticity and ductility, are perhaps best explained with the help of the well known stress-strain curve [Fig. 1.1] of a specimen of structural mild steel subjected to a gradually increasing axial tensile force. As the applied force is gradually increased with a corresponding increase in the material stress, the unit strain also increases proportionately so that the stress-strain curve is a straight line (Hooke’s Law) upto a point A beyond which the strain ceases to be linearly proportional to the stress. Point A defines the Proportional Limit of steel. As the stress continues to increase, the curve undergoes a profound change slightly above the point A. This is marked by a large change in the strain value without a corresponding change in stress. The curve flattens to a horizontal line and the material ‘flows’ as the stress remains constant at the point B which defines the Yield Point of steel. This ‘plastic’ stage of the curve continues to a point C at which time the curve again starts on an upward trend which is no longer linear. It peaks out at the point D, the material develops a ‘neck’ (i.e., starts to elongate rapidly locally thereby resulting in a rapid reduction in its area at that point), the curve drops down until the specimen breaks at the point E on the curve. The following states may be seen in the behaviour of steel described above:

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STRUCTURAL DESIGN IN STEEL

Elastic Plastic Strain hardening D

E

A Stress, I

B

O A P r

C

Unload F

F

Strain, A Fig. 1.1

(a) The Elastic Stage from the origin to the point B signifies the domain in which Hooke’s Law is applicable. The working or allowable stresses in steel structures are constrained to remain within this domain regardless of the method of analysis and design used. If the load is gradually decreased from anywhere in this region, the curve retraces itself back to the origin with no permanent residual strain. (b) The Plastic Stage extends from the point B to the point C. Having become plastic, the steel is capable of deforming substantially at the same applied load. If the specimen is gradually unloaded from anywhere in this region, the resultant curve is a straight line parallel to the line OB, but intersects the strain axis at a point P (see the dotted line in Fig. 1.1), which means that the removal of the load in the plastic stage leaves the specimen with a permanent “set” or strain. (c) The strain hardening stage of the curve starts at the point C at which time the material actually starts gaining strength on account of internal molecular changes brought about by its rapid elongation. It may be seen that the ultimate strength of the material is much higher than even its yield point. The difference between the two may be looked at as reserve strength which it possesses beyond the point at which it starts to yield. The structural mild steel commonly used in India conforms to Indian specifications IS : 226 and is designated as St 42-S. Two types of high strength steel conforming to IS:961 and designated as St 58-HT and St 58-HTW are also available although their use is limited to structures where their higher strength and superior resistance to corrosion are desirable. Whereas the use of high strength steels does mean a reduction in the dead weight of the structure, they are considerably costlier than structural mild steel. The relevant Indian specifications may be consulted for more information about them. This book would limit itself to St 42-S steel exclusively. Table 1.3 lists various properties of the structural steels mentioned above. The high strength steels are included in the table for comparison with the mild steel.

13

INTRODUCTION

TABLE 1.2: Allowable Str esses in Str uctural Steel Stresses Structural No.

Type of Strees

1 2 3 4 5 6 7

Axial Tensile Stress Axial Compressive Stress Flexural Tensile Stress Flexural Compressive Stress Average Shear Stress Maximum Shear Stress Bearing Stress

Symbol

Allowable Stress

Pt Pc Pbt Pbc Pq(avg) Pq(max) Pbrg

0.6 P 0.6 P 0.66 P 0.66 P 0.4 P 0.45 P 0.75 P

TABLE 1.3: Mechanical Pr operties of Str uctural Steels Properties Structural

Type

IS : 226 (St 42-S)

Steel Product

Nominal Thickness (mm)

Guaranteed Minimum Yield Stress (kg/mm2)

Tensile Strength Minimum (kg/mm2)

Plates, Sections and Flats

6 to 20 21 to 40 Over 40

26.0 26.0 23.0

42 42 42

10 to 20 Over 20

26.0 24.0

42 42

6 to 28 29 to 45 46 to 63 Over 63

36.0 35.0 33.0 30.0

58.0 58.0 58.0 55.0

6 to 16 17 to 32 33 to 63 Over 63

36.0 35.0 34.0 29.0

55.0 55.0 52.0 50.0

Bars (round, square, hexagonal) other than rivet bars.

IS : 961 (St 58-HT)

IS : 961 (St 55-HTW)

Plates, sections, flats and bars other than rivet bars.

Same as above

GGG